United StatesDepartment ofAgriculture

ForestService

ForestProductsLaboratory

Madison,Wisconsin

AgricultureHandbookNo. 188

Dry KilnOperator'sManual

Dry KilnOperator’sManual

Edited byWilliam T. Simpson,Research Forest Products Technologist

United States Department of AgricultureForest ServiceForest Products Laboratory 1

Madison, Wisconsin

Revised August 1991

Agriculture Handbook 188

1The Forest Products Laboratory is maintained in cooperation with theUniversity of Wisconsin.

This publication reports research involving pesticides. It does not containrecommendations for their use, nor does it imply that the uses discussed herehave been registered. All uses of pesticides must be registered byappropriate State and/or Federal agencies before they can be recommended.

CAUTION, Pesticides can be injurious to humans, domestic animals, desirableplants, and fish or other wildlife-if they are not handled or appliedproperly. Use all pesticides selectively and carefully. Follow recommendedpractices for the disposal of surplus pesticides aand pesticide containers.

Preface Acknowledgments

The purpose of this manual is to describe both the ba-sic and practical aspects of kiln drying lumber. Themanual is intended for several types of audiences.First and foremost, it is a practical guide for the kilnoperator-a reference manual to turn to when questionsarise. It is also intended for mill managers, so that theycan see the importance and complexity of lumber dry-ing and thus be able to offer kiln operators the supportthey need to do their job well. Finally, the manual isintended as a classroom text-either for a short courseon lumber drying or for the wood technology curricu-lum in universities or technical colleges.

This manual is a revision of the 1961 edition byEdmund F. Rasmussen. Forest Service staff whocontributed to that original edition were Raymond C.Rietz, Edward C. Peck, John M. McMillen, Harvey H.Smith, and A.C. Knauss. It is a credit to these menthat the 1961 edition has been in wide use and demandfor the past 28 years. It is also to their credit that eventhough the manual is out of date in many parts, wewere able to use the basic framework of the originaledition to build on.

The Forest Products Laboratory staff involved in thisrevision were William T. Simpson (who wrote the intro-duction and had overall responsibility for coordination),R. Sidney Boone, James C. Ward, and John L. Tscher-nitz. Each person was responsible for revising certainchapters or parts of chapters. This assignment of re-sponsibilities is indicated on the chapter-opening pages.Chapters 5 and 7 of the original manual were combinedin this revision. Chapter 11, Energy in Kiln Drying,is a new chapter and was written by John L. Tscher-nitz. In addition to this assignment of chapters, therewere many formal and informal meetings among us toexchange ideas.

Many people helped in the revision. We visited manymills to make sure we understood current and develop-ing kiln-drying technology as practiced in industry, andwe thank all the people who allowed us to visit. Pro-fessor John L. Hill of the University of New Hampshireprovided the background for the section of chapter 6on the statistical basis for kiln samples. Kiln manufac-turers were also very helpful in spending time with usand providing photographs and schematics of dryingequipment. In particular, we wish to thank Coe Man-ufacturing Company, Hemco (Harvey Engineering andManufacturing Corp.), Irvington–Moore, Nyle Corpo-ration, Uraken Canada, Ltd., and Wagner ElectronicProducts, Inc., for their help. We also thank ProfessorCharles J. Kozlik, retired from Oregon State Univer-sity, for arranging and accompanying several of us on aplant tour in the Northwest.

The use of trade or firm names in this publication is forreader information and does not imply endorsement bythe U.S. Department of Agriculture of any product orservice.

Contents

Page

Introduction vi

1 Properties of wood related to drying 1

2 Kiln types and features 43

3 Dry kiln auxiliary equipment 75

4 Inspection and maintenance of dry kilnsand equipment 87

5 Stacking and loading lumber for kiln drying 103

6 Kiln samples 117

7 Kiln schedules 133

8 Drying defects 179

9 Operating a dry kiln 207

10 Log and lumber storage 219

11 Energy in kiln drying 239

Glossary 257

Index 269

Introduction

The modern dry kiln is a unique product of research,development, and experience. It is the only practicalmeans now in wide use for rapid, high-volume drying oflumber to conditions necessary for maximum service-ability in housing, furniture, millwork, and many otherwood products. As part of our charge to help furtherthe efficient utilization of our nation’s timber resource,Forest Service research and development in lumber dry-ing has made a significant contribution to the technol-ogy. The Forest Products Laboratory (FPL) has beenconducting research in lumber drying since it was es-tablished in 1910. Early work by Harry Tiemann (TheKiln Drying of Lumber: A Practical and TheoreticalTreatise, J.B. Lippincott Company, Philadelphia, PA,1917) at FPL established lumber kiln-drying technologyand the first lumber dry kiln design. Tiemann’s bookcan really he considered the first drying manual. Sev-eral other FPL drying manuals followed before the 1961manual by Rasmussen.

A well-designed and properly operated dry kiln can ina few days or weeks turn green lumber fresh from theforest into a dry, stable material necessary for success-ful industrial enterprises in today’s highly competitivemarkets. The more critical the drying requirements,the more firmly the dry kiln becomes established as anintegral part of the lumber mill, the furniture factory,or the millwork plant. For many wood products, kiln-dried lumber is essential.

Dried lumber has many advantages over green lumberfor producers and consumers alike. Removal of excesswater reduces weight and thus shipping and handlingcosts. Proper drying confines shrinking and swelling ofwood in use to manageable amounts under all but ex-treme conditions of relative humidity. Properly driedlumber can be cut to precise dimensions and machinedmore easily and efficiently; wood parts can he more se-curely fitted and fastened together with nails, screws,bolts, and adhesives; warping, splitting, checking, andother harmful effects of uncontrolled drying are largelyeliminated; paint, varnish, and other finishes are moreeffectively applied and maintained; and decay hazardsare eliminated if the wood is subsequently treated orprotected from excessive moisture regain.

Efficient kiln drying of lumber is therefore of key im-portance in the utilization of our forest resource. Onone hand, it helps to assure continued markets for woodproducts by increasing their service life, improving theirperformance, and contributing to consumer satisfac-

tion. On the other hand, it helps to conserve our forestresource by reducing waste in manufacture and extend-ing service life and usefulness of products. Both areessential in using timber wisely, which has long been anaccepted tenet of forest management policy.

The full benefits of modern kiln-drying technology canbe gained only when certain prerequisites are observed.Mill management must recognize the importance of ef-ficient operation to quality of product, and operatorsmust be well trained and encouraged to apply the besttechniques. Quality should not be sacrificed for quan-tity in the production of kiln-dried lumber. The highvalue of our timber resource makes it uneconomical todo so.

Terms used in this manual to describe dry kilns andtheir components, drying characteristics of wood, andkiln operational procedures are generally accepted andused throughout the industry. For clarification and tohelp the newcomer with common terminology, a glos-sary of terms is included after the last chapter.

vi

Chapter 1Properties of WoodRelated to Drying

Commercial wood species 1Hardwoods and softwoods 2Structural features of wood 2

Sapwood and heartwood 4Pith 4Annual growth rings 4Wood rays 4Grain and texture 5Color 5Variations in structure 5

Commercial lumber grades 6Hardwood lumber grades 6Softwood lumber grades 6

Wood-moisture relations 7Free and bound water 8Fiber saturation point 8Equilibrium moisture content 8

How wood dries 9Forces that move water 9Factors that influence drying rate 10

Lumber thickness 10Specific gravity and weight of wood 10

Shrinkage of wood 11Average shrinkage values 12Shrinkage variability 12

Drying stresses 12Electrical properties 13Thermal properties 15

Specific heat 15Thermal conductivity 15Thermal expansion 16

Literature cited 16Sources of additional information 16Tables 17Appendix-Equations for relating

temperature, humidity, andmoisture content 39Wet-bulb temperature and relative

humidi ty 39Relative humidity and equilibrium

moisture content 40Psychrometric charts 41

Chapter 1 was revised by William T. Simpson,Supervisory Research Forest Products Technologist.

Lumber drying is one of the most time- and energy-consuming steps in processing wood products. Theanatomical structure of wood limits how rapidly wa-ter can move through and out of wood. In addition,the sensitivity of the structure to stresses set up in dry-ing limits the drying rate; rapid drying causes defectssuch as surface and internal checks, collapse, splits,and warp. Drying time and susceptibility to many dry-ing defects increase at a rate that is more than pro-portional to wood thickness. The variability of woodproperties further complicates drying. Each species hasdifferent properties, and even within species, variabilityin drying rate and sensitivity to drying defects imposelimitations on the development of standard drying pro-cedures. The interactions of wood, water, heat, andstress during drying are complex. The purpose of thischapter is to describe some of the fundamental prop-erties of wood that are relevant to lumber drying. Wewill discuss commercial wood species, wood structure,lumber grades, water movement in wood, how wooddries, specific gravity and weight of wood, wood shrink-age, stress development during drying, and electricaland thermal properties of wood.

Commercial Wood Species

More than 100 commercially important species of treesgrow in the United States. A similar number of speciesare imported into the United States, and the potentialfor additional imported species grows. The lumber pro-duced from all of these species varies greatly in dryingcharacteristics. The most commonly used commercialnames for lumber and the corresponding species namesaccepted by the Forest Service for the trees from whichthe lumber is cut are given in table 1-1 for domesticspecies and table 1-2 for tropical species. Table 1-1 wasadapted from the standard nomenclature of domestichardwoods and softwoods developed by the Ameri-can Society for Testing and Materials (1981). Tropi-cal species follow the nomenclature used by Chudnoff(1984). While the commonly used lumber names aregenerally satisfactory for the buying and selling of lum-ber, they sometimes refer to lumber from a number ofspecies that differ in green moisture content, shrinkage,or drying characteristics. In the tables and indexes ofphysical properties and drying schedules given in thisand other chapters, the woods are arranged alphabet-ically by the common species names accepted by theForest Service.

1

Hardwoods and Softwoods

Trees can be divided into two classes, hardwoods andsoftwoods. The hardwoods, such as birch, maple, andoak, have broad leaves. Some softwoods or conifers,such as the cedars, have scalelike leaves, while others,such as pine, Douglas-fir, and spruce, have needlelikeleaves.

The terms hardwood and softwood are not directly as-sociated with the hardness or softness of the wood. Infact, such hardwood trees as cottonwood, basswood,and yellow-poplar have softer wood than such soft-woods as longleaf pine and Douglas-fir.

Figure 1-1—Cross section of a white oak tree trunk. A,Cambium layer (microscopic) is inside inner bark andforms wood and bark cells. B, Inner bark is moist andsoft, and contains living tissue; the inner bark carriesprepared food from leaves to all growing parts of tree.C, Outer bark containing corky layers is composed ofdry dead tissue; it gives general protection against ex-ternal injuries. Inner and outer bark are separated by abark cambium. D, Sapwood, which contains both livingand dead tissues, is the light-colored wood beneath thebark; it carries sap from roots to leaves. E, Heartwood(inactive) is formed by a gradual change in the sap-wood. F, Pith is the soft tissue about which the firstwood growth takes place in the newly formed twigs.G, Wood rays connect the various layers from pith tobark for storage and transfer of food. (MC88 9016)

Structural Features of Wood

The structure of wood and the location and amountof water in wood influence its drying characteristics.Wood is composed of bark, sapwood, heartwood, andpith (fig. 1-1). Each wood cell has a cavity (lumen)and walls composed of several layers arranged in differ-ent ways (fig. 1-2). The cell wall constituents are cel-lulose, hemicelluloses, and lignin. Most of the tubelikecells are oriented parallel to the long axis of the treeand are termed fibers, tracheids, or vessels, dependingon their particular anatomical characteristics and func-tion. Another type of cell, the wood ray, lies on radiallines from the center of the tree outward and perpen-dicular to the length of the tree. Figures 1-3 and 1-4 il-lustrate the arrangement of cells in softwoods and hard-woods, which have a similar but not identical anatomy.

One particular type of anatomical element, the pit, isimportant in water flow. A pit is a small, valve-likeopening that connects adjacent wood cells and thus isan important pathway for the flow of water. Pits oftenbecome encrusted with substances or otherwise cloggedso that water flow through them is very slow. Severaltypes of pits are shown in figure 1-5.

Pits in softwoods often become aspirated as drying pro-gresses. In aspiration, the torus is displaced so that itcovers the pit aperture. In effect, the valves close dur-ing drying so that water flow through them is inhibited.The result is a decrease in drying rate.

Figure 1-2—Cross section of a wood cell showing theseveral layers in the cell wall.(ML88 5567)

Figure 1-3—Wood structure of a softwood with resinducts. (ML88 5568)1. Cross-sectional face 7. Wood ray2. Radial face 8. Fusiform ray3. Tangential face 9. Vertical resin duct4. Growth ring 10. Horizontal resin duct5. Earlywood 11. Bordered pit6. Latewood 12. Simple pit

Figure 1-5—Pit cross sections. (a) Bordered pit (withtorus in softwoods); (b) simple pit; and (c) half-bordered pit. (ML88 5569)

Figure 1-4—Wood structure of a hardwood.(ML88 5570)1. Cross-sectional face 6. Latewood2. Radial face 7. Wood ray3. Tangential face 8. Vessel4. Growth ring 9. Sieve plate5. Earlywood

3

Sapwood and Heartwood

Sapwood and heartwood (fig. 1-1) affect the drying ofwood. The sapwood layer next to the bark containsliving cells that actively transport fluids necessary tothe life of the tree. As the tree grows and increases indiameter by adding new layers of sapwood, the innerlayers die. This inner wood, called heartwood, becomesinfiltrated with gums, resins, and other material. Sap-wood of softwood species is usually higher in moisturecontent than heartwood; sapwood moisture content inhardwood species is usually somewhat higher than orabout equal to that of heartwood. The infiltration ofgums and other material in heartwood make it more re-sistant to moisture flow (less permeable) than sapwood,and thus heartwood usually requires longer drying time.The lower permeability of heartwood also makes itmore susceptible to certain drying defects (ch. 8), andso it requires milder drying conditions. Heartwood isusually darker than sapwood. However, because thechange in color may occur slowly over a period of sev-era1 years, a band of heartwood may be indistinguish-able from adjacent sapwood; nevertheless, the heart-wood will not dry easily because it is less permeable.Heartwood is also usually more resistant to decay and.some stains than sapwood.

Pith

The pith of a tree (fig. 1-1F) is usually near the cen-ter of the tree and is laid down by the growing tip. Itis usually very small. Pith sometimes cracks duringdrying.

Annual Growth Rings

Diameter growth of a tree in temperate climates is rep-resented by rings that usually can be easily seen on theend of a log as concentric circles around the pith. Thecloser the rings are to the pith, the smaller their radiiof curvature. Each annual growth ring is composed ofan inner part called earlywood (springwood), whichis formed early in the growing season, and an outerpart, called latewood (summerwood), which is formedlater. When lumber is cut from a log, the annual ringsare cut across in one direction or another and form acharacteristic pattern on the broad face of the boards(fig. 1-6). In tropical woods, where there may be morethan one active growing period annually, growth ringscannot be considered annual rings. In the majority oftropical species, however, there is no noticeable begin-ning or end of successive growth periods, so the typicalpattern of rings shown in figure 1-6 does not occur.

Figure 1-6—Annual growth rings. Quartersawn board(left) shows edge of annual rings on its broad face;flatsawn board (right) shows side of rings. (M 554)

The orientation of growth rings to the faces and edgesof boards depends on how lumber is cut from a log.Lumber can be cut from a log in the two ways shown infigure 1-6. Sawing tangent to the annual rings producesflatsawn lumber, also called plainsawn, flat-grained, orslash-grained lumber. Sawing perpendicular to the an-nual rings produces quartersawn lumber, also callededge-grained or vertical-grained lumber. The angleof cut to the annual rings often lies somewhere in be-tween. In commercial practice, lumber with rings atangles of 45° to 90° to the wide surface is called quar-tersawn, and lumber with rings at angles of 0° to 45°is called flatsawn. Hardwood lumber in which annualrings make angles of 30° to 60° to the wide face issometimes called bastard sawn.

Either flatsawn or quartersawn lumber is generally suit-able for most purposes. However, each type of sawnlumber responds differently in drying. Flatsawn lumberis less susceptible to collapse, shrinks and swells less inthickness, and dries faster than quartersawn. Quarter-sawn lumber shrinks and swells less in width, and hasless twist, cup, and surface checks than flatsawn. Thesedrying defects are discussed in chapter 8.

Wood Rays

Wood rays appear as ribbonlike strands on the face ofquartersawn boards (fig. 1-6) and as short lines on theface of flatsawn boards in species with large rays. Be-

cause rays are weak and dry faster than the surround-ing wood cells, surface, end, and honeycomb checksusually occur in or next to them. Species such as oakand beech, which have large rays, require special careduring the early stages of drying to avoid checks.

Grain and Texture

The physical characteristics of various species that havesome bearing on drying are loosely termed grain andtexture. The terms fine grained and coarse grained re-fer to ring pattern, either the prominence of the late-wood band or the width of the rings. When used inconnection with wood cells, grain refers only to the di-rection of the cells or fibers. In straight-grained wood,the fibers run generally parallel to the length of theboard, and in cross-grained or spiral-grained, they runat an angle. The terms end grain and side grain arecommonly used in discussing moisture loss and dryingdefects. A cross section of a log or board has an end-grain surface. Any other section (radial, tangential, orintermediate) has a side-grain surface.

Texture usually refers to the diameter of individualcells. Fine-textured wood has small cells and coarse-textured, large cells. If all the cells of a wood are ap-proximately the same size, the wood is usually calleduniform textured. Uniform-textured woods in gen-eral are less likely to develop drying defects thannonuniform-textured woods. The word texture shouldnot be used in describing hardness of wood.

Color

As a tree grows, the white or straw-colored sapwoodgradually changes to heartwood, and the formation ofextractives changes the color of most species. Holly,basswood, cottonwood, and magnolia, however, areexamples of hardwoods in which the wood undergoeslittle or no change in color. Spruces and true firs areexamples of softwoods that do not change color greatly.

The temperatures used in kiln drying sometimes darkenwood, especially in high-temperature drying. Changesin the color of heartwood during drying are usually oflittle concern, but those that occur in sapwood are of-ten significant. Chemical stains can occur when greensapwood of some species is kiln dried. The sapwood ofhickory tends to turn pinkish when kiln dried (low ini-tial temperatures must be used to preserve its white-ness) and paper birch sapwood may turn brownish.Hard maple sapwood is prone to darkening if dried attemperatures that are too high. Whiteness of the sap-wood is often a very desirable feature of these species,and darkening reduces their value.

Beneficial color changes can be brought about bysteaming wood before drying. Walnut, for example,is steamed in vats to darken the sapwood before dryingso that it more nearly matches the color of the heart-wood. Sapwood of sweetgum can be steamed to pro-duce a salmon color that at one time was desirable forsome products. Red alder is also steamed to produce auniform honey-brown color of sapwood and heartwood.

Several other types of stain are considered drying de-fects, and they are discussed in chapter 8.

Variations in Structure

Lumber commonly contains variations in wood struc-ture, such as spiral grain, knots, compression wood,tension wood, and juvenile wood.

Cross grain in lumber may result either from the wayin which the log is sawed (diagonal grain) or from spi-ral grain that occurred in the growing tree. Whenspiral grain alternately runs in one direction and an-other in successive groups of growth rings, interlockedgrain results. Lumber containing diagonal, spiral, orinterlocked grain shrinks more in length than straight-grained lumber. Such lumber may bow, crook, andtwist during drying.

Knots are sections of tree branches appearing inboards. Because of shrinkage, some kinds of knots maydrop out during drying; more often, however, they areloosened or checked during drying and drop out of theboard during handling or machining. These are calledincased knots, and they result from the growth of trunkwood around dead branches. Intergrown knots, causedby the intergrowth of trunk wood and living branches,are much less likely to drop out of dried lumber.

Compression wood occurs in softwoods mainly on thelower side of leaning trees but sometimes in other partsof the tree trunk. Because this wood shrinks morealong the length of boards than normal wood, boardsthat contain both compression and normal wood maybow, crook, and twist during drying. If this warpingis restrained, the compression wood may fracture andform crossbreaks in the lumber.

Tension wood occurs in hardwoods, mainly on the up-per side of leaning trees but sometimes in other partsof the trunk. Lumber containing this wood will shrinkmore longitudinally than normal wood, causing warpduring drying.

Juvenile wood occurs in a cylinder around the pith.Once juvenile wood is formed, it does not mature-itis in the tree and lumber forever. However, as growthprogresses, the new wood, as it is formed, graduallyacquires more mature wood characteristics. Juvenile

5

wood varies with species and occurs in the first 5 to20 years of growth. The structural and physical proper-ties of juvenile wood are considered inferior. From thestandpoint of drying, the main problem is that juvenilewood shrinks more along the grain than mature wood,and warp is likely to occur during drying. Juvenilewood is more prevalent in fast-grown plantation treesthan in slower grown stands. Species that are grown involume in plantations, such as southern pine, presentwarp problems in drying.

Commercial Lumber Grades

When a log is sawed into lumber, the quality of theboards varies. The objective of grading is to catego-rize each board by quality so that it meets the require-ments of the intended end uses. The grade of a boardis usually based on the number, character, and locationof features that may lower its strength, utility, appear-ance, or durability. Common visible features that affectgrade are knots, checks, pitch pockets, shake, warp, andstain. Some of these features are a natural part of thetree and some can be caused by poor drying and stor-age practices.

Hardwood Lumber Grades

Most hardwood lumber is graded according to rulesadopted by the National Hardwood Lumber Associa-tion. The grade of a board is determined by the pro-portion that can be cut into a certain number and sizeof smaller pieces clear of defects on at least one side.The grade is based on the amount of usable cuttings inthe board rather than on the number or size of grade-determining features that characterize most softwoodgrades.

The highest cutting grade is termed Firsts and the nextgrade Seconds. Firsts and Seconds are usually com-bined into one grade, FAS. The third grade is termedSelects, followed by No. 1 Common, No. 2 Common,Sound Wormy, No. 3A Common, and No. 3B Com-mon. Standard grades are described in table 1-3, whichillustrates the grade-determining criteria of boardlength and width, surface measure of clear cuttings,percentage of board that must yield clear cuttings, andmaximum number and size of cuttings allowed.

Hardwood lumber is usually manufactured to standardsizes. Standard lengths are in 1-ft increments from 4to 16 ft. Hardwood lumber is usually manufactured torandom width, but there are minimum widths for eachgrade as follows:

Firsts . . . . . . . . . . . . . . 6 inSeconds . . . . . . . . . . . . . 6 inSelects . . . . . . . . . . . . . . 4 inNos. 1, 2, 3A, 3B Common . . . . . 3 in

6

Standard thicknesses for rough and surfaced-two-sides(S2S) lumber are given in table 1-4.

This brief summary of grades is not complete and isonly intended to offer a general view of how hardwoodlumber is graded. The official grading rules of the Na-tional Hardwood Lumber Association should be con-sulted for complete details. There are also grading rulesfor dimension stock and special finished products suchas flooring.

Softwood Lumber Grades

Softwood lumber grades can be divided into two cate-gories based on use: for construction and for remanu-facture. Construction lumber is expected to functionas graded and sized after the primary processing stepsof sawing, drying, and planing. Lumber for remanufac-ture is further modified in size and/or shape before use.There are many individual grading rules for differentsoftwood species. The U.S. Department of Commercehas published the American Softwood Lumber Stan-dard PS-20-70, which is an optional standard, in anattempt to reduce the differences in grading rules.

Construction lumber can be divided into three generalcategories: stress-graded, non-stress-graded, and ap-pearance lumber. Stress-graded and non-stress-gradedlumber are used where structural integrity is the primeconcern; structural integrity is of secondary impor-tance in appearance lumber. Almost all softwood lum-ber nominally 2 to 4 in thick is stress graded. Thisis the lumber that is typically used as 2 by 4 studs,joists, rafters, and truss members. Grading is basedon the premise that lumber has lower strength thanclear wood; characteristics used for grading are density(usually judged by ring count), decay, slope of grain,knots, shake, checks and splits, wane, and pitch pock-ets. These characteristics can be visually assessed.

Lumber intended for general building and utility pur-poses with little or no remanufacture is typically non-stress graded. Boards are one of the most commonnon-stress-graded products. The common grades areseparated into several different categories that varywith species and grading associations. First-gradeboards are usually graded primarily for serviceability,although appearance is also a consideration. Typicaluses are siding, cornice, shelving, and paneling. Second-and lower-grade boards are permitted more and largerknots and are suitable for such products as subfloors,sheathing, and concrete forms.

Appearance lumber is often nonstress graded but formsa separate category because of the importance of ap-pearance. Secondary manufacture is usually restricted

to onsite fitting and cutting. Typical products are trim,siding, flooring, casing, and steps. Most appearancegrades are described by combinations of letters such asB&BTR and C&BTR, although such terms as selectand clear are used for some species. The upper gradesallow a few minor imperfections such as small planerskips, checks, stain, and pin knots. The number andsize of imperfections increase as the grade drops.

Lumber intended for further manufacture in plants asopposed to onsite modifications is usually graded asfactory or shop lumber. It forms the basic raw mate-rial for many secondary operations such as furnitureand mill work. Factory Select and Select Shop are typ-ically the highest grades, followed by No. 1, No. 2,and No. 3. Grade characteristics are influenced by thewidth, length, and thickness of the piece and are basedon the amount of high-quality material that can be cutfrom it.

There are several other grading systems for specialtyproducts such as ladders, pencils, tanks, laminatingstock, and industrial clears.

Moisture content is often specified in softwood lum-ber grades. For many products, the moisture contentmust be within certain limits and the grade stampmust include the moisture content at the time of sur-facing. Lumber surfaced green is usually required to bestamped S-GRN. Most softwood lumber is dried to be-low 19 percent moisture content, and when surfaced atthis moisture content it is stamped S-DRY. Sometimesthe maximum allowable moisture content is 15 percent,and this is stamped as MC-15 or KD.

Wood-Moisture Relations

All wood in growing trees contains a considerable quan-tity of water, commonly called sap. Although sap con-tains some materials in solution, from the drying stand-point sap can be considered plain water. Most of thiswater should be removed to obtain satisfactory servicefor most uses of wood. All wood loses or gains moisturein an attempt to reach a state of balance or equilib-rium with the conditions of the surrounding air. Thisstate of balance depends on the relative humidity andtemperature of the surrounding air. Therefore, someknowledge of wood-moisture relations is helpful in un-derstanding what happens to wood during drying, stor-age, fabrication, and use.

The amount of moisture in wood is termed the mois-ture content. It can be expressed as a percentage ofeither dry or wet weight. For most purposes, the mois-ture content of lumber is based on dry weight, but themoisture content of wood fuel is usually based on wet

weight. Moisture content on dry and wet basis is de-fined as follows:

On dry basis,

Moisture content (percent)

Weight of water in wood=

Weight of totally dry wood× 100

On wet basis,

Moisture content (percent)

=Weight of water in wood

Weight of dry wood and water× 100

These two ways of expressing moisture content can berelated by

Moisture content (dry)

Moisture content (wet)=

100 – Moisture content (wet)× 100

In this manual we will deal only with the dry basis. Formost species, the common and accurate method of de-termining moisture content is the ovendrying method,or oven test. This method is inaccurate for species witha high extractives content. In ovendrying (described inch. 6), all the water is evaporated from a wood sectionby heating. Knowing the wood weight before and afterovendrying allows calculation of moisture content.

The amount of water in green or wet wood variesgreatly, depending mainly on species. The moisturecontent of some species may be as low as 30 percent,whereas that of others may be as high as 200 percent.Large variations may occur not only between speciesbut also within the same species and even in the sametree. In softwood species, sapwood usually containsmore water than heartwood. In species such as red-wood, the butt logs of trees may contain more waterthan the top logs. Some species contain an abnormaltype of heartwood, called wetwood or sometimes sinkerstock, that is sometimes higher in green moisture con-tent than normal wood of the species. In addition tothe higher moisture content, wetwood is slower to drythan normal wood and often more susceptible to suchdrying defects as honeycomb and collapse.

Contrary to popular belief, the amount of water ingreen wood does not vary greatly with the season of theyear in which the trees are cut. Moisture content valuesfor green wood of various species is given in table 1-5.

7

Free and Bound Water

Water is held in wood as free water or bound water.Free water is contained in the cell cavities (fig. 1-2);bound water is held within the cell walls. Free wa-ter is held within the cell cavities less tightly thanthe bound water is held within the cell walls. Con-sequently, slightly more energy is required to removebound water than free water. Free water does not af-fect as many wood properties as bound water, but doesaffect thermal conductivity and permeability. Boundwater affects many physical and mechanical properties,and its removal causes changes that affect the use ofthe wood.

Fiber Saturation Point

The fiber saturation point is defined as the moisturecontent at which the cell walls are saturated but nofree water remains in the cell cavities. Moisture con-tent of the individual cell walls at the fiber saturationpoint is usually about 30 percent, but may be lower forsome species. Care must be used in judging whethera piece of wood is at the fiber saturation point. Theterm really refers to individual cells rather than boardsor other pieces of wood. The mechanisms of how wooddries will be discussed later, but basically wood driesfrom the outside to the inside. Thus, during drying, theoutside part of a board might be at 15 percent mois-ture content while the inside might still be at 45 per-cent. The average moisture content of the entire boardmight be 30 percent, but it is erroneous to consider theboard to be at the fiber saturation point. There will bea continuous variation or gradient of moisture contentfrom the outside to the inside of the board from 15 to45 percent, and only some cells will be exactly at thefiber saturation point of 30 percent.

The fiber saturation point is important in the dryingof wood for the following reasons: (1) more energyis required to evaporate water from a cell wall thanfrom the cell cavity (approximately 5 percent moreat 15 percent moisture content and 15 percent moreat 6 percent moisture content); (2) a wood cell willnot shrink until it reaches the fiber saturation point;and (3) large changes in many physical and mechani-cal properties of wood begin to take place at the fibersaturation point.

Equilibrium Moisture Content

Wood loses or gains moisture until the amount it con-tains is in balance with that in the surrounding atmos-phere. The amount of moisture at this point of bal-ance is called the equilibrium moisture content (EMC).The EMC depends mainly on the relative humidity andtemperature of the surrounding air, although species

Figure 1-7—Relation of the equilibrium moisture con-tent of wood to the relative humidity of the surround-ing atmosphere at three temperatures. (ML88 5572)

and previous moisture history have a slight effect onEMC. The relationship of EMC to relative humidityand temperature is shown in figure 1-7. If, for example,wood is kept in air at 141 °F and 65 percent relativehumidity, it will eventually either gain or lose mois-ture until it reaches approximately 10 percent moisturecontent.

Kiln drying usually requires control of EMC condi-tions, that is, temperature and relative humidity, inthe kiln. Thus both temperature and relative humid-ity have to be measured. Two thermometers-dry bulband wet bulb--are used to obtain temperature and rel-ative humidity. The dry-bulb thermometer measurestemperature in the usual way, and the result is calledthe dry-bulb temperature. The sensor of the wet-bulbthermometer is kept wet with a wick cover, from whichwater evaporates at a rate determined by the relativehumidity and temperature of the air. The drier the air,the faster the rate of evaporation. This evaporation hasa cooling effect that increases as the rate of evapora-tion increases. Thus, the drier the air, the greater thecooling effect and the lower the temperature indicatedby the wet-bulb thermometer. The difference betweendry- and wet-bulb temperatures, called the wet-bulbdepression, is thus a measure of the relative humidity ofthe air.

The relationship between relative humidity, tempera-ture, and EMC is shown in table 1-6 for temperaturesbelow 212 °F, and in table 1-7 for temperatures above212 °F. For example, assume that the dry-bulb tem-perature in a kiln is 150 °F and the wet-bulb temper-ature 130 °F. The wet-bulb depression then is 20 °F.Wet-bulb depression temperatures are shown across thetop of tables 1-6 and 1-7 and dry-bulb temperatures onthe extreme left of the table. To find the EMC at theassumed conditions, (1) locate 20 °F wet-bulb depres-

s

sion column and (2) follow this column downward untilit intersects the 150 °F dry-bulb temperature line. TheEMC value, 8 percent, is the underscored value. Notethat the relative humidity value (not underscored),57 percent, is given directly above the EMC value.Wet- and dry-bulb temperatures, EMC, and psychro-metric relations are further discussed in the appendixto this chapter.

How Wood Dries

Water in wood normally moves from higher to lowerzones of moisture content. This fact supports the com-mon statement that “wood dries from the outside in,”which means that the surface of the wood must be drierthan the interior if moisture is to be removed. Dry-ing can be broken down into two phases: movementof water from the interior to the surface of wood, andremoval of water from the surface. Moisture moves tothe surface more slowly in heartwood than in sapwood,primarily because extractives plug the pits of heart-wood. In drying, the surface fibers of heartwood ofmost species reach moisture equilibrium with the sur-rounding air soon after drying begins. This is the be-ginning of the development of a typical moisture gradi-ent (fig. 1-8), that is, the difference in moisture contentbetween the inner and outer portions of a board. Thesurface fibers of sapwood also tend to reach moistureequilibrium with the surrounding air if the air circula-tion is fast enough to evaporate water from the surfaceas fast as it comes to the surface. If the air circulationis too slow, a longer time is required for the surfaces ofsapwood to reach moisture equilibrium. This is one rea-son why air circulation is so important in kiln drying.If it is too slow, drying is also slower than necessaryand mold might even develop on the surface of lumber.If it is too fast, electrical energy in running the fansis wasted, and in certain species surface checking maydevelop if wet-bulb depression and air velocity are notcoordinated.

Water moves through wood as liquid or vapor throughseveral kinds of passageways. These are the cavities offibers and vessels, ray cells, pit chambers and their pitmembrane openings, resin ducts of certain softwoods,other intercellular spaces, and transitory cell wallpassageways (Panshin and de Zeeuw 1980). Most waterlost by wood during drying moves through cell cavitiesand pits. It moves in these passageways in all direc-tions, both along and with the grain. Lighter speciesin general dry faster than heavier species because theirstructure contains more openings per unit volume.

Figure 1-8—Typical moisture gradient in lumber duringdrying at times increasing from t1 to t3. (ML88 5571)

Forces That Move Water

When wood is drying, several forces may be acting si-multaneously to move water (Siau 1984):

1. Capillary action causes free water to flow throughthe cell cavities and pits.

2. Differences in relative humidity cause water vaporto move through the cell cavities by diffusion, whichmoves water from areas of high to areas of low rel-ative humidity. Cell walls are the source of watervapor; that is, water evaporates from the cell wallsinto the cell cavities.

3. Differences in moisture content cause bound waterto move through the cell walls by diffusion, whichmoves water from area of high to areas of low mois-ture content. Generally, any water molecule thatmoves through wood by diffusion moves throughboth cell walls and cell cavities. Water may evap-orate from a cell wall into a cell cavity, move acrossthe cell cavity, be readsorbed on the opposite cellwall, move through the cell wall by diffusion, and soon until it reaches the surface of the board.

9

When green wood starts to dry, evaporation of waterfrom the surface cells sets up capillary forces that exerta pull on the free water in the zones of wood beneaththe surface, and a flow results. This is similar to themovement of water in a wick. Much free water in sap-wood moves in this way. In comparison to diffusion,capillary movement is fast.

Longitudinal diffusion is about 10 to 15 times fasterthan lateral (radial or tangential) diffusion. Radial dif-fusion, perpendicular to the growth rings, is somewhatfaster than tangential diffusion, parallel to the rings.This explains why flatsawn lumber dries faster thanquartersawn lumber. Although longitudinal diffusion is10 to 15 times faster than lateral diffusion, it is of prac-tical importance only in short items. Common lumberis so much longer than it is thick that most of the wa-ter removed during drying does so through the thick-ness direction, leaving from the wide face of a board.In lumber where width and thickness are not greatlydifferent, such as in squares, significant drying occurs inboth the thickness and width directions.

The rate ‘of diffusion depends to a large extent uponthe permeability of the cell walls and their thickness.Thus permeable species dry faster than impermeableones, and the rate of diffusion decreases as the specificgravity increases.

Because moisture moves more freely in sapwood thanheartwood, both by diffusion and by capillary flow,sapwood generally dries faster than heartwood underthe same drying conditions. The heartwood of manyspecies, however, is lower in moisture content than sap-wood, and may reach final moisture content faster.

Factors That Influence Drying Rate

The rate at which moisture moves in wood depends onthe relative humidity of the surrounding air, the steep-ness of the moisture gradient, and the temperature ofthe wood. The lower the relative humidity, the greaterthe capillary flow. Low relative humidity also stimu-lates diffusion by lowering the moisture content at thesurface, thereby steepening the moisture gradient andincreasing diffusion rate. The higher the temperature ofthe wood, the faster moisture will move from the wetterinterior to the drier surface. If relative humidity is toolow in the early stages of drying, excessive surface andend checking may result. And if the temperature is toohigh, collapse, honeycomb, or strength reduction mayoccur (see ch. 8).

Lumber Thickness

Drying rate is also affected by thickness. Drying timeincreases with thickness and at a rate that is more thanproportional to thickness. For example, if thickness isdoubled, drying time is more than doubled. Theoreti-cally, if drying were controlled completely by diffusion,drying time would increase by a factor of four if thick-ness were doubled. But because of the other mecha-nisms involved in drying, drying time increases betweenthree and four times. Thickness variation in lumbercaused by poor sawing can lead to excessive moisturecontent variation after drying or excessive kiln time toequalize the variation. For example, the kiln-dryingtime for l-in-thick red oak will vary by about 4 percentfor each 1/32-in variation in thickness.

Specific Gravity and Weight of Wood

Specific gravity is a physical property of wood that isa guide to ease of drying as well as an index of weight(table 1-8). In general, the heavier the wood, the slowerthe drying rate and the greater the likelihood of devel-oping defects during drying. Specific gravity is definedas the ratio of the weight of a body to the weight of anequal volume of water. The specific gravity of wood isusually based on the volume of the wood at some speci-fied moisture content and its weight when ovendry:

Specific gravityOvendry weight of wood

=Weight of equal volume of water

Thus, if the specific gravity of a piece of green woodis 0.5, the ovendry weight of the wood substance in acubic foot of the green wood is one-half the weight ofa cubic foot of water. The higher the specific gravityof wood, the greater the amount of ovendry wood perunit volume of green wood. Thus, at the same moisturecontent, high specific gravity species contain more wa-ter than low specific gravity species. The green weightof 1 ft3 of wood can be calculated from the followingformula:

Green weight = Specific gravity

× (Moisture content + 100)

× 62.4/100 lb

For example, the green weight of 1 ft3 of a species ofspecific gravity 0.4 at 75 percent moisture content is43.7 lb. The ovendry weight (by substituting 0 formoisture content in the formula) is 25 lb, and thus18.7 lb of water are present. At a specific gravity of0.6 at 75 percent moisture content, the green weight is

10

65.5 lb, the ovendry weight 37.4 lb, and the weight ofwater 28.1 lb. Thus, there are 9.4 lb more water at aspecific gravity of 0.6 than at 0.4.

As the above formula indicates, weight of wood de-pends on its specific gravity and moisture content. Cal-culated weights for lumber are given in table 1-9. Thevalues for weights per thousand board feet apply to athousand feet, surface measure, of boards exactly 1 inthick (actual board feet) and not to a thousand boardfeet lumber scale. These weights were determined inthe way described by Panshin and de Zeeuw (1980) andthe resulting weight per cubic foot at the given mois-ture content multiplied by 83.3, the number of cubicfeet in a thousand board feet. Note that two correctionfactors are given for calculating weights at moisturecontents not shown in table 1-9. These factors are sim-ply added to table values to calculate weights betweentable values. The correction factor for below 30 per-cent moisture content takes into account the volumet-ric shrinkage that occurs below 30 percent moisturecontent. The correction factor above 30 percent mois-ture content does not require a shrinkage correctioncomponent.

Since the weights in table 1-9 are based on actual boardfeet-a thousand lineal feet of lumber exactly 1 in thickand 12 in wide-they must be adjusted upward forrough lumber greater than 1 in thick and downwardfor surfaced lumber less than 1 in thick.

Example: What is the weight of 1,000 fbm of nominal1 by 8 ponderosa pine lumber at 6 percent moisturecontent dressed to 25/32 in thick by 7-1/2 in wide?

The downward adjustment factor is calculated asfollows:

From table 1-9, the weight of 1,000 fbm, actual, of thissize ponderosa pine is 2,271 lb. With the downwardadjustment the weight is

2,271 × 0.732 = 1,662 lb

Example: What is the weight of 1,000 fbm of nominal4/4 rough, random width, northern red oak lumber at75 percent moisture content? Assume the target sawingthickness is 37/32 in.

The upward adjustment factor is calculated as follows:

Table 1-9 does not have a column for 75 percent mois-ture content, so the correction factor in column 2 mustbe used. The weight at 60 percent moisture contentin table 1-9 is 4,666 lb. Using the correction factor of29.1 lb per 1 percent moisture content, the weight of1,000 fbm, actual, is

(75 – 60) × 29.1 + 4,666 = 5,103 lb

And with the upward adjustment factor, the weight ofthe nominal 4/4 lumber is

5,103 × 1.156 = 5,899 lb

Shrinkage of wood

Shrinkage of wood is the basic cause of many prob-lems that occur in wood during drying and also in ser-vice. When water begins to leave the cell walls at 25 to30 percent moisture content, the walls begin to shrink.Even after drying, wood will shrink and swell in serviceas relative humidity varies (table 1-6). Drying stressesdevelop because wood shrinks by different amounts inthe radial, tangential, and longitudinal directions andbecause during drying, shrinkage starts in the outerfibers before it starts in the inner fibers. These stressescan cause cracks and warp to develop.

When wood is dried to 15 percent moisture content,about one-half of the total possible shrinkage has oc-curred; when dried to 8 percent, nearly three-fourthsof the possible shrinkage has occurred. Figure 1-9 il-lustrates how Douglas-fir shrinks with loss of moisture.While these curves are not straight, the relationshipbetween moisture content and shrinkage is generallyapproximated a a straight-line relationship.

Moisture content (percent)

Figure 1-9—Typical relation of moisture content toshrinkage of Douglas-fir. Although the curves are notstraight lines, they may be considered as such for prac-tical shrinkage calculations. (ML88 5573)

11

Average Shrinkage Values

Table 1-10 gives average shrinkage values for variousspecies of wood. These values are given in percentagesof the green dimension.

Shrinkage (percent)Green dimension – Dry dimension

= × 100Green dimension

Wood shrinks about 1.5 to 2 times as much parallel tothe growth rings (tangential) as it does at a right angleto the growth rings (radial). The shrinkage along thegrain (longitudinal) is small (0.2 percent or less for nor-ma1 wood). Characteristic shrinkage patterns of boardsare shown in figure 1-10.

Table 1-10 gives shrinkage values at only 20, 6, and0 percent moisture content. Knowing the total shrink-age of a species at 0 percent moisture content, thepercent shrinkage at any moisture content below30 percent can be calculated. Since shrinkage curvesare reasonably close to straight lines from 30 percent(approximate fiber saturation point) to 0 percent mois-ture content, each 1 percent change in moisture contactbelow 30 percent is equal to 1/30 of the total shrinkagefrom 30 to 0 percent.

where SM is percent shrinkage from green to moistwecontent M and S0 is total shrinkage to 0 percent mois-ture content from table 1-10.

Example: What is the tangential shrinkage of westernhemlock from green to 12 percent moisture content?

From table 1-10, the shrinkage of western hemlock to0 percent moisture content is 7.8 percent. From theabove equation

Shrinkage Variability

Shrinkage differs not only with respect to the length,width, and thickness of a board, but even in materialcut from the same species and from the same tree. Thevalues listed in table 1-10 are only representative values

Figure 1-10—Characteristic shrinkage and distortion offlats, squares, and rounds as affected by the direction ofannual growth rings. The dimensional changes shownare somewhat exaggerated. (M 12494)

for the species, and individual observations of shrinkagemay differ from them.

On the average, hardwoods shrink more than soft-woods. In general, species of high specific gravityshrink more than ones of low specific gravity, but thereare exceptions. Basswood, a light species, has highshrinkage, while the heavier black locust has more mod-erate shrinkage. The amount of shrinkage and the dif-ference between radial and tangential shrinkage have adirect influence on the development of drying defects.Species that are high in extractive content–like tropi-cal species such as true mahogany-have relatively lowshrinkage.

Longitudinal shrinkage is variable. While it is usuallyless than 0.2 percent from green to ovendry, reactionwood and juvenile wood can shrink as much as 1 to1.5 percent. As an increasing amount of young-growthplantation trees with juvenile wood is harvested, thevariability of longitudinal shrinkage and its influence onwarp become more of a problem.

Drying Stresses

The effect of drying stresses on the development of dry-ing defects is discussed in chapter 8. Drying stressesare the main cause of nonstain-related drying defects.Understanding these stresses provides a means for pre-venting them. There are two causes of drying stresses:hydrostatic tension and differential shrinkage. Hydro-static tension forces develop during the flow of capillarywater. As water evaporates from cell cavities near thesurface, it exerts a pull on water deeper in the wood.This tension pull is inward on the walls of cells whosecavities are full of water, and the result can be an in-

12

Figure 1-11—End view of board showing developmentof drying stresses early (a) and later (b) in drying.(ML88 5574)

ward collapse of the cell wall. The danger of collapseis greatest early in drying when many cell cavities arefull of water, and if the temperature is high, collapse ismore likely to occur.

Differential shrinkage between the shell and core oflumber also causes drying defects. Early in the dry-ing process, the fibers in the shell (the outer portionof the board) dry first and begin to shrink. However,the core has not yet begun to dry and shrink, and con-sequently the core prevents the shell from shrinking.Thus, the shell goes into tension and the core into com-pression, as illustrated in figure 1-11. If the shell driestoo rapidly, it is stressed beyond the elastic limit anddries in a permanently stretched (set) condition with-out attaining full shrinkage. Sometimes surface checksoccur during this early stage of drying, and they can bea serious defect for many uses. As drying progresses,the core begins to dry and attempts to shrink. How-ever, the shell is set in a permanently expanded condi-tion and prevents normal shrinkage of the core. Thiscauses the stresses to reverse-the core goes into ten-sion and the shell into compression. The change in theshell and core stresses and moisture content duringdrying is shown in figure 1-12. These internal tensionstresses may be severe enough to cause internal cracks(honeycomb) to occur.

Thickness

Figure 1-12—Moisture–stress relationship during sixstages of kiln drying 2-in red oak. (ML88 5575)

Differential shrinkage caused by differences in radial,tangential, and longitudinal shrinkage is a major causeof warp. The distortions shown in figure 1-10 are dueto differential shrinkage. When juvenile or reactionwood is present on one side or face of a board and nor-mal wood is present on the opposite face, the differencein their longitudinal shrinkage will also cause warp.

Electrical Properties

Electrical properties of wood vary enough with mois-ture content that they can be used to measure mois-ture content reasonably accurately and very quickly.Those electrical properties of wood that indicate levelof moisture content are resistance to the flow of electri-cal current and dielectric properties. These propertiesare utilized in electric moisture meters to estimate themoisture content of wood (James 1988).

The direct current electrical resistance of wood variesgreatly with moisture content, especially below thefiber saturation point. It decreases greatly as mois-ture content increases (table 1-11). Resistance alsovaries with species, is greater across the grain than

13

Figure 1-13—Temperature corrections for reading of calibration temperatures near 70 °F, adequate correc-resistance-type moisture meters, based on combined tions can be obtained simply by shifting the tempera-data from several investigators. Find meter reading on ture scale so that the true calibration temperature coin-vertical axis, follow horizontally to vertical line corre- cides with 70 °F on the percent scale. For example, for

spending to the temperature of the wood, and inter- meters calibrated at 80 °F, add 10 °F to each point onpolate corrected reading from family of curves. Exam- the temperature scale (shift the scale 10 °F toward theple: If meter indicated 18 percent on wood at 120 °F, left), and use the chart as before. After temperaturecorrected reading would be 14 percent. This chart is correction, apply the appropriate species correction.based on a calibration temperature of 70 °F. For other (ML88 5576)

Figure 1-14—Approximate temperature correctionsfor capacitive admittance meter; data taken using a“Sentry” hand meter with calibration setting of 20or greater. Solid lines are for the meter itself at roomtemperature; broken lines are for the meter at the sametemperature as the lumber. (ML88 5578)

Figure 1-15—Approximate temperature corrections forreadings of power-loss type moisture meters; data takenusing a Moisture Register model L. Locate the pointwhose coordinates are the observed scale reading andthe specimen temperature, and trace back parallel tothe curves to the calibration temperature of the meter(usually 80°F). The vertical coordinate here is the cor-rected scale reading, which is then converted to mois-ture content using the usual species conversion tables.Solid lines are for the meter itself at room temperature;broken lines are for the meter at the same temperatureas the lumber. (ML88 5579)

14

along it, and is affected by temperature. Resistanceis not greatly affected by specific gravity. Commercialresistance moisture meters are often calibrated for onespecies, but are supplied with a species correction table.The meters are usually calibrated for 70 °F and alsorequire a temperature correction chart (fig. 1-13). Re-sistance meters use probes that must be driven into thewood for measurement.

Meters that use the dielectric properties of wood areclassified as one of two types-capacitance and power-loss (James 1988). With these instruments, electrodesare pressed against the wood, and high-frequency elec-tric energy is applied. The electrodes do not penetratethe wood. The amount of power absorbed depends onthe moisture content of the wood. Species correctiontables and temperature correction charts (figs. 1-14 andl-15) are also necessary.

Thermal Properties

Thermal properties are relevant to wood drying becausethey are related to energy requirements and the timerequired to heat wood to drying temperature. Specificheat of a material is the ratio of the heat capacity ofthe material to that of water. It is a measure of the en-ergy required to raise the temperature of the material.Thermal conductivity is a measure of the rate of heatflow through a material. The coefficient of thermal ex-pansion is a measure of the change of dimension causedby temperature change.

Specific Heat

The specific heat of wood depends on the temperatureand moisture content of the wood, but is practicallyindependent of density or species. Specific heat of drywood can be approximately related to temperature T,in degrees Fahrenheit, by the following formula:

Specific heat = 0.25 + 0.0006T

When wood contains water, the specific heat increasesbecause the specific heat of water is larger than that ofdry wood. If the specific heat of water is taken as one,the specific heat of wood at moisture content m, wherem is percent moisture content divided by 100, is

Specific heat =0.25 + 0.0006T+ m

1 +m

Example: Estimate the energy in British thermal units(Btu) required to raise the temperature of 50,000 fbmof nominal 4/4 northern red oak at 75 percent moisturecontent from 60 to 110 °F.

This is a continuation of the earlier example on weightof wood, where the weight of 1,000 fbm of 4/4 northernred oak at 75 percent was 5,103 lb. Thus 50,000 fbmweigh 255,150 lb. The specific heat over the intervalbetween 60 and 110 °F can be approximated by usingthe average temperature as follows:

T =60+ 110

2= 85°F

Thus, the specific heat is

The energy required is the product of the weight, thespecific heat, and the temperature rise as follows:

Thermal Conductivity

The thermal conductivity of wood is affected by den-sity, moisture content, extractive content, grain direc-tion, temperature, and structural irregularities such asknots. It is nearly the same in the radial and tangen-tial directions but two to three times greater parallel tothe grain. It increases as the density, moisture content,temperature, and extractive content increase. Thermalconductivity below 40 percent moisture content can beapproximated by

and above 40 percent moisture content by

where k is thermal conductivity in Btu·in/h·ft2. °Fand G is specific gravity based on volume at M percentmoisture content and ovendry weight.

15

Thermal Expansion

The thermal expansion of wood is so small that it isovershadowed by shrinkage and swelling. It is far lessthan dimensional changes associated with changes inmoisture content, and conditions that would causethermal expansion would also cause moisture-relatedshrinkage. The coefficient of thermal expansion is de-fined as the unit increase in dimension per degree in-crease in temperature. The coefficient of ovendry woodin the longitudinal direction is apparently independentof specific gravity and species. In both hardwoods andsoftwoods, it ranges from 0.0000017 to 0.0000025 inchper inch per degree Fahrenheit.

The coefficients of thermal expansion in the radial andtangential directions are 5 to 10 times greater than inthe longitudinal direction and are thus of more practi-cal interest. They depend on specific gravity, and forovendry wood can be approximated by the followingequations over the specific gravity range of 0.1 to 0.8:

Literature Cited

American Society for Testing and Materials. 1981.Standard nomenclature of domestic hardwoods andsoftwoods. ASTM D 1165-80. Philadelphia, PA: Ameri-can Society for Testing and Materials: 388-398.

Chudnoff, Martin. 1984. Tropical timbers of the world.Agric. Handb. 607. Washington, DC: U.S. Departmentof Agriculture. 466 p.

Hawkins, G. A. 1978. Thermal properties of substancesand thermodynamics. In: Mark’s standard handbookfor mechanical engineers. 8th ed. New York: McGraw-Hill: 4-28 to 4-40.

James, William L. 1988. Electric moisture meters forwood. Gen. Tech. Rep. FPL 6. Madison, WI: U.S.Department of Agriculture, Forest Service, Forest Prod-ucts Laboratory. 17 p.

Panshin, A. J.; de Zeeuw, Carl. 1980. Textbook ofwood technology. 4th ed. New York: McGraw-Hill.722 p.

Siau, John F. 1984. Transport processes in wood.Berlin: Springer-Verlag. 245 p.

U.S. Department of Commerce. 1986. American soft-wood lumber standard. Product Standard PS-20-70.Washington, DC: U.S. Department of Commerce. 26 p.

Sources of Additional Information

Cech, M. Y.; Pfaff, F. 1977. Kiln operator’s manual foreastern Canada. Report OPX192E. Ottawa, Ontario:Eastern Forest Products Laboratory. 189 p.

Little, Elbert L. Jr. 1979. Checklist of United Statestrees. Agric. Handb. 541. Washington, DC: U.S. De-partment of Agriculture. 375 p.

Skaar, Christen. 1972. Water in wood. Syracuse, NY:Syracuse University Press. 218 p.

Stamm, Alfred J. 1964. Wood and cellulose scienceNew York: The Ronald Press. 549 p.

U.S. Department of Agriculture. 1987. Wood Hand-book. Agric. Handb. 72. Washington, DC: U.S. De-partment of Agriculture. 466 p.

Ward, James C.; Pang, W. Y. 1980. Wetwood in trees:a timber resource problem. Gen. Tech. Rep. PNW112. Portland, OR: U.S. Department of Agriculture,Forest Service, Pacific Northwest Forest and Range Ex-periment Station. 56 p.

Radial coefficient

Tangential coefficient

where G is specific gravity. Thermal expansion coeffi-cients can be considered independent of temperatureover the range of –60° to 130°F.

The thermal expansion properties of wood containingwater are difficult to define. When wood with moistureis heated, it tends to expand because of normal thermalexpansion and at the same time to shrink because ofdrying that occurs with the rise in temperature. Unlesswood is below about 3 to 4 percent moisture content,the shrinkage will be greater than the thermal expan-sion. The question is sometimes asked if thermal ex-pansion can cause checking in lumber. Because thermalexpansion is so small, it is doubtful that it can causechecking.

16

Table 1-1—Commercial species grown in the United States

Commercial name for lumber Common tree name

HARDWOODS

AlderRed alder

AppleAsh

Black ash1

Oregon ashWhite ash

Aspen2

Basswood3

BeechBirch4

Box elderBuckeye

ButternutCherryChestnut

Cottonwood

Dogwood

Elder, see Box elderElm

Rock elm

Soft elm5

Gum6

Hackberry

Hickory7

HollylronwoodLocust

MadroneMagnolia

MapleHard maple8

Oregon mapleSoft maple8

red alderapple

black ashOregon ashblue ashgreen ashwhite ashbigtooth aspenquaking aspenAmerican basswoodwhite basswoodbeech, Americangray birchpaper birchriver birchsweet birchyellow birchboxelderOhio buckeyeyellow buckeyebutternutblack cherryAmerican chestnutbalsam poplarblack cottonwoodeastern cottonwoodplains cottonwoodswamp cottonwoodflowering dogwoodPacific dogwood

cedar elmrock elmSeptember elmwinged elmAmerican elmslippery elmsweetgumhackberrysugarberrymockernut hickorypignut hickorysand hickoryshagbark hickoryshellbark hickoryAmerican hollyeastern hophornbeamblack locusthoneylocustPacific madronesouthern magnoliasweetbaycucumber tree

black maplesugar maplebigleaf maplered maplesilver maple

Botanical name

Alnus rubraMalus spp.

Fraxinus nigraF. latifoliaF. quadrangulataF. pennsylvanicaF. americanaPopulus grandidentataP. tremuloidesTilia americanaT. heterophylleFagus grandifoliaBetula populifoliaB. papyriferaB. nigraB. lentaB. alleghaniensisAcer negundoAesculus glabraA. octandraJuglans cinereaPrunus serotinaCastanea dentataPopulus balsamiferaP. trichocarpaP. deltoidesP. sargentiiP. heterophyllaCornus floridaC. nuttallii

Ulmus crassifoliaU. thomasiiU. serotinaU. alataU. americanaU. rubraLiquidambar styracifluaCeltis occidentalisC. laevigataCarya tomentosaC. glabraC. pallidaC. ovataC. laciniosaIlex opacaOstyra virginianaRobinia pseudoacaciaGleditsia triacanthosArbutus menziesiiMagnolia grandifloraM. virginianaM. acuminata

Acer nigrumA. saccharumA. macrophyllumA. rubrumA. saccharinum

17

Table 1-1—Commercial species grown in the United States—continued

Commercial name for lumber Common tree name

Myrtle, see Oregon myrtleOak

Red oak

White oak

Oregon myrtleOsage orangePecan7

PersimmonSassafrasSycamoreTanoakTupelo9

WalnutWillow

Yellow poplar

CedarAlaska cedarIncense cedarPort Orford cedarEastern red cedar

Western red cedarNorthern white cedarSouthern white cedar

Cypress10

FirBalsam fir11

Douglas fir12

Noble fir

HARDWOODS—continued

black oakblackjack oakCalifornia black oakcherrybark oaklaurel oaknorthern pin oaknorthern red oakNuttall oakpin oakscarlet oakshingle oakShumard oaksouthern red oakturkey oakwillow oakArizona white oakblue oakbur oakCalifornia white oakchestnut oakchinkapin oakEmory oakGambel oakMexican blue oaklive oakOregon white oakovercup oakpost oakswamp chestnut oakswamp white oakwhite oakCalifornia-laurelOsage-orangebitternut hickorynutmeg hickorywater hickorypecancommon persimmonsassafrasAmerican sycamoretanoakblack tupeloOgeechee tupeloswamp tupelowater tupeloblack walnutblack willowpeachleaf willowyellow-poplar

SOFTWOODS

Alaska-cedarincense-cedarPort-Orford-cedareastern redcedarsouthern redcedarwestern redcedarnorthern white-cedarAtlantic white-cedarbaldcypresspondcypress

balsam firFraser firDouglas-firInland Douglas-firnoble fir

Botanical name

Quercus velutinaQ. marilandicaQ. kellogiiQ. falcata var. pagodaefoliaQ. laurifoliaQ. ellipsoidalisQ. rubraQ. nuttalliQ. palustrisQ. coccineaQ. imbricariaQ. shumardiiQ. falcataQ. laevisQ. phellosQ. arizonicaQ. douglasiiQ. macrocarpaQ. lobataQ. prinusQ. muehlenbergiiQ. emoryiQ. gambeliiQ. oblongifoliaQ. virginianaQ. garryanaQ. lyrataQ. stellataQ. michauxiiQ. bicolorQ. albaUmbellularia californicaMaclura pomiferaCarya cordiformisC. myristicaeformisC. aquaticaC. illinoensisDiospyros virginianaSassafras albidumPlatanus occidentalisLithocarpus densiflorusNyssa sylvaticaN. ogecheN. silvatica var. bifloraN. aquaticaJuglans nigraSalix nigraS. amygdaloidesLiriodendron tulipifera

Chamaecyparis nootkatensisLibocedrus decurrensChamaecyparis lawsonianaJuniperus virginianaJ. silicicolaThuja plicataT. occidentalisChamaecyparis thyoidesTaxodium distichumT. distichum var. nutans

Abies balsameaA. fraseriPseudotsuga menziesiiP. menziesii var. glaucaAbies procera

18

Table 1-1—Commercial species grown In the United States—concluded

Commercial name for lumber Common tree name Botanical name

Fir (continued)White fir

Hemlock

SOFTWOODS—continued

California red firgrand firnoble firPacific silver firsubalpine firwhite fir

A. magnificaA. grandisA. proceraA. amabilisA. lasiocarpaA. concolor

Eastern hemlock

Mountain hemlockWest coast hemlock

JuniperWestern juniper

Larch

Carolina hemlockeastern hemlockmountain hemlockwestern hemlock

alligator juniperRocky Mountain juniperUtah juniperwestern juniper

Tsuga carolinianaT. canadensisT. medensianaT. heterophylla

Juniperus deppeanaJ. scopulorumJ. osteospermaJ. occidentalis

Western larch western larch Larix occidentalisPine

Jack pine jack pine Pinus banksianaLodgepole pine lodgepole pine P. contortaNorway pine red pine P. resinosaPonderosa pineSugar pine

ponderosa pine P. ponderosasugar pine P. lambertiana

Idaho white pine western white pine P. monticolaNorthern white pine eastern white pine P. strobusLongleaf pine13 longleaf pine P. palustris

slash pine P. elliottiiSouthern pine loblobbly pine Pinus taeda

longleaf pine P. palustrispitch pine P. rigidapond pine P. serotinashortleaf pine P. echinataslash pine P. elliottiiVirginia pine

RedwoodP. virginiana

redwoodSpruce

Sequoia sempervirens

Eastern spruce black spruce Picea marianared spruce P. rubenswhite spruce

Engelmann spruceP. glauca

blue spruce P. pungensEngelmann spruce

Sitka spruceP. engelmannii

Sitka spruce P. sitchensisTamarack tamarack Larix laricinaYew

Pacific yew Pacific yew Taxus brevifolia

1Black ash is known commercially in some consuming centers as brown ash, and is also sometimes designated as such in specifications.2Aspen lumber is sometimes designated as poppIe.3For some commercial uses where a white appearance is required, the sapwood of American basswood ( Tilia americana) is specified under the designation

“white basswood.” This commercial-use designation should not be confused with the species (T. heterophylla) having the common name white basswood.4The principal lumber species is yellow birch. It may be designated either sap birch (all sapwood) or red birch (all hardwood) or it may be unselected. Sweet birch

is sold without distinction from yellow birch. Paper birch is a softer wood used principally for turnings and novelties and is widely know as white birch. The remainingbirches are of minor commercial importance.

5Soft elm lumber is sometimes designated as white elm. A special type of slowly grown material is sometimes designated commercially as gray elm. Slippery elmis called red elm in some localities, although that term is also used for two other elms.

6Usually designated either as red gum or as sap gum, as the case may be, or as gum or sweetgum when not selected for color. (For black gum, see tupelo,footnote 9.)

7The impossibility of distinguishing between hickory ad pecan lumber for accurate species identification is recognized. Three of the four major Carya species inthe pecan group have the word “hickory” in their name.

8When hard maple or soft maple is specified to be white, the specification generally is interpreted as being a requirement for sapwood, although it sometimesmay take on the special meaning of being all sapwood with a minimum of natural color.

9The impossibility of distinguishing between black tupelo (blackgum), swamp tupelo, and water tupelo lumber for accurate species identification is recognized.10Cypress includes types designated as red cypress, white cypress, and yellow cypress. Red cypress is frequently classified and sold separately from the other

types.11Balsam fir lumber is sometimes designated either as eastern fir or as balsam.12Douglas fir may be specified either as Coast Region Douglas for or as Inland Region Douglas for, but if the particular type is not so specified or is not otherwise

indicated through the grade specifications, either or both types will be allowed.13The commercial requirements for longleaf pine lumber are that not only must it be produced from trees of the botanical species of Pinus elliottii and P. palustris,

but each piece in addition must average either on one end or the other not less than six annual rings per inch and not less than one-third summerwood. Longleaf pinelumber is sometimes designated as pitch pine in the export trade.

19

Table 1-2—Tropical wood species

Common name (other common names)1 Botanical name2

Afrormosia (kokrodua, assamela)Albarco (jequitiba, abarco, bacu, cerú, tauary)Andiroba (crabwood, cedro macho, carapa)Angelique (basralocus)Apitong (keruing, eng, in, yang, heng, keroeing)Avodire (blimah-pu, apapaye, lusamba, apaya)Balata (bulletwood, chicozapote, ausubo)Balsa (corcho, gatillo, enea, pung, lana)Banak (baboen, sangre, palo de sangre)Benge (mutenye, mbenge)Bubinga (essingang, ovang, kevazingo, waka)Caribbean pine (pino, ocote)Cativo (amansamujer, camibar, muramo, curucai)Ceiba (silk-cotton-tree, kapok-tree)Cocobolo (granadillo, funera, palo negro)Courbaril (cuapinol, guapinol, locust)Cuangare (virola, fruta dorado, miguelario)Cypress, Mexican (cipres)Degame (lemonwood, camarón, palo camarón, surr,a)Determa (red louro, wana, wane, grignon rouge)Ebony, East Indian (kaya malam, kaya arang)Ebony, African (mgiriti, msindi, omenowa)Gmelina (gumhar, yemane)Goncalo alves (palo de cera, palo de culebra)Greenheart (demerara greenheart, bibiru)Hura (possumwood, arbol del diablo, haba)llomba (gboyei, qualele, walele, otie, akomu)Imbuia (Brazilian walnut, canella imbuia)Ipe (bethabara, lapacho, amapa, cortez)lroko (semli, odoum, rokko, oroko, abang)JarrahJelutong (jelutong bukit)Kapur (keladan, kapoer, Borneo camphorwood)KarriKempas (impas, mengris)Keruing (apitong, eng, in, yang, heng, keroeing)Lauan, red, light red, and white (maranti)Lignumvitae (guayacán, palo santo)Limba (afara, ofram, fraké, akom, korina)Mahogany, AfricanMahogany, true (Honduras mahogany. caoba)Manni (chewstick, barillo, cerillo, machare)Merbau (ipil, tat-talun, lumpha, lumpho, kwila)Mersawa (palosapis, pengiran)Mora (nato, nato rojo, mora de Guayana)Obeche (arere, samba, ayous, wawa, abachi)Ocote pine (pino, ocote)Okoume (gaboon, angouma, moukoumi, N’Koumi)Opepe (kusia, badi, bilinga, akondoc, kilingi)Parana pine (pinheiro do paraná)Pau Marfim (marfim, pau liso, guatambú)Peroba de campos (white peroba, ipe peroba)Peroba rosa (amarello, amargoso, ibira-romí)Primavera (duranga, San Juan, palo blanca)Purpleheart (amaranth, palo morado, morado)Ramin (melawis, garu buaja, lanutan-bagio)Roble (encino, oak, ahuati, cucharillo)Roble (mayflower, amapa, roble blanco)Rosewood, Indian (shisham)Rosewood, Brazilian (jacarandá)Rubberwood (árbol de caucho, sibi-sibi)Sande (cow-tree, mastate, avichuri)Santa Maria (jacareuba, barí)Sapele (aboudikro, penkwa, muyovu)Sepetir (sindur, supa, kayu galu, makata)Spanish cedar (cedro, acajou rouge)Sucupira (alcornoque, sapupira)Sucupira (botonallare, peonía, tatabu)Teak (kyun, teck, teca)Wallaba (palo machete, bijlhout, wapa, apá)

Pericopsis elata (Af)Cariniana spp. (LA)Carapa guianensis (LA]Dicorynia guianensis (LA)Dipterocarpus spp. (As)Turraeanthus africanus (Af)Manilkara bidentata (LA)Ochroma pyramidale (LA)Virola spp. (LA)Guibourtia arnoldiana (Af)Guibourtia spp. (Af)Pinus caribaea (LA)Prioria copaifera (LA)Ceiba pentandra (LA)Dalbergia retusa (LA)Hymenaea courbaril (LA)Dialyanthera spp. (LA)Cupressus lusitanica (LA)Calycophyllum candidissium (LA)Ocotea rubra (LA)Diospyros ebenum (As)Diospyros spp. (Af)Gmelina arborea (As)Astronium graveolens (LA)Ocotea rodiaei (LA)Hura crepitans (LA)Pycanthus angolensis (Af)Phoebe porosa (LA)Tabebuia spp. (lapacho group) (LA)Chlorophora excelsa and regia (Af)Eucalyptus marginata (As)Dyera costulate (As)Drybalanops spp. (As)Eucalyptus diversicolor (As)Koompassia malaccensis (As)Dipterocarpus spp. (As)Shorea spp. (As)Guaiacum spp. (LA)Terminalia superba (Af)Khaya spp. (Af)Swietenia macrophylla (LA)Symphonia globulifera (LA)lntsia bijuga and palembanica (As)Anisoptera spp. (As)Mora spp. (LA)Triplochiton scleroxylon (Af)Pinus oocarpa (LA)Aucoumea klaineana (Af)Nauclea spp. (Af)Araucaria angustifolia (LA)Balfourodendron riedelianum (LA)Paratecoma peroba (LA)Aspidosperma spp. (LA)Cybistax donnell-smithii (LA)Peltogyne spp. (LA)Gonystylus spp. (As)Quercus spp. (LA)Tabebuia spp. Roble group (LA)Dalbergia latifolia (As)Dalbergia nigra (LA)Hevea brasilliensis (LA, As)Brosimum spp. Utile group (LA)Calophyllum brasiliense (LA)Entandrophragma cylindricum (Af)Pseudosindora and Sindora spp. (As)Cedrela spp. (LA)Bowdichia spp. (LA)Diplotropis purpurea (LA)Tectona grandis (As]Eperua spp. (LA)

1Additional common names are listed in Chudnoff (1984).2Af is Africa; As, Asia and Oceania; and LA, Latin America.

20

Table 1-3—Standard hardwood cutting grades1

21

Table 1-4—Standard thickness for rough and surfaced (S2S) hardwood lumber

Nominal rough thickness (in) S2S thickness (in) Nominal rough thickness (in) S2S thickness (in)

Table 1-5—Average moisture content of green wood

Moisture content’ (percent)

SpeciesHeart- Sap-wood wood

Mixedheartwood

andsapwood

Moisture content’ (percent)

SpeciesHeart- Sap-wood wood

Mixedheartwood

andsapwood

1Based on ovendry weight.

22

Table 1-6—Relative humidity and equilibrium moisture content at various dry-bulb temperatures and wet-bulb depressionsbelow 212°F.

Dry-bulbtemper-ature(°F)

Relative humidity’ and equilibrium moisture content2 (%) at variouswet-bulb depression temperatures (°F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

23

Table 1-6—Relative humidity and equilibrium moisture content at various dry-bulbtemperatures and wet-bulb depressions below 212°F—continued

Dry-bulbtemper-ature

Relative humidity1 and equilibrium moisture content2 (%) at variouswet-bulb depression temperatures (°F)

(°F) 19 20 21 22 23 24 25 26 27 28 29 30 32 34 36 38 40 45 50

24

Table 1-6—Relative humidity and equilibrium moisture content at various dry-bulb temperatures and wet-bulb depressionsbelow 212°F—continued

Dry-bulb Relative humidity1 and equilibrium moisture content2 (%) at varioustemper- wet-bulb depression temperature (°F)ature(°F) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

25

Table 1-6—Relative humidity and equilibrium moisture content at various dry-bulb temperaturesand wet-bulb depressions below 212°F—concluded

Dry-bulb Relative humidity1 and equilibrium moisture content 2 (%) at varioustemper- wet-bulb depression temperatures (°F)ature

(°F) 19 20 21 22 23 24 25 26 27 28 29 30 32 34 36 38 40 45 50

1Relative humidity values not italic.2Equilibrium moisture content values italic.

26

Table 1-7—Relative humidity and equilibrium moisture content at various dry-bulb temperatures andwet-bulb depressions above 212°F.

Dry-bulbtemper-ature

(°F)

Relative humidity1 and equilibrium moisture content2 (%) at variouswet-bulb depression temperatures (°F)

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

27

Table 1-7—Relative humidity and equilibrium moisture content at various dry-bulb temperatures andwet-bulb depressions above 212oF—concluded

Dry-bulbtemper-ature

(°F) 19 20

Relative humidity1 and equilibrium moisture content2 (%) at variouswet-bulb depression temperatures (°F)

22 24 26 28 30 35 40 45 50 55 60 70 80

28

Table 1-8—Specific gravity of wood

Species

Averagespecificgravity Species

SOFTWOODS HARDWOODS—continued

Averagespecificgravity’

29

Table 1-8—Specific gravity of wood-concluded

Species

Averagespecificgravity1

Species

IMPORTED2 IMPORTED2—continued

Averagespecificgravity1

30

Table 1-9—Calculated weights of wood per thousand board feet actual measure

Approximatecorrection factor

per 1,000 board feetfor each 1 percent

moisture content change

Species

Below Above30 percent 30 percentmoisture moisturecontent content

Weight (lb) per 1,000 actual board feet of various moisture content levels

6% 15% 25% 40% 60% 80%

SOFTWOODS

31

Table 1-9—Calculated weights of wood per thousand board feet actual measure-continued

Approximatecorrection factor

per 1,000 board feetfor each 1 percent

moisture content change

Species

Below Above30 percent 30 percentmoisture moisturecontent content.

Weight (lb) per 1,000 actual board feet of various moisture content levels

8% 15% 25% 40% 60% 80%

HARDWOODS—continued

32

Table 1-9—Calculated weights of wood per thousand board feet actual measure—continued

Approximatecorrection factor

per 1,000 board feetfor each 1 percent

moisture content change

Species

Below Above30 percent 30 percent

moisture moisturecontent content

Weight (lb) per 1,000 actual board feet of various moisture content levels

6% 15% 25% 40% 60% 80%

HARDWOODS—continued

33

Table 1-9—Calculated weights of wood per thousand board feet actual measure—concluded

Approximatecorrection factor

per 1,000 board feetfor each 1 percent

moisture content change

Species

Below Above30 percent 30 percent

moisture moisturecontent content

Weight (lb) per 1,000 actual board feet of various moisture content levels

6% 15% 25% 40% 60% 80%

IMPORTED—continued

34

Table 1-10—Shrinkage values of wood, based on its dimensions when green

Shrinkage (percent)

Dried to 20-percent Dried to 6-percent Dried to 0-percentmoisture content1 moisture content2 moisture content

Species Radial Tangential Radial Tangential Radial Tangential Volumetric

SOFTWOODS

35

Table 1-10—Shrinkage values of wood, based on its dimensions when green—continued

Shrinkage (percent)

Dried to 20-percent Dried to 6-percentmoisture content1 moisture content2

Species Radial Tangential Radial Tangential

HARDWOODS—continued

Dried to 0-percentmoisture content

Radial Tangential Volumetric

36

Table 1-10—Shrinkage values of wood, based on its dimensions when green—continued

Shrinkage (percent)

Dried to 20-percent Dried to 6-percentmoisture content1 moisture content2

Species Radial Tangential Radial Tangential

Dried to 0-percentmoisture content

Radial Tangential Volumetric

37

Table 1-10—Shrinkage values of wood, based on its dimensions when green-concluded

Species

Dried to 20-percentmoisture content1

Radial Tangential

Shrinkage (percent)

Dried to 6-percentmoisture content2

Radial Tangential

Dried to O-percentmoisture content

Radial Tangential Volumetric

IMPORTED—continued

Table 1-11—Average electrical resistance along the grain, for selected species, as measured at 80°F between two pairs of needleelectrodes 1-1/4 inches apart and driven to a depth of 5/16 inch

Electrical resistance (megohms) at different levels of moisture content (percent)

Species 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

38

Appendix—Equations for RelatingTemperature, Humidity,and Moisture Content

In this appendix, we present a series of equations thatrelate wet- and dry-bulb temperatures to specific andrelative humidities, and equations that relate EMC torelative humidity (RH) and temperature. A psychro-metric chart and an example of how to calculate spe-cific and relative humidities are included.

Wet-bulb Temperature andRelative Humidity

When unsaturated air is brought in contact with wa-ter, the air is humidified and cooled. If the system isoperated so that no heat is gained or lost to the sur-roundings, the process is adiabatic. Thus, if the wa-ter remains at constant temperature, the latent heat ofevaporation must equal the sensible heat released bythe air in cooling. If the temperature reached by theair when it becomes saturated is the same as the watertemperature, this temperature is called adiabatic sat-uration temperature or the thermodynamic wet-bulbtemperature.

When unsaturated air is passed over a wetted ther-mometer bulb, so that water evaporates from the wet-ted surface and causes the thermometer bulb to cool,an equilibrium temperature (called the true wet-bulbtemperature) is reached. At this point, the rate of heattransfer from the wetted surface is equal to the rate atwhich the wetted surface loses heat in the form of la-tent heat of evaporation. The thermodynamic wet-bulband true wet-bulb temperatures are not necessarilyequal, but in the range of 215°F to 300°F the differ-ence between these temperatures is negligible. Thus,the RH values based on the difference between the dry-bulb temperature and the thermodynamic wet-bulbtemperature do not differ significantly from RH valuesbased on the difference between the dry-bulb tempera-ture and the true wet-bulb temperature. The maximumdifference is 0.54 percent RH; on the average, the differ-ence is +0.25 percent RH.

(3)

Relative humidity can be calculated from the adiabaticsaturation temperature by the following procedure

(Hawkins 1978). By writing energy and mass balancesfor the process of adiabatic saturation

(1)

where

Y is specific humidity (lb water/lb dry air),

Ys specific humidity for saturation at Ts (lb water/lbdry air),

Tdb dry-bulb temperature (°F),

Ts adiabatic saturation temperature (°F),

and

(2)

where

ρS is vapor pressure at Ts (inHg) and

ρt total pressure (inHg).

To calculate relative vapor pressure at Tdb, it is neces-sary to calculate partial pressure ρ at Tdb as follows:

and relative vapor pressure h is

where ρ* is saturated vapor pressure at Tdb (inHg).

The RH is then defined as

RH = h × 100

(4)

(5)

Example: Given Tdb = 140°F, Ts = 120°F, andρt = 29.92 inHg, calculate the specific and relativehumidities.

39

Step 1: Find the specific humidity at Ys from equation(2). To do this we need to know the ρs of water at Ts.From table 1-A-1, ρs at 120°F is 3.446 inHg.

Thus, from equation (2)

Step 2: Calculate Y at Tdb = 140°F. Fromequation (1)

Step 3: Calculate ρ at the dry-bulb temperature fromequation (3).

Step 4: To calculate relative vapor pressure h at thedry-bulb temperature, we need to know the saturatedvapor pressure ρ* at Tdb. From table 1-A-1, ρ* atTdb = 140°F is 5.881 inHg. From equation (4)

Relative Humidity andEquilibrium Moisture Content

The EMC and RH temperature relationships of tables1-6 and 1-7 can be expressed in equation form, whichis sometimes more convenient than table form. Usefulequations can be derived from theories for the adsorp-tion of water on hygroscopic materials. One such equa-tion that works particularly well is

where

M is moisture content (percent),

h relative vapor pressure, and

(7)(8)

(9)(10)

Equations (6) to (10) represent least squares regres-sion fits of the data in tables 1-6 and 1-7. As such,they give estimates close to but not exactly the sameas those in the tables. For example, the EMC atTdb = 140°F and Ts = 120°F from table 1-6 is 8.0percent. From equations (6) to (10), the EMC is 8.4percent at 55.1 percent RH.

40

Figure 1-A-1—Psychrometric chart. (ML88 5580) Figure 1-A-2—Lines of constant equilibrium moisturecontent. (ML88 5577)

Psychrometric Charts

Psychrometric charts are another useful way to repre-sent wet- and dry-bulb temperatures and absolute andrelative humidities. Figure 1-A-1 is a typical psychro-metric chart showing the relationship between thesefour variables. Using the example Tdb = 140°F andTs = 120°F, the specific humidity at the intersectionof these two temperature lines is approximately 0.075lb/lb and the RH, 55 percent. Figure 1-A-2 shows therelationship between wet- and dry-bulb temperaturesand EMC. At the intersection of 140°F dry-bulb tem-perature and 120°F wet-bulb temperature, the EMC is8 percent.

41

Chapter 2Kiln Types and Features

Foundations and floors 51Heating systems 51

Indirect heating 52Direct heating 53

Steam traps and control valves 54steam traps 54Control valves 56

Air-circulation systems 56Kiln fans 56Baffles 58Plenum chamber 59

Venting and humidification systemsVent ing 60Humidification 60

Equipment to control drying conditionsAutomatic control equipment 61

Classification systems 43Operational techniques 43

Compartment kilns 43Progressive kilns 48

Temperatures of operation 48Low-temperature kilns 49Conventional-temperature kilns 49Elevated-temperature kilns 49High-temperature kilns 49

Type of heating and energy source 49Steam 49Direct fire 49Electricity 50Hot water and hot oil 50Solar 50

General construction features 50Construction materials 50

Aluminum 50Concrete block, poured concrete, and brickWood and plywood 51

60Classification by operational techniques distinguishesbetween the more common compartment-type kiln andthe less common progressive-type kiln.

61 Compartment Kilns

Semiautomatic control systems 61Fully automatic control systems 64Zone control 65

Manual control equipment 66Temperature-measuring devices 66Humidity-measuring devices 66

Specialized drying approaches and kiln typesDehumidification kilns 66Predryers 68Solar dry kilns 69Vacuum drying 70

Literature cited 73Sources of additional information 73Table 73

A lumber dry kiln consists of one or more chambersdesigned to provide and control the environmental con-ditions of heat, humidity, and air circulation necessaryfor the proper drying of wood. As the development ofthe modern dry kiln has progressed, a number of de-sign modifications have been explored in relation to themechanism of heat supply, arrangement and type offans, control of relative humidity or wet-bulb temper-ature, and use of various materials for construction ofthe chamber .

The design of a kiln has an important bearing on itsoperation and drying efficiency. A properly designedand operated kiln will dry most species of lumber orother wood products to any specified moisture contentbetween 3 and 19 percent in a reasonably short timewithout appreciable losses caused by drying defects.

50 Classification Systems

Dry kilns can be classified in a number of differentways. In this manual, we have chosen a system thatclassifies by (1) operational techniques, (2) tempera-tures of operation, and (3) type of heating and energysource. Other possible classifications might include fanarrangement and method of loading the kiln.

Operational Techniques

66

Compartment-type kilns (figs. 2-1 to 2-8) are designedfor a batch process in which the kiln is completelyloaded or charged with lumber in one operation, andthe lumber remains stationary during the entire dry-ing cycle. Temperature and relative humidity are keptas uniform as possible throughout the kiln, and theycan be closely controlled over a wide range of tempera-ture and humidity. Temperature and relative humidity

Chapter 2 was revised by R. Sidney Boone,Research Forest Products Technologist, andWilliam T. Simpson, Supervisory ResearchForest Products Technologist.

43

in dry kilns. (ML88 5604)Figure 2-1—Some plans for location of fans and baffles

are changed as the wood dries based on a schedule thattakes into account the moisture content and/or the dry-ing rate of the stock being dried. Drying schedules varyby species, thickness, grade, and end use of material asdiscussed in detail in chapter 7. All modern dry kilnsuse some type of forced-air circulation system, with airmoving through the load perpendicular to the length ofthe lumber and parallel to the stickers. Although somecross-circulation kilns (airflow parallel to the length ofthe lumber and perpendicular to the stickers) can stillbe found, kilns have not been built using this techniquefor several decades. The natural draft circulation sys-tem, which took advantage of the principle that heatedair rises, is now considered inefficient and is of historicinterest only (Rasmussen 1961). A more detailed dis-cussion of the different types of air circulation systemscan be found later in this chapter under the headingGeneral Construction Features.

Compartment kilns can be classified by the method ofloading. Perhaps the largest number of kilns are of thetrack-loaded type. The lumber is stacked on kiln trucks

that are rolled into and out of the kiln on tracks. Themajority of the softwood lumber in the United States isdried in track-loaded kilns. The other method of load-ing involves moving stacks or packages of lumber di-rectly into and out of the kiln with a lift truck. Theseare generally called package-loaded kilns, although theyare frequently called side-loaded kilns in the westernsoftwood region. The majority of the hardwood lumberin the United States is dried in package-loaded kilns.

Track-loaded kilns commonly have one or two setsof tracks and occasionally three sets, and are knownas single-, double-, or triple-track kilns, respectively(figs. 2-2 to 2-5). The width of the stack of lumber pertrack is typically 6 to 9 feet. In kilns more than onetrack wide, some provision for reheating the air is madebefore it passses through the next stack of lumber. Thelength of a track kiln is usually some multiple of thelengths of the lumber being dried correlated with theamount of lumber production required. Kiln lengthsvary from about 40 to 120 ft; those used for hardwooddrying are typically 40 to 66 ft long and those used for

44

Figure 2.2—Lineshaft, double-track, compartment kilnwith alternately opposing fans. Vents are over fan shaftbetween fans. Vent on high-pressure side of fans be-comes fresh air inlet when direction of circulation isreversed. (ML88 5595)

Figure 2-3—Double-track kiln with fans directly con-netted to motors. Lumber stacks are loaded endwise,and boards are stacked edge-to-edge. Air flows parallelto stickers. (ML88 5594)

45

Figure 2-4—Double-tray-loaded aluminum pre-fabricated kiln with doors at both ends of kiln.(MC88 9017)

softwood, typically 66 to 120 ft long. Lumber-holdingcapacity can vary from around 25,000 fbm (4/4 basis)to 220,000 fbm (8/4 basis).

Track kilns may have doors at one end or, more com-monly, at both ends so that unloading and loading the

kiln require a minimum amount of time. Kiln trucksloaded with green lumber are pushed into the kiln im-mediately after the dried lumber is removed from thekiln. A covered shed is frequently built over the “dry”end of the kiln to protect the dried lumber from in-clement weather while it is cooling and awaiting fur-ther processing. A cover over the “green” end of thekiln will protect the top courses of freshly sawn lumberfrom degrading in the sun as a result of uncontrolleddrying and from rain or snow. Figure 2-8 shows a kilnwith protective cover at both the dry and green ends.Frequently cited advantages of track kilns include shortdowntime for loading and unloading and more uniformdrying primarily because of narrower load widths. Dis-advantages include greater building cost, because trackkilns require more land area than package kilns espe-cially if kiln has tracks at both ends, and the addedexpense of track and kiln trucks.

Package-loaded kilns are generally smaller than track-loaded kilns and have a different configuration forloading the lumber (figs. 2-6, 2-7). Large doors per-mit the stickered and stacked lumber to be loaded intothe kiln with a lift truck. Most package kilns are de-signed to hold 24 ft of lumber from front to back of

Figure 2-5—Direct-fired, double-track-loaded high tem-perature kiln in which hot products of combustion aredischarged directly into the airstream circulating withinthe kiln. (ML88 5605)

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Figure 2-6—Package-loaded kiln with fans connecteddirectly to motors. (ML88 5598)

Figure 2-7—Lift truck delivering package of stickeredlumber to package-loaded kiln.(MC88 9024)

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Figure 2-8—Track-loaded, concrete block kiln withdoors and protective cover at both ends of kiln.(MC88 9023)

kiln, although some are designed for a depth of 16 ft oflumber. Since airflow in package kilns is from front toback, or vice versa, the length of air travel through theload is also 24 ft. No provision is generally made for re-heating the air as it passes through the load. Lumber-holding capacity of package kilns varies from around25,000 to 90,000 fbm (4/4 basis). Some frequently citedadvantages of package kilns include lower building costand use of less land area. Disadvantages include longdowntime for loading and unloading and generally lessuniform drying if initial wood moisture content is above25 percent. Using shorter air-travel distances and hav-ing all lumber at about the same moisture content in-crease drying uniformity. If starting moisture content isbelow 25 percent, uniformity of final moisture contentof lumber in package kilns is usually little different fromthat of lumber in track kilns.

Progressive Kilns

Progressive-type kilns are designed for a continuousprocess in which the loads of stacked lumber enter thegreen end of the kiln and are moved forward, usually ona daily basis, through progressively more severe dry-ing conditions until exiting the dry end of the kiln.Each move forward is accompanied by the removal ofa completed load from the dry end and the addition ofa fresh green load at the green end. The temperatureincreases and the humidity decreases as charges movefrom one zone to the next along the length of the kiln.The desired schedule effect is obtained in this way. Toachieve the necessary range of drying conditions, pro-gressive kilns vary in length depending on the species

and the initial and final moisture content of lumber be-ing dried. Because of the relatively continuous move-ment required in this approach, progressive kilns areusually of the track-loading type. As with compartmentkilns, the early models relied on natural draft circula-tion, but forced circulation using either internal fans orexternal blowers soon became the preferred method ofair circulation.

Progressive kilns lack flexibility in drying kiln chargesthat vary in species, dimension, or moisture content.They do not provide the close control of conditions re-quired by most hardwood operations or the speed ofdrying required by most softwood operations. For thesereasons, there are relatively few progressive kilns oper-ating in the United States, and no new ones have beenconstructed in several years.

Temperatures of Operation

Most lumber dry kilns are designed to operate withina specified range of temperatures. This range dependslargely on the species to be dried and quality and enduse of final products. Also considered are amount ofproduction expected, source of energy, and limitationsof certain components of the system, such as compres-sors and electric motors. A common classification ofkilns based on maximum operating temperatures is asfollows:

Low-temperature kiln . . . . . . . 120 °FConventional-temperature kiln . . . 180 °FElevated-temperature kiln . . . . . 211 °FHigh-temperature kiln . . . . above 212 °F

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Regardless of the temperatures used, the basic require-ments of controlled heat, humidity, and air circulationapply. Therefore, kilns of different temperature classi-fication differ primarily in terms of the source of heatenergy and the type of materials and equipment used inthe kiln structure.

Low-Temperature Kilns

Low-temperature kilns typically operate in the rangeof 70 to 120 °F, though some may not exceed 110 °F.This classification typically includes fan dryers, predry-ers, shed dryers, and some types of vacuum, dehumidifi-cation, and steam-heated kilns.

Conventional-Temperature Kilns

Conventional-temperature kilns typically operate in therange of 110 to 180 °F. The majority of hardwood lum-ber and sizeable amounts of softwood lumber are driedto final moisture content in kilns operating in this tem-perature range. These include steam-heated kilns andthose designs of dehumidification kilns that operate upto 160 °F. The bulk of the kiln schedules available forthe various species and thicknesses are for kilns operat-ing at “conventional temperature.”

Elevated-Temperature Kilns

Elevated-temperature kilns typically operate in therange of 110 to 211 °F. The final dry-bulb temperaturein a schedule for use in an elevated-temperature kilnis commonly 190 or 200 °F and occasionally as high as210 °F. Many western softwood operations and somesouthern pine operations have kilns operating in thisrange. A few easy-to-dry hardwood species may useelevated temperatures in the final step of the schedule.

High-Temperature Kilns

High-temperature kilns typically operate for most ofthe drying schedule at temperatures above 212 °F, usu-ally in the range of 230 to 280 °F. Perhaps the major-ity of southern pine lumber and increasing amounts ofwestern softwood lumber are dried in high-temperaturekilns. These kilns are more often used for dryingconstruction-grade lumber where some surface check-ing and end splitting are acceptable in the grade, ratherthan upper-grade lumber where these defects are lessacceptable. A very small amount of hardwood lumberis dried at high temperatures.

Type of Heating and Energy Source

The type of heating of lumber dry kilns and the energysource for that heat can be divided into the followingcategories: steam, direct fire (hot air), electricity, hotwater and hot oil, and solar. Heat is required in a drykiln for four purposes: (1) to warm the wood and thewater in the wood; (2) to evaporate moisture from thewood; (3) to replace the heat lost from the kiln struc-ture by conduction or radiation; and (4) in kilns withvents, to warm the fresh air entering the kiln.

Steam

Steam has long been the most widely used heatingmedium for kiln drying of lumber. Steam is movedfrom the boiler into the kiln by pipes, and the heat isthen transferred to the circulating air in the kiln. His-torically, many lumber processing operations requiredsteam for a variety of applications, and it was there-fore natural to include sufficient boiler capacity forkiln-drying operations. With the increasing popular-ity of electrically powered sawmills, the dry kilns arefrequently the principal user of steam at an installation.In the early days of dry kilns, burning of wood wastein the boiler was the standard procedure. As oil andnatural gas became more available and less expensive,most operations switched to these energy sources fortheir boilers. Since the “oil scare” and rising prices ofthe 1970’s, there has been a return to burning of woodwaste to generate steam. A more detailed discussion ofboilers, including such items as sizing and horsepower,can be found in chapter 11. For a more complete dis-cussion of heat transfer surfaces and how temperaturesare achieved and controlled in a kiln, see Heating Sys-tems section later in this chapter.

Direct Fire

Direct-fired heating systems differ from steam heatingsystems in that the heated air for the kiln originatesdirectly from the burning of oil, natural gas, or woodwaste. The heated air produced from the burning ofthe fuel is passed through a mixing or blending cham-ber to control the temperature and volume of air goinginto the kiln (fig. 2-5). Direct-fired systems have beenused extensively for high-temperature drying of soft-woods, especially southern pine. The required temper-atures are easily achieved and controlled, and any dis-coloration of the wood caused by combustion gases is oflittle consequence in most softwood operations. Direct-fired systems are seldom used for drying hardwoods,primarily because these systems do not provide theclose control of relative humidities generally requiredfor proper hardwood drying.

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Electricity

The use of electric power to heat a dry kiln is currentlymost often thought of as related to dehumidificationdrying systems or the type of vacuum drying systemsusing electric energy (radiofrequency, microwave, orelectric resistance blankets). I” dehumidification sys-tems, electricity is used to power the compressor orheat pump and the strip heaters that are frequentlyused to bring the kiln up to a minimum temperaturefor efficient operation of the compressors. For smallkilns drying 500 to 1,000 fbm, designs using electricstrip heaters have been suggested (Rice 1977).

Aluminum

Hot Water and Hot Oil

Some kilns are heated by hot water rather than steam.These systems have much lower drying efficiency andare not commonly found in typical commercial opera-tions. However, hot water heating systems are some-times found in smaller homemade or do-it-yourself in-stallations where steam generation is regarded as eitherimpractical or too expensive.

Few lumber dry kilns in the United States use the hotoil system, although interest in using this system hasincreased since the mid-1980’s, particularly in plantsthat have both particleboard presses and dry kilns.

Solar

In the United States and Canada, use of solar energyto heat a lumber dry kiln is limited to small operationsor hobbyists where drying large quantities of lumberon a tight production schedule is not required. Inter-est in totally solar-heated kilns or solar-assisted kilnsis much higher in tropical countries, especially thosewhere more traditional forms of energy are very expen-sive or are not readily available.

General Construction Features

Construction Materials

Dry kilns are constructed of a number of materials, in-cluding aluminum prefabricated panels, concrete block,poured concrete, brick, wood, and plywood. Variouskinds of vapor barriers are used to restrict movementof water vapor from inside the kiln into the struturalmembers and panels and thus prevent deteriorationof the structure. To have acceptable efficiency, kilnsmust be reasonably well insulated against loss of heatthrough the structure. In addition, doors and otheropenings must fit tightly to minimize loss of heat andhumidity. The choice of building materials is frequentlygoverned by such things as operational temperaturesrequired for the species and thicknesses to be dried, life

expectancy of the kiln, capital investment, insurance,source of energy, and type of heating system.

Many kilns constructed in the last decade use prefab-ricated aluminum panels with fiberglass or some formof rigid foam insulation. The panels are joined togetherand bolted to structural load-bearing members of ei-ther steel or aluminum (figs. 2-4 to 2-7). fill-lengthwall and roof panels are manufactured (prefabricated)in standard dimensions for rapid installation on siteand to give flexibility in kiln size. All connecting jointsshould be designed to minimize heat losses and to allowfor expansion and contraction of the metal with chang-ing temperature. This ability to withstand expansionand contraction without damaging the structure makesaluminum the preferred construction material for kilnsexpected to be operated at high temperatures (above212 °F).

Kiln doors are of similar lightweight, insulated, alu-minum panel construction, mounted in a steel or alu-minum frame, with additional bracing for strength andrigidity. Most doors are moved by hangers, which areconnected to rollers operating on a rail over the dooropening. Some type of flexible gasket is generally usedaround the opening to minimize air infiltration andleakage.

Because aluminum is extremely resistant to corrosion,no special vapor sealants or moisture barriers are re-quired. However, regular inspections are needed toensure that no leaks develop in the joints, and anypunctures or tears in the skin of the panel need to berepaired to prevent moisture from the kiln atmospherepassing through to the insulation and reducing its ef-fectiveness. If a steel supporting structure is used,usual precautions of applying a good paint or sealermust be observed to protect the steel from the cor-rosive atmosphere found in most kiln environments.Particular attention should be paid to locations wherethe steel support structure comes into contact withsources of cold temperature (where condensation willoccur on the steel), such as around doors and the first12 to 18 inches above floor level of the vertical supportcolumns.

Concrete Block, PouredConcrete, and Brick

Concrete block, poured concrete, and brick, which aresometimes known collectively as masonry, have histor-ically been used for construction of low-temperature,conventional-temperature, and elevated-temperaturedry kilns (figs. 2-2, 2-3, 2-8). Concrete block filled withsome type of insulation material, such as vermiculiteor rigid foam, is currently the most common type of

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masonry kiln. Kilns with poured concrete walls are oc-casionally seen, but the use of brick has largely fallenfrom use. Masonry kilns may have either load-bearingor nonload-bearing walls. Where walls are nonloadbearing, the block or brick is laid between structuralsteel members that support the roof beams or trusses.Masonry materials should be of high quality, takinginto consideration such factors as durability, insulat-ing properties, and resistance to moisture, humidity,and temperature fluctuations. A high quality mortarmust also be used. To protect the masonry against hu-midity and condensation and to reduce heat and vaportransmissions, the interior walls and ceiling must begiven one or two coats of a specially formulated heat-and vapor-resistant kiln paint or coating. Some designssuggest an inside coating of lightweight concrete to im-prove insulation and to retard moisture movement intothe concrete block. Such designs also require a vapor-resistant coating. Expansion and contraction of thesemasonry materials during routine kiln operation cancause cracks, which should be sealed promptly to pre-vent further deterioration of the wall and roof. Largelybecause of this expansion and contraction, masonrymaterials are not usually chosen when constructinghigh-temperature kilns.

Roofing materials for masonry kilns are frequently pre-fabricated aluminum panels or a “built-up” roof con-sisting of a layered composite of roofers’ felt, vapor bar-rier, and insulation on top of wood, reinforced concreteslabs, or metal decking.

Kiln doors on newer masonry kilns are frequently thesame type of aluminum prefabricated panel doors asthose used on aluminum prefabricated kilns. Someolder kilns may have doors constructed of insulatedwood panels; however, these doors are heavy and de-teriorate with time.

Wood and Plywood

The use of wood for kiln construction is usually lim-ited to low-temperature applications where inexpen-sive, short-term installations are planned, and wheresmall, possibly homemade facilities are considered ade-quate. Plywood interiors in metal or wooden buildingsare fairly common in dehumidification kilns. Construc-tion for dehumidification kilns requires insulation val-ues of R-20 or more for walls and roof; higher valuesare needed in colder climates. Vapor barriers must beextremely tight for efficient operation, and great caremust be given when installing to ensure proper joints.

Foundations and Floors

Kilns must be built on a firm foundation to preventshifting and settlement. The structural misalignmentand cracks caused by settling of the foundation are‘more serious in kilns than in many other types of con-

struction. Misalignment of kilns throws the track sys-tem out of line in track-loaded kilns, which creates se-rious problems when moving kiln trucks. Misalignmentof kilns with lineshaft fan systems can also cause wearand maintenance problems in the fan system. Settlingof the structure can cause cracks, which cause heat lossand problems in humidity control.

Foundation footings and walls are almost invariablymade of concrete. Their width or bearing area is deter-mined by the character of the soil and by the loads tobe imposed upon them.

Most kiln floors are made of poured concrete, usually6 in thick. Placing some form of insulation under theconcrete floor is an increasingly common practice. Thisreduces heat loss and helps to prevent condensation ofwater on the kiln floor in the early part of the kiln runwhen the relative humidity of the air is high and thefloor may be cold.

In some cases, a thick layer of crushed stone may beused. In package-loaded kilns, which use lift trucks toload and unload lumber, floors made of crushed stoneare difficult to maintain. Uneven floors can cause lum-ber stacks to lean, resulting in poor drying, or to fall,damaging the structure or injuring workers. Anotherdisadvantage of crushed stone is that heat is more read-ily lost to the soil or, alternatively, moisture from thesoil enters the kiln when kiln humidity is low. However,crushed stone does permit rapid drainage when conden-sation and water from melting snow or ice accumulatein the kiln during warmup.

Heating Systems

The drying of lumber requires the removal of largequantities of water from the wood. For example, dry-ing southern pine dimension stock green from the sawto 15 percent moisture content requires the removal of1.92 lb (0.23 gal) of water per board foot (24.9 lb/ft3).Drying l-in-thick red oak lumber green from the sawto 7 percent moisture content requires the removal of1.83 lb (0.22 gal) of water per board foot (22 lb/ft3).Since the heat of evaporation of water is approximately1,000 Btu/lb, great quantities of heat energy must begenerated and transferred to the circulating air and tothe wood in the drying process. This section discussesthe mechanism of heat energy transfer from the gen-erating source into the kiln and types of heat transfersurfaces.

The principal methods of conducting heat into the kilnare (1) indirect, where a hot fluid (commonly steam)flows into the kiln through pipes and radiates heat tothe kiln atmosphere through a suitable radiating sur-face, and (2) direct, where hot gases are discharged di-rectly into the kiln atmosphere.

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Figure 2-9—Headers with heating coils. (ML88 5599)

Indirect Heating

Perhaps the best examples of hot fluids used in indi-rect heating systems are steam, hot water, and hot oil.Steam systems are by far the most common in lumberdrying, though systems using hot water or hot oil areoccasionally found.

Steam.-Steam is used at various pressures. Sincethe temperature of steam varies with different levels ofpressure, more radiating surface is required to main-tain a given heat transfer rate or operating temperaturewith low-pressure steam than with high-pressure steam(see ch. 11 for a more detailed discussion of energy).

Steam is transported from the boiler to one or morekilns through large insulated pipes, often called themain feedline. At the kiln, steam enters one or moredistribution header pipes, from which each bank ofheating pipes originates (fig. 2-9). A condensate headeris located at the opposite end of the bank of pipes.Plain iron pipes were the standard material for radi-ating surfaces for many years, but now finned pipeheating coils are used almost exclusively (figs. 2-10,2-11). Depending on diameter and other factors, finnedpipes are considered to have from four to eight timesthe radiating capacity of conventional black iron pipes.Finned pipes are made of iron, aluminum, or copperpiping, which are wound with thin metal strips or at-tached to discs by welding or pressing to increase theheat transfer surface. Fins are made of various materi-als. Heavy gauge steel is the most rigid and serviceablebut is subject to corrosion, and aluminum is an excel-lent heat conductor but much more subject to dam-age. The heat transfer rate of aluminum fins is twice asgreat as that of steel fins. Copper is an excellent con-ductor but is generally considered too expensive forextensive use in lumber dry kilns and is easily damagedbecause of its softness.

Figure 2-10—Return-bend heating coil made with finpipe. (M 106142)

The return-bend heating system has historically beenthe most common arrangement of steam pipes within akiln (fig. 2-10). In this system, the banks of pipes leavethe distributing header, extend the length of shorterkilns, and return to a discharge (condensate) header.In longer kilns (over 66 ft), a return-bend header is ateach end of the kiln, such that returns meet in the mid-dle of the kiln (fig. 2-10, bottom).

It is now considered better practice to divide the heat-ing coils into banks of shorter length, single-pass coils(fig. 2-11) rather than return-bend coils. These shortbanks can be separately valved and thus produce moreuniform temperatures along their length than do longcoils.

As heat is transferred from steam through the coilsto the kiln atmosphere, the temperature of the steamdrops. It cools to the point of condensation, and water(condensate) begins to gather along the length of thecoil, providing the opportunity for uneven heating inthe kiln. Thus, all horizontal coils should be installedwith a downward pitch varying from 1/8 to 1/4 in perfoot of coil length to allow for drainage of condensate.

In multiple-track kilns where the circulating air passesthrough more than one truckload of lumber, it is goodpractice to install booster or reheat coils between thetracks (figs. 2-1 to 2-3, 2-12). The coils may be ar-ranged either vertically or horizontally and serve tomaintain a more uniform temperature within the kiln.

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Figure 2-11—Horizontal single-pass header coils andenlarged view of coils. (MC88 9027)

Hot water and hot oil—In hot water and hot oilsystems, the liquid is circulated by pumps throughheating coils similar to those used in a steam kiln. Thelower amount of heat available from hot water (whereno latent heat is present) in comparison with steamrequires a greater radiating surface. Maximum temper-ature attainable in the kiln is about 180 °F, which isadequate for many operations. However, few of thesesystems are currently in use in the United States. Hotoil systems work on the principle of pumping heatedoil through the heating coils in the kiln, though tem-peratures considerably higher than hot water can beattained.

Direct Heating

In direct-heated kilns, the hot gases produced by burn-ing gas, oil, or wood waste are discharged directly intothe kiln. These hot gases frequently pass through amixing or blending chamber to control temperature andvolume of air entering the kiln.

Burners commonly have electrically or pneumaticallymodulated fuel valves, which operate in connectionwith the recorder-controller. The fuel and air supply

Figure 2-12—Booster coils. (a) Vertical boosteror reheat coils between loads in track-loaded kiln.(b) Booster coils in horizontal position.(MC88 9032, MC88 9033)

for combustion is regulated to maintain the desiredkiln temperature. Some designs use several burner noz-zles, which can be operated individually or the seriesmodulated over a wide turndown range. Many burnersare designed to utilize wood waste and oil or gas inter-changeably.

In the blending chamber the hot products of combus-tion are mixed with the circulating air, raising its tem-perature to the point where subsequent mixing in thekiln will produce the required temperature as governedby the dry-bulb control mechanism. Temperature-limitswitches on the inlet and discharge ends of the combus-tion chamber shut down burners if they overheat. Thedischarge air is usually limited to a maximum of 425 to450 °F. A centrifugal blower forces the heated air fromthe burner through ducts to a plenum chamber, whichdistributes the air to the circulation fans (fig. 2-5).Most kiln air makes repeated circuits through the lum-

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ber piles, and only a portion is returned to the heatingchamber, usually by means of a collecting plenum run-ning the full length of the kiln. As mentioned earlier,in some designs the heat energy is transmitted from theburner through a heat exchanger to the circulating airto prevent combustion gases from entering the kiln.

Steam Traps and Control Valves

Steam traps and control valves are used to conservesteam and regulate its flow through the heating coils.

Steam Traps

In any steam kiln, large volumes of condensate formas steam cools when heat energy is transferred fromthe coils to the surrounding atmosphere. For every1,000 Btu of heat delivered, approximately 1 lb of wa-ter condenses in the steam lines. This condensate,initially at the temperature of the steam, must havea controlled discharge, otherwise the temperature ofthe coils would drop as they fill with condensate thuspreventing the entry of the higher temperature steam.Steam traps operate like automatic valves to controlthe flow and discharge of steam.

Steam traps are installed in the drain lines to removecondensate without the loss of steam. Another func-tion of steam traps is to release trapped air mixed withthe steam. Steam traps should be installed downstreamfrom and below the coils. For best operation, a strainermust be placed upstream of the trap to remove dirt andoil, and a check valve must be placed downstream ofthe trap to prevent back pressure or reverse condensateflow. A blowdown valve should be provided to peri-odically clean out scale and debris from the line. Allheating coils should be individually trapped to preventthe condensate from short circuiting from one coil toanother. The return line to the boiler must be largeenough to handle peak loads of condensate.

Proper sizing of steam traps for dry kilns is extremelyimportant and is more difficult in a dry kiln operationthan in many other applications of steam traps. It isjust as harmful to oversize a trap as it is to undersize.Undersizing a trap retards the discharge of conden-sate, which results in a slow and waterlogged heatingsystem. Oversizing a trap causes a discharge of somesteam with each discharge of condensate, which inter-feres with efficient operation of the heating system andwastes energy.

Steam traps generally used on dry kilns are of threetypes: mechanical or gravity, thermostatic, and thermo-dynamic.

Figure 2-14—Thermostatically controlled steam trap.(ML88 5596)

The mechanical or gravity-type traps often used on drykiln heating systems are of the inverted bucket or open-bucket design. The inverted bucket design (fig. 2-13)has generally superseded the open-bucket type and isthe most commonly used mechanical trap. As steamcondenses in the heating system, the condensate flowsinto the trap. When the trap is filled, the condensatedischarges through the outlet pipe. As soon as the sys-tem is free of condensate, steam enters the invertedbucket. The pressure of steam causes the bucket torise against the valve arm until the valve closes the dis-charge port. Air trapped in the bucket escapes througha vent in the top of the trap. Condensate again be-gins to flow into the trap, displacing the steam in thebucket. This reduces the buoyancy of the bucket un-til it again rests on the bottom of the trap. The dis-charge valve then opens and allows the condensate tobe discharged. Since the air in the top of the trap escapes before the condensate does, air binding is kept to

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Figure 2-15—Impulse steam trap. (MC88 9038)

a minimum. Because bucket-type traps contain liquidcondensate or water, it is important to provide ade-quate insulation in colder climates to prevent freezingof the water and damage to the trap.

In a typical thermostatic trap (fig. 2-14), a bellows thatexpands or contracts with changes in temperature isattached to a valve stem and valve. As the bellows ex-pands or contracts, it closes or opens the valve. Whenthe heating system is first turned on, the coils and trapare cold and contain air and water. At this point, thebellows are contracted and the valve is open. As steamenters the heating system, it displaces the water and airand forces them through the open valve. When all theair and water have been discharged, the trap is filledwith live steam. By then the trap temperature has in-creased enough to cause the bellows to expand, closingthe valve and preventing loss of steam through the trapoutlet. After the valve is closed, condensate again be-gins to accumulate and cool the bellows. This contractsthe bellows enough to open the discharge valve, and thecycle is repeated.

The third type of trap is the thermodynamic or impulsedesign (fig. 2-15). The flow of condensate through thistrap is controlled by differences in pressure betweenthe inlet chamber and the control chamber. When thesteam is off and the trap is filled with air, the pressure

is the same in the inlet as in the control chamber, andthe control valve rests firmly against the valve seat.When condensate enters the trap, the pressure in theinlet chamber becomes greater than that in the controlchamber. The pressure on the underside of the controldisk lifts the control valve free of the valve seat, and airand condensate pass through the valve opening into thedischarge line.

The control cylinder has a reverse taper that adjuststhe flow of condensate around the control disk and intothe control chamber, until the pressures above and be-low the disk are balanced. The temperature of the con-densate then increases because of the hot steam be-hind it. The hot condensate entering the lower pres-sure control chamber flashes into steam, which increasesin volume and retards the flow of condensate throughthe control-valve orifice. When the downward pres-sure on the upper surface of the valve and valve diskexceeds the upward pressure on the rim of the valvedisk, the valve is forced downward, shutting off the flowof condensate through the main orifice. The tempera-ture in the control chamber then drops, and the cycle isrepeated.

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Figure 2-16—Fans in lineshaft arrangement showingdisc fans and zig-zag baffle-shroud system, which di-

Control Valves

Both manually and automatically operated valves areused to control the flow of steam into the coils. Pres-sure regulators and reducing valves are also used tocontrol the pressure of the steam.

Steam flow is regulated by automatically controlled air-operated or electrically operated control valves coupledto the recorder-controller (see section on Equipmentto Control Drying Conditions). Hand-operated gatevalves are usually installed upstream of the controlvalves for “on-off” control of the steam supply. Handvalves are also advantageous on the feed and drain linesof individual heating-coil banks, especially in hardwooddrying operations. These hand valves enable operatorsto close certain banks for better control at lower tem-peratures, thereby reducing excessive fluctuations intemperature due to overshoot of the dry bulb when allbanks are open. The ability to isolate banks of coilsalso permits damaged or leaking ones to be removedand repaired without disturbing the remainder of theheating system.

rects the air through the lumber in either direction de-pending on fan and motor rotation. (MC88 9022)

Air-Circulation Systems

To dry lumber, air of controlled temperature and hu-midity must be passed uniformly over its surface. Thiscirculating air is the “workhorse” of the dry kiln. Assuch, the air performs two functions: it carries heatto the wood to effect evaporation, and it removes theevaporated water vapor. Effective and uniform circu-lation of air involves several factors: the size, location,and speed of the fans to drive the air; provision for re-versal of air circulation; installation and use of bafflesto direct the air through the load; and placement ofstickers within the load to facilitate the movement ofair across each piece of lumber.

Kiln Fans

In modern kilns, fans can be classified in two broadcategories: internal fan kilns, that is, fans located in-side the kiln itself; and external blower kilns, a systemwhere the fan or blower is located outside the kiln andthe air is conducted into the kiln through ducts.

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Figure 2-17—Control room for battery of lineshaftkilns, showing motor and pulley on lineshaft, recorder-controller, air-operated control valves to headers,

Before discussing different types of fans in these twocategories, it may be helpful to review the followinglaws regarding fans: (1) the volume of air moved variesdirectly with the fan speed in revolutions per minute(rpm), (2) the static pressure varies with the squareof the fan speed, and (3) the horsepower varies as thecube of the fan speed and directly as the air density.For more detailed discussion of fan engineering andpower consumption, see chapter 11.

Internal fans.-For internal fan kilns, there are twoprincipal arrangements of the fans: lineshaft and cross-shaft. In both of these arrangements, the fans are typ-ically placed overhead, with a false ceiling or deckbetween the fans and the load of lumber but not ex-tending beyond the edge of the lumber (figs. 2-1 to 2-3,2-5, 2-6).

In the traditional lineshaft arrangement, a series ofmultibladed disc fans (up to 84 in. in diameter in somelarge softwood kilns) is mounted on a single shaft run-ning the full length of the kiln. The fans are alternatelya left- and right-hand design. They are housed in a zig-zag baffle-shroud system that directs the air across thekiln (figs. 2-2, 2-16). So that air circulation may be re-versed efficiently, the fans are designed to operate in ei-ther direction. The motor, usually 50 to 75 horsepower,

hand-operated valves, and air-motor controlling vents.(MC88 9021)

is generally located in the operating room or controlroom at the end of the kiln (fig. 2-17). This type oflineshaft arrangement provides for moving large vol-umes of air at low speeds (up to 400 ft/min throughthe load) with a minimum of power, and it is particu-larly suited to drying lumber with low initial moisturecontent or a species that needs to be dried slowly.

In a more recent adaptation to the lineshaft arrange-ment, propeller-type fans are mounted on the lineshaft(fig. 2-18). This modification can deliver upwards of800 ft/min through the load, and the propeller-typefans are considerably more efficient per motor horse-power than disc fans. When changing from disc topropeller-type fans in retrofit operations, it may be nec-essary to change the type of bearings used for the shaft.

In the cross-shaft arrangement, fans are mounted onindividual shafts aligned across the width of the kiln(figs. 2-3, 2-5). Each fan is driven by an individual mo-tor (usually about 7.5 hp) either belt driven or directconnected. The motor may be mounted inside or out-side the kiln. Motors mounted inside the kiln must beof special construction to withstand high temperatures,especially in kilns operating above 200 °F. With exter-nally mounted motors, consideration should be givento offering some protection from the weather, particu-

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Figure 2-18—Propeller-type fans mounted on lineshaft.(MC88 9020)

larly in colder climates where freezing of condensed wa-ter vapor on the motor or shaft may present problems.Either multiblade-disc or propeller-type fans are com-monly used for cross-shaft kilns. They can deliver largevolumes of air at speeds considerably higher than thefans found in the traditional lineshaft kilns. With themodern trend to higher air velocities, especially desir-able in the high-temperature kilns, propeller-type fansare becoming increasingly popular. These fans havetwo to six blades, some of which have adjustable pitch;are made of cast aluminum; operate at high revolutionsper minute; and are capable of producing air velocitiesof 1,500 ft/min or more (fig. 2-19).

Traditionally, kilns have been designed such that fanspeeds and thus the velocity of air through the load oflumber do not change during the time of the kiln run.However, for the most efficient drying, higher airspeedsare needed during the early stages of drying when thewood is wet and large quantities of water need to beevaporated. Later in the drying schedule, lower air-speeds are adequate as the wood becomes drier and lessmoisture needs to be evaporated. As electrical energycosts have increased over the last decade, there hasbeen increasing interest in installing control equipmenton fan motors so that fan speeds can be adjusted dur-ing the run, thus saving on energy costs. The amountof savings appears to be higher in softwood drying,

which generally starts with relatively high moisturecontent woods that can be dried rapidly with minimaldrying degrade. In hardwood drying, which uses milderschedules and slower drying, less energy costs are ap-parently saved through reduction in fan speeds in thelater stages of the kiln run. Perhaps the greater advan-tage of variable fan speeds is to provide flexibility inairspeed requirements for those operations that dry anumber of species that differ markedly in airspeed re-quirement, such as pine, maple, and oak. Continuinginterest and research in the area of variable speed fansis expected as electrical energy costs rise and cost ofcontrol equipment becomes more competitive.

External fans.—External blower systems, though notas widely used as internal fan systems, offer anotherapproach to air circulation. These commonly use onlyone motor and blower to move air into the kiln. In thissystem, air is drawn from the discharge side of the loadthrough large ducts to an external centrifugal blower,from which the air is passed over the heater, humidifiedto the proper level, and redistributed in the kiln by an-other set of ducts to the high-pressure side of the load.The disadvantages of this approach are the low air ve-locities caused by the length of the necessary ductworkand the fact that the direction of air circulation is dif-ficult, if not impossible, to reverse. The advantage ofthis approach is that the air circulation system (themajor moving parts of a kiln) is concentrated in an eas-ily accessible place and can be readily serviced.

Baffles

To achieve uniform and, where desired, rapid drying,the properly heated and humidified air must be uni-formly directed to and through the lumber. To do thiseffectively, all alternate flow paths must be blocked sothat airflow over, under, and around the load is pre-vented. The best practical way to do this is by usinghinged baffles. The lack of effective use of baffling isone of the major causes of uneven or too slow drying.Airflow under the load in a track-loaded kiln may beprevented by having baffles hinged to the floor that canbe turned up against the kiln trucks to prevent air by-passing under the load. An alternative is to constructthe floor of the kiln with a trough just wide enough toaccommodate the rails and trucks and high enough sothat the lowest course of lumber just clears the level ofthe floor (fig. 2-2). The use of ceiling-hinged baffles ar-ranged so their lower free edge rests on the top of theload is an effective way of preventing airflow over thetop of the load. As the load shrinks during drying, thebaffles must have the ability to move down to keep con-tact with the load (fig. 2-6). Airflow around the ends ofthe load can be prevented by mounting bifold-hingedbaffles in or near kiln corners, ensuring contact withthe ends or corners of the load. A real effort shouldbe made to construct all kiln loads so that no holes or

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Figure 2-19—Propeller-type fans of cast aluminum in cross-shaft arrangement. (MC88 9019)

gaps occur between stacks because of mixed lumberlengths or stacks of uneven height.

Considerable care must be taken by personnel unload-ing the kiln to make sure all hinged baffles (floor, ceil-ing, and end) have been moved away from load beforestarting to move the load out of the kiln. Failure to doso results in baffles being ripped off or damaged. If baf-fles are damaged, they should be replaced immediatelyso that uniform air circulation can be maintained.

Plenum Chamber

The proper design and use of the plenum chamber orplenum space are necessary for adequate and uniformair circulation in a kiln. The plenum chamber is thespace between the lumber and the wall on either sideof a track-loaded kiln or between the lumber and thedoor or wall in a package-loaded kiln (figs. 2-2, 2-6).This area provides space for the fans to build up slightair pressure before passing through the courses of lum-

ber, thereby improving the uniformity of air distribu-tion through the load. When the fans reverse direction,the positive pressure reverses sides; the other side isalways under slightly negative pressure. The plenumchambers should be wide enough so that the staticpressure built up in them is sufficient to ensure uni-form air flow across the loads from bottom to top. Afrequently heard rule-of-thumb for estimating plenumwidth is that the width of the plenum should be equalto the sum of the sticker openings. Thus, if the sumof the sticker openings from top to bottom on one sideof the load is 60 in, the plenum on that side should beabout 60 in wide. A properly designed and loaded kilnwill have adequate plenum space.

It would be a mistake in loading package kilns to putan extra row of packages in what should be the plenumspace on the door side. This results in improper andnonuniform air circulation, and it is a practice to bestrongly discouraged.

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Venting and Humidification Systems

As mentioned before, drying of lumber requires the re-moval of large quantities of water from the wood. Inconventional kilns, the water is carried from the sur-face of the wood by the air passing over the wood. Toachieve proper drying of lumber, the amount of mois-ture in the kiln atmosphere (humidity) must be pre-cisely controlled. When the humidity inside the kilnis higher than desired, the excess moisture is ventedto the outside atmosphere and replaced with air fromthe outside. When the humidity inside the kiln islower than desired, additional moisture is added tothe kiln atmosphere by a steam spray or water spray-atomization.

Venting

Excess kiln moisture can be vented in one of two ways:(1) by static venting with the fans required for air cir-culation in the kiln or (2) by pressure venting with anadditional fan and ductwork.

In static venting, vents are placed in the roof on theintake and exhaust sides of the fans. When the ventsare opened, fresh air is drawn in on the suction sideof the fan and moist air forced out on the pressureside (figs. 2-1 to 2-3, 2-5). When the direction of rota-tion of the fans is reversed, the flow of air through thevents is also reversed. The size and number of ventsrequired depend on the species to be dried, that is,the amount of water to be removed from the wood.Species with large quantities of water, such as mostpines and poplars, require more ventilation than specieswith lower initial moisture content, such as oak or hardmaple or woods that have been air dried or partiallydried in a predryer. Kilns may have one or two linesof vents running the length of the kiln depending onthe fan arrangement (lineshaft (fig. 2-2) or cross-shaft(figs. 2-3, 2-5, 2-6)). Each line is automatically openedand closed by pneumatically or electrically powered mo-tors activated by the recorder-controller system. Insome cases, an additional row or two of manually oper-ated vents are located on the roof. Opening these ventscan provide additional venting when drying speciesthat require large venting capacities (such as sugarpine or white pine). Static venting is the most commonmethod of venting currently used in dry kilns.

In the pressure or powered venting systems, roof open-ings are replaced with two identical metal ducts placedinside the kiln, running the full length in the zoneabove the fan deck. These ducts vent to the atmo-sphere through louvered openings. Adjustable open-ings along the length of each duct regulate the volumeof air discharged into or withdrawn from the kiln; thusair is distributed uniformly throughout the kiln. A fanunit at the end of each duct acts interchangeably as

intake or exhaust, depending on the direction of aircirculation. When the fans are reversed, the ventingsystem also reverses, with the louvers actuated to openimmediately before the fans start. These systems aredesigned to exhaust moist air that has already passedthrough the lumber and bring in drier air on the pres-sure side of the fan. The greater capital cost of such asystem is claimed to be more than offset by lower main-tenance costs.

The vent system in any kiln exhausts more air volumethan it draws into the kiln to accommodate the expan-sion in volume of cooler incoming air as it is heated tothe higher kiln temperatures. In the case of poweredventing, this is accomplished by the design of the fanblade airfoil. The venting system is regulated by therecorder-controller mechanism as in normal roof vent-ing. In some direct-fired kilns, the centrifugal blowerproduces a type of powered ventilation by venting mois-ture through a damper in the return-air duct to theblower.

When venting is used to control excess moisture inthe air, substantial amounts of heat energy are thrownaway or wasted. This phenomenon has been recognizedfor some time, but the energy crisis of the 1970’s in-creased the interest in developing heat exchangers oreconomizers to use or reclaim some of the energy ex-hausted in vent air (Rosen 1979). By the mid-1980’s,at least one system had been developed that has proveneconomically feasible in western softwood kilns. Theair-to-air heat exchanger has replaced the need for tra-ditional venting; it preheats the incoming or makeupair to the kiln. It seems likely that other systems or im-provements to this system will be forthcoming, and theeffectiveness and savings in energy costs to heat kilnswill increase over the next decade.

Humidification

Control of the wet-bulb temperature or humidity inthe kiln is important during the drying, equalizing, andconditioning stages of the drying operation. Close con-trol of wet-bulb temperatures is especially importantin the early stages of drying hardwood species that areprone to surface checks, such as oak and beech, and tominimize surface and end checking in the upper gradesof softwood species. Close control of wet-bulb tempera-tures is also important during the conditioning phase atthe conclusion of the kiln run of any species requiringthis stress-relief treatment.

As mentioned earlier, when the humidity or the wet-bulb temperature of the kiln atmosphere is lower thandesired, additional moisture is added. In steam-heatedkilns, humidity is usually supplied as steam spray fromthe same source that supplies the heating coils. Steamis ejected through special nozzles on a steam spray

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line located in the airstream adjacent to the circula-tion fans, so the spray is mixed with the circulating airbefore it reaches the lumber (figs. 2-2, 2-6). As withthe heating system, steam spray is regulated by therecorder-controller.

If high-pressure steam is used for heating the kiln andis available for humidification, it should not be useddirectly to humidify the kiln. Use of high-pressuresteam adds a considerable amount of heat to the kilnin addition to increasing the humidity. This may causefluctuation or overshoot of the dry-bulb temperaturesuch that it is difficult or impossible to maintain thewet-bulb depression desired. This may be especiallytroublesome during conditioning when controlling wet-bulb depression is critical and adding large quantitiesof steam is necessary to increase the wet-bulb temper-ature. Steam pressure for the steam spray line should,therefore, be reduced to about 15 lb/in2-gauge by apressure regulator some distance before the line en-ters the kiln. If permitted by safety regulations, theportion of the line between the regulator and the kilnshould be left uninsulated so that the superheat in thesteam can dissipate, and the steam for humidificationwill be cooled to near saturation (250-260 °F). Anotheralternative is to install a desuperheater in the sprayline. This device injects water as a fine spray or mistinto the steam spray line, thereby removing the super-heat and reducing the temperature of the steam to nearsaturation.

In some installations that do not have a source ofsteam for humidification, water sprays are sometimesused. The water should be injected into the kiln in theform of a fine mist. It is highly desirable to heat thespray water since cold water has an appreciable cool-ing effect in the kiln and can cause fluctuation of thedry-bulb temperature and poor control of drying condi-tions. In some kilns, water sprays are used in conjunc-tion with steam sprays, but extra care must be taken toprevent water droplets from falling on the lumber andcreating stains.

For close control of wet-bulb temperatures in direct-fired kilns where no steam is available from the centralboiler, a small boiler may need to be installed to gener-ate the large volumes of low-pressure steam required forproper conditioning of the lumber.

Equipment toControl Drying Conditions

While drying conditions in most commercial dry kilnsare controlled by automatic or semiautomatic con-trollers, manual control is sometimes used in smallerinstallations or home-designed equipment.

Automatic Control Equipment

Automatic systems can be further divided into semiau-tomatic and fully automatic. Semiautomatic systemsrecord and control on set points that are changed fromtime to time during the kiln run by an operator. Infully automatic systems, process control informationis entered at the start of the kiln run, and any neededchanges are made automatically by the equipment dur-ing the kiln run.

Several process control techniques, some using special-ized equipment, are available for use with either thesemiautomatic or the fully automatic control equip-ment. They include zone control (see Zone Controlsection), variable frequency speed control for fans (seeKiln Fans section), and in-kiln moisture meters. In-kilnmoisture meters are generally of two types: (1) the re-sistance meter, in which electrodes (pins) are drivenor screwed into boards in the charge of lumber, and(2) the capacitive admittance meter, in which the elec-trode (a strip of metal) lies flat on the surface of thelumber. The electrode is slipped into the load parallelto the stickers. Electric signals on both systems areconverted to moisture content values and read on ameter. Both types of meters are subject to tempera-ture corrections and are not considered very reliable atmoisture contents above 30 percent. In-kiln resistancemeters are frequently used to monitor moisture contentof drying lumber below 30 percent and may be used tocontrol kiln schedules. Capacitive admittance metersare most commonly used in softwood kilns to monitormoisture content of drying lumber below 30 percentand are frequently used to determine when a charge isfinished. Some capacitive admittance meters are con-nected to the controller to shut the kiln down when apredetermined moisture content is achieved.

Semiautomatic Control Systems

Semiautomatic dry kiln control systems are typicallycharacterized by having a recorder-controller. This in-strument continuously measures and records on a chartthe conditions prevailing in the kiln and controls theheat and humidity to conform to the conditions presetby the kiln operator. As drying progresses, the oper-ator changes the instrument set-points to the desiredconditions in the kiln. This may be done based on timeelapsed since the start of the run or on the currentmoisture content of the wood as measured by a sam-

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pling technique such as weighing sample boards. Thefirst is more typical of a softwood drying operation, andthe latter is more commonly used in drying hardwoods.Once a dry-bulb and a wet-bulb temperature have beenset, the instrument automatically controls the condi-tions until they are reset.

Signals indicating the current conditions in the kiln arereceived at the recorder-controller from sensors locatedin the kiln. There is typically only one wet-bulb tem-perature sensor in a kiln but multiple dry-bulb temper-ature sensors. This is because the wet-bulb tempera-ture is essentially the same throughout the kiln, but thedry-bulb temperature may vary considerably over thelength and height of the kiln. The instrument comparesthese kiln conditions to the instrument set-point condi-tions, Changes in the kiln conditions are made throughsignals to air-operated valves that open or close heatingsystems, valves that open or close vents electrically, andhumidification systems as necessary to bring the kiln toset-point conditions.

For many years, the recorder-controller and its com-panion valve systems worked in an on-off mode; thatis, the controller told the valve to be completely openor completely closed. This method often wastes energyand does not offer as close control of kiln conditions asmay be desired. More recently the use of proportionalvalves and controllers has become the accepted prac-tice in most kiln operations. In this approach valves areopen to varying degrees depending on how far the kilnenvironment deviates from set-point conditions, thusoffering more precise control and saving energy.

Sensors currently used in lumber dry kilns are of threetypes. The traditional sensor used for over 50 years isthe gas-filled or liquid-vapor system. A more recentintroduction is an electric system using a resistancetemperature detector (RTD). A third type of sensoris used to measure equilibrium moisture content (EMC)of the kiln atmosphere. This sensor measures EMC di-rectly by electric resistance measurements across elec-trodes clamped to a small wood specimen or cellulosepad (EMC wafer) mounted in the kiln.

Liquid-vapor or gas-filled systems consist of four mainparts: (1) the temperature-sensing bulbs inside thekiln, (2) the armor-protected capillary tubes connect-ing the bulbs with the recorder-controller, (3) the he-lical movement (Bourdon tubes) inside the recorder-controller that provides the mechanical force to movethe pens on the recorder chart and the air relay por-tion of the controller, and (4) the clock movement thatturns the recording chart.

The dry-bulb and wet-bulb temperature-sensing unitsare connected individually by long capillary tubesto the Bourdon tubes inside the recorder-controller

Figure 2-20—Internal view of three-pen, gas-filledrecorder-controller (Moore type) showing Bourdontubes, air relays, clock, gauges, and dials. (MC88 9018)

(fig. 2-20), which is normally located in the kiln controlroom. The bulbs and capillary and Bourdon tubes aresealed with a volatile liquid (butane) and its vapor. In-creasing kiln temperature causes an increase of pressurein the liquid-vapor system; the capillary tube transmitsthe pressure change into the helical or Bourdon tube,causing it to expand. This movement is transmittedto the pen arm, which moves radially outward on therecording chart to indicate the increase in temperature.When the temperature in the dry kiln decreases, thereverse process takes place.

A typical dry kiln is usually equipped with one wetbulb and two or more pairs of dry bulbs. The wet bulbmeasures the wet-bulb temperature in the kiln result-ing from the cooling effect of evaporation on the moistwick and controls the humidity in the kiln through theinstrument. The paired or dual dry-bulb system (twobulbs connected to a common capillary tube) measuresand controls the temperature of the kiln environmenton the entering-air side of the lumber load in the kiln.The entering-air side of the load will always be the hot-ter side. When the air circulation reverses, the oppositeside becomes hotter, and the bulb on the opposite side

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of the load becomes the controlling bulb. Larger kilnshave two or more pairs of dual-control bulbs to bettercontrol the temperature in different zones of the kiln.

The controlling function of the liquid-vapor instru-ment is a pneumatic system of operating valves thatcontrol the amount of steam entering the dry kiln. In-side the recorder-controller case of a Moore instrument,the capillary tube from each bulb system is divided,with one lead going to the recording function and theother to a second Bourdon tube. Foxboro and Honey-well liquid-vapor instruments do not split this capillarybut achieve the same results using mechanical linkages.As pressure changes within the system, needle-type airvalves are brought into play, thereby accurately con-trolling heat input into the kiln and also controllingventing and spray or humidification.

Although the gas-filled or liquid-vapor control systemhas been time proven to be very dependable and ade-quately accurate, it does have some disadvantages whencompared to the newer electronic recorder-controllers.

Electronic recorder-controllers use platinum RTD-type bulbs for sensors of both dry-bulb and wet-bulbtemperatures and are connected to the instrument by16-gauge, three-conductor lead wire. The recordingfunction of the RTD control system contains an elec-tronic servo module that measures resistance changesof a RTD and positions the pen accordingly on thechart. The instrument contains a separate servo mod-ule for each measuring system (fig. 2-21). For example,a three-pen RTD electronic control system will havethree servo module units, one for the wet bulb and twofor the dry bulbs.

The principal element of the controlling system is ei-ther a modulating or an off-on pneumatic control unit,which tracks the measured variable through movementof the pen linkage. When the measured variable crossesthe set point, the control unit actuates a pneumatic orelectrical relay, which in turn sends an air signal to thecontrol valves, activating them as required.

The heart of the RTD electronic system is the servomodule assembly, which contains an electronic bridgecircuit, balancing amplifier, slide wire, and direct-current balance motor. One of the elements of the elec-tronic bridge circuit is a resistance bulb that senses thedry-bulb or wet-bulb temperature. An external relayswitches in the appropriate resistance bulb when fansreverse, thus assuring measurement of entering airtemperature.

Figure 2-21—RTD sensor and instrument.(M87 0167, M88 132-4)

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The advantages and disadvantages of the liquid-vaporand RTD control systems are as follows:

1. The RTD electronic controller has a fast responsetime, with a nominal period of 4 s for total pentravel over the total chart radius (about 4-3/8 in).This is extremely fast compared to a liquid-vaporcontroller, especially if the capillary system is over50 ft long. The graduations on the recording charton liquid-vapor instruments are nonlinear; the spacesbetween lines are closer nearer the hub and fartherapart on the edge. The graduations on the record-ing charts of the RTD controllers are linear over theentire range, thus making the task of setting andreading temperatures easier.

2. The RTD system is not limited by the length of leadrequired. The control instrument can be mountedat distances of up to 2,000 ft with no loss of accu-racy or response time. Any temperature changescaused by variation in lead length are compensatedfor automatically. By comparison, the liquid-vaporcontrollers are generally limited to capillary lengthsof about 100 ft, and the capillaries may be affectedby temperature changes between the sensing bulbsand the control instrument.

3. Temperature ranges can be easily changed on theRTD electronic controller by simply removing theexisting range card and replacing it with a newrange card. The liquid-vapor system requires remov-ing the instrument from the kiln and returning it tothe manufacturer or repair facility, where the sys-tem has to be refilled and recalibrated with specialequipment.

4. The liquid-vapor instruments are sensitive to bulblocations related to the instrument mounting (higheror lower). If these distances change for any reason,calibration is affected. The RTD system is not af-fected by bulb location, and sensing bulbs can bemoved at any time without affecting the calibrationof the instrument.

5. If any damage occurs to the sensing system of theRTD controller, it can be repaired at the site. Sens-ing bulbs can be replaced in a matter of minutes,and damage to lead wires can be repaired withoutany change in calibration or accuracy of the instru-ment. Liquid-vapor systems require removing theinstrument from the kiln with all capillary lines andbulbs intact and returning the instrument to themanufacturer or repair facility.

6. Perhaps the biggest advantage of the RTD electroniccontroller is the ease of calibration. Unlike the time-consuming two-person operation of using bucketsof hot water or hot oil and an etched stem ther-mometer required for calibrating the liquid-vaporsystem (see ch. 4), the calibration of the RTD con-troller is a very simple one-man operation using a

decade box. A given amount of electrical resistancecan be applied to the instrument for various tem-perature ranges, and a direct readout on the chartindicates either proper or improper calibration. Ad-justments are done very easily at the front of theinstrument by simply adjusting the appropriate link-age. Note that this technique calibrates only theinstrument, not the RTD sensor. The sensor is gen-erally assumed to be accurate. Proper resistance inthe RTD sensor can be checked against an electronicbridge. To check the total system, sensors and in-strument, it is suggested that the sensor(s) be placedin an ice-water slurry (32 °F) and then boiling wa-ter (212 °F) and the respective values read on theinstrument chart.

Fully Automatic Control Systems

In this manual, fully automatic control means the pro-cess control information or other drying schedule in-formation is entered at the start of the kiln run. Anychanges in temperature or humidity are made auto-matically by the controller during the kiln run. Thesechanges may also include determination of final targetmoisture content and shutdown of the kiln. Overridechanges are possible with these systems, but seldomused. This procedure differs from semiautomatic con-trol in which the recorder-controller effectively main-tains preset conditions but does not change set points,which must be changed by the operator. Fully auto-matic systems range from cam-operated controllers,used in some regions for several decades, to controllersbased on load cells that weigh the load or part of it, tothe rather recently introduced computerized controllersthat measure or infer changing lumber moisture contentin the kiln.

Cam controllers represent the earliest attempt at fullyautomatic control. They are a form of time-basedschedule and depend on the assumption that for agiven species, thickness, and grade of lumber, the loadmoisture content and hence the conditions in the kilnwill depend on the length of time drying has been inprogress. Two specially cut cams are required, one tocontrol the dry-bulb temperature, the other to controlthe wet-bulb temperature. Different cams have to becut for different species and thicknesses.

The advantages of cam controllers include the follow-ing: (1) schedules are predetermined and monitoringis minimal; (2) schedules can be ramped or movedsmoothly from one set of conditions to the next ratherthan arranged in steps, which cause abrupt changes inconditions and which may waste energy or put extraloads on the boiler; and (3) cams can be cut to givevery predictable, reproducible results, based on expe-rience in drying a given thickness(es) of given speciesstarting at similar initial moisture contents.

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The disadvantages of cam controllers include the follow-ing: (1) there is no direct link between the controllerand the moisture content of the lumber at any giventime during the kiln run—a load drying more slowlythan usual could easily be shut down while at a mois-ture content higher than desired; conversely, a load dry-ing more rapidly than usual could easily be overdried;(2) a recorder-controller rigged for following cams isnot readily converted to other forms of set-point de-termination; (3) care and experience are necessary tocut accurate cams; and (4) failure to monitor at fre-quent intervals may result in not implementing neces-sary changes in response to unforeseen factors such asboiler shutdown, steam leak, or loss of water to the wetbulb.

Load-cell systems are available that weigh the load ora portion of it and make changes in the schedule as thelumber dries. Selected boards are sampled to determineinitial or “green” weight in the usual way by cutting,weighing, ovendrying, and reweighing moisture sections(ch. 6); these values are averaged or weighted. This in-formation together with details of the schedule to befollowed are preprogrammed into the controller at thestart of the run, and the system takes complete con-trol of the drying operation. The main disadvantages ofthis approach are the problems of sampling and deter-mining reliable initial moisture content values, dryingon the average moisture content of the load or portionsampled, and lacking an indication of board-to-boardvariation in moisture content in the load during dry-ing. A preferred approach would be to use very smallload cells to follow the weight loss of individual sampleboards and to make these data available to the controlsystem by board or in groups of boards.

Since about the middle 1980’s, computerized controllershave been introduced in both softwood and hardwoodoperations. The introduction of desk-top-sized personalcomputers has provided a big boost to computerizedcontrol systems. Computerized control systems canrange from those that are little more than electric camtime-based systems to those that measure the mois-ture content of the wood in certain ranges and infer themoisture content of the wood in other moisture con-tent ranges. Since there is currently no reliably accu-rate method of measuring wood moisture content abovethe fiber saturation point (about 30 percent) exceptby weighing, values above 30 percent are inferred fromcontrolled temperature and relative humidity condi-tions in the kiln. Moisture values below fiber saturationpoint are determined by measuring the electrical resis-tance between metal pins or electrodes driven into theboard. Pins may be of different lengths so that mois-ture contents in the core and near the surface may bemonitored and some idea of gradient may be deter-mined. Some systems not only closely monitor and con-trol temperature and humidity conditions in the kiln

but also make changes in fan speeds and monitor orcontrol energy consumption. One computer may con-trol from 1 to as many as 8 to 10 dry kilns.

Computerized kiln controllers will likely find wider ac-ceptance in the lumber industry in the future. Manycurrent installations have shown that computerizationcan make the operation of dry kilns easier and can re-duce the cost of producing high-quality lumber. Thetechnology is advancing rapidly, and as we learn tosense more variables such as shrinkage, stress, woodtemperature, and moisture content, we will add tothe precision with which computer controllers can drylumber.

Zone Control

Zone control is a process control technique for equal-izing dry-bulb temperatures throughout the kiln andcan be used with either semiautomatic or fully auto-matic control systems. Cool spots in the kiln have longbeen noted for uneven drying, producing lumber that ishigher in moisture content than desired. Hot spots tendto produce lumber that is drier than desired. In zonecontrol, the kiln is divided into several zones, with tem-perature sensors coupled to control valves or dampersin the heating system. Zones typically run along thelength of the kiln; some designs have vertical zones aswell. The number of independently controlled zones canvary from 2 up to 24. Historically, zone control withgas-filled recorder-controllers meant that long (66 ftor longer) kilns were divided into two zones: one zonefor each end, or one zone control for operating the re-heat coils in a double-track kiln and another zone con-trol for operating the overhead heating coils. Comput-erized control with electronic RTD sensors has madeit possible to control a much larger number of zones.With computerized control, paired sensors measure the,temperature drop across the load (TDAL or ∆T) andseek, through their circuits with control equipment, tokeep the drying rate at the same level in all zones. Thistechnique is successful and rather widespread in newerhigh-temperature softwood kilns in both the southernand western United States. It is expected to becomemore common in older remodeled or retrofitted soft-wood kilns and some hardwood operations. However, inconventional-temperature hardwood kilns, the TDAL isusually so small that trying to control using this vari-able is not very promising.

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Manual Control Equipment

Some form of automatic kiln control is commonly usedon commercial kilns in the United States and Canada.However, manual control is possible and is generallyof interest to very small operations, often using homedesigned equipment, and to operations in which a per-son monitors the kiln on nearly a full-time basis. Forsuccessful manual control of drying conditions, the dry-and wet-bulb temperatures must be known. If thesetemperatures differ from those desired, the valves thatregulate the flow of steam (heat) and spray (humidity)into the kiln must be adjusted until the desired temper-ature readings are obtained. The appropriate amountof venting must also be watched and adjusted. To keepthe temperature or temperatures reasonably close tothose desired requires considerable operator time formonitoring and making minor adjustments to valves.

Temperature-Measuring Devices

The temperature-measuring devices commonly usedfor manual control are of two classes, indicating andrecording. Glass-stemmed indicating thermometers arefrequently used. The most satisfactory glass-stemmedthermometers have the graduations etched on the stem.Thermometers with separate scales stamped on an at-tached metal strip are not very satisfactory, since anyshifting of the strip with relation to the thermometertube will result in incorrect readings. Indicating dig-ital thermometers have largely replaced the pressure-spring type mentioned in the earlier edition of thismanual. The sensor for these digital thermometers maybe either a thermocouple or a RTD. Type–T (copper-constantan) thermocouple wire is suggested for mostdry kiln use.

Glass-stemmed indicating thermometers of the maxi-mum type are also used to obtain dry-bulb tempera-tures. Maximum thermometers show the highest tem-perature to which they have been exposed. After eachreading, they must be shaken down like clinical ther-mometers. Care should be taken in using maximumthermometers to allow enough time for the mercury toreach a peak temperature.

Digital thermometers with the capability to be cou-pled to a printer are also available when written recordsof the temperature are desired over a period of time.As with indicating-type thermometers, the sensor ofrecording thermometers can be either a thermocoupleor an RTD.

Humidity-Measuring Devices

To follow standardized kiln schedules with manual con-trol requires a knowledge of the wet-bulb temperatureor the relative humidity of the air circulating in the

kiln. This can be done by using an instrument thatreads relative humidity directly or with wet-bulb ther-mometry, which reads the wet-bulb temperature. Thedifference between the dry-bulb temperature and thewet-bulb temperature is the wet-bulb depression. Byknowing these values and by using a psychrometricchart, relative humidity can be calculated (see appendixto ch. 1). As discussed in some detail in the appendixto chapter 1, wet-bulb sensors must be continuouslywetted and located in a position in the kiln where suffi-cient airflow over the sock or wick will provide adequateevaporation of the water and thereby cooling so thataccurate wet-bulb temperatures can be determined.The wet-bulb wick should be changed after every kilncharge or more frequently if it becomes hard or crustyand is not wicking properly. If kiln conditions are tobe controlled by monitoring relative humidity and dry-bulb temperatures, then a high-quality relative humid-ity sensor should be obtained. Inexpensive sensors ormeters of the type commonly found in hardware storesare not recommended as they do not stay in calibra-tion well and can rather quickly give misleading or er-roneous readings. Wet- and dry-bulb hygrometers aresometimes used for manual control. These provide wet-and dry-bulb temperatures from the same instrumentand are illustrated in chapter 3 under Equipment forDetermining Temperatures.

Specialized Drying Approachesand Kiln Types

Dehumidification Kilns

Dehumidification kilns have been mentioned in severalplaces in this chapter. In many respects, these kilns aresimilar to steam-heated or direct-fired kilns, but theydiffer enough to warrant a separate description. De-humidification kilns have several advantages: a boilermay not be required (except as required for stress reliefor desired for warmup); they are more energy efficient,offering good control in drying refractory species thatrequire a low initial dry-bulb temperature as well ashigh relative humidity; and a low-cost kiln structure isadequate for some applications. Disadvantages are thatdehumidification kilns operate primarily on electricalenergy, which in some regions may be more expensivethan gas, oil, or wood residue (even though these kilnsare more energy efficient than other types of kilns);maximum temperatures are limited to about 160 °Fand in some units to about 120 °F; and, in some cases,there may be concern over chemicals in the condensate.

Air-circulation systems are essentially the same asthose used in steam or direct-fired kilns. The entiredehumidification unit may be located outside the kilnin an equipment room and blowers used to circulate airbetween the dehumidifier and the kiln. Another com-mon arrangement is a split system with the compres-

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Figure 2-22—View of a typical dehumidification kilnand schematic of typical dehumidification drying sys-tem. (ML88 5625)

sor and control panel in a separate equipment roomand the blower and coil cabinet inside the kiln. In somesmaller systems (less than 10,000 fbm), the entire dehu-midification unit may be inside the kiln. Air circulationwithin the drying compartment is provided as in theother types of kilns. While earlier designs typically hadair velocities in the lower range, the industry has grad-ually increased the air velocity to a level comparable tothat used in a conventional-temperature hardwood kiln.A typical dehumidification kiln is shown in figure 2-22.

The major difference between dehumidification kilnsand other types of kilns is the method by which wateris removed from the kiln air. The majority of the wa-ter is condensed on the coils of the dehumidifier andremoved as liquid, rather than being vented to the out-side atmosphere. Many larger dehumidification systems

have provisions for periodically venting excess heat,and some moisture is vented in the process of vent-ing heat, but only a small part of the total moisture inthe air is vented. These two characteristics account forthe greater energy efficiency of dehumidification kilns.First, since little moist air is vented to the outside ofthe kiln, the energy contained in the warm, moist airis not lost. Second, when the moisture in the air con-denses on the cold coils of the dehumidifier, the heat ofvaporization is recovered. Most dehumidification kilnsare built so that this recovered energy is used in dry-ing the lumber. The same approximately 1,000 Btu ofenergy per pound of water required to evaporate thewater from the lumber in the first place is recovered inthis condensation.

Kiln control systems on dehumidification kilns of about5,000 fbm and larger are similar to those of other kilns.They typically use RTD dry- and wet-bulb sensors andrecorder-controllers. Controllers for smaller systemsmay use a timer to control the percentage of time thecompressor operates or a humidistat to activate thecompressor. Most large systems (over 10,000 fbm) usedfor drying hardwood lumber are installed with a boilerfor warmup and conditioning or stress relief. Smallersystems often have electrical resistance heating ele-ments that are used to bring the kiln up to operatingtemperature to the point where the compressor cansupply enough energy to maintain the desired dryingconditions. These heating elements can also be used toattain the higher temperatures often called for near theend of the drying schedule.

Materials of construction vary from wood to masonryto prefabricated aluminum panels. The main criteriaare that the drying compartment be well insulated sothat maximum benefit can be derived from the energyefficiency and that the compartment be both airtightand moisture resistant. For medium to large kilns, in-sulation values of R-20 for walls and R-30 for roofs arerecommended. Slightly lower values may be acceptablein warmer climates. For smaller kilns (10,000 fbm ca-pacity or less) with less compression-generated heat,higher R values are required. Very serviceable and low-cost kilns can be built with simple wood-frame con-struction, in both large and small sizes.

In general, one can expect that drying stresses will bepresent after dehumidification kiln drying, as they areafter drying in other type kilns. When drying lumberfor uses where drying stress must be relieved, specialprovisions must be made if the system is not equippedwith a boiler. A small-capacity electric or gas-firedboiler can be incorporated in the kiln for this purpose.

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Figure 2-23—Typical predryer. (ML88 5600)

It is very important to properly size the compressor forthe thickness and species to be dried in the dehumid-ifier. If the compressor is too small, there is a risk ofstain, increased warp, and checking. If the compressoris too large, humidities in the kiln can cycle excessively,possibly resulting in a lack of heat.

Predryers

Predryers are large low-temperature dryers used to drygreen lumber to a moisture content of around 25 per-cent prior to drying to a lower final moisture content ina kiln. Also called warehouse dryers, these large freespan buildings range in lumber-holding capacity from50,000 to over 1,000,000 fbm and are typically sized atfour times kiln capacity (fig. 2-23). Generally, predryersare forklift loaded, although track loading may be pre-ferred in some cases. Most predryers are preengineeredbuildings of structural steel with 1 to 2 in of rigid foaminsulation between painted steel or aluminum sheath-ing. They commonly have concrete floors. Tempera-ture and relative humidity are controlled with temper-ature set-points typically ranging from 75 to 100 °Fand with relative humidity maintained between 60 and90 percent.

Predryers for controlling air-drying conditions havebeen used successfully for over 25 years by some com-panies in the northern latitudes of the United Stateswhere natural air-drying conditions are unfavorable formany months, from both the standpoint of defect de-velopment and length of air-drying time. However, inrecent years high lumber prices and high interest rateshave produced financial incentives strong enough tointerest lumber producers in other areas, especiallyhardwood producers who had typically air dried theirlumber 60 to 90 days or longer before final drying inthe kiln.

The advantages of predrying over air drying in the yardare brighter lumber, more uniform moisture contentof dried lumber, and reduction of drying defects, all inabout one-third less time. Inventory can be reduced byone-third to one-half, freeing capital and yard space.Several species and thicknesses can be mixed in thesedryers. Thus, lumber of different moisture contents,species, and thicknesses may be in the predryer at thesame time; drier lumber can be moved frequently outto the kilns and newly acquired green lumber can bemoved in. The lumber is usually arranged by blocks ina zone to group similar species, thicknesses, and levels

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of moisture content. Disadvantages of predrying overair drying are largely associated with costs of building,energy, and maintenance.

Predryers are typically heated with steam and finnedcoils. Humidity is controlled by external venting whenhumidities are too high and by using moisture releasedfrom the lumber to maintain humidities as high asrequired. Larger predryers are divided into two andsometimes three zones; conditions are controlled sep-arately in each zone by a recorder-controller similarto those described earlier in this chapter. In some in-stallations, temperature and humidity conditions aremaintained with dehumidification units.

Temperatures are usually sensed by electronic RTDs,and humidity is sensed by wet-bulb thermometry orwith relative humidity sensors using the cellulose pador similar sensor. Placement of the sensors above theload in the rafters is often criticized because the sam-pled air is not as representative of the air entering thestacks as one would like. However, the free-span con-struction of the structure does not provide much choicein where to place the sensors.

Air circulation is usually provided by overhead fans ar-ranged in a row horizontally over the plenum betweenthe two rows of lumber (fig. 2-22). The air is directeddown into the plenum by belt-driven or directly drivenfans, and then it is passed through the stacks of lum-ber. One criticism of predryers has been uneven airdistribution, resulting in uneven drying from top tobottom of the stacks. Various forms of baffle systemshave been suggested to improve distribution and unifor-mity of airflow. Exhaust ventilation should be designedso that it does not direct the humid exhaust air downonto the roof; this has been reported to cause localizeddeterioration of the roof. Rather, exhaust ventilationshould consist of “upblast” units that direct vented airstraight up and way from the building. The makeupair enters through louvers in the walls and can be pre-heated if needed.

The concept of using predryers rather than air dryinghas gained wide acceptance in the hardwood indus-try, though it may not be a technique that works welland is profitable for all operations and installations.The techniques are still evolving, and many changes arelikely to be seen in the next few years.

Solar Dry Kilns

Interest in solar dry kilns was low until the energy con-cerns of the mid-1970’s. The advantage of solar kilnsis the free and often abundant energy available, butthe disadvantage is that there is a cost to collectingfree energy. Free energy is also low-intensity energy,which often limits the operating temperature of a kiln

to approximately 130 °F unless prohibitively expensivespecial solar collectors are used. Despite the cost of col-lecting the energy, another advantage of solar kilns isthat relatively small, simple, and inexpensive kilns arepossible, and this level of technology is often well suitedfor small operations.

The average annual solar energy available on a horizon-tal surface in the United States ranges from 1,000 to2,000 Btu per day per square foot of collector area.Average amounts for several locations are given intable 2-1. Tilting the collector surface perpendicularto the sun maximizes the intensity of the direct solarradiation and minimize losses caused by the reflectionof the direct radiation. The general rule for maximiz-ing solar radiation on a year-round basis is to tilt thecollector at an angle to the ground equal to the lati-tude. If solar radiation is to be maximized in the sum-mer months in locations where latitude and ambienttemperature make winter drying impractical, direct ra-diation can be maximized by reducing the tilt angle toabout 15° less than the latitude. In the northern hemi-sphere, the collector should face directly south.

Solar kilns can operate by direct solar collection (green-house type) or by indirect solar collection where thecollector is isolated in some way from the drying com-partment. They can also operate with solar energyalone or with supplemental energy. The four types ofsolar kilns are as follows:

1. Direct collection (greenhouse)

a. Solar only, which is characterized by wide diur-nal and day-to-day changes in temperature andrelative humidity

b. Solar with supplemental energy, which is charac-terized by the ability to follow a drying scheduleand has large nighttime heat losses because of thelow insulating ability of the transparent cover

2. Indirect collection (isolated drying compartment)

a. Solar only, where the diurnal change in temper-ature and relative humidity can be reduced byenergy storage and reduced heat losses at night

b. Solar with supplemental energy, where sched-uled drying is possible and nighttime losses areminimized

Generalized solar kiln designs are shown in figure 2-24.Possible collector surfaces are south-facing walls, eastand west walls, and a roof. Solar collection is eitherdirect (fig. 2-24a,c) or indirect (fig. 2-24b,d). The col-lector surface is either uninsulated (fig. 2-24a,b) or in-sulated at night (fig. 2-24c,d).

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Figure 2-24—Generalized solar kiln design types.(a) Uninsulated dryer and collector are one unit.Airflow mixes on both sides of absorber panel (Ab).(b) Uninsulated, improved design. Airflow mixes withinchamber. (c) Insulated (ln) externally. Airflow mixeson both sides of Ab, day and night. (d) Insulated (ln)internally. Air flows over the front of the absorberpanel (Ab) when damper (Da) is open. (ML88 5603)

Figure 2-25—Solar kiln design for northern latitudes,showing inexpensive control system. (ML88 5602)

In the simplest uninsulated form (fig. 2-24a), air flowson both sides of the absorber surfaces (Ab). The dry-ing chamber and collector surfaces are one unit as in atrue greenhouse structure. A somewhat improved de-sign (fig. 2-24h) isolates the collector surfaces (R and S)and the outer absorber surface (Ab) from the dryingchamber. The energy absorbed on the absorber surfaceflows through the absorber to its inner surface where itis transferred to the circulating kiln air. Both of thesesystems suffer large nighttime heat losses.

For the insulated designs, two variations are possible.In the simpler of these designs (fig. 2-24c), airflow issimilar to that in figure 2-24a except that diurnal insu-lation (ln) is accomplished by external means such asshutters or blankets. Collector and absorber surfacesare also isolated in the design shown in figure 2-24d,with the drying air acting as the medium for heattransfer. When the dampers (Da) are open, the airflows over the black absorber surface (Ab) and backinto the dryer chamber. When the dampers are closed,nighttime airflow is interrupted, thereby reducing night-time (and cloudy day) heat losses because the absorberhas an insulated back (ln). A more detailed schematicof this type of solar kiln is shown in figure 2-25. Inanother common variation of this insulated-type so-lar kiln, the solar collector is detached from the dryingcompartment, and blowers transfer the heated air fromthe collector to the drying compartment (fig. 2-26).

At present, solar drying is not widely used in theUnited States. The main uses are hobbyists or smallwoodworking shops that do not require large dryingcapacity and that do not wish to make large capital in-vestments in drying equipment.

Vacuum Drying

Vacuum drying of lumber is not a new idea, and, infact, it has been considered since the turn of the cen-tury. However, vacuum drying did not come into useuntil the 1970’s because it was considered uneconomi-cal. The principal attraction of vacuum drying is thatthe lowered boiling temperature of water in a partialvacuum allows free water to be vaporized and removedat temperatures below 212 °F almost as fast as it canat high-temperature drying at above 212 °F at atmos-pheric pressure. Drying rate is therefore increased with-out the dangers of defects that would surely develop insome species during drying above 212 °F. Vacuum dry-ing is essentially high-temperature drying at low tem-peratures. During the early 1970’s, the economic out-look for vacuum drying became more favorable, largelybecause of the increased costs of holding large inven-tories of lumber during long drying processes. This isparticularly true in the drying of thick, refractory, high-

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Figure 2-26—Schematic diagram of solar wood-residuedry kiln. A, furnace room; B, intake air enters col-lector; C, solar blower; D, manifold ducts for solar-heated air; E, internal fans; Fc, differential temper-ature sensor-collector; Fd, differential temperaturesensor–dryer; G, humidifier; H, return-air duct from

value species, which can be safely dried in a vacuumkiln in a small fraction of the time required in a con-ventional kiln.

The main difference between the several types of vac-uum kilns currently on the market is the way in whichheat is transferred to the lumber. Convective heattransfer in a partial vacuum is almost nonexistent. Inone common type of vacuum kiln, there are alternatevacuum and atmospheric pressure cycles. Heat is ap-plied to the lumber convectively at atmospheric pres-

dryer to collector (dampered at night); J, entry pointof intake air; K, exhaust vents; RH1, humidistat for ex-haust vents K: RH2, humidistat for shutting kiln offat high humidity; RH3, humidistats for humidifier G.(ML88 5601)

sure, and then a vacuum cycle is applied to remove wa-ter at low temperature. These cycles are alternatedthroughout the drying. Another common type of vac-uum kiln maintains a vacuum throughout the entiredrying process, and the heat is transferred to the lum-ber by direct contact with steam-heated platens or byelectrically heated conductive blankets that contact thelumber (fig. 2-27). A third type employs high-frequencyelectrical energy to heat the lumber. In all types, wateris removed from the drying chamber by pumps.

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Figure 2-27—Vacuum-kiln type in which heat is sup-plied to the lumber by contact with electrically heatedblankets. (M85 0351-10)

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Literature Cited

Rasmussen, E. F. 1961. Dry kiln operator’s manual.Agric. Handb. No. 188. Washington, DC: U.S. Depart-ment of Agriculture. 197 p,

Rice, W. W. 1977. Dry kiln: A design to season500 board feet. Fine Woodworking. Spring: 39-43,

Rosen, H. N. 1979. Potential for energy recovery fromhumid air streams. Res. Pap. NC-170. St. Paul, MN:U.S. Department of Agriculture, Forest Service, NorthCentral Forest Experiment Station. 10 p.

Sources of Additional Information

Branhell, G.; Wellwood, R. W. 1976. Kiln drying ofwestern Canadian lumber. Western Forest ProductsLaboratory Information Report VP-X-159. Vancou-ver. 112 p.

Cech, M. Y.; Pfaff, F. 1977. Kiln operator’s manual foreastern Canada. Eastern Forest Products LaboratoryReport OPX192E. Ottawa. 189 p.

Knight, E. 1970. Kiln drying western softwoods.Moore-Oregon, Portland. (Out of print.) 77 p.

McMiIlen, J. M.; Wengert, E. M. 1978. Drying easternhardwood lumber. Agric. Handb. No. 528. Washing-ton, DC: U.S. Department of Agriculture. 104 p.

Tschernitz, J. L. 1986. Solar energy for wood dryingusing direct or indirect collection with supplementalheating. Res. Pap. FPL-477. Madison, WI: U.S. De-partment of Agriculture, Forest Service, Forest Prod-ucts Laboratory. 81 p.

Wengert, E. M.; Oliveira, L. C. 1985. Solar heated,lumber dry kiln designs. Blacksburg, VA: Departmentof Forest Products, Virginia Polytechnic Institute andState University. 91 p.

Table 2-1—Annual average of daily solar radiationavailable at various locations in the United States

CitySolar radiation

(Btu/ft2-day)

Fort Worth, TXGrand Junction, COGreensboro, NCIndianapolis, INLexington, KYLittle Rock, ARShreveport, LA

Albuquerque, N MAmes, IAAtlanta, GABoise, IDBoston, MACorvallis, ORDavis. CA

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Chapter 3Dry Kiln AuxiliaryEquipment

Equipment for determining moisture content 75Balances and scales 75

Triple-beam balance 75Electronic top-loading balance 75Self-calculating balance 76Indicating balance 78Self-calculating scale 78

Saws 79Drying ovens 79

Electrically heated ovens 79Steam-heated ovens 80

Electric moisture meters 80Resistance moisture meters 81Dielectric power loss moisture meters 81

Distillation equipment 82Equipment for determining temperatures 82

Electric digital thermometers 83Etched-stem thermometers 84Hygrometers 84

Equipment for determining air movement 85Literature cited 86

Chapter 3 was revised by R. Sidney Boone,Research Forest Products Technologist.

Certain auxiliary equipment is needed to operate a drykiln in the most economical manner and to obtain gooddrying results. Drying schedules based upon moisturecontent cannot be successfully applied unless the mois-ture content of the stock is known. Therefore, equip-ment should be available for determining the moisturecontent of the stock. Equipment should also be avail-able for determining the temperature, humidity, andvelocity of air in the kiln to maintain uniform condi-tions for fast drying.

Equipment for DeterminingMoisture Content

Such items as balances, scales, saws, drying ovens, andelectric moisture meters are used in determining themoisture content of wood. Distillation equipment isused for accurate determination of moisture content ofwoods that hold relatively large amounts of oil, resins,wood preservative, or fire-retardant chemicals.

Balances and Scales

Triple-Beam Balance

One of the most commonly used types of balances forweighing small moisture sections is the triple-beambalance, shown in figure 3-1. Balances best suited forweighing the recommended sizes of moisture sections(see preparation of kiln samples and moisture sec-tions in ch. 6) should have a maximum capacity of atleast 1,000 g and weigh to an accuracy of at least 0.1 g(0.01 g is preferable).

Electronic Top-Loading Balance

Electronic top-loading balances are available in a widerange of weighing capacities, precisions, styles, andprice ranges. Models with printers that provide a writ-ten record of the weights and portable battery-operatedmodels are also available. Two types suitable for weigh-ing small moisture sections are shown in figure 3-2. Be-cause of the size of the pieces to be weighed and theprecision to which they need to be weighed, the samebalance cannot be used to weigh the small moisturesections and the much larger sample boards (see prepa-ration of kiln samples and moisture sections in ch. 6).For weighing moisture sections, the balance shouldhave a maximum capacity of at least 1,000 g and weigh

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Figure 3-1—Two types of triple-beam balances suitablefor weighing moisture sections. (M87 0198, M87 0169)

to at least 0.1 g (0.01 g is preferable). A type of bal-ance suitable for weighing sample boards is shown infigure 3-3. For weighing sample boards, the balanceshould have a maximum capacity of at least 15,000 gand weigh to 1.0 g. Operations drying wide boards ofhigher density hard woods should consider having amaximum capacity of 20,000 to 30,000 g.

Self-Calculating Balance

To calculate moisture content, it is necessary to knowthe original and the ovendry weights of the wood sec-tions. The loss in weight is divided by the ovendryweight (see ch. 6 for procedure). Self-calculating bal-ances, similar to the one shown in the upper part offigure 3-4, have been developed to speed up these calcu-lations or to eliminate them entirely. As shown in thelower part of figure 3-4, the moisture readings can beestimated to the nearest 0.5 percent when the valuesare less than 10 percent, and to the nearest 1.0 per-

Figure 3-2—Electronic top-loading balances suitable forweighing small moisture sections. (M87 020,M87 0175)

Figure 3-3—Electronic top-loading balance for weighingsample boards. (M87 0174)

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Figure 3-4—Self-calculating moisture balance. Top:Triple-beam balance with special scale on specimenpan used to calculate moisture content of moisture sec-tion after ovendrying. Bottom: Specimen pan is carriedon revolving indicator that indicates moisture contentdirectly on scale. (M 90343)

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Figure 3-5—Indicating balance. (M87 0199)

2

3.

Figure 3-6—Self-calculating scale or guide for determin-ing moisture content of kiln sample. (M87 0168)

4.

cent when the values are more than 10 percent. A pre-scribed sequence of operating steps, supplied by themanufacturer, must be followed in carrying out a mois-ture content determination for moisture sections withthis balance.

Indicating Balance

Indicating balances such as that shown in figure 3-5can be used for weighing sample boards. Weights mustbe placed on the pan on the left side and may need tobe changed with the weighing of each sample board.Care must be taken to account for the total amount ofweight on the pan as well as the reading on the indica-tor, which can be read to the nearest graduation, 1 gor, on some models, 0.01 lb.

Self-calculating scale

Another type of scale used to determine the daily orcurrent moisture content of kiln samples is the moistureguide (fig. 3-6). This scale has a movable weight on thelong arm of a graduated beam. Attached to the shortarm of the beam is a semicircular plate graduated interms of percentage of moisture content. Above thisplate is a movable indicator arm with a hook.

If the moisture guide is to be used with reasonableaccuracy, certain procedures must be followed:

1. Immediately after the moisture content sections havebeen cut and weighed, apply end coating to the kilnsample, and hang the sample from the hook on themovable indicator arm, with the indicator set atzero. Move the sliding weight on the long beam to apoint that brings the beam into balance. Record thevalue of the balancing point on the kiln sample, andplace the sample in the kiln with the load of lumberit represents.

Ovendry the moisture content sections to constantweight and calculate their moisture content values.

When the moisture content of the sections has beenobtained, remove the kiln sample board from thekiln, hang it on the movable indicator hook with theindicator set at zero, and move the sliding weight onthe long beam to the setting determined in step 1.Then place metal weights, such as washers or leadslugs, on the end of the kiln sample until the longbeam balances.

With added metal weights in place, set the movableindicator arm to the moisture content value of thesections determined in step 2, and move the slidingweight on the long beam until balance is again ob-tained. Erase or cross out the previously recordedvalue on the kiln sample and record the new balancevalue. This new value will be the setting of the slid-ing weight on the long beam used for all subsequentmoisture determinations.

5. Remove the metal weights from the sample. Withthe sliding weight set at the new value obtained instep 4, move the indicator arm until the long beambalances. The current moisture content of the kilnsample can then be read on the semicircular plate.

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6. Subsequent moisture content values of the samplesare obtained by setting the sliding weight on thelong beam at the new balance value obtained instep 4, hanging the kiln sample on the movable in-dicator hook, and moving the indicator arm untilthe long beam is balanced. The current moisturecontent is read on the semicircular plate.

Saws

Band, table, radial arm, swing, and portable saws aregenerally used for cutting moisture sections. Handsaws are not recommended. A band saw is particu-larly suitable for slotting and slicing small sections formoisture-distribution and casehardening tests. Sawsshould be sharp, have the proper set, and be providedwith suitable safety devices. Saws that are not sharp orhave improper set tend to overheat or burn the wood,thereby changing the moisture content of the section.

Drying Ovens

Several kinds of ovens are used for drying moisture sec-tions. Drying ovens should be large enough to provideadequate open spaces between the sections of wood be-ing dried. The temperature of the oven should be con-trolled with a thermostat or other means so it will staywithin the desired setting (212 to 218 °F, 215 ± 3 °F).Excessive temperature will char the sections and mayalso start fires. Temperatures below 212 °F will notdrive off all the water in the sections. The oven shouldhave ventilators on the top or sides and bottom toallow the evaporating moisture to escape.

Electrically Heated Ovens

Electrically heated ovens are commonly used in kilndrying (fig. 3-7). Ovens containing fans to circulate theair and speed up drying are generally recommended, es-pecially if large numbers of moisture sections are driedfrequently. Natural draft ovens, those depending onthe heat rising to create air circulation, are usually lessefficient and require more time to remove all the mois-ture from the sections. Check the thermostat settingwhen the oven is empty, using a thermometer insertedin the hole provided in the top of the oven. When wetor moist wood is placed in the oven, the temperaturewill fall at first and then rise as the wood dries. Do notreset the thermostat higher after placing the wood inthe oven or it will be above set point when the wood isdry.

Recently, an increasing number of operators have suc-cessfully used home-type microwave ovens to ovendrymoisture sections. Considerable care must be usedwhen drying moisture sections in a microwave oven.Although sections can be dried in minutes rather than

Figure 3-7—Electrically heated ovens for drying mois-ture sections: (a) large floor model, (b) smaller table-top model. (M87 0193-1, M87 0193-16)

hours, it is rather easy to overdry the section (burn itin the center) or to underdry the section (not removeall the moisture), resulting in an inaccurate ovendryweight of the moisture section. One procedure suggestsusing a medium-low to low power setting and an ovenwith a carousel tray to prevent uneven drying (Wengert1984). Suggested time for ovendrying is about 10 minfor dry pieces and about 20 min or longer for greenpieces.

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Steam-Heated Ovens

Steam-heated drying ovens are satisfactory if a suit-able supply of steam is continuously available. Ovens ofthis type are usually homemade and may be equippedfor either natural- or forced-air circulation. The tem-perature in the oven is usually regulated or controlledby a reducing valve on the steam feed line. The re-ducing valve is adjusted to maintain the desired tern-perature (typically 215 ± 3 °F) in accordance with athermometer inserted through the top of the oven. Aswith the electrically heated oven, set the temperaturewhen the oven is empty, not after the moist wood hasbeen placed in the oven. Shelves for the moisture sec-tions should be made of perforated metal or large mesh,heavy wire. Provide ventilators to remove the moisture-laden air.

Electric Moisture Meters

Electric moisture meters, if properly used, provide arapid, convenient, and, for most purposes, sufficientlyaccurate means of determining moisture content whenit is less than 30 percent (James 1988). Woods treatedwith salts for preservation or fire-retardant purposeswill generally, give meter readings that are too high,and the use of electric moisture meters to determinemoisture content is not recommended. Electric mois-ture meters are available as portable hand-held units oras stationary units used to monitor moisture content ofmaterial moving along conveyor lines. In many situa-tions, temperature and species corrections must be ap-

Figure 3-8—Selected models of resistance-type moisturemeters: (a) meter with insulated two-pin electrodes,dial readout; (b) meter with uninsulated two-pin elec-trodes; (c) meter with insulated two-pin electrodes,digital readout. (MC88 9030, M88 0131-13)

plied for accurate readings; correction data are usuallysupplied by the manufacturer of the equipment. Thereare two types of meters commonly available, resistance(or conductance) and dielectric.

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Resistance Moisture Meters

The most common type of portable hand-held mois-ture meter is the resistance-type (also known asconductance-type) meter. Resistance-type meters usepin-type electrodes that penetrate the wood. The use-ful operating range of most meters of this type is be-tween about 7 and 30 percent. Although some instru-ments have scales that read above the fiber saturationpoint (usually taken to be 30 percent moisture con-tent), the accuracy above 30 percent is questionable.Selected models of resistance-type meters are shown infigure 3-8.

When using resistance-type meters on pieces of lumberwith rectangular cross sections, pins should be drivenone-fourth to one-fifth of the thickness of the piece toindicate an average moisture content. For circular crosssections, the depth of the pins should be one-sixth ofthe diameter. For the most accurate readings, orientpins so that the current flows parallel to the grain, withpins driven in the wider face of the piece. Pins drivenparallel to the grain in the narrow face of the piece willgive acceptable readings when ready access to the wideface is not convenient. If readings drift, take the read-ing immediately after the electrode is driven into thespecimen. Actual moisture readings (subject to temper-ature and species correction) appear on the meter dialor readout.

Two-pin electrodes are quite commonly used withlumber, posts, or poles. Electrodes using l-in-longinsulated pins are the type most commonly used(fig. 3-8a,c). Insulated pins are helpful in avoiding falsereadings if wood has been surface wetted with rain,snow, or dew. Also, by using pins that are insulatedexcept at the tip, some indication of moisture contentgradient can be determined as the pins are driven todiffering depths in the wood. To get an estimate of theaverage moisture content of a pole or heavy timbers,extra long pins (2-1/2 in) are available. Short (5/16 in)uninsulated pins are used on models such as shown infigure 3-8b, and when inserted to the proper depth,these pins give accurate average moisture contents forstock up to 2 in thick.

Four-pin electrodes are more commonly used with ve-neer and sawn lumber less than 1 in. in thickness. Aswith two-pin electrodes, accurate readings can be ob-tained on stock up to 2 in thick using short (5/16 in)uninsulated pins.

Dielectric Power Loss Moisture Meters

A hand-held moisture meter of the dielectric power losstype is shown in figure 3-9. The surface-contact elec-trodes are nonpenetrating and may vary in design ac-cording to the material on which they are to be used.

Figure 3-9—A radiofrequency power loss type electricmoisture meter. (M 133689)

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Figure 3-10—A dielectric electric moisture meter withsmooth surface electrode. (M88 0239, M88 0238)

The instrument shown has eight spring-cushioned con-tact points equally spaced on the circumference of acircle. This design is for use primarily on rough lum-ber. The electric field from this electrode penetratesabout 3/4 in, so that specimen thicknesses up to about1-1/2 in may be read. With surface-contact electrodes,the surface layers of the specimen have a predominanteffect on the meter readings. Other electrode config-urations are used for surfaced lumber and veneer. Anexample of a smooth surface electrode is shown infigure 3-10.

The range of these power-loss meters is from 0 to about30 percent moisture content. Some manufacturers offermeters where actual moisture content is read on thedial. For others, the actual moisture content value isnot read directly from the dial, but must be equatedwith moisture content from a separate table.

Stationary meter systems using noncontact sensors areavailable to monitor moisture content of moving lum-ber on a dry chain (fig. 3-11) or at the outfeed from aplaner. Such systems can be equipped to mark or eject,or both, individual pieces that are outside preset mois-ture specifications. Some equipment offers a summaryprintout showing such items as total piece count, aver-

age moisture content of all pieces, and distribution ofmoisture content at specified moisture content intervals

Distillation Equipment

Some woods contain a high percentage of volatile com-pounds or are impregnated with oily preservatives. Thevolatiles will be driven off in the ovendrying process,resulting in an incorrect moisture content value. Dis-tillation equipment should be used for determining themoisture content of such woods (American Society forTesting and Materials 1986).

Equipment for DeterminingTemperatures

Checking temperatures in a dry kiln is frequently nec-essary to determine the causes for nonuniform dryingand the differences in temperature between the areasaround the control bulbs and other areas in the kiln.Occasionally, it may be desirable to verify that thetemperature indicated by the sensor for the recorder-controller is an accurate value. These temperaturemeasurements are usually made on the entering-air side

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Figure 3-12—Hand-held digital thermometer(M87 0171)

Figure 3-11—Stationary inline moisture meter locatedon dry chain. Arrows show location of noncontact sen-sors. Note spray nozzle for marking pieces beyond pre-set limits. (MC88 9035)

of the loads, although at times leaving-air temperaturesare simultaneously obtained so that the temperaturedrop across the load can be determined. Electric digi-tal thermometers, etched-stem glass thermometers, andoccasionally hygrometers are used for this purpose.

Electric Digital Thermometers

Electric digital thermometers, using either thermocou-ples or resistance temperature detectors (RTD) assensors or probes, are rapidly becoming a commonway of measuring temperatures in dry kilns. They areavailable as portable hand-held models (fig. 3-12), oras panel or bench-top models, which can be mountedin the control room (fig. 3-13). For those designs us-ing thermocouple sensors, type T (copper—constantan)thermocouple wire is commonly used in dry kiln envi-ronments, although type J (iron—constantan) or typeK (chromel—alumel) is sometimes used. Thermocou-ple connections at the sensor should be soldered orfused together to make the junction. Some commer-cially prepared thermocouple sensors are enclosed in ametal sheath and look somewhat like RTD sensors. Re-sistance temperature detectors are usually of the plat-inum type, with all leads enclosed in a metal sheath(fig. 3-14).

Figure 3-13—Panel-mounted digital thermometer(M87 0197)

Figure 3-14—Resistance temperature detector (RTD)sensor. (M87 0167)

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The thermometer component may be considered a typeof potentiometer that converts the electrical signal fromthe sensor to a digital readout. These electrical ther-mometers can be obtained to work with thermocouplesensors, RTD sensors, or both, and depending on de-sign, can monitor from 1 to 12 probes or sensors. Somealso offer the capability to have printers attached sowritten records of the data can be obtained. It shouldbe pointed out that the accuracy of thermocouple sen-sors can vary as much as ±2 °F and when used withthe less expensive thermometers, the combined accu-racy can vary as much as ±3.6 °F. By using more ex-pensive thermometers, this variability can be reducedto around ±1.5 °F. Resistance temperature detectorsensors are considered more accurate than thermocou-ples, and by using them with moderately priced ther-mometers, accuracy of around ±0.6 °F can be achieved.This information should be kept in mind when com-paring temperature values from sensors located in thesame area of the kiln or when comparing thermocouplereadings to readings from the recorder-controller.

Etched-Stem Thermometers

Mercury-in-glass thermometers with the temperaturescale etched on the glass stem are frequently used tocheck dry kiln temperature. They should be placed inthe kiln at the locations to be checked and not movedwhen temperature readings are taken. Obviously, itis necessary for the kiln operator to go into the kilnto make these temperature readings. Note: In a low-temperature or conventional-temperature kiln, if thewet bulb is above 120 °F, one should wear protectiveclothing on all exposed skin (including neck, arms,and hands) and a face mask to provide cooled air.It is not recommended that elevated-temperature orhigh-temperature kilns be entered while the kilns arerunning.

The temperature survey of low- and conventional-temperature kilns can be quickly made if several ther-mometers are placed at the different zones in the kilnwhere temperature checks are desired. The possibilityof breaking glass thermometers is reduced by puttingthem in metal sheaths (fig. 3-15). The sheathed ther-

mometer is suitable for making dry-bulb measure-ments, but if wet-bulb temperatures are also beingmeasured, the sheath must be removed so that a wickcan be applied directly over the mercury bulb of thethermometer.

Maximum thermometers are also used for checkingkiln temperatures. By mounting two maximum ther-mometers on a frame and supplying one with a wickand a water supply, both the maximum wet- and dry-bulb readings can be obtained. Care should be taken inchoosing the location in the kiln for the thermometersas they will read the hottest temperature sensed, evenfor very brief periods, and thus there is a tendency fora biased high reading.

Hygrometers

Hygrometers are instruments for measuring the dry-bulb and wet-bulb temperatures of circulated air.These instruments vary from the hand-held and hand-operated sling psychrometer (fig. 3-16) to stationarymounted dry- and wet-bulb thermometers (fig. 3-17).Instruments that directly read relative humidity mayalso be considered hygrometers.

Figure 3-16—Sling psychrometer for determiningrelative humidity. (M87 0196)

Hygrometers similar to that shown in figure 3-18 aresometimes used to check kiln temperatures. Such hy-grometers may use etched-stem thermometers or thosewith the calibrations on adjacent metal strips. Onebulb must be continuously supplied with water to getwet-bulb readings. Using these data with the psychro-metric chart in the appendix to chapter 1, equilibriummoisture content and relative humidity values can bedetermined. Sling psychrometers are helpful in spotchecking wet- and dry-bulb temperatures (thereby de-termining relative humidity and electric moisture con-tent) in a dry kiln or storage area. Hygrothermographsare instruments that measure and record temperatureand relative humidity. They are helpful in providing acontinuous written record of conditions in storage shedsor other areas where the temperature does not exceedabout 120 °F (fig. 3-19).

Figure 3-15—Etched-stem glass thermometer in metalprotecting case. (M87 0195)

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Figure 3-19—Hygrothermograph for measuringand recording temperature and relative humidity(M87 0172)

Figure 3-17—Stationary mounted dry- and wet-bulbthermometers. (M 137003)

Equipment for DeterminingA i r M o v e m e n t

Since the direction and rate of airflow are important inthe operation of a dry kiln, means of determining thesefactors are necessary. Rate of airflow may be measuredwith anemometers and the direction of flow may beinferred from these measurements.

Anemometers are instruments for measuring the veloc-ity or force of air. Several types of anemometers, alsocalled air meters, can be used to determine the velocityof air in dry kilns. One commonly used type is called ahot-wire or thermal anemometer (fig. 3-20). The wirein the probe is heated by electricity from a batteryin the unit. The amount of cooling of the hot wire isproportional to the velocity of the air passing over thewire. Velocities are indicated directly on a scale cali-brated in feet per minute.

In another commonly used type of anemometer, the airenters the instrument through a port or shutter, andvelocity is read directly in feet per minute on a cali-brated dial. This type, known as a deflection anemome-ter, is shown in figure 3-21.

Another type of anemometer occasionally used in drykilns is the rotating vane anemometer. The sensor ofthis instrument is a disk fan mounted on pivot bear-ings and provided with a revolution counter. Air veloci-ties, in feet per minute, are read directly on a dial or insome models on a digital readout.

Figure 3-18—Hygrometers: left, wet- and dry-bulb hy-grometer made from two etched-stem glass thermome-ters; right, wet- and dry-bulb hygrometer with maxi-mum thermometer. (M 86250, M 90337)

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Literature Cited

American Society for Testing and Materials. 1987.Standard test methods for moisture content of wood.ASTM D 2016-74. (Reaffirmed in 1983). Philadel-phia, PA: American Society of Testing and Materials:436-449.

James, W. J. 1988. Electric moisture meters for woodGen. Tech. Rep. FPL-6. Madison, WI: U.S. Depart-ment of Agriculture, Forest Service, Forest ProductsLaboratory. 17 p.

Wengert, E. M. 1984. Using a home microwave ovenfor ovendrying. Wood Drying New Digest B-4,11,Forest Products Research Society; Apr. 1 p.

Figure 3-20—Hot-wire air meter. (M87 0194-18)

Figure 3-21—Deflection anemometer. A type of airvelocity meter. (M87 0194-13)

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Chapter 4Inspection andMaintenance ofDry Kilns and EquipmentKiln structure 87

Walls, roofs, and ceilings 87Prefabricated aluminum panels 88Masonry 88

Doors 89Floors 89Rails and rail supports 89

Recording-controlling instruments 90Proper location of control sensors or bulbs 90

Dry-bulb sensors 90Wet-bulb sensors 90Equilibrium moisture content and

relative humidity sensors 91Care of recording-controlling instruments 91Calibration of recording-controlling instruments 91

Heating systems 92Steam-heated kilns 92

Improperly insulated feedlines 92Leaking pipes and unions 92Sagging and distorted pipes 93Defective valves and regulators 93Faulty pressure gauges 93Faulty automatic and manual valves 93Faulty steam traps 93

Direct-fired kilns 94Humidification systems 95

steam spray 95water spray 95

Venting systems 95Air-circulation systems 95Kiln trucks 96Use of protective coatings 97Housekeeping and maintenance around dry kilns 97Locating problems in kiln maintenance

and operation 97Tables 99Appendix. Kiln inspection checklist 100

Chapter 4 was revised by R. Sidney Boone,Research Forest Products Technologist, andWilliam T. Simpson, Supervisory ResearchForest Products Technologist.

Adequate kiln maintenance is as essential to efficientdry kiln operation as good design and construction.Adequate maintenance can be accomplished onlythrough regular, frequent inspections of the kiln andauxiliary equipment. If inspections reveal the need forrepairs or replacements, they should be made as soonas possible to avoid drying problems.

Regular, systematic inspections should cover such itemsas the kiln structure; doors; floor; tracks; control equip-ment; heating, spraying, and venting system; trucks;lumber-handling equipment; and general housekeeping.To make sure that inspections are thorough, the op-erator should note the condition of the kiln structureand the equipment on a checklist. The checklist at theend of this chapter can be made to fit any specific kilninstallation.

Kiln Structure

Dry kilns are required to withstand much harsher con-ditions than those conditions that ordinary buildingsare subjected to, regardless of the materials used inconstruction. Kilns must withstand not only extremeexternal weather conditions but also even more extremeinternal conditions. Relative humidity can vary from5 to 95 percent, and temperatures can change from-20 °F (ambient) to 250 °F during operation of a high-temperature kiln. In addition, vapors that arise fromthe woods being dried are often corrosive. Structuralcomponents of the kiln and the internal protective coat-ings must be capable of withstanding this broad rangeof operating environments.

Walls, Roofs, and Ceilings

The majority of today’s commercially built steam-heated or direct-fired dry kilns are made of either(1) prefabricated aluminum panels on a steel or alu-minum structural frame or (2) masonry, primarily con-crete block or light-weight aggregate block and some-times precast concrete. Although aluminum prefab-ricated kilns require a larger capital investment, theamount of maintenance required is considerably lessthan that required by a concrete block structure.

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Prefabricated Aluminum Panels

The prefabricated aluminum panel for walls and roofsof dry kilns was developed in the mid-1950’s, and by1960 it had received rather widespread acceptance.Early designs used the type of insulation board thenin common use in residential and commercial buildings.Later designs used fiberglass insulation, and more re-cent designs use a rigid foam insulation. The designand construction of panels for walls, roofs, and doorsare frequently the same, although in some cases pan-els for roofs are thicker and have more insulation. Thealuminum panels generally are not affected by the ex-pansion and contraction that occurs in a kiln cycle,even those cycles in which dry-bulb temperatures goas high as 250 to 300 °F. At operating temperaturesabove about 215 °F, some manufacturers prefer fiber-glass insulation; others prefer rigid foam insulation inthe aluminum panels.

Insulation values for aluminum panel kilns range froman R value of about 16 for 2-in-thick panels to about32 for 4-in-thick panels. In contrast, concrete blockkilns have R values ranging from only 1 to 3, depend-ing on thickness, type of aggregate, and whether or notthe cores are filled with insulation. A wood-frame wallwill have an R value of 4 to 5 when the stud space isnot filled with insulation and a value of about 12 whenfilled with insulation.

Maintenance of aluminum panels usually requires onlyensuring that moisture does not get past the skins andwet the insulation, reducing its value. This means re-pairing (sealing) any holes or tears in the skins as soonas they are noticed. Care should also be taken to re-pair any separation between the aluminum skin andthe metal frame around each panel. Moisture can enterthe interior of the panel and wet the insulation throughthis avenue as well as through holes in the skins. Weepholes are usually put in the bottom of the panels fordraining water that may build up in the panel, but theweep holes must be kept open and free from sawdust ordirt.

Steel components of a dry kiln must be protected fromwater vapor as well as corrosive vapors that are emit-ted from certain woods during drying, such as oak andhemlock. This is commonly done by painting or spray-ing a vapor- and corrosion-resistant paint or coating onthe steel members. Recoating is usually necessary every2 to 5 years. Suitable coatings can be obtained fromdry kiln manufacturers.

Masonry

Masonry kilns may develop cracks from expansion andcontraction caused by temperature changes inside thekiln during the drying run. This problem is sometimesexaggerated by a large temperature difference betweenthe inside and outside environments. Most concrete,including concrete blocks, is rather porous and can ad-sorb large quantities of water vapor from the kiln atmo-sphere. If cracks are not sealed when small, they willincrease in size, which leads to excessive heat and va-por losses and premature failure of the entire structure.Large cracks may also cause cold zones in the kiln thatslow up drying and permit mold and stains to developon the lumber located in those zones.

Proper maintenance of concrete kilns or concrete partsof a kiln consists of prompt recognition and repair ofproblem areas. Some good maintenance practices forconcrete kiln structures are as follows:

1. For all kilns constructed of masonry or wood (orlined with plywood), coat the inside surfaces witha vapor- and corrosion-resistant material before thekiln is used and whenever required thereafter. Usu-ally recoating is necessary every 2 to 5 years. Suit-able coatings can be obtained from dry kiln manu-facturers or other knowledgeable suppliers. Neverput vapor-resistant coatings on the exterior surfacesof masonry or wooden dry kilns, though a water-repellent coating can be applied if desired.

2. As soon as possible, seal cracks that develop in thestructure as a result of repeated expansion and con-traction of the building material. If the cracks aresmall, a coating of kiln paint may be sufficient, butlarger cracks should be filled with mastic, mortar,or cement. Coat the mortar or cement fillers with akiln paint after they have set.

3. Cracks that develop because of settling of the struc-ture can be temporarily repaired in the same man-ner as expansion and contraction cracks. To re-duce future maintenance costs, however, determinethe cause of the settling and correct it as soon aspossible.

4. Openings in the kiln structure for steam lines, tub-ing, fan shafts, and the like should be as small aspossible. Insert sleeves in the openings and plug thespace not occupied by pipe with epoxy or siliconecompounds or some similar material.

5. Promptly caulk with a nonhardening filler any openjoints and splits that occur in wood or plywooddry kilns. Refasten all loosened boards as soon aspossible.

6. Use noncorrosive metal fastenings if possible.

7. Immediately repair or replace failed supportingmembers of the structure

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Steel components of a dry kiln must be protected fromwater vapor as well as corrosive vapors that are emit-ted from certain woods during drying, such as oak andhemlock. This is commonly done by painting or spray-ing a vapor- and corrosion-resistant paint or coating onthe steel members. Recoating is usually necessary every2 to 5 years. Suitable coatings can be obtained fromdry kiln manufacturers.

Doors

Doors are frequently the weakest and most troublesomepart of a kiln structure. They are often damaged whenthey are opened or closed carelessly, when a forklift op-erator does not pay attention when loading or unload-ing the kiln, or when an improperly blocked truckloadof lumber in a track-loaded kiln rolls into the doors.The common use of prefabricated aluminum doors,on both aluminum prefabricated kilns and masonrykilns, has solved many of the problems associated withdoors on the dry kilns of the 1930’s, 1940’s, and 1950’s.During that time, it was difficult to design and build alarge door that was strong, lightweight, easy to handle,well insulated, and resistant to corrosion.

1.

2.

3.

4.

Doors, door hangers, stops, rollers, roller tracks, andgaskets that are poorly maintained cause excessivelosses of heat and vapor and are difficult to open andclose. Lower temperatures occur near damaged orpoorly fitted doors because of cold air infiltration,and drying is slower in that zone. In high-temperaturekilns, large amounts of condensate form on the frame-work near the doors when they are opened at the com-pletion of the kiln run. Steel members should be ad-equately protected to prevent excessive corrosion andrust. Neglect of doors and door equipment may alsocreate a hazard to workers. Some good maintenancepractices for door and door equipment are as follows:

1. Immediately repair or replace damaged doorhangers, rollers, and roller tracks.

2. Lubricate parts in accordance with the manufac-turer’s recommendations.

3. Repair or replace torn or missing gasket material orgaskets that no longer provide an adequate seal.

4. Instruct or warn lift truck operators to be alert tominimizing damage to doors (also to walls andbaffles) when loading or unloading the kiln.

5. In package-loaded kilns, ensure that piles are stableand will not tip over into doors or walls.

6. In track-loaded kilns, block wheels of standingloaded kiln trucks, so that the trucks cannot rollinto the kiln door.

Floors

The floors of most commercial dry kilns are con-structed of concrete. In some small kilns, usuallyoperated on a part-time basis and perhaps homedesigned, the floor may be crushed stone, lumber,or even dirt or sand. All types of floors requiremaintenance.

Good maintenance practices for kiln floors include thefollowing:

Provide a waterproofing treatment on new concretefloors to prevent spalling or scaling. Treat againwhen necessary.

Repair and seal cracks in concrete floors thatdevelop because of settling or expansion andcontraction of the concrete.

For stone, dirt, or sand floors, maintain an even floorlevel, filling holes and leveling as needed.

Provide proper drainage of site so that rain andsurface runoff do not flood kiln floor.

Rails and Rail Supports

Generally, rails and rail supports in kilns with fans orblowers located above or on the sides of the dryingcompartment are not troublesome, since the rails areusually well supported and anchored. Weak rails orrail supports in old converted natural-circulation kilnsand in older design forced-circulation kilns, where thefans or the air-supply ducts are located below tracklevel, may collapse or spread under heavy loads. Fail-ure of the rails or rail fastenings can seriously damagekiln equipment, injure workers, and result in lost dryingtime.

Good maintenance practices for rails and rail supportsinclude the following:

1.

2.

3.

4.

Immediately replace or tighten broken or loose railfastenings.

Promptly realign spread rails and securely fastenthem to the rail supports.

Leave a break in the rails under the doors to min-imize rail corrosion caused by condensate drippingfrom the doors.

As needed, apply corrosion-resistant paints to therails, metal rail supports, and rail fastenings.

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Recording–Controlling Instruments

Accurate control of both the dry- and wet-bulb temper-ature (or dry-bulb and relative humidity) is essentialfor efficient kiln operation. The most common and thebest method of control is the use of semiautomatic orfully automatic recording-controlling instruments. (SeeEquipment to Control Drying Conditions section inch. 2 for a detailed description of control instruments.)Although these instruments are usually efficient, theyare at times troublesome. Some problems are associ-ated with improper location of the sensors or bulbs.Once sensors and bulbs are properly located, problemsmay arise from improper calibration, faulty flow of wa-ter to the wet-bulb pan, and dirty wet-bulb wicks. Theefficiency of air-operated instruments may be seriouslyimpaired by oil, water, and dirt in the compressed air.Approximately 25 percent of pneumatic instrument fail-ures can be attributed to a contaminated air supply.

Proper Location of Control Sensors or Bulbs

Dry-Bulb Sensors

To accurately sense temperature, the sensor must bemounted in the main airstream flowing in the plenum.It cannot be too close to the wall or to the load of lum-ber. Care must also be taken not to place the sensorsand capillary tubes too close to steam pipes or othersources of heat that may give false readings. The tradi-tional approach has been to have at least two dry-bulbsensors and one wet-bulb sensor in a kiln. Longer kilnsusually have four dry-bulb sensors and one wet-bulbsensor. This is known as a dual end control system; oneset of dry-bulb sensors is located about one-fourth toone-third the length of the kiln from one end and theother set, one-fourth to one-third the distance from theother end. The dry-bulb sensors are mounted on oppo-site walls of the kiln and are hooked up such that thebulb on the entering-air (hottest) side of the load pro-vides the signal that is sent to the instrument. Whenthe airflow is reversed by the fans, the bulb on the op-posite side of the kiln becomes the controlling bulb.Dry bulbs that are improperly located may result invery high temperatures that increase drying losses orin very low temperatures that prolong drying time andresult in mold and stain.

degrade. Using brackets furnished by the kiln manu-facturers and following the instructions for installationshould ensure satisfactory readings from the sensors.

Wet-Bulb Sensors

The wet-bulb sensor must be located so that air circu-lates around it at all times. Experience suggests thatair speeds as low as 150 ft/min are satisfactory, butmore reliable readings are obtained at air speeds of 300to 600 ft/min or higher. Only one wet-bulb sensor islocated in each kiln because the wet-bulb temperatureis essentially the same throughout the kiln and is notas variable as the dry-bulb temperature. The wet-bulbsensor should be located about the middle of the kilnlengthwise and at a height above the floor that allowsconvenient inspection of wick and water level. Improperlocation and care of the wet-bulb sensor result in poorcontrol of the wet-bulb temperature. When the kilnis operating, the wick must be kept wet by a constantsupply of clean water to the water pan or reservoir. Adry or partially dry wick will result in an actual wet-bulb temperature in the kiln lower than that recordedor indicated on the instrument. The instrument thensignals the vents to open in an effort to reduce humid-ity. Therefore, occasional cleaning of the water reser-voir and flushing of the supply line are recommended.The flow of water to the water reservoir is usually con-trolled by a needle valve, which needs to be regulatedfrom time to time. If the flow of water is too rapid, itstemperature may be too low when it reaches the bulb,and the thermometer can give a false reading.

In some steam-heated kilns, condensate from the drainend of the coils is used to supply water to the wet-bulb wick. The condensate is piped from the drain linethrough a coiled copper tube for cooling and then pipedto the water reservoir. In using this system, care mustbe taken to assure that the water is adequately cooled,for water that is too hot will also give false readings.

Although it is common to locate one of the dry-bulbsensors near the wet-bulb sensor, never locate the dry-bulb sensor below the wet-bulb sensor. If the wet-bulbwater reservoir overflows, or if liquid water falls on thedry-bulb sensor, it will be cooled and will thus sense atemperature that is lower than the actual temperature.This false lower temperature causes the heat valves toopen, resulting in kiln overheating and the potential for

The wet-bulb sensor itself should never, under any cir-cumstances, touch the water in the reservoir nor shouldwater drip directly on the sensor.

The water reservoir should be equipped with an over-flow line that has its discharge end outside the kiln,and the water supply should be regulated so thatthe discharge is a very slow drip, not a steady flow.The overflow line must be kept open to prevent wa-ter spilling over the top of the pan into the kiln. If thekiln is shut down for a day or more, shut off the watersupply; if the temperature in the kiln is likely to dropbelow freezing during this time, drain the water linesand water reservoir. After a shutdown, the wick shouldbe replaced when the kiln is restarted.

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A dirty or badly encrusted wick affects wet-bulb con-trol. The wet-bulb wick should be made of highly ab-sorbent cloth. Replace the wick frequently with a newor laundered one whenever the kiln is loaded with anew charge of lumber or more frequently if necessary.

The wet-bulb sensor in a gas-filled or liquid-vapor sys-tem is frequently plated to minimize corrosion. Whenchanging the wick, check the bulb for pitting or othersurface deterioration. When necessary, have the bulbreplated or replaced by the instrument manufacturer.

Because of the cooling effect of evaporation, the wet-bulb temperature in a dry kiln is usually lower thanthe dry-bulb temperature; at no time can it be higher.If the reading is higher, the instrument is out ofcalibration.

Equilibrium Moisture Content andRelative Humidity Sensors

Some recorder-controllers and some drying systems usesensors that tense equilibrium moisture content or rel-ative humidity directly, rather than indirectly throughwet-bulb thermometry. While the manufacturer’s in-structions should be followed to the letter, the samegeneral principles for locating dry-bulb sensors apply towet-bulb sensors.

Care of Recording–Controlling Instruments

The period of reliable performance of control instru-ments can be greatly increased by proper care. Theparts of a recorder-controller are precision built andcan be easily damaged. However, they are well pro-tected against injury and dust, and they will give trou-blefree service for many years if the case is not leftopen too long at a time. Replace broken cover glass im-mediately. Never use compressed air, brushes, or clothto clean off dust that may settle within the instrumentcase.

Generally, repairs of gas-filled or liquid-vapor systemsshould not be attempted in the field. Instrument repairand cleaning require special tools, skills, and equip-ment. Such work should be done at the manufacturer’splant or by an authorized serviceperson.

The only part of the control instrument that requireslubrication is the clock, and this should not be donetoo frequently. Never lubricate the pivot points on thelinkage arms.

The compressed air flowing into an air-operated instru-ment must be free of oil and moisture. The quality ofthe compressed air is very important. For this reason,the air is passed through a filter dripwell or trap before

entering the instrument. The trapped oil or moisture isblown from the dripwell or trap at least once daily byopening a blowoff valve. Usually the elements in filtersmust be replaced once a year or more frequently if theybecome discolored.

Repairs to electronic or computerized recorder-controllers should be made only by an experiencedtechnician or authorized serviceperson. The skills andtools needed are different from those used in gas-filledsystems. For those instruments controlling air-actuatedvalves, the compressed air supply must be clean andprotected from oil and moisture. The clock must belubricated occasionally. The slide wires on the servomotors may need to be cleaned if pen response becomessluggish.

Calibration of Recording–Controlling Instruments

When instruments are out of calibration, the actualdrying conditions within the kiln differ from thoserecorded on the chart, and serious kiln-drying defectsor increased drying time may result. Because a newinstrument may be jarred during shipment, check cal-ibration at two or three points over total range at thetime of installation. Thereafter, check it for accuracyfrequently by using thermometers.

Recalibration of a recorder-controller found to be in er-ror is not difficult, but it should be done carefully. Theequipment required includes a liquid container and anaccurate temperature-measuring device. Because thedifference in height between bulbs of gas-filled systemsand the recorder-controller case affects the recordedtemperature, verify that the bulbs are at the correctheight in relation to the instrument by checking the no-tation on the information plate inside the instrumentcase. Calibrate the instrument with the bulbs at aboutthe same height above or below the instrument case asthey will be in service. Height does not affect resistancetemperature detectors (RTD). Two people are requiredfor the calibration-one at the sensor in the liquid con-tainer and one at the instrument.

The procedure for calibration is as follows:

1. Fill the liquid container with water or oil at least asdeep as the sensors are long, so that the sensors canbe completely submerged. Heat the water to 200 °For the oil to about 280 °F, and place the containernear the sensors.

2. Remove the sensors from their fastenings and com-pletely submerge in the heated liquid. If the dry-and wet-bulb sensors are located together in thekiln, calibrate them together. Avoid sharp bends inthe tubing of gas-filled systems. The sensors should

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not touch the sides or bottoms of the container. In adual dry-bulb system, only one sensor usually needscalibration. If there has been a difference in thetemperatures recorded by the dry-bulb thermome-ters at fan reversal during kiln operation, check eachsensor separately. The person stationed at the liquidcontainer should gently and constantly stir the liquidduring calibration.

3. After about 10 min, the person at the liquid con-tainer should take a temperature reading of the hotliquid with the thermometer or other device. Theperson at the instrument then records this read-ing together with the corresponding temperatureindicated by the instrument.

4. Record these two temperatures every 20 °F as theliquid gradually cools. If cool liquid is added to re-duce calibration time in gas-filled systems, let 5 to10 min elapse before temperatures are taken, sothat the temperature change is reflected at the in-strument, Resistance systems stabilize almost im-mediately. Make periodic check readings until theliquid temperature drops to below the lowest kilntemperatures used at the plant.

5. If the indicated temperatures on the instrumentchart are consistently lower or higher than the wa-ter temperatures by a constant amount, adjust therecorder pen arms upward or downward by thatamount by turning the small screw located on thepen arm or the pen arm pivot. If the differencesbetween the indicated temperatures and the watertemperatures are not constant, a trained technicianshould make the adjustment. A correction chart canbe made so that the instrument can be used in theinterim until it is adjusted.

6. The next step, the adjustment of the control-settingindicator, should be made only by a knowledgeable,experienced person. The indicator is adjusted whilethe compressed air or electricity is on. Lower thetemperature-setting indicator to a temperature be-low that indicated by the pen on the chart and thenmove the indicator slowly upward until the motorvalve it controls begins to open. Record the temper-ature shown by the setting indicator. Then move thesetting indicator slowly downward until the motorvalve begins to close and record the indicated tem-perature. If the average of the two recorded temper-atures is different than the temperature indicatedby the pen, move the control-setting indicator-by means of adjustment screws on the indicator-upward or downward by the amount of thedifference.

Some kiln operators prefer not to adjust the instrumentpens or control-setting indicators. Instead, they list thecalibration data and place this list near the face of theinstrument. These data are used as a guide for settingthe instrument in kiln runs.

Resistance sensors can be calibrated as outlined above,or they can be calibrated quickly with precision electri-cal resistors. (For a more complete discussion, see thesection on semiautomatic control systems in ch. 2.)

Dry kiln operators should be familiar with the man-ufacturer’s instructions for the care and maintenanceof recorder-controllers. If the instrument should fail,trained service people should be contacted for adviceand service.

Heating Systems

A correctly designed and properly maintained heatingsystem produces uniform drying conditions in a kiln.Unfortunately, the maintenance of heating systems isoften neglected, and the consequent nonuniform dryingconditions cause kiln degrade, extended drying time,nonuniform moisture in the lumber, and increased dry-ing cost. On the other hand, frequent inspection andprompt corrective action can minimize, if not eliminate,many adverse effects.

Steam-Heated Kilns

Problems that occur with steam-heated kilns includeimproperly insulated feedlines, leaking pipes andunions, sagging and distorted pipes, defective valvesand regulators, faulty pressure gauges, faulty automaticand manual control valves, and faulty steam traps.

Improperly Insulated Feedlines

Insulate all main feedlines from the boiler to thekiln to reduce losses in steam temperature, pres-sure, and consumption. In control rooms or otherareas frequented by workers, steam lines, headers, andvalves should be insulated for safety. The insulation onmany steam feedlines is either improperly installed ordamaged. Replace deteriorated or damaged insulationas soon as possible.

Leaking Pipes and Unions

Leaking pipes, caused by corrosion or mechanical dam-age, increase steam consumption. If the leak occurswithin the kiln, this will affect the wet-bulb tempera-ture. Repair or replace leaking pipes. When necessary,clean all pipes and fittings.

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Sagging and Distorted Pipes

Feedline and coil supports frequently fail, causing thepipes to become distorted and to sag. Condensate andscale accumulate in the sagged pipes and eventuallyplug them. Sagging coils will become water logged,thereby drastically reducing their ability to transferheat to the kiln. Straighten or replace sagging and dis-torted pipes. Protect pipe supports against corrosion,and reinforce or replace them when examination showsthey are failing.

Defective Valves and Regulators

Fluctuations in steam pressure caused by faultypressure-reducing valves and regulators result innonuniform drying conditions. If adjustment does notcorrect the condition, repair or replace the defectiveparts.

Faulty Pressure Gauges

The pressure gauges used in conjunction with the re-ducing valves and regulators occasionally go out of cal-ibration. Recalibrate the gauges at intervals against agauge known to be accurate or replace them.

Faulty Automatic and Manual Valves

Automatic valves that control steam flow may leak orfail to open or close properly. Failure of an air-operatedmotor valve to open is usually associated with a leakin the air supply line, a damaged diaphragm, or anovertight packing nut that causes mechanical bind-ing. Some valves have a compression spring that fa-cilitates opening; others have springs that facilitateclosing. The springs should be checked periodicallyfor proper adjustment and functioning. Failure of elec-trically operated valves may be associated with powerfailure, damaged wiring, or faulty motor. A valve thatleaks because of worn parts or the presence of scale onthe seat can usually be detected by a slow, continuousrise in temperature above the set point. A valve thatis slow to open or fails to open can be detected by aslow drop in temperature below the set point when theinstrument is calling for heat.

Repair or replace faulty valves. Keep a spare mo-tor valve on hand as well as extra motor valve parts,including diaphragms, springs, packing compound,valve stems, and valve seats. If leaks occur around thevalve-stem packing nut, tighten the nut or replace thepacking.

Manual valves are used extensively on steam heatingsystems. These valves, which are usually of the gatetype, should be operated wide open or completelyclosed. Open or close the valves occasionally to keepthem from rusting or corroding in the open or closedposition. If leaks occur around the valve-stem packingnut, follow the procedure outlined for faulty automaticvalves. Keep replacement valves and spare parts onhand.

Faulty Steam Traps

Consult kiln manufacturers, engineers, and steam-trapmanufacturers on trap installations to minimize fail-ures in the trapping system. The following summarywill assist the operator in locating and correcting trapproblems.

The failure of a steam trap to discharge may be due to(1) excessive operating pressures, (2) failure of conden-sate to reach the trap, (3) a plugged bucket vent (inthe case of bucket traps), (4) dirt in the trap, (5) wornor defective parts, or (6) excessive back pressures inthe condensate return line. Excessive operating pres-sures in the steam feedline may be caused by the failureof the reducing valve or pressure regulator, by inaccu-rate readings on the pressure gauge, or by the raising ofsteam pressures beyond the operating range of the trap.Failure of the condensate to reach the trap may be dueto a closed motor valve on the feedline, a closed manualvalve in the line between the coils and the trap, openor leaking bypass valves that allow the condensate toflow around the trap, or water-logged steam lines. Dirt,rust, or scale in the condensate may plug the bucketvent. This problem can be minimized by installing astrainer ahead of the trap and cleaning it at frequentintervals. A strainer will also prevent the trap bodyfrom becoming filled with dirt. Install blowoff valves onall traps, and blow out the traps for a short period eachday the kiln is in operation.

Continuous discharge of water from a trap can becaused by the inadequate size of the trap or trap ori-fice (that is, an opening too small for the steam pres-sure used), rust or scale under the seat in a disc trap, aworn seat that prevents proper closing, or a rusted bel-lows. These difficulties can be prevented by installing atrap that has been sized correctly and is large enoughto handle the peak condensate load, which will usuallyoccur during the warmup period.

If the trap blows live steam, the discharge valve maynot be seating. A bucket-type trap that blows livesteam may have lost its prime. A badly worn valveseat or dirt lodged between the valve and valve seatwill cause improper seating of the valve. A trap that

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loses its prime is usually subjected to sudden or fre-quent drops in steam pressure. If this occurs frequently,install a good check valve ahead of the trap. Maintain-ing a fairly constant supply of steam pressure will alsominimize this problem.

Worn or defective trap parts may cause complete fail-ure. Some parts can be easily replaced on the job withvery little, if any, loss in operating time. Replace-ment is even simpler if a bypass line has been installedaround the trap. When a defective trap cannot be re-paired on the job, replace it with a new or recondi-tioned trap. Repair the defective trap at the first op-portunity. Annual cleaning and overhaul of all traps isrecommended.

Trap failure can be detected by observing dischargefrom the trap, obtaining temperatures on the supplyand discharge sides, or listening to the action of thetrap. The discharge action of most traps can be ob-served from test outlets. These should be opened fre-quently. If steam discharges continuously from a cor-rectly sized trap, the trap is not functioning properly;determine the cause and correct it. Do not confuseflash steam with live steam. Flash steam, which is dueto pressure changes, is white as it leaves the test valve.Live steam generally appears in a continuous flow, andit is transparent as it leaves the test valve.

By listening carefully to traps during operation, trapscan be checked without visual observation of the con-densate discharged. This method is, therefore, muchmore convenient when working with a closed conden-sate return system. The necessary equipment consistsof an industrial stethoscope or a homemade listeningdevice such as a 2-ft length of 3/16-in steel rod in a filehandle, a piece of wood dowel, or a screwdriver (table4-1). With a little practice, the operation of the inter-nal components of the trap can be heard with any ofthese homemade devices merely by placing one end ofthe tool against the trap bonnet and the other end toyour ear.

A steam trap is essentially an automatic condensatevalve, the only function of which is to pass condensateand hold back steam. This definition implies that asignificant temperature differential exists between theupstream and downstream sides of a properly function-ing trap. Trap performance, therefore, can be checkedby measuring temperatures on the pipeline immediatelyupstream and downstream of the trap. Two require-ments for this method are a simple contact pyrometerfor making the measurements on the surface of the pipeand a knowledge of line pressure upstream and down-stream of the trap. For each steam pressure, there is acorresponding steam temperature. Table 4-2 shows typ-ical pipe surface temperature readings corresponding toseveral operating pressures.

Let us assume the upstream pressure in the piping sys-tem is 150 lb/in2-gauge, and the pressure downstreamof the trap is 15 lb/in2-gauge. The pyrometer measuresan upstream temperature of 335 °F and a downstreamtemperature of 225 °F. (File or wire-brush the pipe atpoints of measurement to provide good contacts for thetip of the pyrometer.) Table 4-2 shows that for an up-stream pressure of 150 lb/in2-gauge, a pyrometer read-ing between 348 °F and 329 °F should be obtained. Fora downstream pressure of 15 lb/in2-gauge, a pyrome-ter reading of between 238 °F and 225 °F is desirable.We can conclude, therefore, that the trap is functioningproperly.

Now let us assume the same pressures, but a pyrometerreading of 335 °F upstream and 300 °F downstreamof the trap. The insufficient spread between the twotemperatures indicates that live steam is passing intothe condensate return line. The trap has failed whileopen, and it needs to be repaired or replaced.

In still another example, suppose the pyrometer read-ings are 210 °F on both sides of the trap. Such a read-ing is all right downstream, where we know the pres-sure is 15 lb/in2-gauge. However, this reading is toolow upstream where we know the pressure is 150 lb/in2-gauge. The low upstream temperature probably indi-cates a restriction in the line that is reducing the pres-sure to the trap. A clogged strainer may be the culprit;blow out the trap before looking any further for a causefor the problem.

Although these examples deal with a closed return sys-tem, the temperature measurement method can also beused to check traps that discharge to the atmosphere.In this situation, of course, the downstream pressure isalways atmospheric.

Direct-Fired Kilns

In direct-fired kilns, the hot gases produced by burninggas, oil, or wood waste are discharged directly into thekiln. Burners commonly have electrically or pneumati-cally modulated fuel valves. Temperature-limit switchesare located on the inlet and discharge ends of the com-bustion chamber and are set to shut down the burnersif they overheat beyond the predetermined set point.Careful attention should be paid to proper monitor-ing and maintenance of all sensors, temperature-limitswitches, and safety equipment associated with theburner. Manufacturer’s recommendations and instruc-tions and State safety codes should be closely followed.

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Humidification Systems

Steam Spray

Steam sprays supply moisture to the kiln atmospherewhen required to maintain the desired relative humid-ity. Saturated or “wet” steam is preferable to super-heated or “dry” steam for this purpose. Using low pres-sure steam or installing a desuperheater in the steamline are common ways of obtaining saturated steam(see discussion of humidification in ch. 2). Manual orautomatic valves controlling the flow of steam sprayinto a kiln require the same maintenance as those usedin steam-heating systems. Follow the inspection andmaintenance procedures as discussed for heating sys-tems. A flow of steam or condensate from the steamspray line when the valves are closed indicates leakagethrough the control valve. A falling wet-bulb temper-ature when the control instrument is calling for steamspray indicates there is an inadequate supply of sprayinto the kiln or the steam spray motor valve has failedto open. Repair or replace defective valves immediately.

The steam spray lines usually slant downward from thefeed end. Usually a small drain line discharging out-side the kiln is provided to drain off the condensatethat collects at the low end. Keep this drain line open.Inspect the steam spray line itself periodically to seethat the discharge holes or nozzles are open and thatthe pipe has not been bent or turned so that the spraydischarges onto the lumber or the instrument controlbulbs.

Water Spray

Occasionally water spray lines are installed in kilns tosupply moisture when required for humidification. Gen-erally, water spray cannot supply sufficient water vaporrequired for effective conditioning treatments. Inspectthe valves frequently that control the flow of water intothe spray line and repair or replace defective valves im-mediately. Open plugged spray holes or nozzles andrepair or replace damaged lines.

Venting systems

Most kilns are provided with ventilators for exhaust-ing hot, moist air from the kiln and taking in fresh air.Excessive venting increases heating and humidificationrequirements, and it should be avoided by proper ad-justment and maintenance of the venting system. Aneffective and low-cost method for preventing excessiveventing is the installation of an air exhaust valve on theair line at the vent control valve.

The controller and the vent systems should be adjustedso that venting and spraying cannot occur at the sametime. This obviously wastes energy, and in cold cli-mates the spray can condense on contact with coldair and cause accelerated corrosion of any steel sur-face with which the condensate or “rain” comes intocontact.

Although vents can be manually or automatically op-erated, automatic ones are recommended. To preventexcessive venting, frequently inspect the system andkeep it in good repair. This generally means goingon the kiln roof rather than observing the vents fromgroundlevel. The inspection and maintenance of ventsrequire the following:

1. Keep the linkage system connecting two or morevent lids or dampers lubricated and inspect it pe-riodically for damage and excessive wear at pivotpoints. Straighten, repair, or replace bent, broken,or excessively worn pins, hinges, rods, chains, andlevers.

2. Inspect the vent lids or dampers when they are in aclosed position. If the lids or dampers are partiallyopen, adjust the linkage so that the lids or dampersfit tightly. This adjustment can be made quickly andeasily on most kilns.

3. Install gaskets around vent openings if there is ex-cessive leakage when the vent lids are closed.

4. Avoid overventing. Adjust the linkage so that thelids or dampers are open just wide enough to obtainthe desired venting. High winds will often keep ventlids open even if no air is supplied to the controlvalve. This can be corrected with a counterweight.

5. Examine air lines or electric circuits connecting thevent mechanism to the control instrument for airleaks and short circuits.

6. Keep the compressed air used to operate the ventmechanism dry and free of oil. Water in the airsupply line may freeze the motor valve during coldweather. If dry compressed air cannot be obtained,protect the air supply line against freezing.

Air-Circulation Systems

The uniform circulation of air in a kiln is extremely im-portant for proper drying, and it is dependent on well-maintained air-circulation equipment. Any failure ordamage to the component parts of the air-circulationsystem extends drying time and may also result innonuniform drying. Therefore, the maintenance andcare of the component parts of the air-circulationsystem are essential.

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The items to be checked in the periodical inspection ofthe air-circulation system and some of the maintenanceprocedures include the following:

1. Fan motors

a. Lubricate fan motors in accordance with themanufacturer’s instructions. Replace leakybearing seals.

b. Keep windings and armatures free of dust. Drycompressed air may be used for blowing out dust.

c. Keep motor mounts and anchor bolts tight.

d. Protect fan motors located outside the kiln fromthe weather.

e. Properly ventilate the control room to avoidoverheating fan motors.

f. In the kiln, use fan motors designed for hightemperatures and high relative humidities.

g. Protect fan motors against overloading. Relaysshould be set to kick out under small overload.

h. Repair or replace damaged or badly worn motors.

i. Have a qualified electrician inspect all elements ofthe electrical circuits periodically and keep themin good condition.

2. Fan shafts

a. Lubricate shaft bearings according to the man-ufacturer’s instructions and replace leaking oilseals.

b. Keep bearing supports tight and aligned with theshaft. Misalignment may overload the fan motorand damage the fan shaft and bearings.

c. Keep fans shafts aligned, both horizontally andvertically.

d. Keep friction and babbitt bearings tight.

e. Replace damaged or badly worn bearings.

f. Replace or repair badly worn keys or keyways.

g. Keep shaft couplings tight.

h. Replace damaged fan shafts.

3. Pulleys and belts

a,. Keep pulleys tight on the shafts.

b. Replace badly worn or damaged pulleys toprevent excessive belt wear or belt slippage.

c. Tighten belts according to manufacturer’srecommendations. Do not overtighten.

d. Replace badly stretched or damaged belts.

e. Keep all belts uniformly tensioned or tight onmultibelt systems.

4.

5.

6.

7.

Fans

a. Repair minor damage to fans; replace badlydamaged fans.

b. Keep fans tight on fan shafts.

c. See that the clearance between the tips of fanblades and the fan shroud conforms to themanufacturer’s recommendations.

d. Ensure that all fans are rotating in the same di-rection and that all reverse at the proper time.This is especially important to check in cross-shaft fan arrangement.

Caution: Exercise extreme care when fans mustbe inspected while they are running. Do notstand on fan deck when fans are running; rather,stand on ladder and look over edge of fan deck.Serious injuries have resulted from carelessnessduring the inspection of moving fans.

Fan baffles and floor

a. Repair or replace damaged fan baffles and floors.

b. Keep anchor bolts in fan baffles tight to minimizevibration and possible damage to fans.

Load baffle system (includes top, floor, and endbaffles)

a. Repair or replace damaged baffles.

b. Lubricate baffle hinges.

c. Maintain pulleys and cables on hinged bafflesystems in good condition.

Oil lines, connections, and bearings

a. Leaking oil lines, connections, and bearings in-crease safety and fire hazards, create an adverseworking environment, and may stain the lumber.

b. Make a systematic inspection for oil leaks andtighten loose connections.

c. Repair or replace damaged lines.

Kiln Trucks

Frequent inspection and proper maintenance of kilntrucks can minimize downtime and accidents. Properlubrication will help extend truck life. Recommendedmaintenance procedures are as follows:

1. Repair or replace damaged truck frames, axles, andbearings promptly.

2. Keep bolts and rivets in truck frames tight.

3. Repair or replace damaged metal or wood crosssupports.

4. Provide enough trucks so that no truck is loadedover its capacity.

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Use of Protective Coatings

Since ferrous (iron) metal in a dry kiln will rust or cor-rode, frequent inspection of metal parts is essential.Remove rust and coat the affected surface with a suit-able protective paint. Such paints can be obtained fromdry kiln manufacturers. (If manufacturers do not havethese paints, they may furnish names of suppliers.)Typical areas of rapid corrosion are around doors, thelower 16 to 24 in of structural support columns (H orI beams) in aluminum prefabricated kilns, any locationwhere a steel column or beam attaches to or extendsthrough the kiln floor or wall, and any other locationwhere condensation can occur for a prolonged period.Heat- and vapor-resistant kiln paint or coating is nec-essary for the inside of block and concrete kilns to pro-tect masonry against humidity and condensation andto reduce heat and vapor transmissions. Do not ap-ply to the outside of masonry kilns as the moisture willbe trapped in the wall and speed deterioration of thestructure.

Housekeeping and MaintenanceAround Dry Kilns

Good housekeeping around dry kilns is essential. Thepossibility of injuries, damage to equipment, derailmentof kiln trucks, and fires can be minimized by keepingthe dry kiln, operating room, and surrounding areaclean and free of safety and fire hazards. Good house-keeping practices include the following:

1. Immediately pick up stickers that have fallen fromloads of lumber and place them in convenientlylocated sticker racks.

2. Pick up lumber that has fallen from loads and repileit on the loads or return it to the storage area.

3. Remove sawdust and other debris that collects onkiln roofs or sifts into the kiln.

4. Keep kiln walkways free of debris.

5. If possible, push any stickers or lumber that projectinto walkways back into the load to prevent in-juries to workers. Boards projecting into plenumspaces or between vertical stacks of lumber can alsocause nonuniform air velocities through the loads oflumber.

6. Stop oil or grease leaks around bearings, fans, blow-ers, and motors, and wipe up spilled oil or grease assoon as possible. Use drip pans to catch oil or greasethat drips from bearings. Place oily or greasy ragsin closed containers.

7. Keep control rooms clean, free of accumulated de-bris. and well ventilated at all times.

8.

9.

10.

Keep transfers, tracks, and tramways on the loadingand unloading ends of dry kilns in good alignmentand repair.

Inspect stairways and ladders frequently and replaceweak members at once.

Keep walkways along roof in good repair to provideaccess for inspection of vents, vent motor valves,vent linkages, oil cups for bearings, and other partsof the kiln.

Locating Problems in KilnMaintenance and Operation

To assist the dry kiln operator in rapidly finding thecauses of poor drying, the common sources of troubleare outlined in this section.

If the dry-bulb temperature does not reach the setpoint in a reasonable length of time, the causes maybe as follows:

1. Steam pressure is too low.

2. Heat transfer is insufficient.

3. Heating coil is damaged, waterlogged, air-bound, orplugged.

4. Manual valves on steam supply or drain lines areclosed or only partially open.

5. Automatic motor valve fails to open.

6. Steam trap is defective.

7. Valves are open on bypass line around steam trap.

8. Back pressures in return line to boiler areexcessive.

9. Venting is excessive.

10. Leakage from kiln structure and around doors isexcessive.

11. Recorder-controller system is malfunctioningbecause

a. air or electrical signal fails to travel fromcontroller to motor valve or

b. sensor bulb (gas-filled or RTD) is not workingproperly.

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If dry-bulb temperature continues to climb above theset point, the causes may be as follows:

1. Automatic motor valve is leaking.

2. Motor valve remains open.

3. Heat is being transferred through a common wallfrom an adjacent kiln.

4. Heat from steam spray is excessive (more commonat low wet-bulb depressions or during conditioningphase of schedule).

If the wet-bulb temperature fails to reach the set pointin a reasonable length of time, the causes may be asfollows:

1. Insufficient steam is entering the spray line because

a. steam supply to spray system is insufficient,

b. automatic motor valve fails to open,

c. manual valve on feedline is closed or onlypartially open, or

d. holes or nozzles in spray line are plugged.

2. Leakage of heat and vapor from kiln structure oraround doors is excessive.

3. Venting is excessive.

If the wet-bulb temperature continues to rise above theset point, the causes may be as follows:

1. Motor valve on steam spray line is leaking.

2. Motor valve on steam spray line remains open.

3. Water is standing on kiln floor.

4. Steam or water lines in kiln are leaking.

5. Valve in bypass line around motor valve is open.

6. Venting is insufficient.

7. Wet-bulb wick is dry, dirty, or crusty.

If the lumber is not uniformly dried or has excessivedegrade associated with hot or cold zones within thekiln, the causes may be as follows:

1. Hot zones may be caused by

a. higher than average air velocities across heatingcoils because of faulty stacking and inadequatebaffling,

b. leakage of heat through a damaged wall commonto two kilns, or

c. leakage in heating coils.

2. Cold zones may be caused by

a.

b.

c.

d.

e.

infiltration of colder air through cracks in the kilnwall or around doors,

damaged fans or fan motors,

short circuiting of the air because of faultystacking or inadequate baffling,

improper drainage of condensate from coils, or

downdrafts through the vents.

Incorrect recording of dry- and wet-bulb temperaturesmay be caused by

1. control instrument that is out of calibration ordamaged,

2. improper air circulation over control bulbs,

3. exposure of control bulbs or capillary lines to directradiation from heating coils and feedlines or heatfrom steam spray,

4. water on the dry bulb,

5. dirty or dry wet-bulb wick or wet-bulb wick made ofimproper cloth,

6. too fast or too slow waterflow to wet-bulb waterpan,

7. absence of wick on the wet bulb,

8. misplacement of wick on dry bulb instead of wetbulb,

9. wrong recorder chart, or

10. excess capillary tubing on gas-filled or liquid-vaporsystems rolled up in kiln (best to roll up excesscapillary tubing in control room rather than kiln).

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Table 4-1—Operating sounds of various types of traps

Operating sounds of Operating soundsTrap type properly functioning trap of failed trap

Disc (impulse Opening and snap-closing Normally failsor thermodynamic) of disc while open—

cycles in excessof 60/min

Mechanical(bucket)

Cycling sound ofbucket as it opensand closes

Fails while open—sound of steamblowing through

Fails whileclosed—no sound

Thermostatic Sound of periodic dis- Fails whilecharge if on medium- closed—no soundto-high load;possibly no sound iflight load (throttleddischarge)

Table 4-2—Pipe surface temperatures at various steampressures

Pipe surface termperatureSteam pressure Steam temperature range(lb/in2-gauge) (°F) (°F)

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A p p e n d i x -Kiln Inspection Checklist

(Where maintenance or replacement is recommended, indicate kiln number.)

I. Kiln Structure

1. Doors and door hangers, present condition:

Do door hangers operate properly:

Do doors fit properly:

Do gaskets adequately seal door:

What maintenance a replacement is recommended:

2. Walls, present condition:

IS protective coating adequate (masonry kilns):

Are cracks repaired or holes patched:

What maintenance or replacement is recommended:

3. Structural steel members, present condition:

Is protective coating adequate:

What maintenance or replacement is recommended:

4. Roof or ceiling, present condition:

Is protective coating adequate to minimize corrosion and vapor transmission:

What maintenance or replacement is recommended:

5. Floors and walkways, present condition:

What maintenance or replacement is recommended:

6. Rails and supports, present condition:

What maintenance or replacement is recommended:

II. Control system

1. Recorder-controller, present condition:

Is correct chart paper on instrument:

Is recorder-controller properly calibrated:

Are capillary tubes protected:

Are leads and connections of RTD adequately protected:

Are bulbs or sensors properly located and mounted for accurate reading of kilnconditions:

Does cellulose EMC wafer need replacing:

What maintenance or replacement is recommended:

2. Water supply:

Is water supply line to wet bulb open:

Is wet-bulb water pan clean:

Is water supply unusually hot or cold:

Is drain line from water pan open:

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Is wet-bulb wick replaced regularly:

What maintenance or replacement is recommended:

3. Air supply:

Is compressed air supply at correct pressure, clean, and uninterrupted:

Is compressor in good condition:

Are water and grease traps in good condition:

What maintenance or replacement is recommended:

III. Heating and Humidifying System

1. Steam feedlines and headers, present condition:

Are feedlines and headers properly insulated:

What maintenance or replacement is recommended:

2. Heating coils or ducts, present condition:

Are all pipes open to full flow of steam:

What is the condition of supports:

Is ductwork bent or otherwise damaged:

What maintenance or replacement is recommended:

3. Traps, present condition:

Are traps in best possible location:

What maintenance or replacement is recommended:

4. Condensate return line, present condition:

Are condensate pumps working properly:

Is line properly sized for volume carried:

What maintenance or replacement is recommended:

5. Automatic and manual control valves, present condition:

Are automatic control valves working properly:

Are springs and diaphragms working properly:

Are manual blowdown-valves provided for traps:

Are manual valves provided for shutting off individual coils:

Are check valves working properly:

What maintenance or replacement is recommended:

6. Spray lines, present condition:

Are spray holes or nozzles open:

Does condensate from spray line drip on lumber:

Is spray line properly trapped:

What maintenance or replacement is recommended:

7. Vents, present condition:

Do all vents open and close properly:

Do air motors and linkages work properly:

What maintenance or replacement is recommended:

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IV. Air Circulation System

1. Fans and motors, present condition:

What is the condition of electrical connections and switches:

Are fans slipping on shafts:

Are all fans turning in proper (same) direction:

What maintenance or replacement is recommended:

2. Shafts and bearings, present condition:

Are motors and shaft bearings properly lubricated:

What maintenance or replacement is recommended:

3. Fan baffles, cowling, and fan floor, present condition:

What maintenance or replacement is recommended:

4. Load baffles, present condition:

Can load baffles be improved:

What maintenance or replacement is recommended:

5. Air passageways (including ductwork in direct-fired kilns):

Are air passageways open and unobstructed:

Could air movement be improved:

What maintenance or replacement is recommended:

V. General Condition of Yard, Kilns, and Control Room

Does grading and surface of yard provide for good drainage directed away from kiln(s): —

Are alleys adequate for maneuvering lift truck:

Are kiln trucks in good condition:

What maintenance or replacement is recommended:

Is control room neat and clean:

Are good kiln records kept:

Are kilns and surrounding area neat and clean:

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Chapter 5Stacking and LoadingLumber for Kiln Drying

Sorting 103Species 103Moisture content 104Heartwood and sapwood 104Wetwood 104Grain 104Grade 104Thickness 105Length 106Sorters 106

Stickering lumber 106Sticker material 107Moisture content of stickers 107Sticker size 107

Width 107Thickness 107

Load supports 107Sticker location, spacing, and alignment 108

Location 108Spacing 108Alignment 109Auxiliary stickers 109

Sticker guides 109Care of stickers 110

Box piling random-length lumber 110Mechanical stacking and unstacking equipment 110

Stackers 110Unstackers 112

Stacking lumber for various types of dry kilns 112Kiln samples 112

Kiln sample pockets built into stack 113Kiln sample pockets cut into stack 113Kiln samples in bolster space 113

Protecting stacked lumber 113Weights and restraining devices 113Loading and baffling dry kilns 114

Track-loaded kilns 115Package-loaded kilns 116

Literature cited 116Sources of additional information 116

Much of the degrade, waste, and moisture content vari-ation that occurs during kiln drying results from poorstacking and loading. Well-stacked lumber and prop-erly loaded and baffled kilns result in faster and moreuniform drying, less warp, and less sticker loss. Stack-ing and loading procedures vary widely for hardwoodsand softwoods, differences in plant layouts, type ofmaterial to be dried, and types of kilns and stackingequipment. However, certain principles apply to allstacking and loading. The purpose of this chapter isto describe these principles.

Sorting

Sorting lumber before drying simplifies stacking andalso aids in placing material of similar drying charac-teristics in the same kiln charge. The extent of sortingdepends on practical considerations--some sorts are al-most unavoidable, whereas others are sometimes omit-ted. Lumber can be sorted by species, moisture con-tent, heartwood and sapwood, wetwood, grain, grade,thickness, and length.

Species

Some species of wood have markedly different dryingcharacteristics than others. For example, the time re-quired to kiln dry green 4/4 red oak to a final mois-ture content of 7 percent is two to three times thatrequired to kiln dry 4/4 hard maple. Furthermore, amilder drying schedule must be used for the oak toavoid drying defects. If these two species were driedin the same kiln charge, the hard maple would have tobe dried by the milder oak schedule. Consequently, thehard maple would be in the kiln longer than necessary,

Chapter 5 was revised by William T. Simpson,Supervisory Research Forest Products Technologist.

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which increases drying costs. On the other hand, thedrying characteristics of hard maple and yellow birchdo not differ greatly. Thus, if the lumber has the samethickness and moisture content, these species can bedried together economically. Similarly, in softwoods thecedars and redwood might take three to five times aslong to dry as Douglas-fir or the true firs. So, when-ever possible and practical, a kiln charge should consistof the same species or of species with similar dryingcharacteristics.

Moisture Content

It is not desirable to mix air-dried, partially air-dried,and green lumber in the same kiln charge. Wetter lum-ber requires milder initial drying conditions and longerdrying time than drier lumber. For example, 4/4 cy-press air dried to a moisture content of 25 percent canbe kiln dried to 7 percent moisture content in aboutone-half the time required to dry green lumber to thesame final moisture content. Similarly, 4/4 red oak canhe kiln dried from 25 to 7 percent moisture content inabout one-quarter the time required to kiln dry red oakfrom green to 7 percent.

Because of the many variables involved, specific recom-mendations cannot be made as to the maximum allow-able difference in initial moisture content between thedriest and wettest lumber in a kiln charge. This dif-ference must be determined by each kiln operator onthe basis of production needs and the quality of dryingdesired. In general, the difference in moisture contentbetween the driest and wettest lumber in a kiln chargeshould be smaller (1) for air-dried or partially air-driedlumber than for green lumber, (2) for shorter expecteddrying times than for longer times, and (3) for a narrowrange in the desired final moisture content.

Heartwood and Sapwood

In many hardwood species, sapwood has a higher greenmoisture content than heartwood (ch. 1, table 1-4). Inaddition, sapwood generally dries faster and has fewerdefects than heartwood. Thus, sapwood and heartwoodusually reach the final moisture content at different

times, Because of these differences, it would often beadvantageous to separate heartwood from sapwood, butthis is usually impractical.

Wetwood

Wetwood, sinker stock, and wet pockets are terms usedto describe wood that has a green moisture contenthigher than that of the normal wood of the species(see ch. 8 for additional discussion). The higher mois-ture content is sometimes confined to areas that aresurrounded by normal wood. The condition is usu-ally confined to heartwood, but also often occurs inthe transition zone between sapwood and heartwood.In addition to higher green moisture content, wetwoodusually dries considerably slower and often with moredrying defects than normal wood of the species. Thenet result is that final moisture content after kiln dry-ing is quite variable, and drying defects sometimes oc-cur when wetwood and normal wood are kiln dried to-gether. Some affected species are the hemlocks, truefirs, aspen, oak, and cottonwood. For example, typi-cal kiln-drying times to 19 percent moisture content forwestern hemlock dimension lumber are 78 h for normalheartwood (65 percent green moisture content), 115 hfor sapwood (170 percent green moisture content), and160 h for wetwood (145 percent green moisture con-tent). Sorting techniques on the green chain, whichtransports lumber between the edger and the lumberstacker, are possible, but they are not always accurateor practical. Faster and more reliable techniques needto be developed for effective, practical sorts.

Grain

Flatsawn lumber (ch. 1, fig. 1-6) generally dries fasterthan quartersawn lumber, but it is more susceptible tosuch drying defects as surface checks, end checks, andhoneycomb. For practical purposes, large quantities ofquartersawn lumber may be segregated from flatsawnlumber and dried under relatively severe kiln condi-tions, using a shortened drying time.

Grade

The upper grades of lumber, both hardwood and soft-wood, are generally used in products that requirehigher strength, closer control of final moisture content,and better appearance than the lower grades. There-fore, higher grade lumber is usually sorted out and kilndried by different schedules than the lower grades.

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Thickness

Sorting for thickness is essential. Uniform thickness oflumber simplifies stacking and drying. It also reduceswarping in the lumber as well as breakage and distor-tion of stickers. Warping of lumber and sticker distor-tion resulting from stacking lumber with different thick-nesses are shown in figure 5-1a. Cupping and twistingin the thinner boards is caused by lack of contact be-tween the boards and stickers. Without the restraintof the weight of the lumber pile, boards are very likelyto warp. Warping and distortion also disrupt airflowthrough the lumber pile, resulting in nonuniform dry-ing. A stack of uniformly thick and well-piled lumber isshown in figure 5-lb. Figure 5-1—(a) Stacking lumber of different thicknesses

in the same stack results in warping of lumber and de-formation and breakage of stickers. (b) Lumber of uni-form thickness and stickers remain flat during drying.(M 115549, M87 929)

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Figure 5-2—Effect of thickness and initial moisture con-tent on kiln-drying time of red oak. (ML88 5562)

Another reason for sorting for thickness is the variationin drying time with thickness. Figure 5-2 shows theeffect of thickness on kiln-drying time of red oak. Forexample, kiln drying from green to 7 percent moisturecontent might range from 18 days for 4/4 lumber to32 days for 6/4 lumber to 55 days for 8/4 lumber.

Lumber that is miscut is likely to vary considerably inthickness across the width and along the length of thepiece. Not only is miscut lumber difficult to stack, thethinner part of boards cannot be kept flat. Moreover,the thicker parts dry more slowly and may developmore defects than thinner parts. Lumber is sometimespresurfaced or skip-dressed to attain a uniform thick-ness, which pays off in reduced warp and more uniformmoisture content.

Length

One of the best and easiest methods for sorting is tostack lumber of a single length on kiln trucks or inpackages (fig. 5-lb). If the stickers are well supportedand in good alignment, such stacking results in flat-ter and straighter lumber. Overhanging ends of longerboards in a truckload of mixed-length lumber are likelyto warp during drying. Stacking lumber of uniformlength is a common practice among softwood produc-ers and some larger hardwood producers. Most hard-wood producers, however, use box piling, which will bedescribed later in this chapter.

Sorters

Lumber is often sorted by grade and size on the greenchain between the edging and stacking operations.Different categories of boards are held in bin or slingsorters until the lumber is stacked for drying. Differ-ent types of sorters are the slant bin, vertical bin, sling,and buggy. A sling and a slant-bin sorter are shown infigure 5-3.

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Figure 5-3—Sling sorter (a) and slant-bin sorter (b) forholding sorted lumber prior to stacking for the dry kiln.(MC88 9029, MC88 9031)

Stickering Lumber

The purpose of stickers is to separate each board sur-face so that air can flow over each surface and evap-orate water. Stickers must be selected and placed sothat they give adequate support to minimize warpingof the lumber and breakage and distortion of stickers.In most applications, stickers should also be chosen tominimize stains that sometimes develop in the lumberthat contacts the stickers. Important considerations ofstickering include species and grade of wood used forstickers, moisture content of stickers, sticker size andplacement in stack, and load supports.

Sticker Material

Many stickers are required in a kiln-drying opera-tion, and replacement is costly. Practical measures forlengthening sticker life are therefore worthwhile. Stick-ers are often made from clear, straight-grained lumberrather than from low-grade lumber. The initial cost ofsuch stickers may be higher, but their longer servicelife usually offsets this cost. Straight-grained stickersmade from the harder woods stay straighter, break less,and generally last longer than irregular-grained stickersfrom softer woods.

Species such as hickory, hard maple, beech, oak,Douglas-fir, and larch make good stickers. However, forpractical reasons, the species being dried at the plantare usually used for stickers.

Moisture Content of Stickers

Stickers should be made from kiln-dried lumber. Theyshould be protected from readsorbing moisture duringstorage or holding between kiln charges. This reducesthe chance of sticker stain, which is a discoloration onthe surface of or deeper within the lumber where itcontacts the sticker. Kiln drying the stickers kills moldspores that cause the stain, and protection from read-sorption minimizes pickup of new spores. The use ofheartwood for stickers also reduces staining. In addi-tion, kiln drying reduces the distortion and thicknessshrinkage of stickers that could occur in use.

Sticker Size

Width

Wide stickers slow the drying of lumber in the areasof contact; these areas may remain at a higher mois-ture content than areas of the lumber not in contactwith the stickers. If stickers are too narrow, the lumberor stickers are liable to be crushed. Stickers for hard-woods are usually 1-1/4 to 1-1/2 in wide and shouldnot exceed 1-1/2 in. Stickers for softwoods are gener-ally about 2 in wide and sometimes up to 3 in wide forsofter species such as sugar, white, and ponderosa pine.

Thickness

Stickers are usually 3/4 to 1 in thick, although1/2-in-thick stickers are sometimes used. The thinnerstickers increase the capacity of a kiln and may be ad-equate for slow-drying species. They increase air ve-locity through the lumber stack and tend to make air-flow more uniform. However, the increased number oflayers of boards in a kiln causes a decrease in the vol-ume of air passing over each board face. In fast-dryingspecies, the volume of air per unit time may be inade-

quate to hold the amount of moisture evaporating fromthe board surfaces. In some species, such as easternwhite pine, this lower evaporation rate may cause stain-ing of the surface. In addition, thinner stickers breakand deform more readily, and sagging boards are likelyto obstruct airflow.

Regardless of size of thickness, all stickers within akiln charge should be surfaced to a uniform thicknessThickness and width should be sufficiently differentto avoid sticker misorientation (for example, a stickerplaced on edge rather than flat).

Load Supports

Unless lumber stacks are properly supported, sag-ging and distortion in the lower courses will result(fig. 5-4a). On the other hand, lumber stacked as infigure 5-4b will not sag because the load supportsare directly under tiers of stickers. The schematic infigure 5-5 shows the proper alignment of load supportsand stickers.

Figure 5-4—(a) An insufficient number of load sup-ports, improperly placed, causes sagging in this stackof lumber. (b) Properly aligned load supports pre-vent sagging of the stack and distortion of the lumber.(M 115545, M 115696)

Figure 5-5—Typical lumber truck showing alignment ofload supports and stickers. (ML88 5566)

The thinner the lumber, the greater the number of loadsupports required. A bottom course of thick dunnagesometimes can be used instead of additional supports.The usual spacing for load supports is 2 ft. The dis-tance between load supports can be increased for thicklumber, but it is better to use too many than too fewsupports.

Sticker Location, Spacing, and Alignment

Good location, spacing, and alignment of stickers re-duce warping and minimize end checking and splitting.

Location

Stickers should be placed flush with or very near theends of boards whenever possible (fig. 5-5). This willminimize warping at the ends of boards and will alsoretard end drying to some extent, thus helping tominimize end checking and splitting.

Spacing

Optimum sticker spacing is governed by the lumber’stendency to warp, its thickness, and its resistance tocrushing. In general, hardwoods require closer stickerspacing than softwoods. Some particularly warp-pronespecies like sweetgum and the elms benefit by spacingof less than 2 ft. The stickers of hardwoods that arethinner than 1 in should also be spaced less than 2 ftapart. Also, to avoid crushing stickers between the bot-tom courses of heavy loads, sticker spacing may need

Figure 5-6—Package of lumber raised by forklift truck.Short tiers of stickers above point of contact with forksreduce sag in the lower courses of lumber and help

prevent the end stickers from falling out of the stack,(M 115553)

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to be reduced. Modern lumber-stacking machines typ-ically have sticker guides adjustable in 1-ft increments.However, these machines are commonly operated withthe guides set for 2-ft sticker spacing for both hard-woods and softwoods.

Alignment

For the best control of warp during drying, the tiers ofstickers should be aligned vertically (figs. 5-4b and 5-5).Misaligned stickers (fig. 5-4a), particularly in stacks ofgreen lumber, invariably cause nonuniform distributionof weight and result in sharp kinks in the lumber wherethe stickers contact it. The thinner the lumber, thegreater the possibility of kinking. Considerable wasteresults from incorrectly aligned stickers in 4/4 lumber.

Auxiliary Stickers

Packages of lumber are commonly transported arounddry kilns with forklift trucks and straddle carriers.When lifted by this kind of equipment, lumber in thelower courses of a package often sags, and stickers atthe ends may fall out. One way to avoid this problemis to use short tiers of stickers above the forks (fig. 5-6)or the carrier bunks. The number of stickers neededin these extra tiers depends upon the thickness of thelumber and the weight of the package. Usually stick-ers are interlaid between the bottom 6 to 10 courses oflumber.

Sticker Guides

Sticker guides are devices that force stickers to beplaced in exactly the same vertical alignment tier aftertier. They ensure good spacing and alignment of stick-ers and are used almost universally. Sticker guides varyin type and are used in both manual and automaticstacking. One type used in manual stacking is shownin figure 5-7. Vertical channel irons equal in length tothe height of the load are positioned along each guideat points corresponding to the desired sticker spac-ing. The stickers, cut about 2 in longer than the de-sired width of the load, are held in place by the guidechannels.

Semiautomatic stackers have built-in sticker guides(figs. 5-8, 5-9). They are similar to the guide describedabove except that they do not need to be as tall as thestack because the level of the top course changes as thestack is built, and there is no need to pivot the guideaway from the stack. Most plants use semiautomatic orautomatic stackers.

Figure 5-7—Sticker guides for stacking lumber on kilntrucks. (M 134969)

Figure 5-8—Semiautomatic stacker. Lumber stack islocated on hydraulic lift so that the level of the topcourse is always at working level for the sticker crew.Stickers are placed by hand. (MC88 9013)

Figure 5-9—Semiautomatic stacker. Lumber stack islocated on hydraulic lift so that the level of the topcourse is always at working level for the sticker crew.Stacker arm moves course to a position over stickersand lowers it into position. (MC88 9012)

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Figure 5-10—The handling of stickers is facilitatedif they are racked for transport by carrier or forklifttruck. (MC88 9011)

Care of Stickers

The primary concerns in the care of stickers are to pre-vent breakage in handling, distortion in handling orstorage, and excessive pickup of water in storage thatcould support the growth of molds that could causestain. Large plants often invest considerable moneyin sticker-handling equipment, such as conveyors, toreduce handling costs without damaging the stickers.Smaller plants can construct racks or other holding de-vices (fig. 5-10) that will reduce sticker losses and pos-sibly handling costs. Stickers should always be storedunder cover to keep them dry and free of stain anddecay.

Box Piling Random-Length Lumber

At many plants, particularly those that dry hardwoods,segregation of lumber by length is not practical. Thebox-piling method of stacking is recommended and canbe done manually or mechanically. Random-lengthlumber that is box piled will dry straighter, flatter,more uniformly, and with less sticker loss than lumberthat is not box piled.

In box piling, the length of the outside boards in eachcourse is equal to the full length of the stack. Thus,in figure 5-11, boards numbered 1 and 7 in all coursesare as long as the stack. Other full-length boards,when available, are usually placed near the center ofthe courses, such as board 4 in course A and B. Theshorter boards in the same course are alternately placedwith one end even with one or the other end of theload. The shorter boards in all courses in the sametier of boards are all placed with one end even withthe same end of the load. For example, in figure 5-11all even-numbered short boards (with the exception ofboard 6, course D) are placed even with the front endof the load and all odd-numbered boards even with therear end. Occasionally, two narrow, short boards, suchas boards 5 and 6 of course D, are placed over a widerboard, such as board 5 of course C. Also, two or moreshort boards can sometimes be laid end to end in thesame tier of lumber. For example, 6- and 8-ft boardscould be laid end to end in a load of lumber 14 to 16 ftlong.

The column effect obtained by box piling ensues thatall boards are well supported and held down; warp,particularly cup and bow, is thereby lessened, alongwith sticker deformation and breakage. The unsup-ported ends of the short boards within the stack maywarp to some extent.

If enough full-length boards are not available for place-ment on the sides of the load in occasional courses,shorter boards laid end to end can be used. When thisis done, filler blocks should be placed in any gaps be-tween the stickers above and below the course of lum-ber, particularly when the gaps occur at the ends of theload. These blocks will keep the ends and sides of theloads from sagging and will also reduce sticker break-age. The blocks should be the same thickness as thelumber.

Mechanical Stacking andUnstacking Equipment

Most plants use semiautomatic and automatic equip-ment for stacking and unstacking lumber. Several typesof equipment are available, and they ail eliminate anymanual handling of lumber.

Stackers

With both semiautomatic and automatic stackers, asolid package of lumber is placed on a tilting break-down hoist from which the lumber slides onto a con-veyor where the courses are assembled. The stackedlumber, in some cases on the kiln trucks, is placed ona hydraulic lift controlled by the stacker crew and el-

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Figure 5-11—Box piling of random-length lumber.(ML88 5565)

Figure 5-12—Lumber stacker showing hydraulic liftthat keeps the top of the stack at working height.(MC688 9010)

Figure 5-13—Sticker magazines located above auto-matic lumber stacker. (MC88 9009)

evated to a comfortable working height (fig. 5-12). Ascourses of lumber are mechanically moved onto the lift,the load is lowered a distance equal to the thickness ofthe boards, stickers are placed, and another course oflumber is moved into position.

Semiautomatic stackers require that each sticker beplaced by hand (figs. 5-8, 5-9). A stationary guide lo-cated on the lift facilitates alignment of the stickers. Inautomatic stackers, the stickers are typically loaded ina set of magazines located above the load of lumber onthe lift (fig. 5-13). The stickers are positioned automat-ically on the course of lumber.

Unstackers

With one common type of unstacker, the load of driedlumber is placed on a tilting hydraulic lift (fig. 5-14).The lift is raised and tilted, and the top course of lum-ber slides by gravity to the dry chain. The stickers slidedown a ramp to a sticker bin or conveyor. The lift isthen raised to the next course of lumber and the cyclerepeated.

Stacking Lumber for VariousTypes of Dry Kilns

Modern dry kilns are almost universally internal fankilns where airflow is across the width of the lumberstack. In the past, there were other types of kilns, suchas natural circulation and external blower kilns andkilns where air flowed along the length of the stacks orsometimes even vertically. For optimum airflow, stack-ing procedures had to conform to the type of kiln used.Since few of these older type kilns remain in service,stacking procedures for them will not be discussed here.The earlier version of this manual (Rasmussen 1961)contains these stacking procedures.

Kiln Samples

Kiln samples, that is, boards used to estimate theprogress of drying, will be described in detail in chap-ter 6. Although other process-control techniques arebeginning to be applied, the use of sample boards isstill widespread in hardwood kiln drying. Moreover,some of the newer automatic kiln-control schemes stilldepend on the selection and placement of kiln samples.

Kiln samples are placed in the lumber stack in one ofthree possible ways: (1) kiln sample pockets are builtinto the stack, (2) kiln sample pockets are cut into thestack, and (3) kiln samples are placed in the bolsterspace.

Figure 5-14—Lumber unstacker showing boards slid-ing onto a conveyor and stickers sliding away from thelumber. (MC88 9008)

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Figure 5-15—Box piling of random-length lumber show-ing sample pockets and kiln samples. (M 75804)

Kiln Sample Pockets Built Into Stack

Ideally, kiln samples are placed in pockets in the lum-ber stacks (fig. 5-15). These pockets can be built in atthe time of stacking. Since the kiln samples are usu-ally longer than the space between tiers of stickers, thesticker or stickers immediately above the sample shouldbe shortened by the width of the kiln sample. Other-wise, these stickers will bear on the sample and make itimpossible to remove the sample for periodic weighingand examination. Two short stickers, the width of thesample pocket, are used to support the kiln sample andallow air circulation across both faces.

Kiln Sample Pockets Cut Into Stack

If it is not feasible to make kiln sample pockets at thestacker, the kiln operator can create sample pocketsafter stacking. If appropriate safety precautions aretaken, kiln samples can be cut from stacked lumberwith a small chain saw. Care is necessary because thesmall space requires cutting with the tip of the saw,which can cause kickback of the blade. Guides can beattached to the saw bar to help steady the blade andprevent cutting adjacent boards. Unfortunately, therecommended length of kiln samples is greater thannormal sticker spacing. Removing the cut kiln samplefrom the stack is relatively easy if the sample is shorterthan the sticker spacing because there are no stickersabove and below the sample to hold it in place. How-ever, when the sample is longer than the sticker spac-ing, there will be one point on the sample where it isheld in place above and below by stickers. In this situ-ation, it is necessary to pry or jack up the board abovethe sample to relieve the sticker pressure and remove

the sample. A simple forklike tool can be fitted overthe end of the sticker that is left when the sample is re-moved; the end of the sticker can be snapped off with asideways motion of the tool.

Kiln Samples in Bolster Space

A shortcut taken by some kiln operators is to place kilnsamples in the bolster space between packages of lum-ber in package-loaded kilns. This avoids the necessityof making pockets at the stacker or cutting them intothe stack. However, airflow through the bolster spaceis not the same as airflow through the lumber stack,and the drying rate of kiln samples placed there is notrepresentative of lumber in the stack. Holding racksfor samples can be made with boards above and belowthe sample, thus simulating a sample pocket, but thesamples are not at the same moisture content as boardsin the stack, which will affect their drying rate some-what. Kiln operators who have placed kiln samples inthe bolster space successfully have carefully correlatedthe drying rate of the samples with the drying rate oflumber in the stack. These samples dry a little fasterthan they would if properly placed in the stack, and theoperators adjust kiln conditions accordingly. Particularcare must be taken if the lumber is at a high moisturecontent upon entering the kiln. In this case, airflow isparticularly important in determining drying rate, anddisastrous errors in estimating moisture content of thelumber in the stack are possible. When the lumber en-tering the kiln has been air-dried or predried to 25 per-cent moisture content or below, airflow becomes lessimportant in determining drying rate, and fewer errorsresult from using samples in the bolster space to esti-mate moisture content of lumber in the stack.

Protecting Stacked Lumber

Kiln trucks or packages of lumber stacked for kiln dry-ing are often air dried first or held at the loading endof the kiln for some period of time before entering thekiln. Such lumber should be protected from rain anddirect sunshine to prevent checking and warping of theupper layers. Portable roofs made of simple buildingmaterials, such as corrugated metal, can be used forthis purpose (fig. 5-16). An open shed or roof overthe green end of the kiln is also an effective means ofprotection.

Weights and Restraining Devices

Weights placed on top of a load of lumber or restrain-ing devices that exert pressure are frequently used toreduce warp in the top layers of a stack. Concrete slabsare often used for top weighting, and at least 50 lb/ft2

is necessary for effectiveness. Top loading has beenfound effective in reducing bow and twist, but less so

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Figure 5-16—Corrugated pile covers for kiln trucksof lumber stored outdoors prior to kiln drying.(M 115539)

in reducing crook. Serrated or pinned stickers (stick-ers with metal pins that protrude vertically along thelength of the sticker and are spaced a little furtherapart than the width of board) have been found effec-tive against crook in laboratory experiments, but havenot been used commercially.

Spring-loaded clamps are also used to reduce warp intop layers (fig. 5-17). One common device consists ofwire rope and tension springs attached to each endof light I-beams that extend across the load directlyover the stickers and about 6 in beyond each edge. Thespring is pulled into tension and hooked into a stickeropening about 5 to 6 ft below the top of the load.The spring extension usually accommodates the loadshrinkage, but it sometimes requires adjustment duringdrying.

Loading and Baffling Dry Kilns

Overloading and underloading affect the quality of dry-ing achieved in a given kiln. A capacity load assumesnot only that the lumber is properly stacked in thekiln but also that the loads or packages of lumber areof lengths that provide suitable overall dimensions.That is, the spaces between truckloads and between thecharge and walls and ceiling are those called for by thekiln design. If these spaces are changed by overloadingor underloading, air circulation is changed as well, withconsequent effects on drying time and quality.

The higher the air velocity in a kiln, the greater thepossibility that air will short circuit through gaps.The basic principle of airflow through lumber in a kilnis that the fans cause air pressure to build up in theplenum chamber on one side of the load. This staticpressure causes airflow through the load; ideally the

Figure 5-17—Spring-loaded hold-down clamp for reduc-ing warp in top layers of lumber and spring extendertool. (ML88 5564)

pressure is uniform, so that airflow is also uniform.Any gaps in, around, over, or under the load provideflow paths for air and prevent the buildup of uniformair pressure. Kilns are usually engineered with spe-cific fan characteristics and plenum chambers, and theyshould not be altered without careful consideration.

Although a particular dry kiln is not limited to onlycertain lengths of lumber, a kiln operator must considerhow overloading or underloading affect air circulationand thus plan the loading patterns to the best advan-tage, deviating as little as possible from the overallcharge dimensions best suited for the kiln. When cir-cumstances demand that the loading pattern must be

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Figure 5-18—A method of baffling voids in a charge oflumber in a double-track kiln. (ML88 5563)

considerably changed from the norm, the kiln operatorshould exercise special care to keep air circulation asuniform as possible by adding auxiliary load baffles.

Track-Loaded Kilns

In single-track kilns, the distance between the loadsof lumber and top load baffles should not exceed 4 in.The ends of the kiln trucks of lumber should be but-ted snugly together. If this is not possible because longboards overhang from the ends of the loads, the voidscreated should be blocked. If the kiln charge lacksone or more lumber trucks, the entire charge shouldbe pushed to one end of the kiln, and the empty areashould be closed by solid baffles extending from thetrack level to the kiln ceiling, the fan floor, or the topload baffle. If the kiln has doors on one end only, a

charge that is short one or more kiln trucks should bepushed toward the closed end rather than the door endof the kiln.

More care is required in loading multiple-track kilns(ch. 2, fig. 2-2) than single-track kilns with the sametype of loading. Short circuiting through voids in acharge of lumber in a single-track kiln can be controlledwith solid baffles. In a multiple-track kiln, however, asolid baffle blocking a space in one track of lumber mayreduce airflow through some of the lumber on the othertracks.

A method of baffling voids in a double-track kiln is il-lustrated in figure 5-18. Track 1 has three trucks oflumber, one of which is a short load, and track 2 isfully loaded. The void spaces on both tracks betweenkiln-end D and the loads are small, about 1 ft wide. Atemporary solid baffle extending from the kiln floor tothe fan floor can be installed in this opening if desired,but since the opening is quite small, the value of a baf-fle here is questionable. The larger voids between theshort and long loads on track 1 and between kiln-end Cand the ends of the loads are blocked off by temporarybaffles to prevent excessive short circuiting of the air.The baffles shown on track 1, however, should not besolid. A solid baffle here would block off track 2 fromair circulating through the loads from side A to side B,and the lumber on this track would be shorted of airand dry more slowly than the rest of the charge. Slot-ted or perforated plywood baffles have been used in asituation like this. Snow fence has also been used suc-cessfully. Perforated baffles do not provide the same re-sistance to airflow as a load of lumber, but they reduceshort circuiting considerably. The space on kiln-end C,track 2, is blocked off with a temporary solid baffle.

The low load illustrated on track 2 (section A-A) wouldproduce a large void that would permit excessive shortcircuiting of air if it were not baffled. The slotted orperforated baffle shown between the load baffle and thetop of the low load permits air to move across the loadson both tracks in both directions of airflow with verylittle short circuiting.

If a charge in a double-track kiln is short two lumbertrucks, each track should be loaded one truck short.The trucks should be butted together. Both tracksshould be loaded as closely as possible to the same endof the kiln so that most space occurs at the oppositeend. Then, with both tracks evenly loaded, two solid,temporary baffles can be placed to block out the spaceon each track.

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Figure 5-19—Packages of lumber properly spaced sideby side on kiln trucks for kiln drying. (M 100902)

Sometimes lumber is stacked in packages, and the pack-ages are loaded by forklift and placed two-wide onkiln trucks (fig. 5-19). The sides of adjacent packagesshould be spaced 3 to 4 in apart. If this is not done,the sticker openings between courses of lumber maynot line up as drying progresses because of nonuniformshrinkage between loads. When this happens, the cir-culating air may be blocked off. Note also in figure 5-19that strips of lumber are fastened to the ends of thebolsters separating packages. This practice is recom-mended because large volumes of air can short circuitthrough these openings, which are usually 4 in wide.

Package-Loaded Kilns

Careful placement of packages and baffles is particu-larly important in package-loaded kilns (fig. 2-6) to pre-vent short circuiting of airflow. Short circuiting is morecritical in this type of kiln than in track-loaded kilnsbecause of the generally longer distance air must travelfrom the entering-air to the leaving-air side of the load.In general, the greater the capacity of package-loadedkilns, the more difficult it is to prevent short circuit-ing. When loading a package kiln, the initial back rowof packages should be placed tight against one wall.The second row should be “side-shifted” so it is tightagainst the opposite wall. The placement of packagesshould be alternated from one wall to the other until allrows have been loaded. This will leave a minimum ofair space along either wall for short circuiting of air.

Kiln operators are sometimes tempted to add au ex-tra tier of packages to increase the kiln capacity. Thisnarrows the plenum space from the design width andcauses nonuniform airflow through the load. It is poorpractice and not recommended.

The spaces between adjacent tiers of packages aresometimes made smaller or larger than recommended.If the spaces are too small, circulation through thesticker openings may be impaired because the open-ings are misaligned during load shrinkage. If the spacesare too large, air will short circuit through the openingsor around the ends of the tiers of packages.

The installation of additional top- and side-load bafflesreduces short circuiting over or around the ends of thetiers of packages and increases kiln efficiency. Tempo-rary solid or slotted baffles may be required when largevoids occur in a kiln charge that is short one or morepackages or in which the tiers of packages are incom-plete. Solid baffles should never be placed so that theyblock airflow to any packages.

Because air generally travels a long distance acrossa charge of lumber in package-loaded kilns, blockingthe bolster space (fig. 5-19) helps to prevent the shortcircuiting of air through the bolster space.

Literature Cited

Rasmussen, E.F. 1961. Dry kiln operator’s manual.Agric. Handb. 188. Washington, DC: U.S. Departmentof Agriculture. 197 p.

Sources of Additional Information

Angevine, A. 1970. A tool system for cutting kiln sam-ples from stickered lumber. Forest Products ResearchSociety News Digest, Wood Drying Division. 4 p.

McMillen J. M.; Wengert, E. M. 1978. Drying east-ern hardwood lumber. Agric. Handb. 528. Washing-ton, DC: U.S. Department of Agriculture. 104 p.

Reitz, R. C.; Page, R. H. 1971. Air drying of lumber.Agric. Handb. 402. Washington, DC: U.S. Departmentof Agriculture. 110 p.

Ward, J. C.; Pong, W. Y. 1980. Wetwood in trees: atimber resource problem. Gen. Tech. Rep. PNW-112. Portland, OR: U.S. Department of Agriculture,Forest Service, Pacific Northwest Forest and RangeExperiment Station. 56 p.

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Chapter 6Kiln Samples

Variability of material 118Species 118Thickness 118Moisture content 118Heartwood and sapwood 118Wetwood or sinker stock 118Grain 119

Number of kiln samples 119Moisture content schedules 119Time schedules 119

Checking kiln performance 119Selecting kiln samples 120Preparing kiln samples and moisture sections 120

Cutting kiln samples and moisture sections 120Determining moisture content and weight of

kiln samples and moisture sections 120Weighing moisture sections 120Weighing kiln samples 121Ovendrying moisture sections 121Weighing ovendry moisture sections 121Calculating moisture content of moisture

sections 121Calculating ovendry weight of kiln samples 122

Placing samples in kiln charges 123Using kiln samples during drying 123

Calculating current moisture content of samples 123Using samples for kiln schedule changes 124Using automatic systems 124

Intermediate moisture content tests 124When to make intermediate tests 125How to make intermediate tests 125Intermediate shell and core moisture tests 125

Final moisture content and stress tests 125Recording drying data 127

Forms for recording data 129Graphs of drying data 129

Literature cited 131Source of additional information 131

Chapter 6 was revised by William T. Simpson,Supervisory Research Forest Products Technologist.

Lumber is dried by kiln schedules, which are combi-nations of temperature and relative humidity appliedat various times or at various moisture content levelsduring drying (see ch. 7). When moisture content lev-els are the determining factor for adjusting tempera-ture and relative humidity in the kiln, some means ofestimating moisture content of the lumber in the kilnduring drying is necessary. These estimates are madewith kiln sample boards, which are weighed or other-wise sensed during drying.

Kiln samples are not used in drying softwood dimensionlumber and are rarely used for drying lumber for higherquality softwood products. Kiln samples are usuallyused in hardwood lumber drying because incorrect kilnconditions have more severe consequences for hardwoodlumber than for softwood lumber.

Traditionally, kiln samples have been removed from thekiln periodically and weighed manually for moisturecontent estimates. This manual procedure is still usedin the majority of hardwood operations, but automatedmethods are beginning to be developed. One suchmethod utilizes probes that are inserted into sampleboards to measure electrical resistance as an estimate ofmoisture content (ch. 1, table 1-11). This electrical re-sistance signal can then be fed into a computer controlsystem that makes scheduled changes in kiln conditionsautomatically. Another system uses miniature load cellsthat can continuously weigh individual sample boards;the weights are fed into a computer control system.

Whether kiln samples are monitored manually or au-tomatically, the same principles of selection and place-ment apply. The main principle of selection is that thekiln samples be representative of the lumber in the kiln,including the extremes of expected drying behavior.It is impractical to monitor moisture content of everyboard in a kiln, so the samples chosen must representthe lumber and its variability. The main principles ofplacement are that the samples are spread throughoutthe kiln at various heights and distances from the endsof lumber stacks and that the samples are subject tothe same airflow as the lumber.

The handling of kiln samples requires additional oper-ator time, and some lumber is lost when kiln samplesare taken. These disadvantages are more than offset byseveral advantages. The selection, preparation, place-ment, and weighing of kiln samples, if properly done,

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provide information that enables a kiln operator to(1) reduce drying defects, (2) obtain better control ofthe final moisture content, (3) reduce drying time andimprove lumber quality, (4) develop time schedules,and (5) locate sources of trouble that affect kiln per-formance. All these advantages add up to lower dryingcosts and more uniformly dried lumber.

This chapter covers selection and preparation of kilnsamples; the number of samples required in a kilncharge; determination of moisture content and ovendryweight of samples; how to use samples during drying;how to make intermediate moisture content estimates;tests for residual drying stresses; and recording andplotting of data.

Variability of Material

To make full use of known drying techniques and equip-ment and to assure good drying in the shortest time,each kiln charge should consist of lumber with similardrying characteristics. Differences between boards willinvariably exist despite measures to minimize them,and kiln sample selection must include these differ-ences. The following variables should be consideredin selecting kiln samples: (1) species, (2) thickness,(3) moisture content, (4) heartwood and sapwood,(5) wetwood or sinker stock, and (6) grain (flatsawnor quartersawn).

Species

Wood of both native and imported species has a widerange of physical properties that can influence ease ofdrying (ch. 1 and USDA 1987). These properties in-clude specific gravity, shrinkage, moisture diffusion andpermeability, strength perpendicular to the grain, andsize, distribution, and characteristics of anatomical el-ements. Such woods as basswood, yellow-poplar, andthe pines are relatively easy to dry, with few or no se-rious drying defects. Others, such as the oaks, blackwalnut, and redwood, are more likely to check, honey-comb, and collapse during kiln drying. Consequently, itis usually advisable to dry only one species at a time ina kiln, or, at most, a few species that have similar dry-ing characteristics. If mixed species are dried together,kiln samples should be taken from all species.

Thickness

When lumber dries, the moisture evaporates from allsurfaces but principally from the wide faces of boards.Thickness, therefore, is the most critical dimension.The thicker the lumber, the longer the drying time andthe more difficult it is to dry without creating defects(ch. 1). Lumber of different thicknesses cannot be dried

in the same kiln charge without either prolonging thedrying time of the thin lumber or risking drying defectsin the thick lumber.

Kiln operators should recognize miscut lumber and ei-ther dress it to uniform thickness or choose kiln sam-ples accordingly. Nominal l-in-thick lumber can varyfrom 3/4 in to over 1 in thick, even in the same board.The thinner parts will dry faster than the thick parts,resulting in uneven final moisture content or dryingdefects.

Moisture Content

The extent to which lumber has been air dried orpredried before it is put in a kiln must also be consid-ered, because moisture content often governs the dryingconditions that can be used. If all the free water has al-ready been removed, more severe drying conditions canbe used in the initial stages of kiln drying, with littleor no danger of producing drying defects. Furthermore,a uniform initial moisture content greatly acceleratesdrying to a uniform final moisture content. If boardsvary considerably in initial moisture content, the kilnsamples should reflect this variation.

Heartwood and Sapwood

Sapwood usually dries considerably faster than heart-wood. Resins, tannins, oils, and other extractives re-tard the movement of moisture in the heartwood. Ty-loses and other obstructions may be present in thepores of the heartwood of some species, principallywhite oak and the locusts. Sometimes, it is practicalto segregate the heartwood and sapwood boards. Thegreen moisture content of sapwood is usually higherthan that of heartwood, particularly in the softwoods.For these reasons, heartwood lumber may not reach thedesired final moisture content as soon as sapwood, orvice versa. Choice of kiln samples should be guided bythe relative proportions of heartwood and sapwood in akiln charge.

Wetwood or Sinker Stock

Wetwood or sinker stock (Ward and Pong 1980) is acondition (bacterial infection) that develops in the liv-ing tree and causes entire boards or parts of boards tobe higher in initial moisture content, slower drying, ormore susceptible to drying defects. Hemlocks, true firs,red oaks, aspen, and cottonwood develop this kind ofwood. Ideally, wetwood should be segregated from nor-mal lumber and dried separately by a different dryingschedule. However, it is not always possible or practicalto do so. From the standpoint of kiln sample selection,only the hardwoods are really relevant. Bacterially in-fected wood often has a disagreeable odor, especially inred oaks, or may be darkened (aspen and cottonwood).

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If wetwood is suspected, the sample boards should beselected accordingly and observed carefully duringdrying to detect any drying defects.

Grain

Quartersawn boards generally dry more slowly thanflatsawn, but they are less susceptible to surface check-ing. Thus, more severe drying conditions can be usedon quartersawn lumber to reduce drying time. It issometimes advantageous to segregate quartersawnand flatsawn lumber in drying. If not, kiln samplesshould reflect the relative amounts of the two grainsin a charge.

Number of Kiln Samples

The number of kiln samples needed for any kiln chargedepends upon the condition and drying characteristicsof the wood being dried, the performance of the drykiln, and the final use of the lumber. There are sev-eral reasons for using kiln samples, which may dictatethe number as well as placement of the samples. Kilnsamples are used when drying lumber by moisture con-tent schedules and for developing time-based dryingschedules.

Moisture Content Schedules

By far the most important purpose of kiln samples is toenable a kiln operator to dry a kiln charge of lumber bya specific moisture content schedule (ch. 7). This typeof schedule calls for changes in drying conditions thatare based on the moisture content of the lumber duringvarious stages of drying.

Because many variables affect drying results, the spe-cific number of kiln samples required when using mois-ture content schedules has not been firmly established.The requisite number is different for different species,initial moisture contents, and kilns, and it is best de-termined through experience. A rule of thumb given inthe earlier version of this manual (Rasmussen 1961) isto use at least four samples in charges of 20,000 fbm orless. For charges of 100,000 fbm or more, 10 to 12 kilnsamples per charge are usually sufficient. More samplesshould be used when (1) drying a charge of lumber ofdifferent species, thicknesses, moisture contents, grain,or mixture of heartwood and sapwood, (2) drying anunfamiliar species, (3) drying costly lumber, (4) ob-taining drying data for modifying a drying schedule ordeveloping a time schedule, and (5) using a dry kilnwhose performance is unknown or erratic.

Since publication of the earlier edition of this manual,research has been conducted to better define the nec-essary number of kiln samples (Fell and Hill 1980).

Although this research is not directly applicable tocommercial practice, it provides some guidelines. Intheir scheme, which applies to hardwoods, Fell and Hillrecommended using 10 to 12 kiln samples to monitormoisture content from green to 40 percent. Twenty to23 samples are recommended for 40 to 12 percent mois-ture content because this range requires a more pre-cise estimate of moisture content. Finally, only 7 to 12samples are recommended for 12 to 6 percent moisturecontent. These recommendations call for considerablymore kiln samples than the rule of thumb guidelines,but they are based on statistical sampling. Operatorsshould use their experience and individual circum-stances to weigh the value of the increased precision,at an increased cost, that comes with increasing thenumber of kiln samples.

Time Schedules

At plants where certain species and thicknesses of lum-ber from the same source are dried regularly, kiln oper-ators can utilize kiln samples to develop time schedulesfor subsequent charges of the same material dried fromand to the same moisture content. This may involveextra sampling work to measure the full range of vari-ables, but after sufficient information and experienceare obtained, kiln samples can be eliminated for futurecharges.

Time schedules are generally used in drying softwoods.It is possible, however, to develop satisfactory timeschedules for some of the more easily dried hardwoods.Some samples should be used occasionally to check theperformance of the kiln and the final moisture contentof the lumber.

Checking Kiln Performance

Studies of kiln performance show that dry-bulb tem-perature and rate of air circulation throughout a kilnmay vary considerably and affect the time and qualityof drying. Variations in temperature and air circula-tion can be determined with testing equipment (ch. 3).But if such equipment is not available, kiln samples canbe used to check variability of kiln performance. Allsamples for this purpose should be cut from the sameboard to minimize variation in drying characteristicsbetween samples. The samples should be placed nearthe top and bottom of the stacks and on both sides, atintervals of 10 to 16 ft along the length of the kiln.

Kiln samples that dry slowly indicate zones of low tem-perature or low air circulation, and those that dryrapidly indicate zones of high temperature or high aircirculation. If the drying rates vary greatly, actionshould be taken to locate and eliminate the cause. Dif-ferences in drying rates between the samples on the

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entering-air and leaving-air sides of the stacks will as-sist the operator in determining how often to reverseair circulation. The greater the difference in drying ratebetween these two, the more frequently the direction ofair circulation should be reversed.

Selecting Kiln Samples

Ideally, segregation of lumber is based on all the factorsthat affect drying rate and quality. Since this is fre-quently not possible or practical, a kiln operator mustbe guided in the selection of kiln samples primarilyby the drying rate of the most critical, slowest dryingmaterial. The largest number of samples should be se-lected from the slowest drying material. Some samplesshould also be selected from the fastest drying material,since these will determine when the equalizing periodshould be started (ch. 7).

The best time to select boards from which kiln sampleswill be cut is during stacking. Some boards are selectedto represent the heavier, wetter, and thicker boardsand to contain a relatively high percentage of heart-wood. Usually one kiln sample is cut from each sampleboard to assure a representative group of kiln samples(fig. 6-1). Some kiln samples are also cut from boardsthat represent the drier and faster drying boards. Suchboards are usually flatsawn, narrow, and scant in thick-ness, contain a high percentage of sapwood, or are drierthan the rest of the lumber at the time of stacking.

Moisture sections

One sample per board

Figure 6-1—Method of cutting and numbering kilnsamples and moisture content sections. (ML88 5587)

Preparing Kiln Samples andMoisture Sections

Kiln samples that will be weighed during drying andmoisture sections are prepared as shown in figure 6-1.Kiln samples should be 30 in or more long. Moisturesections, cut for the purpose of estimating initial mois-ture content, should be about 1 in. in length along thegrain. Knots, bark, pitch, and decay should not be in-cluded in the kiln samples, except when drying lumberof the common grades. The moisture sections that arecut from each end of the kiln sample must be of clear,sound wood. Any bark on the kiln sample or moisturesections should be removed before weighing because itcauses error in the moisture content estimate and inter-feres with drying.

Cutting Kiln Samples andMoisture Sections

Mark the kiln samples and moisture sections for iden-tification, as shown in figure 6-1, before they are cut.Usable lengths of lumber can be salvaged from each endof the board when the kiln samples and moisture sec-tions are cut. If no usable lengths would be left, cutthe samples and moisture sections about 20 in or morefrom the ends of the boards to eliminate the effects ofend drying.

With certain exceptions, moisture sections are not cutless than 1 in along the grain and are cut across the fullwidth of the board. It may be necessary to cut mois-ture sections less than 1 in. in length along the grainif a quick estimate of moisture content is needed. Tominimize errors, take extra precautions in cutting, han-dling, and weighing these thinner sections. In dimen-sion stock 1 in square or less in cross section, moisturesections are cut 2 in or more in length along the grain.A sharp, cool-running saw should be used and the sec-tions weighed immediately. If it is necessary to cut anumber of sections at a time before weighing them, thesections should be wrapped in aluminum foil or a sheetof plastic wrap.

Determining Moisture Content and Weightof Kiln Samples and Moisture Sections

The moisture content of a kiln sample is determinedfrom the moisture sections cut from each end of thesample. The average moisture content of these two sec-tions and the weight of the kiln sample at the time ofcutting are used to calculate the ovendry weight ofthe sample. The ovendry weight and the subsequentweights of the sample obtained at intervals duringdrying-called current weights—are used to calculatethe moisture content at those times.

Weighing Moisture Sections

After cutting the moisture sections, rapidly remove allbark, loose splinters, and sawdust, and weigh the sec-tions immediately. Weigh each section on a balancethat has precision of 0.5 percent of the weight of thesection and that reads in grams. Triple beam or toploading pan balances are suitable for this (ch. 3). Bal-ances with the precision to weigh moisture sections aresomewhat delicate, and they require proper care andmaintenance. The manufacturer’s recommendationsand procedures should be consulted to ensure accuratemeasurement.

To save weighing and calculating time, the two mois-ture sections cut from each kiln sample are sometimesweighed together. This technique, however, does not

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distinguish the difference in moisture content usuallypresent between the two moisture sections; therefore,separate weighings are preferred. After weighing themoisture sections separately or together, mark theweight on each section with an indelible pencil or a felt-tip pen with waterproof ink and record the weight orweights on a data form.

Weighing Kiln Samples

After cutting the kiln samples, remove all bark, loosesplinters, and sawdust. Then, immediately apply anend coating. Most kiln companies offer an end-coatingproduct, and asphalt roofing compounds are effectiveand readily available. Immediately after end coating,weigh the kiln samples on a balance that is sensitiveto 0.01 lb or approximately 5 g. The balance capac-ity should be about 35 lb (15 kg). Weight should beexpressed in either metric units or in pounds and dec-imals of pounds (not ounces). Mark the weight witha waterproof pencil or ink on the kiln sample and alsorecord it on a data form. The weight of the end coat-ing can usually be disregarded. If for some reason thekiln sample has to be shorter than recommended and ismade from a low-density species, it may sometimes bedesirable to consider the weight of the end coating. Ifso, weigh the kiln sample immediately before and afterend coating; the difference, which is the weight of theend coating, should be subtracted from all subsequentsample weights.

Ovendrying Moisture Sections

After weighing the moisture sections, they should bedried until all water has been removed in an oven main-tained at 214 to 221 °F (101 to 105 °C). This usuallytakes 24 to 48 h in a convection oven. To test whetherthe sections are thoroughly dry, weigh a few sections,place them back in the oven for 3 to 4 h, and thenreweigh. If no weight has been lost, the entire groupof sections can be considered completely dry. A typicalelectric oven for drying moisture sections is shown inchapter 3.

The moisture sections should be open piled in the ovento permit air to circulate freely around each section(ch. 3, fig. 3-7). Avoid excessively high temperaturesand prolonged drying because they cause destructivedistillation of the wood. The result is that ovendryweights are too low, and thus the estimate of moisturecontent is too high. If newly cut sections are placed inan oven with partly dried sections, the newly cut sec-tions may cause the drier sections to absorb some mois-ture and unnecessarily prolong drying time.

Microwave ovens can also be used for ovendrying mois-ture sections. Such ovens are much faster than a con-vection oven (moisture sections can generally be dried

in less than 1 h), but care must be taken not to overdryor underdry the sections. In a convection oven thereis a considerable margin of error. If a moisture sec-tion is left in longer than necessary, no great harm isdone, and the ovendry weight will not be affected sig-nificantly. However, if a moisture section is left in a mi-crowave oven even slightly longer than necessary, con-siderable thermal degradation can occur. The indicatedovendry weight of the moisture section will be less thanit should be, and the calculated moisture content willbe too great. This danger can be decreased by using amicrowave oven with variable power settings. Throughexperience, the operator can establish combinations ofspecies, size, and initial moisture content of moisturesection, ovendrying time, and oven power setting thatgive accurate ovendry weights.

Weighing Ovendry Moisture Sections

Ovendried moisture sections are weighed by the sameprocedures as freshly cut moisture sections. However,the sections must be weighed immediately after remov-ing from the oven to prevent moisture adsorption.

Calculating Moisture Contentof Moisture Sections

Moisture content of the moisture sections is calculatedby dividing the weight of the removed water by theovendry weight of the sections and multiplying the quo-tient by 100. Since the weight of the water equals theoriginal weight of the section minus its ovendry weight,the formula for this calculation is

Moisture content in percent

Example: Calculate the average moisture content oftwo moisture sections (fig. 6-1) when

Green weight of moisture section a1 is 98.55 g

Ovendry weight of moisture section a1 is 59.20 g

Green weight of moisture section a2 is 86.92 g

Ovendry weight of moisture section a2 is 55.02 g

Wanted: The average moisture content of moisture sec-tions a1 and a2, Two methods of calculating averagemoisture content in percent can be used.

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Method 1:

Moisture content of section a1

Moisture content of section a2

The average moisture content of moisture sections a1

and a2 is

Method 2:

If the sections are weighed together, the combinedgreen weight of sections a1 and a2 is 185.47 g, and theircombined ovendry weight is 114.22 g. Then

Average moisture content

Although the average moisture content of moisture sec-tions a1 and a2 calculated by method 2 results in aslightly higher value than that obtained by method 1,the calculated ovendry weight of the kiln sample usingeither method will be the same when corrected to thenearest 0.01 lb.

It is sometimes convenient when making calculations touse a shortcut method of calculating moisture contentby the formula

Substituting the weights for moisture section a1 in thisformula

Moisture content in percent of section a1

Calculating Ovendry Weightof Kiln Samples

The moisture content of a kiln sample at the time ofcutting and weighing is assumed to be the same as theaverage of the moisture content values of the two mois-ture sections cut from each end of the sample. Knowingthis value and the weight of the sample at the time thesections were cut, the ovendry weight of the sample canbe calculated by using the following formula:

Ovendry weight of kiln sample

(2)

where MC is moisture content

Example: Calculate the ovendry weight of kiln sampleA-1 (fig. 6-1), which had an original weight of 4.46 lb,using the average moisture content calculated for mois-ture sections a1 and a2, 62.2 percent.

Ovendry weight of kiln sample

A shortcut version of equation (2) that is useful withcalculators is

Ovendry weight of kiln sample

Substituting the weights for kiln sample A-l,

Ovendry weight of kiln sample

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Placing Samples in Kiln Charges

After kiln samples are cut, end coated, and weighed,they are placed in sample pockets as described in chap-ter 5 and illustrated in figure 6-2. Sample pockets areusually placed at several locations along the length ofthe kiln in the sides nearest the walls. Since the kilnsamples are representative of the lumber being dried,they should at all times be exposed to the same dryingconditions or they will give a false estimate of the mois-ture content of the kiln charge. For example, if samplesare cut and weighed several days before the lumber isloaded into the kiln, the samples should be insertedin the loads or packages during the time the lumber isoutside the kiln.

If a mixed kiln charge is being dried, the samples rep-resenting each type of material should be placed in thetruckloads or packages containing that lumber. For ex-ample, if 4/4 and 6/4 pine lumber are being dried inthe same charge, the 4/4 samples should be with the4/4 lumber and the 6/4 samples with the 6/4 lumber.

Some operators of poorly lighted kilns place smallcolored-glass reflectors or reflective tape on the edgesof the samples or the edges of boards above and belowthe sample pocket. These reflectors can be located witha flashlight. To guard against replacing samples in thewrong pocket after weighing, a number or letter corre-sponding to the sample can be written on the edge ofthe board immediately above or below the pocket.

Using Kiln SamplesDuring Drying

As drying progresses, the drying conditions in the kilnare changed on the basis of the moisture content ofthe samples at various times during the run. How fre-quently the samples must be weighed will depend onthe rate of moisture loss; the more rapid the loss, themore frequently samples must be weighed. The samplesmust be returned to their pockets immediately afterweighing.

Calculating Current MoistureContent of Samples

Two weights are required to calculate the current mois-ture content of a sample: the current weight and thecalculated ovendry (OD) weight. The formula used isas follows:

Current moisture content

(3)

Figure 6-2—(a) Schematic showing placement of kilnsamples in sample pockets built in the side of a loadof lumber. The pockets should be deep enough so thatthe kiln samples do not project beyond the edge of theload. (b) Photograph showing kiln sample in place.(ML88 5624, MC88 9028)

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Thus, if the calculated ovendry weight of the sample is2.75 lb and its current weight 4.14 lb, then

Current moisture content

After another day of drying, this sample may weigh3.85 lb. The current moisture content of the samplewill then be

The following shortcut formula can also be used to cal-culate current moisture content:

Current moisture content

Substituting the above values in this formula,

Using Samples for Kiln Schedule Changes

Kiln schedules provide for changes in kiln conditions asdrying progresses. With moisture content schedules,the temperature and relative humidity are changedwhen the moisture content of the kiln samples reachescertain levels as defined by the particular schedule inuse. If the schedules recommended in chapter 7 areused, drying conditions should be changed when theaverage moisture content of the wettest 50 percent ofthe kiln samples equals a given moisture content in theschedule. Sometimes, a kiln operator may change dry-ing conditions according to the wettest one-third of thesamples or the average moisture content of a smallergroup that may be distinctly wetter or more difficultto dry than the others. These are called the control-ling samples. The moisture content of the driest sampledetermines when equalizing should be started (ch. 7).

Using Automatic Systems

When automatic control systems are used, the use ofkiln samples and moisture sections is changed some-what. When electronic probes are used, there is noneed to cut kiln samples or moisture sections. Theprinciples of selecting sample boards still apply, how-ever, because the probes will be inserted in sampleboards. Variation in drying time between sampleboards also needs to be known. When miniature loadcells are used, kiln samples and moisture sections arestill necessary. However, since the weights of the sam-ple boards are taken automatically and continuously,there is no need to enter the kiln to get kiln samples.Computer interface and control do not require manualcalculation of current moisture content.

The use of electronic probes that estimate moisturecontent from electrical resistance is growing. Suchprobes offer automatic control, but they currentlyhave some limitations. The change in electrical resis-tance with moisture contents above 30 percent is small,so that the probes are limited in accuracy above thislevel of moisture content. Currently, charges in kilnsthat use these control systems are dried to 30 percentmoisture content using home other control principle,and probes are able to control from 30 percent to finalmoisture content.

Intermediate MoistureContent Tests

If the moisture content of the moisture sections doesnot truly represent that of the kiln sample, the calcu-lated ovendry weight of the kiln sample will be wrong.This may mislead the operator into changing kiln con-ditions at the wrong time, with such serious conse-quences as prolonged drying time, excessive drying de-fects, and nonuniformly dried lumber. For example,if water pockets are present in the moisture sectionsbut not in the sample, the calculated ovendry weightof the sample will be too low and its current moisturecontent too high. This will lead the kiln operator tobelieve that the moisture content is higher than it re-ally is, and scheduled kiln condition changes will bedelayed. The end result is an unnecessary extension ofdrying time. Conversely, if water pockets are presentin the sample but not in the moisture sections, the cal-culated ovendry weight of the sample will be too highand its current moisture content too low. This will leadthe kiln operator to believe that the moisture contentis lower than it really is, and scheduled kiln conditionchanges will be made too soon. The result is an ac-celeration of the kiln schedule that could cause dryingdefects. These potential problems can be avoided bymaking intermediate moisture content estimates.

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When to Make Intermediate Tests

When the calculated moisture content of one or a fewkiln samples is much higher than that of the othersamples, or if their rate of drying appears to be muchslower than the average rate, a moisture check shouldbe made on those samples for a better estimate of theircalculated ovendry weight. The best time for makingan intermediate estimate is when the average mois-ture content of the samples is about 20 to 25 percent.Intermediate estimates can be made on all the sam-ples in the charge if the operator wants an even betterestimate of moisture content.

How to Make Intermediate Tests

Trim a section about 5 in long from one end of the kilnsample. Then, cut a l-in-wide moisture section fromthe newly exposed end of the sample, weigh it imme-diately, and ovendry. Coat the freshly cut end of theshortened sample and weigh it immediately. The newweight of the sample is the new “original” weight usedin equation (2). After weighing the sample, place itin its pocket in the kiln charge. As soon as the mois-ture section has been dried, weigh it and calculate itsmoisture content with equation (1). Substitute the newmoisture content value, together with the new origi-nal weight of the sample in equation (2), to obtain anew calculated ovendry weight. Use the new calcu-lated ovendry weight in equation (3) to obtain the cur-rent moisture content of the sample in all subsequentweighings.

A moisture content check may be desirable near theend of the kiln run to obtain a better estimate of whento start equalizing.

Intermediate Shell andCore Moisture Tests

Moisture content gradients are discussed in chapter 1.Sometimes, it is useful for the kiln operator to knowthe moisture contents of the shell (the outer part of theboard) and the core (the inner part of the board). Forexample, in a species that is susceptible to drying de-fects, such as oak, it is quite important to delay raisingthe temperature in the kiln to the high temperaturesof the last few steps in the kiln schedule until the coremoisture content is 25 percent or below. Otherwise,honeycomb is likely to develop (ch. 8). However, whenthe core is at 25 percent moisture content, the averagemoisture content for the whole piece will be somethingless than 25 percent and thus will not always be a reli-able indicator of the core moisture content. Therefore,shell and core moisture content estimates are sometimesuseful. A typical moisture section, 1 in along the grain,is cut and then further cuts are made into the shell

Figure 6.-3—Method of cutting section for measuringshell and core moisture content. (ML88 5586)

and core portions, as shown in figure 6-3. The shell andcore are weighed separately and then ovendried so thatthe moisture content can be calculated according toequation (1).

Final Moisture ContentAnd Stress Tests

After the lumber has been dried to the desired finalmoisture content, the drying stresses relieved by a con-ditioning treatment (ch. 7), and the charge removedfrom the kiln, a final moisture content check on thesample boards is often desirable. The average moisturecontent as well as shell and core estimates can be madein the same way as already described.

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Figure 6-4—Method of cutting stress sections for case-hardening tests. Lumber that is less than 1-1/2 in thickis cut into three prongs, and the middle prong is re-moved; lumber that is 1-1/2 in thick or thicker is cutinto six prongs, and the second and fifth prongs areremoved. (ML88 5585)

Figure 6-5—(a) Method of cutting stress sections forsevere casehardening tests. (b) Prongs are offset so thatthey can cross and indicate severity of casehardening.(ML88 5584)

Drying stresses are discussed in chapter 1, the reliefof drying stresses (conditioning) is discussed in chap-ter 7, and the consequences of unrelieved stresses arediscussed in chapter 8. Here, we describe how to pre-pare and interpret stress sections. There are two basicways to prepare stress sections. Both methods work onthe principle that the stresses, that is, tension in thecore and compression in the shell, will become unbal-anced when a saw cut is made. Figure 6-4 shows oneway to cut stress sections and illustrates the reaction ofsections that are casehardened (sections with residualdrying strews). When the sections have stress, the twoouter prongs pinch in because the tension stress in thecore is released by the saw cut. Thus, the inner faces ofthe prongs shorten because of the release of stresses.

In situations where drying stresses are severe, theprongs as cut in figure 6-4 will touch and in fact snaptogether tightly. Because they touch, it is difficult tojudge the severity of the stresses. The second methodof making the casehardening test visually distinguishesbetween severe and moderate drying stresses. Thestress section is sawed to allow diagonally oppositeprongs to bypass each other by an amount related tothe severity of drying stresses (fig. 6-5b). The saw-ing diagram for preparing these sections is shown infigure 6-5a. After cutting the section from the sampleboard, saw on lines P and Q but do not remove the sec-tion loosened by these cuts. Saw along line R, whichis approximately midway in the section’s width. Sawdiagonally along S and its diagonally opposite coun-terpart. Remove the diagonally opposite prongs andthe loose center section to allow free movement for theremaining diagonally opposite prongs. If the dryingstresses are severe, the prongs will cross, as shown infigure 6-6.

Unfortunately, residual drying stresses and moisturegradients sometimes interact and can cause confu-sion. If the core of a cut stress section is not at thesame equilibrium moisture content as the air where itis cut, the moisture content of the core will change, andthe inner face will either shrink and react as if case-hardened, or swell and react as if reverse casehardened(fig. 6-4). Most commonly, the moisture content of thecore is high enough so that the core shrinks when ex-posed to the surrounding air. Then, the inner face cre-ated by the saw cut loses moisture and shrinks. Theresult is that the prongs pinch in as if casehardened.The time required for prong movement is a good indi-cation of whether residual drying stresses or moisturegradients cause prong movement, or if the cause is acombination of these. Residual drying stresses causeprong movement immediately, whereas a change causedby moisture content requires at least several hours tocomplete. If immediate prong movement is observed,

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Figure 6-6—Stress sections showing crossing of prongs when sections are cut by the procedure shown infigure 6-5. (MC88 9034)

followed by additional prong movement, both factorsare the cause. In either case, prong movement pointsto a condition that should be corrected to avoid warpupon resawing or machining (ch. 8). Either additionalstress relief or equalization or both procedures arerequired.

Occasionally, the transverse casehardening test willshow no stress, but the lumber will bow when resawed.Bowing is caused by longitudinal stress resulting fromeither longitudinal tension set in the surface zones orlongitudinal shrinkage differentials caused by reactionwood (tension wood in hardwoods). These stresses aremost likely to be unrelieved when conditioning tem-perature or equilibrium moisture content is too low orwhen conditioning time is too short. The longitudinalstress sticks in figure 6-7 show whether such stressesare present. If longitudinal stresses are a problem, con-ditioning should be at 180°F or higher. The lumbermust have been equalized, and the recording instru-

ment must be in calibration. If longitudinal stresses arestill a problem, the wet-bulb setting can be raised 1 °Fover the recommended value. Also, the conditioning pe-riod can be extended about 4 h per inch of thickness.If tension wood stresses are very severe, they may notyield to any conditioning treatment.

Recording Drying Data

Good recordkeeping of the details of kiln runs can beuseful to the kiln operator in several ways: (1) for mod-ifying drying schedules on subsequent charges to obtainfaster drying without sacrificing quality, (2) for devel-oping time schedules for certain types of lumber thatare dried frequently, (3) for determining the effect ofseasonal weather conditions on kiln performance anddrying time, and (4) for checking kiln performance forcauses of nonuniform drying or drying defects.

Figure 6-7—Method of cutting sections for final mois-ture content and drying stress tests. MC is moisturecontent. (ML88 5583)

The kinds of data to be recorded will vary with thenature of the drying. More than the usual amount ofdrying data is required in the case of a test run in anew kiln, a new and unfamiliar type of lumber, and anew or modified schedule. Also, good documentationof the kiln run may be useful when precise drying isrequired or high-value lumber is dried. The data caninclude lumber species, grade, origin (of both the lum-ber (sawmill) and the trees (geographical location) itwas cut from), grain (flatsawn or quartersawn), per-centage of sapwood, number of rings per inch, moisturecontent, and thickness; date of sawing; intermediatehandling between sawing and drying; drying data (ini-tial), schedule, time, and defects; handling and storageafter drying; and shipping date. Any other informationthat the kiln operator considers relevant should also benoted.

Figure 6-8—Form used for recording kiln sample data in a dry kiln run of 4/4 air-dried soft maple. Data for3 of 10 kiln samples are shown. (ML88 5582)

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Moisture and Stress Record

Figure 6-9—Form for recording final moisture content and drying stress data for three kiln samples.(ML87 5321)

Forms for Recording Data Graphs of Drying Data

Kiln sample data should be recorded on suitable forms,such as ones supplied by kiln manufacturers. Many kilnoperators develop their own forms to fit their specificneeds. Two forms are shown in figures 6-8 and 6-9.Drying data obtained for each sample during thekiln run can be entered on a kiln sample record form(fig. 6-8). Other data such as kiln number, lumbervolume, species, thickness, and starting and endingdates for the run can be entered as required in a head-ing. The form in figure 6-8 is for only three sampleboards. This form also shows data recorded for inter-mediate moisture content estimates and the moistureregained during the conditioning treatment. The weightof the end coating used on the kiln samples can also berecorded, if required.

Data for the final moisture and drying stress tests canbe recorded on a form like the one shown in figure 6-9.The degree of casehardening present in the lumber isnoted on this form. Supplemental moisture data ob-tained with a moisture meter should also be recorded.

Graphs of drying data show at a glance the time re-quired to reach certain moisture contents. A plot ofthe moisture contents of several kiln samples is shownin the lower portion of figure 6-10 for 4/4 northern redoak. The curve illustrates the steady loss of moistureover the entire drying period. Curves plotted from dataobtained from each sample are useful for checking kilnperformance and the reliability of the moisture con-tents of the kiln samples. For example, if the moistureloss data from some samples in several charges in thesame zones in a kiln consistently indicate a slower orfaster drying rate than that of the other samples in thecharges, this is evidence of a cold or hot zone or differ-ent air circulation in that location. The source of trou-ble can usually be found and corrected. On the otherhand, if it is known or if an investigation shows thatthe cause is not associated with a cold or hot zone ordifferent air circulation, the calculated ovendry weightof the kiln sample may be inaccurate and an intermedi-ate moisture content estimate is needed.

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Figure 6-10—Graph showing kiln-drying schedule andmoisture content at various times during drying of4/4 northern red oak. EMC is equilibrium moisturecontent. (ML88 5581)

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Literature Cited

Fell, J. D.; Hill, J. L. 1980. Sampling levels for hard-wood kiln-drying control. Forest Products Journal.30(3): 32-36.

Rasmussen, E. F. 1961. Dry kiln operator’s manual.Agric. Handb. 188. Washington, DC: U.S. Departmentof Agriculture. 197 p.

U.S. Department of Agriculture. 1987. Wood hand-book: Wood as an engineering material. Agric. Handb.72. Washington, DC: U.S. Department of Agriculture.466 p.

Source of Additional Information

Ward, J. C.; Pang, W. Y. 1980. Wetwood in trees: atimber resource problem. Gen. Tech. Rep. PNW-112.Portland, OR: U.S. Department of Agriculture, ForestService, Pacific Northwest Forest and Range Experi-ment Station.

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Chapter 7Kiln Schedules

Hardwood schedules 135General hardwood schedules 135

Moisture content basis 135Material considerations 135Recommended schedules for steam-heated

kilns 135Assembly of a drying schedule 136Examples of assembled schedules 137Use of schedules for air-dried or predried

lumber 137Modifications to general hardwood schedules 138

Shifting wet-bulb depression schedules 138Using H-type wet-bulb depression schedules 138Shifting temperature schedules 139Changes within the schedule 139

Special hardwood schedules 140Maximum strength schedules 140Alternate schedules for some species 140Time schedules 140High-temperature schedules 140Schedules for imported species 140Schedule for presurfaced northern red oak 140

Softwood schedules 141Softwood moisture content schedules 141

Moisture content basis 141Material considerations 142Moisture content schedules 142Kiln drying air-dried lumber 142Modifying softwood moisture content

schedules 142Commercial softwood time schedules 142

Conventional-temperature kiln schedules 143High-temperature kiln schedules 143

Softwood schedules for special purposes 143Brown-stain control 143Setting pitch and retaining cedar oil 143Lumber treated with waterborne preservatives

or fire retardants 144Maximum strength schedules 144Bevel siding, venetian blinds, and other

resawed products 144Bundled short-length items 144Large timbers and poles 144Tank stock 145Knotty pine lumber 145

Dehumidification kiln schedules 145Sterilizing, equalizing, and conditioning

treatments 145

Sterilizing treatments 145Mold 145Fungal stain and decay 146Insects 146

Equalizing and conditioning treatments 146Equalizing treatment 147Conditioning treatment 147

Kiln-drying time 147Literature cited 148Sources of additional information 148Tables 149

Chapter 7 was revised by William T. Simpson,Supervisory Research Forest Products Technologist,and R. Sidney Boone, Research ForestProducts Technologist.

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A kiln schedule is a carefully worked-out compromisebetween the need to dry lumber as fast as possible and,at the same time, to avoid severe drying conditions thatwill cause drying defects (ch. 8). It is a series of dry-and wet-bulb temperatures that establish the temper-ature and relative humidity in the kiln and are appliedat various stages of the drying process. Temperaturesare chosen to strike this compromise of a satisfactorydrying rate and avoidance of objectionable drying de-fects. The stresses that develop during drying (ch. 1)constitute the limiting factor that determines the kilnschedule. The schedules must be developed so thatthe drying stresses do not exceed the strength of thewood at any given temperature and moisture content.Otherwise, the wood will crack either on the surfaceor internally, or be crushed by forces that collapse thewood cells. Wood generally becomes stronger as mois-ture content decreases, and, to a lesser extent, it be-comes weaker as temperature increases. The net resultis that as wood dries, it becomes stronger because ofthe decreasing moisture content and can tolerate higherdrying temperatures and lower relative humidities with-out cracking. This is a fortunate circumstance becauseas wood dries, its drying rate decreases at any giventemperature, and the ability to raise the drying tem-perature helps maintain a reasonably fast drying rate.Thus, rapid drying is achieved in kilns by the use oftemperatures as high as possible and relative humiditiesas low as possible. For hardwoods, relative humiditycan generally be reduced substantially before tempera-ture can be raised substantially.

Drying stresses are related to the difference betweenthe moisture content of the interior and surface of thelumber. The extent of this difference is related to thekiln temperature, relative humidity, and airflow as wellas the characteristics of the species. The larger thedifference in moisture content, the greater the dryingstresses. If the drying stresses become too great, theycan exceed the strength of the wood and cause surfaceand internal cracks. Many kiln schedules are based onaverage moisture content of the wood because it indi-cates the difference in moisture content between theinterior and surface of the wood.

Kiln schedules can be classified as general or special.General schedules are intended for drying lumber in-tended for almost any product and will do a satisfac-tory job. Special schedules are those developed to at-tain certain drying objectives; for example, to reducedrying time, dry chemically treated lumber, or maintainmaximum strength of the lumber for special uses. Be-cause of the many variables in the character of wood,type and condition of kiln, quality of drying required,and cost considerations, no schedule presented in thischapter can be considered ideal. The schedules are presented as guides for kiln operators in developing sched-ules best suited for their own particular operation. In

general, the schedules presented are conservative andcan often be accelerated with care; this chapter alsooutlines procedures for systematically accelerating aschedule.

Commercial kilns use different methods for dryinghardwoods and softwoods. In general, hardwood lum-ber is slower drying and more susceptible to defectsthan softwood lumber. Also, most end uses of kiln-dried hardwood lumber require uniformity of mois-ture content and permit few drying defects. Softwoods,on the other hand, generally dry faster and more uni-formly than hardwoods, and are less susceptible to dry-ing defects. Also, most structural lumber is made fromsoftwoods, and the standards for such lumber are lowerin regard to drying defects and tolerance of moisturecontent. The net result is that hardwoods are gener-ally kiln dried by moisture content schedules; that is,dry- and wet-bulb temperatures are changed when thelumber reaches certain moisture contents. Softwoods,on the other hand, are generally kiln dried by timeschedules—whether the wood is intended for struc-tural lumber or for appearance uses, such as furnitureor millwork. In time schedules, dry- and wet-bulb tem-peratures are changed after certain periods with no es-timate of moisture content as a guide. Moisture contentschedules can often be changed to time schedules af-ter lumber of the same species, thickness, and source isrepeatedly dried in the same kiln.

Satisfactory time schedules have been worked out fordrying softwood lumber of a uniform character in thesame type of kiln. An operator inexperienced in dry-ing softwoods may want to consider a moisture contentschedule as a safer way to get started and then switchover to a time schedule later. Even though moisturecontent schedules are rarely used for softwood lumber,they are included in this manual for the occasions whenthey might be useful.

The schedules listed in this chapter are designed foruse in kilns where the air velocity is approximately400 ft/min. The general schedules are conservativeenough to produce lumber with a minimum of dryingdefects in a reasonably short time. The operator shouldnot make the schedules more conservative unless thereis some specific reason for doing so, such as abnormallumber or poor kiln performance. With properly main-tained kilns, the general schedules can usually be modi-fied to shorten drying time.

The schedules presented in this manual are also pre-sented in the report by Boone et al. (1988) referencedat the end of this chapter. In this report, the kilnschedules are completely written out rather than coded,and thus the report serves as a quick reference sourcefor schedules.

Hardwood Schedules

General Hardwood Schedules

Pilot testing and considerable commercial experiencehave demonstrated that the general schedules devel-oped by the Forest Products Laboratory for steam-heated kilns, which are presented in this chapter, aresatisfactory for drying 2-in and thinner hardwood lum-ber. They form the base from which an operator candevelop the most economical schedule for a specifictype of kiln. Related information on application andmodification of the schedules is also presented togetherwith suggestions for drying thick hardwoods.

Moisture Content Basis

Both drying rate and susceptibility to drying defectsare related to the moisture content of lumber, so kilnschedules are usually based on moisture content. Thesuccessful control of drying defects as well as the main-tenance of the fastest possible drying rate in hardwoodlumber depends on the proper selection and control oftemperature and relative humidity in the kiln.

At the start of drying, a fairly low temperature is re-quired to maintain maximum strength in the fibersnear the surface to help prevent surface checks (ch. 8).The relative humidity should be kept high early in dry-ing to minimize the surface checking caused by the ten-sion stresses that develop in the outer shell of lumber(ch. 1). Even at these mild initial kiln conditions, thelumber will lose moisture rapidly. Therefore, each com-bination of species and thickness (and in some cases,end product) has been classified into a schedule codeof “T” number for temperature and “C” number forwet-bulb depression settings. To maintain a fast dry-ing rate, relative humidity must be lowered graduallyas soon as the moisture content and stress condition ofthe wood will permit. Wood becomes stronger as mois-ture content decreases and can withstand higher dryingstresses. As a general rule, relative humidity can besafely lowered gradually after the green wood has lostabout one-third of its moisture content. The tempera-ture generally cannot be raised, even gradually, untilthe average moisture content reaches about 30 per-cent. These first temperature changes must be grad-ual because at about this moisture content the stressesbegin to reverse; that is, the core of the lumber goesinto tension (ch. 1), and the danger of internal hon-eycomb becomes a concern. When the moisture con-tent at midthickness is below 25 to 30 percent moisturecontent (which means the average moisture content forl-in-thick lumber is about 20 percent), it is generallysafe to make a large increase in dry-bulb tempera-ture in order to maintain a fast drying rate. In thickerlumber of some dense species, it is necessary to bringaverage moisture content down to 15 percent to get

midthickness moisture content down to 25 to 30 per-cent. If the temperature is raised too soon while thecore is still wet and weak, the danger of honeycomb isgreat in some species such as oak. An ample numberof kiln samples should be used to make good estimatesof these critical moisture contents. The recommendedoperating procedure is to take the average moisturecontent of the wetter half of the kiln samples—calledthe controlling samples-as the factor that determineswhen to change drying conditions (ch. 6).

Material Considerations

The general schedules are for hardwoods that are to bedried from the green condition. They can be modifiedto apply to previously air-dried lumber. The schedulesare for the more difficult to dry types of lumber in aspecies-for example, flatsawn heartwood. Because ofthe difference in the moisture content of sapwood andheartwood in many species, most of the kiln samplesshould be taken from the wettest heartwood and theirmoisture content used in applying the kiln schedule.Modifications are suggested later in this chapter forlumber that is all or predominately sapwood.

Recommended Schedules forSteam-Heated Kilns

Schedules for dry-bulb temperatures and wet-bulb de-pressions are given in tables 7-1 and 7-2. Together, thedry-bulb temperature and the wet-bulb depression de-termine the relative humidity and the wood equilibriummoisture content (EMC) (ch. 1, table 1-6).

Table 7-1 lists 14 temperature schedules ranging froma very mild schedule, T1, to a severe schedule, T14.In all cases, initial temperatures are maintained untilthe average moisture content of the controlling samplesreaches 30 percent.

Table 7-2 lists the wet-bulb depression schedules for sixmoisture content classes. These classes are related tothe green moisture content of the species (table 7-3).Another moisture content class, H, will be discussedlater. There are eight numbered wet-bulb depressionschedules; number 1 is the mildest and number 8, themost severe. The wet-bulb temperature to be set onthe recorder-controller is obtained by subtracting thewet-bulb depression from the dry-bulb temperature.

Table 7-4 is an index of recommended schedules for 4/4to 8/4 hardwood lumber and other products. Whilethe same schedule is listed for 4/4, 5/4, and 6/4 lum-ber, these thicknesses obviously will have different dry-ing times and should be dried separately. For drying6/4 lumber of refractory species such as oak, the 8/4schedule may be desirable.

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There are 672 possible schedules in tables 7-1 and 7-2.There is no demonstrated need for so many schedules,nor have they all been tested. They merely represent asystematic way to develop the whole range of degreesof severity in kiln schedules. The combination of ex-perience and judgment then allows one to estimate anappropriate schedule.

Kiln-drying hardwoods thicker than 8/4 from the greencondition is often impractical because of the long kilntime. A common practice is to either air dry the lum-ber before kiln drying or use a predryer before kiln dry-ing. Table 7-5 is an index of suggested schedules for10/4 and thicker hardwood lumber. These schedulesare not as well established as the schedules for thinnerlumber and should be used with caution.

Assembly of a Drying Schedule

A form such as the one in table 7-6 can be used to as-semble a drying schedule as follows:

1. From table 7-4, find the schedule code number forthe lumber to be dried. In table 7-6 the code num-bers are T8-C3 for 4/4 sugar maple. Place the codenumbers at the top of the form.

2. Since the first change in drying conditions involvesthe wet-bulb depression, write the wet-bulb depres-sion step numbers 1 through 6 in column 2.

3. In column 3, write the moisture content values cor-responding to these steps from the appropriate mois-ture content class of table 7-2. In this example, theclass is C, so the values are >40, 40, 35, 30, 25, and20.

4. In column 5, write the wet-bulb depression valuescorresponding to the steps from the appropriate wet-bulb depression schedule number from table 7-2. Inthis example, the number is 3, so the wet-bulb de-pression values are 5, 7, 11, 19, 35, and 50.

5. In column 1, write the temperature step numbers.Since dry-bulb temperature changes are not madeuntil the average moisture content of the control-ling samples reaches 30 percent, repeat tempera-ture step number 1 as often as necessary. In thisexample, it is repeated three times. The mois-ture content at the start of temperature step 5 is15 percent (table 7-1). Therefore, in filling outthe schedule form it is necessary to repeat wet-bulb depression step 6, as shown in table 7-6. Ex-perienced kiln operators usually omit columns 1and 2.

Figure 7-1—Kiln schedule and drying curve for 4/4sugar maple. (ML88 5608)

6. In column 4, write the dry-bulb temperature thatcorresponds to the temperature step number in table7-1. If step 1 is repeated, the initial dry-bulb tem-perature must be repeated, as shown in table 7-6.

7. Subtract the wet-bulb depression from the dry-bulbtemperature in each step to obtain the correspondingwet-bulb temperature. These values are entered incolumn 6.

Columns for relative humidity and EMC, which arehelpful in understanding drying, can be added at theright of the table if desired. These values can be ob-tained from table 1-6 in chapter 1. The T8-C3 schedulefor 4/4 sugar maple and a drying curve obtained in akiln run are illustrated in figure 7-1.

Uniformity of moisture content and relief of dryingstresses are achieved by equalizing and conditioningtreatments near the end of drying, as described later inthis chapter.

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Examples of Assembled Schedules

Some schedules for hardwoods, assembled from ta-bles 7-1 and 7-2, are illustrated in table 7-7. A studyof these will be helpful when assembling schedules forother species.

The schedules listed in tables 7-1 and 7-2 may be con-servative for some types of dry kilns and for some dry-ing requirements. With experience, an operator shouldbe alert to the possibility of modifying schedules to re-duce drying time. Schedule modifications are discussedlater in this chapter.

Use of Schedules for Air-Dried orPredried Lumber

The general schedules for green hardwoods are also rec-ommended for kiln drying lumber that has previouslybeen air dried or dried in a predryer. Most kiln samplesshould be prepared from the wettest and slowest dryingboards, but should include at least one sample from thedriest and fastest drying boards (ch. 6).

For 4/4, 5/4, and 6/4 (except oak) lumber that hasbeen dried to 20 to 30 percent moisture content, thefollowing procedure applies:

1. Bring the dry-bulb temperature up to the value pre-scribed by the schedule for the average moisturecontent of the controlling kiln samples, keeping thevents closed and the steam spray turned off.

2. After the kiln has reached the dry-bulb temperature,set the wet-bulb temperature.

a. If the air-dried or predried lumber has not beenwetted on the surface or exposed to a long pe-riod of high humidity just before entering the kiln,set the wet-bulb temperature as specified by theschedule.

b. If there has been surface wetting or moisture re-gain, set the wet-bulb controller for a 10 °F wet-bulb depression and turn on the steam spray. Letthe kiln run 12 to 18 h at this wet-bulb setting,and then change to the wet-bulb setting specifiedby the schedule.

For 6/4 and 8/4 oak that has been dried to 20 to30 percent moisture content, the following procedureapplies:

1. Bring the dry-bulb temperature up to the value pre-scribed by the schedule for the average moisturecontent of the controlling kiln samples, keeping thevents closed. Use steam spray (manually) only asneeded to keep the wet-bulb depression from exceed-ing 12 °F.

2. After the kiln has reached the dry-bulb temperature,set the wet-bulb temperature.

a. If there has been no surface moisture regain, setthe wet-bulb temperature at the level specified bythe schedule.

b. If there has been surface moisture regain, set thewet-bulb controller for an 8 °F wet-bulb depres-sion and turn on the steam spray. Let the kilnrun for 18 to 24 h at this setting. Then set a12 °F depression for 18 to 24 h before changingto the conditions specified in the schedule.

If the moisture content of lumber going into the kiln ismuch above about 30 percent, the procedure for lum-ber that has been only partially air dried or predriedis slightly different. For 4/4, 5/4, and 6/4 (except oak)lumber, the following procedure applies:

1. Bring the dry-bulb temperature up to the value pre-scribed by the schedule for the average moisture con-tent of the controlling kiln samples. Keep the ventsclosed and use steam spray only as needed to keepthe wet-bulb depression from exceeding 10 °F. Donot allow the depression to become less than 5 °F ormoisture may condense on the lumber.

2. After reaching the prescribed dry-bulb temperature,run each of the first three wet-bulb depression stepsof the whole schedule a minimum of 12 h, but stillobserve the 5 °F minimum wet-bulb depression.Then change to the conditions prescribed for themoisture content of the controlling samples.

For partially dried 6/4 and 8/4 oak, the followingprocedure applies:

1. Bring the dry-bulb temperature up to the value pre-scribed by the schedule for the average moisture con-tent of the controlling kiln samples. Keep the ventsclosed and use steam spray only as needed to keepthe wet-bulb depression from exceeding 8 °F. Do notallow the depression to become less than 5 °F.

2. After the prescribed dry-bulb temperature has beenreached, run each of the first three wet-bulb depres-sion steps of the schedule a minimum of 18 h whilestill observing the 5 °F minimum wet-bulb depres-sion. When the kiln conditions coincide with thoseprescribed by the schedule for the average mois-ture content of the controlling samples, change tothe moisture content basis of operation.

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Figure 7-2—Kiln schedule and drying curve for air-dried 4/4 black cherry that has regained surface mois-ture before entering the kiln. (ML88 5607)

The kiln-drying conditions for 4/4 air-dried blackcherry that has regained surface moisture beforeentering the kiln are shown in figure 7-2.

Air-dried lumber should not be subjected to high hu-midity at the start of kiln drying. This may causesurface checks to open during subsequent drying andthereafter remain open. It may also increase warping.

Modifications to General Hardwood Schedules

Once a kiln operator has dried a certain species anditem by one of the general kiln schedules without caus-ing defects or excessive degrade, modification of theschedule should be considered to reduce drying time.Perhaps the lumber can stand a more severe schedulewithout developing serious defects, or the dried prod-uct does not need to be free of defects. The operatorshould try to develop the fastest drying schedules con-sistent with acceptable amounts and types of defects.Schedules should be modified in a systematic way, for

which good records will be helpful. It must be recog-nized, however, that schedule modification satisfactoryfor lumber from one source and dried in one kiln maynot be satisfactory for lumber from another source anddried in a different kiln.

Kiln schedule modifications required by factors of kilnoperation or performance are dealt with in chapter 9.Drying charges of mixed species are also discussed inchapter 9.

The first move in systematic schedule modification is toshift from one wet-bulb depression schedule to another,the second is to shift temperature schedules, and thethird is to modify certain steps within the schedule.

Shifting Wet-Bulb Depression Schedules

The moisture content classes (table 7-3) are set up sothat a species of wood can be classified in accordancewith the green moisture content of its heartwood. Themoisture content limits of the classes were chosen ona conservative basis. Thus, the first modification thata kiln operator should consider is to shift to a highermoisture class, particularly if the green moisture con-tent is near the upper end of the values in the class.For example, 4/4 northern red oak at 95 percent mois-ture content has been successfully dried in pilot testson the E2 instead of the D2 schedule, with a saving of4 or 5 days in drying time. By going to the E2 sched-ule, the first increase in wet-bulb depression is made at60 percent moisture content rather than at 50 percent.This modification is especially useful when the lumberto be dried is mostly sapwood.

The next modification that should be considered is toshift to the next higher wet-bulb depression sched-ule number. This modification results in an increasedwet-bulb depression at each moisture content level. Itmay cause minor surface and end checks that are gener-ally of little concern for many uses. A drastic change inwet-bulb depression may cause severe surface and endchecks.

Using H-Type Wet-Bulb Depression Schedules

A special moisture content class, designated as H, hasbeen devised to permit more use of the principles thatthe first change in wet-bulb depression can be madewhen one-third of the green moisture content is goneand that additional increases in wet-bulb depressioncan be made soon after. This is particularly useful indrying species with a green moisture content of greaterthan 140 percent, but may also be applied with someadvantage to lumber with a green moisture content of100 percent or more. The H schedules are given in ta-ble 7-8. The wet-bulb depression schedule numbers arethe same as those in table 7-2.

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Figure 7-3—Kiln conditions and drying curve for 1-1/2-in-thick water tupelo heartwood, based on H scheduleT6-H2. (ML88 5606)

To set up a specific H schedule, find the moisture con-tent for the first change in wet-bulb depression by tak-ing two-thirds of the average green moisture content ofthe controlling samples. If, for example, their averagegreen moisture content is 168 percent, the first changepoint is 112 percent. For convenience, this is roundedto 110. Subsequent changes in wet-bulb depressionare made after each 10 percent loss in moisture. AnH schedule developed for 6/4 water tupelo heartwood isshown in figure 7-3. In view of the long drying time inthis particular case, preliminary air drying or predryingshould be considered. However, H schedules are appli-cable to other, faster drying species.

Shifting Temperature Schedules

Temperature is critical in preventing collapse and hon-eycomb, two defects that may not appear until later inthe drying process. Until the kiln operator has gained

experience in drying a particular species and thickness,the recommended temperature schedule number shouldbe followed. The general temperature schedules willsafely dry most lumber used in commercial drying. Ifthe lumber being dried is almost all sapwood or is rela-tively free of natural characteristics that contribute todrying defects, increasing the temperature (T) numberby 1 or 2 to obtain a 10 °F greater initial temperaturegenerally is permissible. For example, 9/4 all-sapwoodsugar maple free of pathological heartwood and mineralstreak has been dried on a T7 temperature scheduleinstead of the recommended T5 schedule. The milderT5 schedule would be used for drying a charge of sugarmaple that had a considerable amount of heartwood ormineral streak.

Changes Within the Schedule

The only significant change that can be made withina wet-bulb depression schedule is a more rapid reduc-tion of wet-bulb temperature during the intermediatestages of drying. The logical approach is to increase thewet-bulb depression in steps 3 and 4 of table 7-2. Thismodification should be approached with caution, andseveral charges should be dried before making furthermodification. If any objectionable amount of checkingoccurs, ease back the wet-bulb depression to the previ-ously satisfactory schedule.

Three types of temperature changes within the Tschedules (table 7-1) can be considered. One is to usea temperature in the initial stage of drying that is be-tween that of steps 1 and 2 of the recommended sched-ule. For some slow-drying species, such as 4/4 red oak,using an initial temperature of 115 °F instead of 110 °Funtil the lumber reaches 30 percent moisture contentmay be satisfactory if experience has shown no surfacechecking at 110 °F. Another type of change is to in-crease the dry-bulb temperature during the intermedi-ate stages of drying. This is the most dangerous changebecause of the possibility of honeycomb in some speciesand should be approached with caution. A third typeof change is to increase the temperature during the laststages of drying. After the average moisture content ofthe controlling samples has reached 15 to 20 percent,temperatures of 200 °F or greater can be used with-out damaging the wood. Research and experience arebeginning to show that many hardwood species thathave been dried to below 15 to 20 percent can be safelydried the rest of the way at temperatures as high as230 °F.

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Special Hardwood Schedules

Although the general hardwood schedules, with minormodifications, will do a good job of drying most speciesfor most end uses, special purpose schedules are advan-tageous in some cases. Some examples follow.

Maximum Strength Schedules

Exposure of wood to temperatures above 150 °F cancause some permanent reduction in strength. At kilntemperatures of 200 °F or less, only long exposurewould cause excessive strength reduction. Thus, thegeneral drying schedules and proper operating proce-dues do not significantly reduce the strength of thelumber; lumber strength is sufficient for most end uses.However, when the wood is to be used for products re-quiring high strength per unit weight, such as aircraft,ladders, and sporting goods, somewhat lower temper-atures should be used in drying. Table 7-9 lists tem-perature schedule code numbers for various softwoodand hardwood species, and table 7-10 lists the actualmaximum drying temperatures at various moisture con-tents recommended for these schedules. For example,from table 7-9, 1-in-thick Sitka spruce has a tempera-ture schedule number of 2. Then, from table 7-10, themaximum drying temperature at 40 percent moisturecontent is 145 °F. Any general schedule used shouldthus be modified to stay below these maximum temper-atures. Wet-bulb depressions should remain the sameas listed in the general schedules.

Alternative Schedules for Some Species

Some species have peculiar drying characteristics orthere is some other reason for a special drying schedule.Some of the more useful schedules are mentioned in thefollowing paragraphs; these and other special schedulesare described in table 7-11.

Hickory.—Upper grades of hickory are sometimes usedfor high-quality specialty products, such as tool handlestock, and require a slightly more conservative schedulethan that listed in table 7-4.

Swamp and water tupelo.—The heartwood and sap-wood of swamp and water tupelo have quite differentdrying characteristics. When the heartwood and sap-wood can be separated, it is advantageous to dry themseparately by different schedules.

Aspen.—Aspen trees sometimes develop a darkenedarea of wet-pocket wood in the center of the tree. Thiswood is slow drying and susceptible to collapse; it isusually present in the lower grade boards sawn fromthe center of the log. The upper grades of lumber sawnfrom the outside of larger logs can still be dried by therecommended general schedule.

Sugar maple.—Some end uses of sugar maple put apremium on the whitest color possible for sapwood, andthe special schedule in table 7-11 will accomplish this.Also sugar maple sometimes has mineral streaks thatare impermeable and subject to collapse and honey-comb during drying.

Red oak.—The red oaks are subject to a bacterial in-fection that invades the living tree and subsequentlycauses the lumber to be more susceptible to drying de-fects. There is little if any visual difference betweenbacterially infected and noninfected lumber. Often,however, infected oak has a characteristic rancid odor.With care, bacterially infected oak can be dried witha minimum of surface checks and honeycomb by usingschedules listed in table 7-11.

Red and white oak—In sawing lumber from logs,the saw usually leaves small tears and fractures in thesurface fibers of a board. These tears are points ofweakness where drying stresses can cause surface checksto occur. If these boards are lightly surfaced, the tearsare removed and the boards are less likely to surfacecheck. As a result. the kiln schedule can be accelerated.

Time Schedules

Hardwood time schedules have been developed for someof the western hardwoods and are listed in table 7-12.

High-Temperature Schedules

High-temperature kiln drying is usually defined as theuse of dry-bulb temperatures above 212 °F, usuallyin the range of 230 to 250 °F. Research and limitedexperience have shown that many of the low-densityhardwoods can be dried at high temperatures whilestill maintaining quality. Schedules for these species areshown in table 7-13.

Schedules for Imported Species

The same principles that govern the selection of sched-ules for domestic species also apply to imported species.The schedules recommended in table 7-14 were gath-ered largely from the world literature on lumber drying.Table 7-14 is arranged by common name, and the scien-tific names can be found in chapter 1, table 1-2.

Schedule for Presurfaced Northern Red Oak

Presurfacing of lumber before kiln drying can result inreduced degrade from warping and practically eliminatesurface checking. The technique, when combined withan accelerated schedule, can lead to 16 to 30 percentsavings in drying time for 4/4 red oak. Other benefitsof presurfacing include increased volume per kiln load

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and reduction of planer jams in the rough mill. Suc-cessful use of this technique depends on uniform air ve-locity of about 400 ft/min, well-baffled loads, accuratetemperature and humidity control, adequate moisturecontent sampling, and a knowledgeable kiln operator.The cost to initiate and use the system is minimal.

The procedure is simple and only requires that therough lumber be surfaced on two sides prior to stack-ing for drying. A double surfacer can be placed nearthe lumber grading station and ahead of the automaticstacker. Conveyors can feed the lumber and take itaway from the planer. An alternative to using a knifeplaner in the line is to install an abrasive planer using24- or 36-grit belts.

Whichever way the planing is done, the machine shouldbe set to remove equal amounts from each side of theboards. For example, lumber sawed 1-1/8 in thick inthe rough can be planed to 1-1/32 in by taking 3/64 infrom each face; if the boards are sawn to reasonablyuniform thickness, 80 percent of the pieces will haveclean faces for their full length.

No change is required in the stacking operation, assum-ing the usual good practices are followed, including uni-form sticker thickness and spacing, good vertical align-ment, box piling, and support for ends of boards. Oneor two extra courses can generally be stacked in a unitpackage of surfaced lumber compared to a package ofrough lumber of the same height. No change is requiredin kiln loading procedures when using surfaced lumber,again assuming good pile support, good alignment, andproper baffling are already practiced.

One major reason for presurfacing lumber before dry-ing is to be able to accelerate the drying and therebyreduce costs. Existing schedules can be used, and itis possible to save about 10 percent in drying time ascompared with drying rough lumber. Part of this sav-ing is due to reducing or eliminating the thickness vari-ation between boards and to the fact that the lumber isslightly thinner than rough stock.

Research work on oak drying has shown that becausesurfacing reduces the potential for checking and split-ting, higher temperatures can be used earlier in thekiln run. McMillen (1969) developed a schedule foraccelerated drying of presurfaced 1-in-thick northernred oak (table 7-13). Tests of this schedule on variousloads of red oak in a variety of kilns have shown that16 to 30 percent drying time can be saved in commer-cial kilns, if the schedule is followed as designed. Interms of kiln days, this means 4/4 oak can be safelydried green from the saw to 7 percent moisture contentin 18 days instead of 21 to 28 days.

Successful use of this schedule depends on the follow-ing:

1. The kiln equipment must be in good repair—accurate calibration of the recorder-controller; goodadjustment of vents, automatic valves, and traps;and proper operation of fan and baffle system.

2. The kiln load must be well stacked and baffled so theair velocity through the load is at least 400 ft/min.

3. Drying must be controlled with well-selected kilnsamples. A minimum of six samples are recom-mended; eight are preferred for better knowledge ofmoisture content distribution.

4. The kiln operator must be confident that the dry-ing information is accurate and must make schedulechanges promptly.

There are two disadvantages to presurfacing lumberprior to drying: (1) since hardwood lumber is gradedin the rough form, surfacing the boards may change thegrade and make any dispute about the original gradedifficult to settle and (2) if a planer is not readily avail-able for presurfacing, the added cost of the machinemay not be justifiable. This could especially be true ifthe rough lumber was very accurately sawn.

Softwood Schedules

Softwood Moisture Content Schedules

The softwood moisture content schedules presented inthis chapter can be used with the kiln sample proce-dure of chapter 6 to dry softwood lumber with a min-imum of drying defects. These schedules are describedfor the sake of the few instances where they might beused and for the sake of maintaining knowledge aboutthem. Because softwoods are generally easy to dry, in-dustry practice has gone to almost exclusive use of timeschedules. Time schedules will be discussed in the nextsection.

Moisture Content Basis

As in the drying of hardwoods, there is a relationshipbetween the moisture content of the lumber and thedrying conditions the lumber can withstand. Althoughthe stress patterns that develop in softwood lumberduring drying differ from those in hardwood lumber,the surface zones do become stressed in tension (so thatsurface checking is a danger) during the early stages ofdrying and ultimately become stressed in compression.However, stress reversal generally does not occur un-til the lumber reaches a moisture content somewherebetween 20 and 15 percent-a little lower than in hard-woods. Therefore, wet-bulb depressions should not bedrastically increased until the lumber reaches this mois-

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ure content level. Gradual changes in wet-bulb depres-sion can be made early in drying, however, in accor-dance with the moisture content of the lumber. Thetemperature and moisture content relationships thatcause collapse and honeycombing in hardwoods affectsoftwoods similarly.

Material Considerations

The difference between sapwood and heartwood mois-ture content is considerable in many softwoods (ch. 1,table 1-5). Generally, the heartwood is more suscepti-ble to drying defects, so most of the schedules are basedon the moisture of the heartwood. In some situations,however, the heartwood dries to a safe moisture con-tent level before the sapwood is dry enough to stand adrastic increase in wet-bulb depression. In these cases,the schedules are based on the moisture content of thesapwood or of a mixture of sapwood and heartwood.

Wetwood or sinker stock can be a problem when dryingsome softwood species such as redwood, hemlock, sugarpine, eastern and western white pine, and the true firs.This is wood that contains so much water and so littleair in the cell cavities that it sometimes sinks in wa-ter. Wetwood dries slowly and is subject to collapseif too high a temperature is used during the initialstages of drying. If practical, it is desirable to sort thegreen softwoods of species prone to wetwood into dif-ferent weight or moisture content classes and dry eachseparately.

The softwood moisture content schedules are intendedfor drying green lumber, but they can be applied topartially air-dried lumber as well.

Moisture Content Schedules

The softwood moisture content schedules are given intables 7-15 and 7-16. These schedules are similar to thegeneral schedules for hardwoods, except for a few im-portant differences. Wet-bulb depressions of 40 °F ormore are avoided until the controlling moisture contentreaches 15 percent. Changes in wet-bulb depression be-tween 15 and 35 °F are made gradually, 5 °F at a time.For drying lower grades, final wet-bulb depressions gen-erally do not exceed 20 °F. The main features of mois-ture content schedules of this type were discussed inthe Hardwood Schedules section in this chapter. In themoisture content method of operation, the initial tem-perature is maintained until the controlling kiln sam-ples have an average moisture content of 30 percent.

Table 7-17 is an index of recommended schedules for4/4, 6/4, and 8/4 softwood lumber, of both upper andlower grades. The schedules for lower grade lumbergenerally call for lower final temperatures and smallerfinal wet-bulb depressions to reduce loosening of knotsand to hold planer splitting to a minimum.

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Table 7-18 is an index of suggested schedules for 10/4and thicker softwoods. The drying time may be toolong for ordinary commercial operations, but the sched-ules are suitable for special cases where thick lumber ofupper grades is to be dried.

Instructions for assembling a softwood moisture contentschedule are the same as those given for hardwoods.

Kiln Drying Air-Dried Lumber

Since preliminary air drying is uncommon for softwoodsthat are to be kiln dried (except for redwood, incensecedar, and western redcedar), recommended schedulesfor kiln drying air-dried lumber have not been devel-oped. The following steps are suggested for the assem-bly of such a schedule.

1. Determine the moisture content of representativesamples of slow- and fast-drying boards (ch. 6) anduse the average moisture content of the wettest halfof the samples as the controlling moisture content.

2. Use the temperature step of the recommendedschedule corresponding to that moisture content(table 7-15).

3. If the controlling moisture content is above 40 per-cent, dry the lumber as green.

4. If the controlling moisture content is 40 percent orless, change the wet-bulb depression as follows:

a. Use a depression of 10 to 15 °F for the initial 8 to16 h.

b. After this period, if the controlling moisture con-tent is between 15 and 25 percent, change thewet-bulb depression to 20 °F.

c. Use a wet-bulb depression of 30 °F or more afterthe lumber reaches 15 percent moisture content.

Modifying Softwood Moisture Content Schedules

The principles described for hardwood schedule modifi-cation generally can be applied to softwoods.

Commercial Softwood Time Schedules

Most western and southern softwood mills use timeschedules to dry both upper and lower grade lumber.The drying conditions are changed at convenient in-tervals, such as every 12 or 24 h or multiples thereof.A wide range of schedules has been developed at in-dividual mills or by individual researchers, and theseschedules are often modified. The schedules given hererepresent schedules that should serve as a satisfactorystarting point for kiln operators. They are intendedas a guide from which an operator can develop thebest schedule for the particular drying requirements

and type of kiln at the mill. Time schedules are depen-dent on the rate of air circulation and kiln performancebecause these affect drying rate. The conventional-temperature schedules in this chapter are based on theperformance of single-track or double-track kilns thatare equipped with booster coils and for a minimum airvelocity of 400 ft/min. The high- temperature sched-ules are intended for kilns with 800 to 1,000 ft/min airvelocity.

Conventional-Temperature Kiln Schedules

The recommended schedules aye indexed in table 7-19,and the schedules themselves are written out intable 7-20. Because the schedules were developed froma wide diversity of actual schedules, the times given inthe last step are for guidance only. The actual time re-quired for individual kiln charges may vary from thetimes given. If at the end of a kiln run the moisturecontent level and the degree of moisture content uni-formity do not meet requirements, modify the scheduleor the equalizing time, or both, on subsequent charges.The length of time of the last step in the schedule is of-ten modified to attain the desired target final moisturecontent. The most common procedure used to adjustdrying time for variations in initial moisture content isto use the same initial and intermediate drying stepsand then to lengthen or shorten the final step to reachthe desired final moisture content. In winter when lum-ber is sometimes quite wet when placed in the kiln, theinitial step is prolonged or is preceded by a milder step.

Lumber from trees that have been dead for some time,such as insect-killed trees, is likely to be lower in mois-ture content and therefore require less drying time thanlumber from trees that were alive at the time of har-vesting. Lumber from dead trees may be more suscepti-ble to surface checking.

High-Temperature Kiln Schedules

The usual range of temperatures for high-temperaturedrying of softwoods is from 230 to 250 °F, although thecurrent trend is for even higher temperatures. High-temperature drying of some softwood species has be-come common in the last 15 to 20 years. Althoughtests have shown that significant strength loss occursin some western species, southern pine apparently ismuch less affected than other species and shows little orno strength loss. The effect of strength loss should beconsidered when selecting a kiln schedule for a productwhere loss of bending or tension strength is important.

Since the mid-1970’s the majority of new kilns builtfor drying southern pine dimension lumber have beenhigh-temperature kilns, and most of these have beendirect-fired rather than steam-heated kilns. Wet-bulbcontrol is not as precise in direct-fired kilns, and con-

ditioning is generally not possible because steam sprayis lacking. However, direct-fired kilns are usually lesscostly to build than steam-heated kilns and generallyperform satisfactorily for southern pine lumber.

The species index of schedules is given in table 7-21,and the actual schedules are written out in tables 7-22and 7-23.

Softwood Schedules for Special Purposes

Some softwood lumber and items require or benefitfrom special precautions or schedules, and the follow-ing sections discuss some of these special needs.

Brown-Stain Control

Brown stain is a discoloration of wood that can occurduring kiln drying as a result of a change in the color ofsubstances normally present in some softwoods. It canbe a significant problem in drying sugar pine, easternand western white pine, ponderosa pine, sinker hem-lock heartwood, and the southern pines. Brown stain ismost prevalent during hot and humid months. It occursin logs that have been stored in water or on sprinkledlog decks for long periods. The storage time betweenwhen lumber is sawed and dried should be kept to aminimum, especially if the lumber is solid piled.

Brown stain can be severe when high dry- and wet-bulbtemperatures are used at the start of the schedule. Ifit is a problem, the initial dry-bulb temperature shouldbe dropped so as not to exceed 120 °F. Use as large awet-bulb depression as the lumber will tolerate withoutexcessive surface and end checking. A suggested sched-ule based on moisture content for eastern and west-ern white pine and sugar pine is given in table 7-24,and schedules based on time are provided in table 7-25.(See following section if setting the pitch is necessary.)

Setting Pitch and Retaining Cedar Oil

Kiln schedules can be modified either to retain oil inwood, as in drying eastern redcedar used for cedarchests, or to set pitch that might exude later from pineand cause paint and finishing problems by bleedingthrough. High temperature in the presence of moistureand steam causes volatile oils and resins to vaporize.Therefore, when drying eastern redcedar, avoid hightemperatures and do not condition the lumber unlessit is absolutely necessary because it will be resawed orsurfaced unequally.

On the other hand, to set pitch it is desirable to driveoff the volatile turpentine and other solvents normallypresent. This can be done most easily at the start ofdrying by using a high temperature. However, if brownstain is a problem, the best compromise is to use the

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anti-brown-stain schedule at the start of drying andfinish with a dry-bulb temperature of at least 160 °F.A temperature of 160 °F is usually satisfactory for 4/4lumber, but the final temperature for thicker lumbershould be at least 170 °F.

Lumber Treated With Waterborne Preservativesor Fire Retardants

Some softwood species, particularly southern pine andDouglas-fir lumber and plywood, are often treated withfire retardants and preservatives. Preliminary dryingis required before either treatment; the lumber can bepredried in the same way as lumber that is not treated.During treatment, however, the lumber or timbers reab-sorb considerable water, and they are often redried af-ter treatment. The chemicals used in treatment usuallyaccelerate the strength-reducing effects of prolonged ex-posure of moist wood to high temperatures. Research isin progress to help set maximum recommended dryingtemperatures for treated wood products where strengthis critical, but until those temperatures are better de-fined the usual recommendation is to not exceed 190 °Ffor wood treated with waterborne preservatives and160 °F for wood treated with fire retardants (Winandy1988). Table 7-26 shows several satisfactory schedulesfor treated Douglas-fir plywood.

Maximum Strength Schedules

Maximum drying temperatures for maintaining max-imum strength were discussed earlier in this chapter.The maximum temperatures for softwoods for eachmoisture level are shown in table 7-10, and the speciescode numbers for finding these temperatures are shownin table 7-9.

Bevel Siding, Venetian Blinds, andOther Resawed Products

Softwood lumber that is to be resawed into bevel sid-ing, venetian blinds, or other products should be prop-erly equalized and conditioned (see section on equaliz-ing and conditioning treatments) to obtain a uniformmoisture content over the cross section and to relievedrying stresses. Otherwise, the resawed halves of theboards will quite likely cup (ch. 8). Before equalizing,use the final wet-bulb depression given in the schedulesto achieve a low average moisture content as soon aspossible.

Bundled Short-Length Items

Most drying of bundled short-length items takes placefrom the end-grain surfaces. Because some of theseitems do not end or surface check readily, kiln sched-

ules for them can be rather severe. Other items, how-ever, still require low dry-bulb temperatures to avoidcollapse.

Because western redcedar shingles produced from wetstock that is logged in low areas may collapse, the shin-gles are dried with an initial dry-bulb temperature ofabout 95 °F. This temperature is gradually increasedover a 10- to 14-day period to 150 °F or higher. Shin-gles produced from stock at a relatively low moisturecontent can be started at 150 °F or higher and finishedat 180 °F. In both cases, wet-bulb temperature is notcontrolled, and the vents are kept open.

Incense cedar pencil stock is usually dried from a greento partially dry condition in the form of 3-in planksor squares and then cut into thin slats and graded.These slats are treated with a small amount of wax,bundled, and treated with a water-soluble dye. Becausethe treatment generally is a full-cell process in whichall cell cavities become filled with liquid, the slats maycollapse under severe drying conditions. Use low tem-peratures and high relative humidities at the start ofdrying and gradually make them more severe as dryingprogresses. Drying times are quite long, usually 23 to30 days.

Pine squares, which are 4/4, 5/4, and 6/4 in cross sec-tion and 24 to 36 in long, are dried in bundles about5 in square. Use a constant kiln temperature of 140 °Fdry bulb and 110 °F wet bulb. Drying time is 13 to14 days. Similar drying conditions can be used on othershort items made of easily dried softwoods.

Large Timbers and Poles

It is not customary to kiln dry large timbers or polesof many species because of the long drying times re-quired. Such wood is usually air-dried or used green.One notable exception is southern pine. Because ofits relative ease of drying and extensive use, success-ful high-temperature schedules have been developedfor southern pine; several schedules are given in ta-ble 7-27 for crossarms and poles. Timbers with crosssections of 4 to 5 in are often used for decking and assuch require proper drying with a minimum of surfacechecks. Schedules for such timbers are given in table7-28. Even more so than with other schedules presentedin this chapter, these specialized schedules representa starting point for the kiln operator to build on. Inmany cases, the objective of kiln drying large timbers isonly to dry the outer shell of the timber to either con-trol surface checking or remove water so that the outershell can be treated with a waterborne preservative.

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Tank Stock

Lumber for tank stock can be dried by the sched-ules used for the upper grades of the same thickness.Since the stock is used in contact with water or aque-ous solutions, it should not be dried lower than 15 to20 percent moisture content. Therefore, equalization(see Equalizing and Conditioning Treatments section)should be done at an equilibrium moisture content(EMC) of about 12 percent.

Knotty Pine Lumber

Knotty pine lumber is often used for decorative pur-poses and thus has higher appearance requirementsthan other low-grade pine lumber. The moisture con-tent or time schedules given for lower grade lumber aregenerally satisfactory for preventing excessive check-ing or loosening of knots during the first stages of dry-ing. Drying time, however, should be prolonged toreach a final moisture content of 7 to 8 percent. Some-what lower relative humidities may be needed to reachthis final moisture content without prolonging drying.The pitch should be set with a final temperature of atleast 160 °F. Conditioning to relieve stresses is alsodesirable.

Dehumidification Kiln Schedules

Dehumidification kilns began gaining use in the UnitedStates in the late 1970’s and have grown in popularitysince then. Because of their relative newness, a widerange of schedules is not available for recommenda-tion. The moisture content schedules recommendedin this chapter should be satisfactory for most pur-poses. The major difference between schedules forsteam-heated and dehumidification kilns is temperaturelimitation. Dehumidification kilns cannot attain thecommon 180 °F final temperature of most conventionalschedules. Early dehumidification kilns were limitedto a maximum temperature of 120 °F, which resultedin prolonged drying times below the fiber saturationpoint. Newer models can operate up to 160 °F and canapproach the drying times of steam-heated kilns.

The schedules for steam-heated kilns can be convertedfor use with a dehumidification kiln, as shown in table7-29. The schedule T4-C2 for 4/4 white oak is con-verted to accommodate a maximum dry-bulb temper-ature of 120 °F. To make the conversion, substitute120 °F for those dry-bulb temperatures above 120 °Fand then maintain an EMC in the dehumidificationschedule step about the same as in the conventionalschedule step. A similar conversion can be made for adehumidification kiln with a 160 °F maximum temper-ature, although the converted schedule will differ onlyin the last step of the schedule. Note that some dehu-

midification kiln manufacturers recommend that theirequipment not be operated at dry-bulb temperaturesabove 160 °F and wet-bulb temperatures above approx-imately 110 to 120 °F.

An ideal application of dehumidification kilns is theiruse in minimizing surface checking in the early stagesof drying refractory species. Low dry-bulb tempera-tures and high relative humidities are sometimes dif-ficult to maintain in steam-heated kilns, particularlyin hot weather. Often, the use of steam spray to in-crease relative humidity only raises the dry-bulb tem-perature without reducing the wet-bulb depression.In a tightly built dehumidification kiln, it is possibleto maintain dry-bulb temperatures of 90 °F or lesswhile still maintaining a relative humidity of 80 per-cent or more. These conditions are quite successful inpreventing surface checking. A general purpose, low-temperature schedule is suggested in table 7-30. Vari-ations of this schedule that apply the general principleof low initial dry-bulb temperature and high humidityfollowed by a gradual increase of dry-bulb tempera-ture and decrease of relative humidity should also besuccessfull.

Sterilizing, Equalizing, andConditioning Treatments

Sterilizing Treatments

A sterilizing treatment can be used in the dry kiln tostop the growth of excessive mold on the surface ofwood under certain conditions (ch. 8). The dry kilncan also be used to sterilize wood that has been in-fected with stain or decay fungi or attacked by wood-destroying insects.

Mold

Mold can develop on green lumber in a kiln operatingat temperatures up to 120 °F. Although the mold gen-erally does not penetrate the wood enough to causeserious stain during kiln drying, it can fill up the airspaces in a load of lumber and seriously interfere withair circulation. Not only does this slow drying as awhole, but the lumber under the mold may honeycomblater in drying when the temperature is raised underthe false belief that moisture content is low enough tosafely raise the temperature.

To sterilize for mold, the kiln charge (green lumberonly) should be steamed at or near 100 percent relativehumidity at a dry-bulb temperature of 130 °F or higherfor 1 h after all parts of the kiln have reached that tem-perature. After steaming, the normal drying scheduleshould begin. Infrequently, two sterilizing treatments

145

may be required about a day apart to stop the devel-opment of mold. If the growth is not heavy enough toblock air circulation, sterilization is not necessary.

Fungal Stain and Decay

The temperatures normally used at the start of kilndrying are usually high enough to stop the growth ofstain or decay organisms that may have infected greenwood during storage or air drying. A temperature of110 °F stops the growth of these organisms but doesnot kill them. Tests show that a temperature of 150 °For higher for at least 24 h should kill all stain and de-cay fungi. As long as the wood is kept below 20 per-cent moisture content, new stain and decay will notstart.

Insects

Both softwoods and hardwoods are attacked by a num-ber of wood-boring insects, whether the wood is greenor dry. Imported lumber or air-dried lumber that hasbeen stored for a long time should be examined for ev-idence of insects. If they are found, a sterilizing treat-ment should be given.

Lyctus (powder-post) beetles and their eggs and larvaeare killed by heating the lumber according to the sched-ule given in table 7-31. The schedule conditions includeallowances for heating the lumber to the center, for coldspots in the kiln, and for additional time as a safetyfactor. To sterilize, use an EMC that is within 2 per-cent above or below the moisture content of the wood.If the wood has less than 8 percent moisture content,a temperature above 140 °F and a relative humiditysomewhat below 60 percent should give satisfactory re-sults, using the times given in table 7-31 for the 130 °Ftemperature. Exact data on temperatures and times re-quired to kill other insects are not available, but thehigher temperature schedule of table 7-31 may beadequate.

Normal kiln drying or temperature sterilization will notprevent future infestation by insects.

Equalizing and Conditioning Treatments

Equalizing and conditioning have been mentionedseveral times in this manual, and the purpose of thissection is to discuss them in detail. Frequently, themoisture content of lumber varies considerably amongboards in a kiln charge. This can be because of natu-ral variability in drying rate or initial moisture content,heartwood and sapwood, or wet pockets in the lumber,or variability in drying conditions in various parts ofthe kiln. Variation in final moisture content can causeserious problems in the subsequent processing and use

of the lumber. The purpose of equalizing is to reducethis variation in moisture content.

The drying stresses discussed in chapter 1 often remainin boards even after drying is complete. These resid-ual drying stresses (often called casehardening althoughthere is no actual hardening of the surface) can causeproblems of warp and saw blade pinching in manufac-ture and use (ch. 8) and should be removed from thelumber for many end uses. The purpose of conditioningis to relieve the residual compressive drying stresses inthe shell by plasticization with high temperature andhigh relative humidity. Conditioning has another bene-ficial effect of producing more uniform moisture contentthroughout the thickness of the boards. Effective equal-izing is necessary before satisfactory conditioning canbe accomplished because the effectiveness of condition-ing depends on moisture content.

Conditioning is not really necessary for softwood di-mension lumber that will be kiln dried to an aver-age moisture content of 15 percent or a maximum of19 percent; furthermore, it is not effective at such ahigh moisture content. Equalizing may be necessary ordesirable for such lumber. On the other hand, equal-izing and conditioning are usually necessary for hard-wood or softwood lumber that will be dried to below11 percent moisture content and used in end productswith stricter requirements.

Equalizing and conditioning treatments also dependon the type of kiln schedule. Equalizing depends onknowledge of the variability of moisture content be-tween boards. The only way to get this information isthrough tests. When a moisture-content-based sched-ule is used with kiln samples, the samples will serve asthe basis for equalizing and can also be used to preparestress sections (ch. 6). When a time-based schedule isused without kiln samples, it is more difficult to deviseeffective equalizing and conditioning treatments. Oneway to devise an equalizing treatment is to use an elec-tric moisture meter during the last stages of drying toestimate variability. If this is done, care must be takento ensure that the correct temperature is applied to themeter reading. The other way to devise an equalizingtreatment to follow a time-based schedule is to develop,by experience, a time-temperature schedule that equal-izes relative humidity. This will later minimize rejectsin processing lumber with surface fuzziness in planingcaused by high moisture content or with planer splitscaused by low moisture content. Similarly, the optionsfor developing conditioning treatments to follow a time-based schedule are to cut stress sections or to ascertainthe need and develop the procedures for conditioningthrough trial and error.

146

The following procedures are based on the use of kilnsamples for equalization and stress sections for condi-tioning. The basic principles can be applied to developprocedures for time-based equalizing and conditioning.The procedures given will be satisfactory for lumberthat is dried to final average moisture content of from5 to 11 percent. Table 7-32 contains basic informationon the moisture content of the kiln samples and thekiln EMC conditions for these treatments. Wet-bulbdepression values required to obtain desired EMC con-ditions are given in chapter 1, table 1-6.

Equalizing Treatment

The procedure for equalizing a kiln charge of lumber,using table 7-32, is as follows:

1. Start equalizing when the driest kiln sample in thecharge has reached an average moisture content2 percent below the desired final average moisturecontent. For example, if the desired final averagemoisture content is 8 percent, start equalizing whenthe driest kiln sample reaches 6 percent.

2. As soon as the driest sample reaches the moisturecontent value stated in step 1, establish an equaliz-ing EMC in the kiln equal to that value. In the ex-ample given in step 1, the equalizing EMC would be6 percent. During equalizing, use as high a dry-bulbtemperature as the drying schedule permits.

3. Continue equalizing until the wettest sample reachesthe desired final average moisture content. In theexample given in step 1, the wettest sample wouldbe dried to 8 percent.

If the equalizing treatment is to be followed by a con-ditioning treatment, it may at times be necessary tolower the temperature to obtain the desired condition-ing EMC condition. If so, begin by lowering the tem-perature 10 °F 12 to 24 h prior to the start of con-ditioning. Also, lower the wet-bulb temperature tomaintain the desired equalizing EMC.

Conditioning Treatment

The conditioning treatment, whether or not precededby an equalizing treatment, should not be started un-til the average moisture content of the wettest samplereaches the desired final average moisture content.

The procedure for conditioning is as follows:

1. The conditioning temperature is the same as the fi-nal step of the drying schedule or the highest tem-perature at which the conditioning EMC can be con-trolled. For softwoods, set the wet-bulb temperatureso the conditioning EMC will be 3 percent abovethe desired final average moisture content. For hard-

woods, the conditioning EMC is 4 percent above thedesired final average moisture content. The wet-bulbdepression that will give the desired conditioningEMC is given in chapter 1, table 1-6. If, at the de-wed conditioning temperature, a wet-bulb depres-sion value is not shown for the desired EMC, choosethe wet-bulb depression value for the nearest higherEMC given for that temperature. Set the desiredwet-bulb temperature for the proper depression butdo not raise the dry-bulb temperature above theequalizing temperature until after the proper wet-bulb temperature is attained.

Example: Assume a hardwood species with a desiredfinal moisture content of 8 percent and a condition-ing temperature of 170 °F. The conditioning EMCfrom table 7-32 is 12 percent. At 170 °F, an 8 °Fwet-bulb depression will give an EMC of 12.4 per-cent (table 1-6). If the lumber is a softwood, theconditioning EMC would be 11 percent and the wet-bulb depression 10 °F.

2. Continue conditioning until satisfactory stress reliefis attained.

The time required for conditioning varies consider-ably with species and lumber thickness, the type ofkiln used, and kiln performance. At a conditioningtemperature of 160 to 180 °F, hardwoods generallyrequire 16 to 24 h for 4/4 lumber and up to 48 h for8/4 lumber. Some 4/4 softwood species can be condi-tioned in as short as 4 h. If conditioning temperaturesare lower than 160. to 180 °F, conditioning time will beprolonged.

The most exact way to determine when conditioning iscomplete is the casehardening test described in chap-ter 6. Conditioning time should not be continued anylonger than necessary because of excessive steam con-sumption and excessive moisture pickup, particularly inlow-density species.

If tests for average moisture content are made imme-diately after the conditioning treatment, the mois-ture content obtained will be about 1 to 1-1/2 percentabove the desired value because of the surface mois-ture regain. After cooling, the average moisture contentshould be close to that desired.

Kiln-Drying Time

The approximate time required to kiln dry softwoodlumber can be estimated from some of the time sched-ules given earlier in the chapter. Table 7-33 lists ap-proximate drying times for 1-in-thick softwood and

147

hardwood species. The times listed are for kiln dryingat conventional temperatures where the final scheduletemperature is approximately 180 °F. Lumber thickerthan 1 inch will take longer to dry than the times givenin table 7-33. The increase in drying time is more thanproportional to the increase in thickness. For exam-ple, if thickness is doubled, the drying time will be in-creased by a factor of about 3 to 3.5.

Literature Cited

Boone, R. S.; Kozlik, C. J.; Bois, P. J.; Wengert, E.M. 1988. Dry kiln schedules for commercial woods-temperate and tropical. Gen. Tech. Rep. FPL-GTR-57. Madison, WI: U.S. Department of Agriculture,Forest Service, Forest Products Laboratory. 158 p.

McMiIIen, J. M. 1969. Accelerated kiln drying ofpresurfaced 1-inch northern red oak. Res. Pap. FPL122. Madison, WI: U.S. Department of Agriculture,Forest Service, Forest Products Laboratory. 29 p.

Winandy, J. E. 1988. Effects of treatment and redryingon mechanical properties of wood. In: Proceedings ofconference on wood protection techniques and the useof treated wood in construction. Madison, WI: ForestProducts Research Society.

Sources of Additional Information

Bramhall, G.; Wellwood, R. W. 1976. Kiln dryingof western Canadian lumber. Information ReportVP-X-159. Canadian Forestry Service, Western ForestProducts Laboratory.

Cech, M. Y.; Pfaff, F. 1977. Kiln operator’s manualfor eastern Canada. Report OPX192E. Eastern ForestProducts Laboratory.

Chudnoff, M. 1984. Tropical timbers of the world.Agric. Handb. 607. Washington, DC: U.S. Departmentof Agriculture.

Gerhards, C. C.; McMillen, J. M. 1976. High tempera-ture drying effects on mechanical properties of softwoodlumber. In: Proceedings of Symposium. Madison, WI:Forest Products Laboratory.

Knight, E. 1970. Kiln drying western softwoods.Moore, OR: Moore Dry Kiln Company of Oregon. (Outof print.)

Koch, P. 1972. Utilization of the southern pines. Agric.Handb. 420. Washington, DC: U.S. Department ofAgriculture.

Kozlik, C. J. 1967. Effect of kiln conditions on thestrength of Douglas-fir and western hemlock. Re-port D-9. Corvallis, OR: Forest Research Laboratory,Oregon State University.

Kozlik, C. J. 1968. Effect of kiln temperatures onstrength of Douglas-fir and western hemlock dimensionlumber. Report D-11. Corvallis, OR: Forest ResearchLaboratory, Oregon State University.

Kozlik, C. J. 1987. Kiln drying incense-cedar squaresfor pencil stock. Forest Products Journal. 37(5): 21-25.

Koslik, C. J.; Ward, J. C. 1981. Properties and kiln-drying characteristics of young-growth western hemlockdimension lumber. Forest Products Journal. 31(6):45-53.

Mackay, J. F. G. 1978. Kiln drying treated plywoodForest Products Journal. 28(3): 19-21.

McMillen, J. M.; Wengert, E. M. 1978. Dryingeastern hardwood lumber. Agric. Handb. 528.Washington, DC: U.S. Department of Agriculture.

Rasmussen, E. F. 1961. Dry kiln operator’s manual.Agric. Handb. 188. Washington, DC: U.S. Departmentof Agriculture.

Rice, W. W. 1971. Field test of a schedule for accel-erated kiln drying presurfaced 1-inch northern redoak. Res. Bull. 595. Amherst, MA: University ofMassachusetts.

Rietz, R. C.; Page, R. H. 1971. Air drying of lumber:A guide to industry practices. Agric. Handb. 402.Washington, DC: U.S. Department of Agriculture.

Simpson, W. T. 1980. Acccelerating the kiln drying ofoak. Res. Pap. FPL 378. Madison, WI: U.S. Depart-ment of Agriculture, Forest Service, Forest ProductsLaboratory. 9 p.

Thompson, W. S.; Stevens, R. R. 1972. Kiln dryingof southern pine poles: Results of laboratory and fieldstudies. Forest Products Journal. 22(3): 17-24.

Ward, J. C.; Simpson, W. T. 1987. Comparison of fourmethods for drying bacterially infected and normalthick red oak. Forest Products Journal. 37(11/12):15-22.

148

Table 7-1—Moisture content schedules for hardwoods

Dry-bulbtemperature

step no.

Moisturecontentat startof step

(percent)

Dry-bulb temperatures (°F) for various temperature schedules

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14

Table 7-2—General wet-bulb depression schedules for hardwoods

Wet-bulb depressions (°F) forMoisture content (percent) at start of step various wet-bulb depression

Wet-bulb for various moisture content classes schedulesdepressionstep no. A B C D E F 1 2 3 4 5 6 7 8

Table 7-3—Moisture content classes for variousgreen moisture content values

Green moisture content(percent) Moisture content class

up to 40

40 to 60

60 to 80

80 to 100

100 to 120

Above 120

149

Table 7-4—Code number index of schedules1 recommended for kiln drying domestic hardwood 4/4 to 8/4 lumber and other products

Species

Alder, redFor darker colorFor lighter color

AppleAsh, blackAsh, green, Oregon.

whiteAspenBasswood

StandardLight color

Beech

Birch, paper

Birch, yellow

BlackgumBoxelderBuckeye, yellowButternutCherry, blackChestnutCottonwood, normalCottonwood, wet

streakDogwoodElm, American and

slipperyElm, rockHackberry

Hickory

HollyHophornbeam

(ironwood)Laurel, California

(Oregon Myrtle)Locust, blackMadroneMagnoliaMaple, bigleaf,

red, silver

Maple, sugar (hard)

Oak, Californiablack2

Oak, red (upland)2

Oak. red (southernlowland)2

Oak, white (upland)2

Oak, white (lowland)2

Osage-orangePecanPersimmon

SassafrasSweetgum (sap gum)

Lumber schedules

4/4, 5/4, and 6/4 6/4 Schedules for other products

Dry-bulbtempera-

tureWet-bulb

depression

Dry-bulbtempera-

tureWet-bulb

depression Name

Dry-bulbtempera- Wet-bulb

ture depression

150

Table 7-4—Code number index of schedules1 recommended for kiln drying domestic hardwood 4/4 to 8/4 lumber andother products-concluded

Lumber schedules

4/4, 5/4, and 6/4 8/4 Schedules for other products

Species

Dry-bulbtempera-

tureWet-bulb

depression

Dry-bulbtempera-

tureWet-bulb

depression Name

Dry-bulbtempera- Wet-bulb

ture depression

Sweetgum (red gum)SycamoreTanoakTupelo, blackTupelo, swampTupelo, waterWalnut, blackWillow, blackYellow-wofar

1Schedules are given in tables 7-1 and 7-2.2All 6/4 oak species should be dried by the 8/4 schedule.3See table 7-11.

Table 7-5—Code number index of schedules suggested for kiln drying thick domestic hardwoods1

Schedules for various thicknesses of lumber2

Species

10/4 lumber

Dry-bulbtempera- Wet-bulb

ture depression

12/4 lumber

Dry-bulbtempera- Wet-bulb

ture depression

16/4 lumber

Dry-bulbtempera- Wet-bulb

ture depression

Alder, redAsh, whiteAspenBirch, yellowBlackgumBoxelderCherryCottonwoodCottonwood, wet

streakElm, AmericanElm, rockHackberryMaple, bigleaf,

red, silverMaple, sugar

(hard)Oak, redOak, whiteSweetgum

(sap gum)Sweetgum

(red gum)SycamoreTupelo, blackWalnut. blackYellow-poplar

1A good end coating should be applied to all stock in most cases.2For squares, use a web-bulb depression number one unit higher than the onesuggested for lumber. Thus, for 3- by 3-in birch, use T3-B3.3After passing 30 percent moisture content, gradually shift to wet-bulbdepression schedule B2.

151

Tabled 7-6—Method of assembly of kiln-drying schedule for green 4/4 sugar maple1

Dry-bulb Wet-bulb Moisture content Dry-bulb Wet-bulbtemperature depression at start of step temperature depression

step no. step no. (percent) (°F) (°F)

Wet-bulb Relativetemperature humidity

(°F) (percent)

Equilibriummoisturecontent

(percent)

‘Schedule Code no. T8-C3

Table 7-7—Examples of general schedules for kiln drying lumber of certain hardwood Species1

Moisture contentat start of step

(percent)

4/4, 5/4, 6/4 lumber schedules 8/4 lumber schedules

Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulbtempera- depress- tempera- tempera- depres-

siontempera-

ture ture ture sion ture

OAK, RED (UPLAND)

SCHEDULE T4-D2 SCHEDULE T3-D1

OAK, WHITE

SCHEDULE T4-C2 SCHEDULE T3-C1

MAPLE, HARD

SCHEDULE T8-C3 SCHEDULE T5-C2

ASH, WHITE; CHERRY

SCHEDULE T8-B4 SCHEDULE T5-83

BLACKGUM

SCHEDULE T12-E5 SCHEDULE T11-D3

153

Table 7-8—H-type wet-bulb depression schedules for hardwoods

Wet-bulb depressions (°F) for variousWet-bulb Moisture content wet-bulb depression schedules

depression at start of stepstep no. (percent) 1 2 3 4 5 6 7 8

Table 7-9—Temperature schedule code numbers for maximum Table 7-10—Maximum drying temperatures for maximumstrength retention strength retention

Species

Schedule numbers according tospecies thickness

1 in 1-1/2 in 2 in 3 in >3 in

Moisture Maximum drying temperature (°F) forcontent various schedules1

(percent)1 2 3 4 5 6 7 8

BaldcypressDouglas-firFir,

noblered

Hemlock, westernPine,

northern whiteponderosaredsugarwestern white

SpruceredSitkawhite

White-cedar, Port-Orford

Ash, commercial whiteBirch, yellowCherry, blackAfrican mahoganyMahogany, trueMaple,

silversugar

Oak,commercial redcommercial while

Sweetgum

SOFTWOODS

HARDWOODS

Yellow-poplarWalnut, black

1Temperature schedule code numbers described in table 7-9.2When the initial moisture content of the stock exceeds 40 percent, the initial

temperature should be maintained until the moisture content reaches 40percent, at which time the temperature may be increased 5°F.

154

Table 7-11—Special schedules for certain hardwood species

Temperatures (°F) for various thicknesses of lumber

Moisture contentat start of step

(percent)

4/4 6/4 8/4

Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

HICKORY-UPPER GRADES FOR SPECIAL PURPOSES

TUPELO, SWAMP—HEARTWOOD

TUPELO, SWAMP—SAPWOOD

TUPELO, WATER—HEARTWOOD

155

Table 7-11—Special schedules for certain hardwood species-continued

Temperatures (°F) for various thicknesses of lumber

Moisture contentat start of step

(percent)

4/4 6/4 8/4

Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

TUPELO, WATER—SAPWOOD

ASPEN—LOW COLLAPSE

SUGAR MAPLE, WHITE COLOR—INITIAL MOISTURE CONTENT BELOW 50 PERCENT

SUGAR MAPLE, WHITE COLOR—INITIAL MOISTURE CONTENT ABOVE 50 PERCENT

Table 7-11—Special schedules for certain hardwood species—continued

Temperatures (°F) for various thicknesses of lumber

Moisture contentat start of step

(percent)

4/4 6/4 8/4

Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

UPLAND RED OAK—PRESURFACED

UPLAND WHITE OAK—PRESURFACED

RED OAK, 4/4 AND 5/4—BACTERIA INFECTED

RED OAK, 6/4—BACTERIA INFECTED

157

Table 7-11—Special schedules for certain hardwood species—continued

Moisture contentat start of step

(percent)

Temperatures (°F) for various thicknesses of lumber

4/4 6/4 8/4

Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

RED OAK, 8/4—BACTERIALLY INFECTED, AIR DRIED OR PREDRIED (DRYING HISTORY UNKNOWN)

RED OAK, 8/4—BACTERIALLY INFECTED, DRIED FROM GREEN IN PREDRYER, THEN KILN DRIED

SOUTHERN LOWLAND RED AND WHITE OAK, 6/4 AND 8/4—AIR DRIED OR PREDRIED TO 25 PERCENT MOISTURE CONTENT

MAPLE—MINIMUM HONEYCOMB IN 6/4 AND 8/4 MINERAL STREAK

NORTHERN RED OAK—PRESURFACED 1-INCH

158

Table 7-11—Special schedules for certain hardwood species—concluded

Moisture contentat start of step

(percent)

Temperatures (°F) for various thicknesses of lumber

4 / 4 6/4 8/4

Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

1See figure 7-3 for changes between 110 and 70 percent moisture content on the H2 schedule.2It may not be possible to achieve 90 °F wet-bulb temperature in hot weather.3Operate with vents closed; no steam spray until equalizing.4For 8/4, continue until wettest sample is 8 percent.5For 8/4, time on this step is about 5 days6The 4/4 schedule also applies to 5/4.7Average moisture content of all samples controls.8Average moisture content of wettest half of samples controls.9This schedule should also be used for mineral-streaked yellow birch.10Kiln samples should be 2 ft longer than normal so that three or four intermediate moisture content tests can be made. For green stock, start with normal kiln

sample proceduire. For air-dried stock, cut both an average section and a "darkest zone" section at the start. Cut out the darkest, wettest appearing portion of the lattersection with a bandsaw. Weight and ovendry this portion separately to determine when temperature of 140 °F and higher can be used. After the final drying conditionhas run 1 day, revert to the full-size kiln sample method to start equalizing and conditioning.

11Begin control on darkest zone of wettest sample.

159

Table 7-12—Time schedules for domestic hardwood lumber species

step Timeno. (h)

Temperature (°F)

Dry bulb Wet bulb

step Timeno. (h)

Temperature (°F)

Dry bulb Wet bulb

ALDER, RED—8/4

ALDER, RED—10/4, 12/4

ASH, OREGON—4/4, 5/4, 6/4

ASH, OREGON—8/4, 10/4, 12/4

ALDER, RED—4/4, 5/4, 6/4 LAUREL, CALIFORNIA OR OREGON MYRTLE—4/4, 5/4, 6/4

MAPLE, BIG LEAF—4/4. 5/4, 6/4

MAPLE, OREGON—8/4

OAK, CALIFORNIA SLACK AND OREGON WHITE,AND TANOAK—4/4, LOWER GRADES

OAK, CALIFORNIA BLACK AND OREGON WHITE—6/4, LOWER GRADES

160

Table 7-13—High-temperature kiln schedules for domestichardwood lumber species

Tempera-turestepno.

Moisturecontent

(percent)Time

(h)

Temperature (°F)

Dry bulb Wet bulb

ALDER, RED—4/4, 5/4

ASPEN AND BALSAM POPLAR—2 BY 4 DIMENSION

BASSWOOD, BLACKGUM, RED MAPLE,SWEETGUM SAPWOOD, AND YELLOW-POPLAR—4/4, 5/4

ASPEN—4/4, 5/4,6/4, 7/4, AND 2-IN DIMENSION

RED ALDER, BASSWOOD, BLACKGUM, REDMAPLE, AND YELLOW-POPLAR—7/4 FLITCHES

161

Table 7-14—Code number index of schedules recommended for kiln drying Imported species

Species(common name)

4/4, 5/4, and 6/4 8/4 lumberlumber schedules schedules

Dry bulb Wet-bulb Dry bulb Wet-bulbtemper- depres- temper- depres-

ature sion1 ature sion1

AfrormosiaAlbarcoAndirobaAngeliqueApitongAvodireBalataBalsaBanakBengeBubingaCaribbean pineCativoCeibaCocoboloCourbarilCuangareCypress, MexicanDegameDetermaEbony, East IndianEbony, AfricanGmelinaGoncalo alvesGreenheartHurallombalmbuiaIpelrokoJarrahJelutongKapurKarriKempas

4/4, 5/4, and 6/4 8/4 lumberlumber schedules schedules

Species(common name) Dry-bulb Wet-bulb Dry-bulb Wet-bulb

temper- depres- temper- depres-ature sion1 ature sion1

KeruingLauan, red and whiteLignumvitaeLimbaMahogany, AfricanMahogany, trueManniMerbauMersawaMoraObecheOcote pineOkoumeOpepeParana pinePau MarfimPeroba de camposPeroba rosaPrimaveraPurpleheartRaminRoble (Quercus)Roble (Tabebuia)Rosewood, IndianRosewood, BrazilianRubberwoodSandeSanta MariaSapeleSepetirSpanish cedarSucupira (Bowdichia)Sucupira (Diplotropis)TeakWallaba

1The Ietter S denotes softwood schedule code number from table 7-15.

162

Table 7-15—Moisture content schedules for softwoods

Moisture Dry-bulb temperatures (°F) for various temperature schedulescontent

Dry-bulb at starttemperature of step T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T 1 3 T 1 4

step no. (percent)

Table7-16—Moisture content wet-bulb depression schedules for softwoods

Moisture content (percent) at start Wet-bulb depressions (°F) forof step for various moisture various wet-bulb depression

Wet-bulb content classes schedulesdepression

step no. A B C D E F 1 2 3 4 5 6 7 8

1Go directly to step 10..

163

Table 7-17—Code number index of moisture content schedules1 recommended for kiln drying 4/4, 6/4, and 8/4 softwood lumber

Schedules for lower grades2 Schedules for upper grades’

Species 4/4 6/4 8/4 4/4 6/4 8/4

BaldcypressCedar

AlaskaAtlantic whiteEastern redcedarIncenseNorthern whitePort-Orfordwestern redcedarLightHeavy

Douglas-fircoast regionInland region

FirBalsamCalifornia redGrandNoblePacific silverSubalpineWhite

HemlockEasternWestern

LarchPine

Eastern whiteRegular

JackLodgepolePonderosa

HeartwoodSapwoodAntibrown-stain

RedSouthern yellowsugar

LightHeavy

Western whiteRegularwater core

RedwoodLightHeavy

SpruceEastern (black, red,

white)EnglemannSitka

Tamarack

1Schedules are given in tables 7-20 and 7-21.2Lower grades include commons, dimension, and box; upper grades include clears, selects, shop, and factory; also tight-knotted paneling.3Maximum wet-bulb depression 25 °F.4Maximum wet-bulb depression 20 °F.

164

Table 7-18—Code number index of moisture content schedules1

suggested for kiln drying thick softwood Iumber2

Species 10/4 12/4 16/4

BaldcypressCedar

Atlantic whiteIncenseNorthern whiteWestern redcedar (light)

Douglas-fir, coast regionFir

BalsamCalifornia redGrandNobleWhite

HemlockEasternWestern

Larch, westernPine

Eastern whitePonderosaRedSouthernWestern white

Redwood (light)Spruce

Eastern (black, red, white)EngelmannSitka

Tamarack

Index for variouslumber thicknesses

1Schedules are given in table 7-20 and 7-21.2Upper grades, including clears, selects, and factory lumber.

165

Table 7-19—lndex of time schedules1 for kiln drying softwood species at conventional temperatures

Schedules for Schedules forlower grades2 upper grades3

Common name (botanical name) Comments4

4/4,5/4 6/4 8/4 4/4,5/4 6/4 8/4 12/4, 16/4

CedarAlaska yellow (Chamaecyparisnootkatensis)

lncense (Libocedrus decurrens)Port-Orford (Chamaecyparislawsoniana)Western juniper (Juniperusoccidentalis)Western redcedar (Thujaplicata)

Douglas-fir (Pseudotsugamenziesii)

Fir, trueAlpine (Abies lasiocarpa)Balsam (A. balsamera)California red (A. magnifica)Grand (A. grandis)Noble (A. nob/is)Pacific silver (A. amabilis)White (A. concolor)

HemlockMountain (Tsuga mertensiana)Western (T. heterophylla)

Light to medium sorts only. Prone tocollapse. For heavy sort, air dryto 20 percent moisture content and

kiln dry with table HC, startingwith step 4.

aUse 12 h for each setting.Decrease dry- and wet-bulb settingsby 10oF for first 46 h.

Upper grades, including laminatedstock, dimension, 4/4 common.

Clears and shop require condition-ing in most cases. Ladder stockrequires lower temperature toprevent strength reduction.

bOmit step 1 and reduce step 3 to 12 h.cReduce step 3 to 12 h.dOmit step 1 for vertical grain.

True fir and hemlock can be driedtogether, but problems with percentoverdry and wets are likely.

e96 to 108 h all widths.f96 h flat grain; start with step 2 for

vertical grain, 60 h.g10 to 14 days for sinker heartwood.

Hemlock and true fir can be dried to-gether, but problems with percentoverdry and wets are likely.Prone to excessive warp andchecking.

h96 to 108 hall widths.i96 h flat grain; start with step 2

for vertical grain, 60 h.j14 days for sinker heartwood.

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Table 7-19—Index of time schedules1 for kiln drying softwood species at conventional temperatures—concluded

Schedules for Schedules forlower grades2 upper grades3

Common name (botanical name) Comments4

4/4,5/4 6/4 8/4 4/4,5/4 6/4 8/4 12/4, 16/4

LarchAlpine (Larix lyalli)Western (L. occidentalis)

PineEastern white (Pinus strobus)Jack (P. banksiana)Jeffrey (P. jetfreyi)Limber (P. flexilis)Lodgepole (P. contorta)Ponderosa (P. ponderosa)Southern

Loblolly (Pinus taeda)Longleaf (P. palustris)Shortleaf (P. echinata)Slash (P. elliottii)

Sugar (P. lambertiana)HeavyLight.

Eastern white (P. strobus)Idaho white/western white

(P. monticola)

Redwood (Sequoia sempervirens)LightHeavy and medium

SpruceSpruceSlack (Picea mariana)Engelmann (P. engelmannii)Red (P. rubens)Sitka (P. sitchensis)White (P. glauca)

Yew, Pacific (Taxus brevifolia)

kOmit first 12 h of schedule

3 by 5 timbers use table PC. 10/4and 12/4 flitches use table OC.

IAir dry to 20 percent moisture con-tent. then dry with table DC.

mAir dry to 20 percent moisture con-tent, then dry with table GC.Prone to collapse.

nReduce last 3 steps of schedule from24 to 18 h each setting.

oAir dry to 20 percent moisture con-tent, then dry with table IC

1See table 7-20 for description of schedules.2Lower grades include commons, dimensions, box, and studs.3Upper grades include clears, selects, shop, and factory.4Comments are cross-referenced to column entries by superscript letters.

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Table 7-20—Time schedules for kiln drying softwood lumber at conventional temperatures

step Timeno. (h)

Temperature (°F)

Dry-bulb Wet-bulbstep Timeno. (h)

Temperature (°F)

Dry-bulb Wet-bulb

SCHEDUAL BC (SP—8/4; STEAM)2

SCHEDULE CC (SP—12/4 DIMENSION; STEAM)2

SCHEDULE DC2

SCHEDULE EC2

SCHEDULE FC2

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SCHEDULE AC (SP1—4/4,5/4; STEAM)2 SCHEDULE GC2

SCHEDULE HC2

SCHEDULE IC2

SCHEDULE JC2

SCHEDULE KC2

Table 7-20—Time schedules for kiln drying softwood lumber at conventional temperatures—continued

no. (h) Dry-bulb Wet-bulbstep Time

Temperature (°F)

SCHEDULE LC2 SCHEDULE QC2

SCHEDULE RC2

SCHEDULE MC2

SCHEDULE NC2

SCHEDULE OC2

SCHEDULE PC2

Step Timeno. (h)

Temperature (°F)

Dry-bulb Wet-bulb

SCHEDULE SC’

SCHEDULE TC2

SCHEDULE UC2

SCHEDULE VC2

169

Table 7-20—Time schedules for kiln drying softwood lumber at conventional temperatures—concluded

Step Timeno. (h)

Temperature (°F)

Dry-bulb Wet-bulbStep Timeno. (h)

Temperature (°F)

Dry-bulb Wet-bulb

SCHEDULE WC’ SCHEDULE ZC4

SCHEDULE AAC4

SCHEDULE XC’

SCHEDULE YC4

SCHEDULE BBC

SCHEDULE CCC

1SP, southern pine.2Equalize and condition as necessary3Spray off; vents working.4No conditioning.

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Table 7-21—Index of time schedules1 for kiln drying softwood lumber at high temperature (>212 °F)

Lumber schedulesSchedules for other

Common name (botanical name) 4/4,5/4 6/4 8/4 products Comment

Ceder, northern white(Thuja occidentalis)

Douglas-fir(Pseudotsuga menziesii)

Fir, trueFir, trueBalsam (Abies balsamera)Balsam (Abies balsamera)California red (A. magnifica)California red (A. magnifica)Grand (A. grandis)Grand (A. grandis)Noble (A. procera)Noble (A. procera)Pacific silver (A. amabilis)Pacific silver (A. amabilis)

Subalpine (A. lasiocarpa)QHWhite (A. concolor)

HemlockMountain (Tsuga mertensiana)Western (T. heterophylla)

Larch, western(Lark occidentalis)

PineJack (Pinus banksiana)Limber (P. flexilis)Lodgepole (P. contorta)Ponderosa (P. ponderosa)Red (Norway) (P. resinosa)SouthernLoblolly (P. taeda)Longleaf (P. palustris)Shortleaf (P. echinata)Slash (P. elliottii)

SpruceBlack (Picea mariana)

Engelmann (P. engelmannii)Red (P. rubens)White (P. glauca)

1See table 7-22 for description of schedules.

171

Table 7-22—Time schedules for kiln drying softwood lumber at high temperatures

Temperature (°F)Step Time stepno. (h) Dry-bulb Wet-bulb no.

Time(h)

Temperature (OF)

Dry-bulb Wet-bulb

SCHEDULE AH1 SCHEDULE HH1

SCHEDULE BH (C&BTR SYP–5/4; DIRECT FIRED)1,2

SCHEDULE MH (LODGEPOLE, JACK PINE, WHITESPRUCE–STUDS)1

SCHEDULE CH (C&BTR SYP–1-IN RANDOM WIDTH;DIRECT FIRED)1

SCHEDULE DH (SYP–2 BY 4-2 BY 10; DIRECT FIRED)1

SCHEDULE NH (WHITE SPRUCE–2-IN DIMENSION;GAS FIRED)1

SCHEDULE EH (SYP–4 BY 4; DIRECT FIRED)1

SCHEDULE OH (DOUGLAS-FIR, LARCH–2- BY 4-IN DIMENSION)1

SCHEDULE FH1

SCHEDULE PH (WESTERN HEMLOCK, AMABILISFIR–2- BY 4-IN DIMENSION)1

SCHEDULE GH1

SCHEDULE QH (ALPINE FIR–2-IN DIMENSION)1

1Equalize and condition as necessary.2C&BTR, common and Better grade; SYP, southern yellow pine.3At 10 percent moisture content, final wet-bulb temperature will be approximately

145°F for direct-fired kilns and approximately 175°F for steam-heated kilns.4At 10 percent moisture content, final wet-bulb temperature will be approximately

150°F for direct fired kilns.5At 15 percent moisture content, final wet-bulb temperature will be approximately

155°F.6At 20 percent moisture content, final wet-bulb temperature will be approximately

140°F.7Pull charge when sapwood and corky heartwood are dry.

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Table 7-23—Time schedules for kiln drying softwood lumber at Table 7-25—Anti-brown-stain time schedules for eastern whitehigh temperatures pine, western white pine, and sugar pine

stepno.

Moisturecontent

(percent)

Temperature (°F)

Dry-bulb Wet-bulb

SCHEDULE IH

SCHEDULE JH

SCHEDULE KH

Table 7-24—Anti-brown-stain moisture content schedules for4/4-6/4 eastern white pine, western white pine, and sugar pine

Moisture contentat start of step

(percent)

Dry-bulbtemperature

(°F)

Wet-bulbdepression

(°F)

Wet-bulbtemperature

(°F)

stepno.

Dry-bulb Wet-bulbTime temperature temperature

(h) (°F) (°F)

4/4-5/4 LUMBER

7/4-8/4 LUMBER

1Spray off, vents open.

Table 7-26—Recommended kiln schedules for Douglas-firplywood treated with chromated copper arsenatepreservative

Schedule’

Temperature (°F)

Dry-bulb Wet-bulb

Drying time toapproximately14 percent(h)

3/4-IN-THICK PLYWOOD

1/2-IN-THICK PLYWOOD

1Two alternative schedules are given for each size of plywood.2Initial wet-bulb temperature–the schedule calls for a 1 °F per h decrease

in wet-bulb temperature as drying progresses.

1Spray value shut.

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Table 7-27—Suggested kiln schedules for large southern Pine timbers and poles

Time in Temperature (°F)each step

(h) Dry-bulb Wet-bulb Comment

3 BY 6- AND 4 BY 8-IN TIMBERS

The 180 °F final step is prolonged until thetimbers reach 18 percent moisture content.

4-1/2-BY 5-1/2-IN CROSSARMS

3-1/2- BY 4-1/2-IN PARTIALLY AIR-DRIED CROSSARMS

No control

No control

Final moisture content at a 1 -in depthis 17 to 22 percent.

UP TO 6- BY 6-IN TIMBERS

Fan reversal every 3 h with 3-min ventingat that time. Dry outer 2 in. to belowfiber saturation point.

6- BY 6-IN AND GREATER TIMBERS AND POLES (SEVERE SCHEDULE)

Fan reversal every 3 h with 3-min ventingat that time. Dry outer 2 in. to belowfiber saturation point.

10-1/2-IN-DIAMETER POLES AND PILING (MILD SCHEDULE)

8- TO 10-IN-DIAMETER POLES AND PILING (ACCELERATED SCHEDULE)

Initial moisture content about 85 percent.Final moisture content 30 percent inouter 3 in.

174

Table 7-28—Time schedules for kiln drying 4- by 5-in roof decking

Stepno.

Time(h)

Dry-bulbtemperature

(°F)

Wet-bulbtemperature

(°F)

WHITE FIR

ENGELMANN SPRUCE

WESTERN REDCEDAR

Table 7-29—Conversion of a schedule from a steam-heated kiln to dehumidification kiln

Moisture contentat start of step

(percent)

EquilibriumTemperature (°F) moisture

Relative humidity contentDry-bulb Wet-bulb (percent) (percent)

4/4 WHITE OAK–T4-C2 FOR STEAM-HEATED KILN

4/4 WHITE OAK–T4-C2 CONVERTED TO DEHUMIDIFICATION SCHEDULE WITH MAXIMUM TEMPERATURE OF 120 °F

175

Table 7-30—General low-temperature schedule for kiln drying refractory species

Moisture contentat start of step

(percent)

EquilibriumTemperature (°F) moisture

Relative humidity contentDry-bulb Wet-bulb (percent) (percent)

Table 7-31—Schedule for killing Lyctus (powder-post) beetles and their eggs

Temperature (°F)

Dry-bulb Wet-bulbtemperature depression

Relativehumidity(percent)

Equilibriummoisture content

(percent)

Thicknessof lumber

(in)

Kiln reaches setconditions

(h)

Equalizing moisturecontent values

(percent)

Desired final Moistureaverage) contentmoisture of driestcontent sample at

(percent) start

Equilibriummoisturecontent

conditionsin kiln

Moisture contentof wettest

sample at end

Conditioning equili-brium moisture con-tent values (percent)

Softwoods Hardwoods

176

Table 7-33—Approximate kiln-drying periods for 1-in lumber1

Species

Time (days) required to kiln dry1-in lumber

20 to 6 percent Green to 6 percentmoisture content moisture content Species

Time (days) required to kiln dryl-in lumber

20 to 6 percent Green to 6 percentmoisture content moisture content

BaldcypressCedar

AlaskaAtlantic whiteEastern redcedarIncenseNorthern whitePort-OrfordWestern redcedar

Douglas-firCoast typeIntermediate typeRocky Mountain type

FirBalsamCalifornia redGrandNoblePacific silverSubalpineWhite

HemlockEasternWestern

Larch, westernPine

Eastern whiteLodgepolePonderosaRedSouthern yellow

LoblollyLongleaf

ShortleafSugar

LightHeavy

Western whiteRedwood

LightHeavy

SpruceEastern, black,

red, whiteEngelmannSitka

Tamarack

SOFTWOODS HARDWOODS

American

HackberryHickoryHolly, AmericanHophornbeam, easternLaurel, CaliforniaLocust, blackMadrone, PacificMagnoliaMahoganyMaple

Red, silver (soft)Sugar (hard)

OakCalifornia black

RedWhite

Osage-orangePersimmon, commonSweetgum

HeartwoodSapwood

Sycamore, AmericanTanoakTupelo

BlackWater

Walnut, blackWillow, blackYellow-poplar

Alder, redApple Ash

BlackWhite

AspenBasswood, AmericanBeech, AmericanBirch

PaperYellow

Buckeye, yellowButternutCherry blackChestnut, AmericanChinkapin, goldenCottonwoodDogwood, floweringElm

Rock

Live

1Because of the many factors affecting drying rate and the lack of specificdata covering each case, wide variation from these values must beexpected. These values represent only a general idea of average dryingperiods and should not be used as time schedules. Some of the dryingtimes shown were obtained from commercial kiln operators.

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Chapter 8Drying Defects

Effect of drying temperatures 180Defect categories 180

Rupture of wood tissue 180surface checks 180End checks and splits 182Collapse 183H o n e y c o m b 1 8 5Ring failure 186Boxed-heart splits 186Checked knots 186Loose knots 186

Warp 187Uneven moisture content 188

Board rejects 188Water pockets 188Control measures 188

Discoloration 189Sapwood discolorations 189Heartwood discolorations 194Discolorations in wood containing wetwood 196Metallic and alkaline stains 197Removal of discoloration from dried wood 197

Drying defects of major concern incommercial woods 197

Relationships between drying defectsand machining 198Planer splits 198Broken knots and knotholes 198Chipped and torn grain 198Raised grain 199Residual drying stresses 199

End checks 199Planer splits 199Warp 199

Literature cited 200Sources of additional information 200Tables 201

Chapter 8 was revised by James C. Ward, ResearchForest Products Technologist, and William T. Simpson,Supervisory Research Forest Products Technologist.

The success of a company and the livelihood of thedry kiln operator may depend on knowing the causesof defects in lumber and methods to prevent their oc-currence. Since some defects are not observed in greenlumber and are first noted after the drying operation,they are often called drying defects even though thedefects may have started in the tree, log, or green lum-ber. Defects that develop in dry wood products duringmachining, gluing, and finishing operations may alsobe blamed on poor drying practices. A drying defectis any characteristic or blemish in a wood product thatoccurs during the drying process and reduces the prod-uct’s intended value. Drying degrade is a more specificterm that implies a drying defect that lowers the gradeof lumber. Every year, drying degrade and other dry-ing defects cost the softwood and hardwood lumberindustries millions of dollars in lost value and lost vol-ume caused by poor product performance. When unex-pected defects appear in dried wood products, theircause is often blamed on the drying operation. Thepurpose of this chapter is to describe the various typesof defects that can occur in dried wood products andto show how these defects are related to the kiln-dryingoperation.

Many features of wood affect its utility when it is pro-cessed into lumber and special products. These includesknots, ring shake, bark, mineral streaks, pitch pockets,compression and tension wood, juvenile wood, and spi-ral or interlocked grain, all of which form in the treeand directly influence the grade and value of each indi-vidual board. Ordinary processing of lumber may re-move some of these natural features through trimmingand thus improve the quality and value of the remain-ing piece.

Defects that reduce the grade and value of lumber oftendevelop during logging, sawmilling, drying, finishing,and mechanical handling. A principal objective is todry the wood economically with as little developmentof defects as possible. The degree of care to exercisein controlling the development of defects depends onthe final use of the lumbar. It is important for the kilnoperator to be familiar with the various defects thatreduce the grade and value of dry wood products, to

know when the defects can be reduced or eliminatedwith proper drying practices, and to recognize whencorrective measures other than drying are required.When drying is used to control defects, it should bedone in a manner consistent with the economy of the

179

overall manufacturing system. Before adopting a dry-ing procedure to control specific drying defects, the kilnoperator should determine whether the procedure willinduce other defects that may lower the value of thelumber.

Effect of Drying Temperatures

High temperatures reduce the strength of wood in twoways. First, there is an immediate and reversible effect.For example, wood is weakened when heated from 75 to240 °F but regains strength if immediately cooled to75 °F. The second effect occurs over time and is per-manent. When wood is heated for long times at hightemperatures, ‘it is permanently weakened; the loss ofstrength remains after the wood is cooled. Roth ef-fects are greater at high moisture content than at lowmoisture content. The permanent effect is caused by acombination of time, temperature, and moisture con- tent. Strength loss increases as any one of these factorsincreases.

The immediate,-reversible effect of high-temperaturedrying is important in the development of drying de-fects that result from breakage or crushing of woodcells. When the drying stresses described in chapter 1become greater than the strength of the wood, thistype of drying defect develops. This is why high tem-peratures early in drying are dangerous. The weakeningeffect of high temperatures coupled with high moisturecontent can cause the wood to fracture or be crushed.

High-temperature drying for long periods, particu-larly early in drying when the moisture content is high,may not result in breakage or crushing-type drying de-fects, but it can cause a permanent loss in strength orother mechanical properties that affect product per-formance in end use. Table 8-1 shows the effect ofhigh-temperature drying (225 to 240 °F) compared toconventional-temperature drying (<180 °F) on stiffness(modulus of elasticity) and bending strength (modulusof rupture) of several species. In general, stiffness is notgreatly reduced by high-temperature drying, hut bend-ing strength may be reduced by as much as 20 percent. Surface Checks

For many uses of wood, some reduction in strength isnot important. In some uses, it is quite important. Forexample, the 20 percent loss in bending strength notedin table 8-1 for Douglas-fir can be a concern in struc-tural lumber. Wood for ladders, aircraft, and sportinggoods requires high strength and toughness retention.

There is evidence that lumber treated with waterbornepreservatives and fire retardants is particularly sensitiveto strength reduction if drying temperatures are toohigh. Temperatures ranging from 140 to 160 °F havelittle effect on mechanical properties. The schedules

180

in chapter 7 (tables 7-9 and 7-10) can be used wherestrength retention is a major concern.

Defect Categories

Most defects or problems that develop in wood prod-ucts during and after drying can be classified under oneof the following categories:

1. Rupture of wood tissue

2. warp

3. Uneven moisture content

4. Discoloration

Defects in any one of these categories are caused byan interaction of wood properties with processing fac-tors. Wood shrinkage is mainly responsible for woodruptures and distortion of shape. Cell structure andchemical extractives in wood contribute to defects asso-ciated with uneven moisture content, undesirable color,and undesirable surface texture. Drying temperature isthe most important processing factor because it can beresponsible for defects in each category.

Rupture of Wood Tissue

Many defects that occur during drying result. from theshrinkage of wood as it dries. In particular, the defectsresult from uneven shrinkage in the different directionsof a board (radial, tangential, or longitudinal) or be-tween different parts of a board, such as the shell andcore. Rupture of wood tissue is one category of dry-ing defects associated with shrinkage. Knowing where,when, and why ruptures occur will enable an opera-tor to take action to keep these defects at a minimum.Kiln drying is frequently blamed for defects that haveoccurred during air drying, but most defects can oc-cur during either process. In kiln drying, defects canbe kept to a minimum by modifying drying conditions,and in air drying, by altering piling procedures.

Surface checks are failures that usually occur in thewood rays on the flatsawn faces of boards (figs. 8-1and 8-2). They occur because drying stresses exceedthe tensile strength of the wood perpendicular to thegrain, and they are caused by tension stresses that de-velop in the outer part, or shell, of boards as they dry(ch. 1). Surface checks can also occur in resin ductsand mineral streaks. They rarely appear on the edgesof flatsawn boards 6/4 or lees in thickness but do ap-pear on the edges of thicker flatsawn or quartersawnboards. Surface checks usually occur early in drying,but in some softwoods the danger persists beyond theinitial stages of drying. They develop because the lum-

Figure 8-1—Surface checks in cherrybark oak. (M 137194)

Figure 8-2—Surface checks in Douglas-fir dimensional lumber. (M 22523)

181

ber surfaces get too dry too quickly as .a result of rela- products, such, as interior parts of furniture, wall studs,tive humidity that is too low. Surface checks can also and some flooring applications, mild surface checkingdevelop during air drying. Thick, wide, flatsawn lum-ber is more susceptible to surface checking than thin,

will not cause any problems in use.

narrow lumber. Lumber that has surface checked during air dryingshould not be wetted or exposed to high relative hu-

Many surface checks, particularly those in hardwoods,close in the later stages of drying This occurs when

midity before or during kiln drying. Such treatments

the stresses reverse and the shell changes from tensionfrequently lengthen, widen, and deepen surface checks.Lumber that has open surface checks after kiln drying

to compression (ch. 1). Closed surface checks are un- should also not be wetted because subsequent exposuredesirable in products requiring high-quality finished to plant conditions will dry out the wetted surface andsurfaces, such as interior trim and molding, cabinets, enlarge the checks.and furniture. The checks will quite likely open to someextent during use because of fluctuations in relative hu-midity that alternately shrink and swell the surface.

End Checks and Splits

Superficial surface checks that will be removed duringmachining are not a problem. In products such as tool

End checks (fig. 8-3), like surface checks, usually oc-

handles, athletic equipment, and some structural mem-cur in the wood rays, but on end-grain surfaces. They

bers, either closed or open surface checks can increasealso occur in the early stages of drying and can be min-

the tendency of the wood to split during use. In someimized by using high relative humidity or by end coat-

Figure 8-3—End checks in oak lumber. (M 3510)

182

End splits often result from the extension of end checksfurther into a board. One way to reduce the extensionof end checks into longer splits is to place stickers atthe extreme ends of the boards. End splits are also of-ten caused by growth stresses and are therefore not adrying defect. End splits can be present in the log orsometimes develop in boards immediately after sawingfrom the log.

Collapse

in cell cavities that are completely filled with water(ch. 1). Both of these conditions occur early in drying,but collapse is not usually visible on the wood surfaceuntil later in the process. Collapse is generally associ-ated with excessively high dry-bulb temperatures earlyin kiln drying, and thus low initial dry-bulb tempera-tures should be used in species susceptible to collapse.

The tendency to end check becomes greater in allspecies as thickness and width increase. Therefore, theend-grain surfaces of thick and wide lumber squares,and gunstocks should be end coated with one of theend coatings available from kiln manufacturers andother sources. To be most effective, end coatings shouldbe applied to freshly cut, unchecked ends of greenwood.

Collapse is a distortion, flattening, or crushing of woodcells. Figure 8-4 shows collapse at the cell level, andfigure 8-5 shows a severe case of collapse at the boardlevel. In these severe cases, collapse usually shows up

Figure 8-4—Photomicrograph showing collapsed wood as grooves or corrugations, a washboarding effect, atcells. (M 69379) thin places in the board. Slight amounts of collapse are

ing. End checks occur because moisture moves muchusually difficult or impossible to detect at the boardlevel and are not a particular problem. Sometimes

faster in the longitudinal direction than in either trans- collapse shows up as excessive shrinkage rather thanverse direction. Therefore, the ends of boards dry faster distinct grooves or corrugations.than the middle and stresses develop at the ends. End-checked lumber should not be wetted or exposed to Collapse may be caused by (1) compressive dryinghigh relative humidity before any further drying, or the stresses in the interior parts of boards that exceed thechecks may be driven further into the board. compressive strength of the wood or (2) liquid tension

Figure 8-5—Severe collapse in western redcedar. (M 111997)

183

Figure 8-6—(a) Cross section of quartersawn (upper) red oak boards showing ring failure, collapse, and someand flatsawn (lower) red oak boards showing honey- honeycomb. (MC88 9025)comb and slight collapse; (b) cross section of flatsawn

Wetwood in particular is susceptible to collapse. Al- commercially in Australia. This treatment basicallythough rare, collapse has been known to occur during consists of steaming the lumber as near as possible toair drying. 212 °F and 100 percent relative humidity. Recondi-

tioning is most effective when the average moistureCollapse is a serious defect and should be avoided if content is about 15 percent, and 4 to 8 h are usuallypossible. The use of special drying schedules planned required. Steaming is corrosive to kilns, and unless col-to minimize this defect is recommended. Some species lapse is a serious problem that cannot be solved by low-susceptible to collapse are generally air dried before ering initial drying temperatures, steaming may not bebeing kiln dried. practical.

In many cases, much excessive shrinkage or washboard-ing caused by collapse can be removed from the lumberby reconditioning or steaming, a treatment first used

184

Figure 8-7—Honeycomb that does not appear on thesurface of a planed red oak board (lower) does appear

Honeycomb

Honeycomb is an internal crack caused by a tensilefailure across the grain of the wood and usually oc-curs in the wood rays (fig. 8-6). This defect developsbecause of the internal tension stresses that developin the core of boards during drying (ch. 1). It occurswhen the core is still at a relatively high moisture con-tent and when drying temperatures are too high for toolong during this critical period. Therefore, honeycombcan be minimized by avoiding high temperatures untilall the free water has been evaporated from the entireboard. This means that the core moisture content ofboards should be below the fiber saturation point be-fore raising temperature because that is where honey-comb develops. When the average moisture content ofentire sample boards is monitored for schedule control,there is no direct estimate of core moisture content.Depending on the steepness of the moisture gradient,which is often unknown in most kiln-control schemes,the core moisture content can be quite high even whenthe average moisture content of the whole sample is

when the board is machined into millwork (upper).(M 140291)

low. The danger is that schedule changes based on av-erage moisture content that call for an increase in dry-bulb temperature can be made too soon while moisturecontent in the core is still high, thus predisposing thewood to honeycomb. Measurements of shell and coremoisture content (ch. 6) should be taken before thesedangerous schedule changes are made.

Deep surface and end checks that have closed tightlyon the surface of lumber but remain open below thesurface often called honeycomb, but they are alsoknown as bottleneck checks.

Honeycomb can result in heavy volume losses of lum-ber. Unfortunately, in many cases the defect is notapparent on the surface, and it is not found until thelumber in is machined (fig. 8-7). Severely honeycombedlumber frequently has a corrugated appearance on thesurface, and the defect is often associated with severecollapse.

185

Ring Failure Checked Knots

Ring failure occurs parallel to annual rings either Checked knots are often considered defects. The checkswithin a growth ring or at the interface between two appear on the end grain of knots in the wood raysrings (fig. 8-6b). It is similar in appearance and of- (fig. 8-9). They are the result of differences in shrinkageten related to shake, which is the same kind of failurethat takes place in the standing tree or when the treeis felled; wood weakened by shake fails because of dry-ing stresses. In wood with ring failure, internal tensionstesses, especially in high-temperature drying, developafter stress reversal. The failure frequently involves sev-eral growth rings, starting in one and breaking alongwood rays to other rings. It can occur as a failure inthe end grain in the initial stages of drying and extendin depth and length as drying progresses. Ring failurecan be kept to a minimum by end coating and by usinghigh initial relative humidity and low dry-bulb temper-ature schedules.

Boxed-Heart Splits

A boxed-heart split is shown in figure 8-8. These splitsstart in the initial stages of drying and become increas-ingly worse as the wood dries. The difference betweentangential and radial shrinkage of the wood surround-ing the pith causes such severe stresses in the faces ofthe piece that the wood is split. It is virtually impossi-ble to prevent this defect.

Figure 8-9—Checked knot in sugar pine. (M88 0157)

parallel to and across the annual rings within knots.Checked knots occur in the initial stages of drying andare aggravated by using too low a relative humidity.These defects can be controlled by using higher rela-tive humidities and by drying to a higher final moisturecontent, but it is almost impossible to prevent them.

Loose Knots

Encased knots invariably loosen during drying(fig. 8-10) because they are not grown into the sur-rounding wood but are held in place by bark andpitch. These knots shrink considerably in both direc-tions of the lumber face (across the width and alongthe length), whereas the board shrinks considerablyin width but very little in length. Consequently, the

Figure 8-8—Boxed-heart split in red oak. (M 115582) Figure 8-10—Loose knot in southern pine. (M 16268)

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dried knot is smaller than the knothole and frequently appear fairly early in drying and becomes progressivelyfalls out during handling or machining. Nothing can be worse as drying continues. Cup is caused by greaterdone to prevent the loosening of dead knots during dry- shrinkage parallel to than across the growth rings. Ining. Fewer dead knots will fall out during machining, general, the greater the difference between tangentialhowever, if the final moisture content of the lumber can and radial shrinkage, the greater the degree of cup.be kept as high as possible before machining. Thinner boards cup less than thicker ones. Because

tangential shrinkage is greater than radial shrinkage,

Warpflatsawn boards cup toward the face that was closestto the bark (ch. 1, fig. 1-10). A flatsawn board cut

Warp in lumber is any deviation of the face or edge of near the bark tends to cup less than a similar board

a board from flatness or any edge that is not at right cut near the pith because the growth ring curvature

angles to the adjacent face or edge (squares). It can is less near the bark. Similarly, flatsawn boards from

cause significant volume and grade loss. All warp can small-diameter trees are more likely to cup than those

be traced to two causes; differences between radial, from large-diameter trees. Due quartersawn boards

tangential, and longitudinal shrinkage in the piece as do not cup. Cup can cause excessive losses of lum-

it dries, or growth stresses. Warp is also aggravated by ber in machining. The pressure of planer rollers often

irregular or distorted grain and the presence of abnor- splits cupped boards. Cup can be reduced by avoiding

mal types of wood such as juvenile and reaction wood, overdrying. Good stacking is the best way to minimize

Most warp that is caused by shrinkage difference can be cup.

minimized by proper stacking procedures (ch. 5). Theeffects of growth stresses are more difficult to control, Bow is a deviation flatwise from a straight line drawn

but certain sawing techniques are effective and will be from end to end of a board. It is associated with lon-

described later.gitudinal shrinkage in juvenile wood near the pith of atree, compression or tension wood that occurs in lean-

The five major types of warp are cup, bow, crook, ing trees, and crossgrain. The cause is the difference

twist, and diamonding (fig. 8-11). Cup is a distortion in longitudinal shrinkage on opposite faces of a board.

of a board in which there is a deviation flatwise from a Assuming that there are no major forms of grain distor-

straight line across the width of a board. It begins to tion on board faces, bow will not occur if the longitudi-nal shrinkage is the same on opposite faces.

Crook is similar to bow except that the deviation isedgewise rather than flatwise. While good stackingpractices also help reduce crook, they are not as effec-tive against this type of warp as they are against cupand bow.

Twist is the turning of the four corners of any face of aboard so that they are no longer in the same plane. Itoccurs in wood containing spiral, wavy, diagonal, dis-torted, or interlocked grain. Lumber containing thesegrain characteristics can sometimes be dried reasonablyflat by wing proper stacking procedures. Twist, bow,and crook have definite allowable limits in the gradingrules for softwood dimension lumber, so it is desirableto minimize these defects.

Diamonding is a form of warp found in squares or thicklumber. In a square, the cross section assumes a dia-mond shape during drying. Diamonding is caused bythe difference between radial and tangential shrinkagein squares in which the growth rings run diagonallyfrom corner to corner. It can be controlled somewhatby sawing patterns and by air drying or predryingbefore kiln drying.

Figure 8-11—Various types of warp that develops inboards during drying. (ML88 5555)

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Uneven Moisture Content the core eventually dries. Resawing boards with wa-ter pockets results in bowing and twisting of the new

Wood is dried to an average moisture content that pieces from additional drying of the exposed cores. Wa-is compatible with subsequent processing operations ter pockets can be a problem with dried stock that isand the use of the final product. Kiln operators in the used for glued cores in the manufacture of doors andUnited States generally aim towards a target moisture panels. Even though the water pockets may be pencilcontent of 15 percent for softwood dimension (construc- thin, they will build up enough steam pressure duringtion) lumber and 6 to 8 percent for softwood and hard- electronic gluing operations to explode and shatter thewood lumber to be manufactured into items such as surface of the pieces. Dielectric moisture meter mea-furniture, flooring, or millwork. Uneven moisture con- surements will be erroneous for wood containing watertent refers to a condition where individual boards in a pockets.kiln charge have a level of moisture content that devi-ates greatly from the target moisture content. Theseboards arc rejected for immediate processing and end

Control Measures

we for two reasons: (1) the average board moisturecontent is either above or below an acceptable range for

Uneven moisture content causes drying problems in

the intended moisture content or (2) the average mois-the kiln when (1). there are wide differences in mois-

ture content of the entire board is within the acceptableture content in the initial kiln charge and (2) boards

moisture content range, but the core of the board has awithin the charge have greatly different permeability.Wide moisture content differences occur when the kiln

water (wet) pocket that cannot be tolerated in the nextprocessing step.

is loaded with a mixture of green and partially driedboards. The problem can also develop when the charge

Board Rejectscontains species such as pine or hemlock where thegreen moisture content of the sapwood is much higherthan the moisture content of the heartwood. Prob-

Most boards are rejected because of moisture content lems with uneven moisture content also occur whenthat is too high, but boards with extremely low mois- She charge contains boards with wetwood or boards ofture content (overdried) can also be troublesome dur- mined species with different permeability.ing later machining operations.-Softwood dimensionlumber with an average moisture content of 19 percent Initial moisture content differences.—When theor less is graded as “dry lumber,” and boards 20 per- kiln is loaded with a mixed charge of boards contain-cent and over in moisture content are defined as un- ing high and low moisture contents, the final dryingseasoned lumber. Softwood dimension lumber dried conditions must be coincidental with the target mois-below 10 percent moisture content is usually considered ture content. The charge will be dried according to theoverdried because it is subject to serious planer splits rate of moisture loss in the wettest boards and equal-and breakage when surfaced. Overdried lumber is not ized to a final acceptable moisture content range thatrejected by lumber-grading associations, but the kiln includes the driest boards. The drier boards will beoperator might receive complaints from operators of in the kiln longer than necessary, which is the pricem-house machining. paid for eliminating wet boards. This procedure is

used when the target moisture content is 8 percentAfter drying, boards with excess moisture content will or lower. It is not practical for drying softwood di-shrink more than boards within the desired moisture mension lumber where the target moisture content iscontent range and may not yield an end product of 15 percent and green moisture content values rangeacceptable size or shape. Satisfactory glue bonds are from 50 percent for heartwood boards to 170 percentdifficult to obtain between “wet” and “dry” elements for sapwood boards. By the time the sapwood reachesin composite products. If the wood moisture content the target moisture content the. heartwood will be over-is too high for the equilibrium moisture content inside dried, and it is not economical to increase the moisturebuildings, then furniture will develop loose joints, cabi- content of the heartwood boards from 8-10 percent tonet doors and shelves will warp, and moldings will have 12-15 percent.

unsightly gaps.When mixed charges of high and low moisture content

Water Pockets boards will not be dried to a target moisture contentof 8 percent or lower, then the lumber should be seg-

Some boards will have acceptable overall average mois- regated into different board sorts and each sort dried

ture content and yet have internal water pockets or separately. In commercial practice, however, sorting for

streaks with moisture contents of 10 percent or more moisture content differences is usually done after kiln

higher than the average. Surfacing of boards containing drying. The boards are identified for moisture content

water pockets can result in surface depressions when on the dry chain with dielectric inline moisture meters

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and the wet boards redried. Redrying can increase dry-ing costs by 25 percent or more. It would be preferableto identify high and low moisture content boards onthe green chain before drying. Presorting green woodcannot be accomplished with inline dielectric mois-ture meters when the wood moisture content is above30 percent.

Although presorting on the green chain is possiblethrough weighing individual boards when heavy andlight board sorts are to be dried separately, this is notdone commercially. Existing mills are not equipped toinstall inline weighing devices or to handle boards ofdifferent sizes and weights but similar moisture contentvalues. Recently, a new technique has been developedby the Canadian Forintek Laboratory in Vancouver,BC, that has promise for commercial presorting of lum-ber by moisture content differences. This Method usesinfrared surface measurements, and through computer-controlled equipment identifies each board by moisturecontent. Moisture content values ranging from below15 percent to above 150 percent can be measured; andthe equipment can be installed on existing green chainproduction lines.

surements could be employed to segregate green hem-firlumber into three board sorts: sapwood, heartwood,and wetwood (Ward et al. 1985). To date, no methodfor commercial presorting of wetwood is available onthe market.

Discoloration

The use of dried wood products can be impaired by dis-colorations, particularly when the end use requires aclear, natural finish. Unwanted discolorations can de-velop in the tree, during storage of logs and green lum-ber, or during drying. Discolorations may also developwhen light, water, or chemicals react with exposed sur-faces of dried wood. This section is mainly concernedwith discolorations that develop in clear, sound woodbefore or during drying. Any discolorations beyondthe control of the drying and related processing op-erations, such as mineral stain and decay in the tree,will be mentioned only when they might form the focalpoints for initiation of drying defects. Drying discol-orations have been traditionally classified in associationwith fungal attack or chemicals in the wood. Current

Permeability differences.— Presorting boards on thegreen chain can solve the problem of permeability dif-ferences when the lumber charge contains a mixture-of species. As a general guide to which species can bedried together and which cannot, the kiln operator canwe the kiln schedules and tables of kiln-drying timesin chapter 7. For example, 4/4 aspen and basswoodare both dried under schedule T12-E7 from green to6 percent moisture content in the same length of time.Two different species that have similar but not identi-

knowledge suggests that this dual classification needsto be broadened somewhat. Some discolorations onceconsidered chemical in origin are caused by bacteria,which can only be detected underhigh-power micro-scopes, Also, the formation of unwanted color will varywith complex interactions of tree species, type of woodtissue, and drying conditions. Successful control of dis-colorations depends upon the ability of the dry kiln op-erator to recognize differences in the wood quality ofthe species being dried and environmental factors thatwill initiate discoloration.

cal drying requirements, such as red and white oak, canstill be dried together. However, the mixed oak charge

To prevent discolorations, the dry kiln operator must

must be dried under the milder white oak schedule us-know the wood species and determine the wood type

ing white oak kiln samples. This procedure can also(sapwood, heartwood, or wetwood). The third and

be used for species with widely differing permeabilitysometimes hardest step is to determine if the causal

although it may not be economically feasbile, For ex-factors are primarily chemical or microbial.

ample, a mixed charge of soft maple and red oak mustbe dried under the milder red oak schedule with oak Sapwood Discolorations

kiln samples. This will double the kiln residence timenormally required for the maple. When the tree is cut, sapwood contains living par-

enchyma cells, which are not present in fully formed

When wetwood or sinker stock is responsible for un- heartwood. Sapwood parenchyma cells may still be

even moisture content and water pockets, presorting alive when the logs are sawed into lumber; as these cells

on the green chain is the best solution, but this is not die, enzymes and chemical by products are produced

easily done with currently available techniques. For that may darken the wood. This darkening is intensi-

species such as hemlock, true fir, white pine, aspen, fied by oxidative heating of, the moist wood or by at-

and cottonwood, the moisture content of wetwood tack by fungal molds or aerobic bacteria. Sapwood also

will be higher than that of heartwood but equivalent contains starches and sugars that provide food for mold

to that of sapwood. Wetwood can be accurately pre- fungi and bacteria,

sorted from normal lumber by hand, but this proce-dure has not been successfully applied to green chains Chemical.— Chemical discolorations are the result of

in high-production mills. Research results from the For- oxidative and ensymatic reactions with chemical con-

est Products Laboratory indicate that electronic mea- stituents in the sapwood. They range in color from

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Figure 8-12—Chemical brown stain in sapwood of Douglas-fir. (Top) Board end exposed, to air; (bottom)internal wood. (M88 0162)

pinkish, bluish, and yellowish hues through gray andreddish-brown to dark brown shades. As a group, hard-woods are more subject to oxidative surface discol-orations than softwoods. In some hardwood speciessuch as alder and dogwood, intense discolorations willappear within an hour after the green wood surface isexposed to the air. Most oxidative discolorations areconfined to within 1/16 in of the outer layer of theboard and can be eliminated by planing.

A chemical brown stain that sometimes occurs in WestCoast Douglas-fir penetrates deeper into the sapwoodduring kiln drying (fig. 8-12). Interior discolorations ofthis type cannot be satisfactorily prevented by treatingthe board surface with antistain chemicals. Miller et al.(1983) found that steaming the green wood to 212 °Finactivated the oxidative enzymes within the board andeffectively eliminated the internal brown stain.

During drying, the degree of sapwood discoloration de-pends upon the chemical constituents of the sapwoodand the drying temperature until the average mois-

ture content of the board is well below the fiber sat-uration point. If drying temperatures are too high,chemical discolorations will penetrate deeply into theboard. Above 140 °F, brown discolorations will becomequite pronounced throughout sapwood boards of maple,beech, birch, and alder that is beiug dried from thegreen condition. Tan, yellowish, or pinkish hues maydevelop in the green sapwood of maple, hickory, andash when dried under kiln schedules that are usuallyrecommended for these species (fig. 8-13). Such seem-ingly mild discolorations are not acceptable for prod-ucts requiring “white stock.” Drying schedules for pro-duciug white stock (ch. 7) usually start with a dry-bulbtemperature less than 110 °F and a 10 °F wetbulb de-pression. Drying temperatures are kept below 130 °Funtil the average moisture content reaches 15 percent.

Distinct brown discolorations will develop in the greensapwood of southern yellow pine at drying tempera-tures above 160 °F. When southern yellow pine is driedat high temperatures in excess of 212 °F, a dark browndiscoloration develops that penetrates to at least 1/8 inbelow the surface (fig:8-14).

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Figure 8-13—Kiln-dried and planed sugar maple withand without discoloration. (Left) General reddish-brown discoloration of sapwood from prolonged logstorage and drying with conventional schedule T8-C3; (right) light sapwood board cut from fresh Jogsand dried with an anti-brown-stain schedule (ch. 7).(M 138652)

Sometimes oxidative discolorations are not evident untilthe outer 1/32- to 1/16-in surface has been planed off.This is because the outer surface of the green boardhas dried to below the fiber saturation point before ox-idative chemical reactions can be completed, but themajor inner portion of the board is still green. Thiscan happen with stacked lumber that begins to air drybefore kiln drying is started.

Deep grayish-brown chemical discolorations may oc-cur in the sapwood of lumber from air-drying yardsand predryers. These low-temperature sapwood discol-orations are an important problem in oak, hickory, ash,maple, tupelo gum, magnolia, persimmon, birch, bass-wood, and Douglas-fir (fig. 8-15). In contrast to chem-ical discolorations that occur with high drying temper-atures, these discolorations develop during very slowdrying or wet storage of the sapwood at relatively lowtemperatures. In this situation, enzymes are producedby slowly dying parenchyma cells, which darken whenoxidized. To avoid this, the green lumber should bestickered immediately after sawing, and drying shouldbe started at temperatures above 70 °F. Good air cir-culation is essential. Heating or steaming the greenlumber at 212 °F has been tried, with limited success,to inactivate the enzymes that contribute to the dark-ening reactions.

Figure 8-14—Brown sapwood stain in 8/4 southernpine kiln dried with a high-temperature schedule.(a) Rough, dry board showing surface darkening of sap-wood but not of heartwood under sticker; (b) closeup ofsapwood surfaced to 1/32 in, 1/16 in, and 1/8 in (leftto right). (MC88 9041, MC88 9040)

Some kiln operators have observed that the tendencyfor sapwood to discolor varies in lumber from differentareas and from trees growing on certain soils such aswet bottomlands.

Fungal.—Fungal stains, often referred to as blue stain,are caused by fungi that grow in the sapwood and useparts of it (such as sugars and starches) for food. Bluestain fungi do not cause decay of the sapwood, andthey cannot grow in heartwood or wetwood that doesnot have the necessary food substances. However, poordrying conditions that favor the growth of blue-stainfungi can lead to infections by decay-producing fungi.With the exception of toughness, blue stain has littleeffect on the strength of the wood.

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Figure 8-15—Gray sapwood stain in southern red oakdried green with humid, low-temperature conditionsand with poor air circulation. (MC88 9037)

To prevent blue stain, it is necessary to produce un-favorable conditions for the fungi. Blue-stain fungiare disseminated by spores, which are produced ingreat abundance and are disseminated by wind and in-

sects, or by direct growth from infected to uninfectedwood. Blue-stain fungi will survive but cannot grow inwood with a moisture content of 20 percent or loweror a temperature of 110 °F. Temperatures higher than150 °F are lethal to the fungi. This means the dry kilnoperator may be able to employ drying schedules forcontrol. In the summer months and in the tropics, theoperator will need to chemically treat the wood withfungicides in addition to using proper kiln schedules.

Chemical fungicides, or biocides, make the sapwood un-suitable as food for blue-stain fungi (fig. 8-16). Sodiumpentachlorophenol (PCP) has been one of the most ef-fective and widely used fungicides for controlling sap-wood stains in lumber, but its use has been recentlycurtailed by the U.S. Environmental Protection Agency(EPA) because of adverse effects on workers and theenvironment. New chemical formulations with lowermammalian toxicity appear promising for the controlof sapwood stain (Cassens and Eslyn 1983, Tsunodaand Nishimoto 1985). For chemical control to be ef-fective, the green lumber must be chemically treatedsoon after sawing fresh logs. Treating lumber from logsthat have laid on the yard for a prolonged period andthat are already infected with fungi will not be effec-tive unless the lumber is kiln dried immediately undertemperatures lethal to the fungi. However, under anyconditions, chemically treated lumber should be stackedon stickers immediately after treatment.

Figure 8-16—Untreated (left) and dip-treated (right)sweetgum lumber after 120 days in an air-drying yard.(M88 0158)

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Figure 8-17—Interior sapwood stain in a section of 6/4white pine lumber. Where lumber is bright on the sur-face but stained inside, conditions were initially suit-able for infection. Later, chemical treatment or accel-erated drying made the conditions unfavorable for fur-ther fungal growth in the outer portion of the piece.(M 39166)

Figure 8-18—Surfaced board of eastern white pinerevealing blue stain that developed during the earlystage of dehumidification drying. The lumber wasfrom winter-cut Jogs and was not chemically treated.(M88 0160)

In warm, moist environments the airborne spores ofsapwood-stain fungi seem to germinate very soon af-ter landing on the surface of green, untreated sapwood.This means that the lumber must be chemically treatedimmediately after the logs are sawed or certainly nolonger than 36 h afterward. Chemical fungicides willusually not soak into the board more than 1/32 in un-der commercial operating methods. Therefore, it is im-portant to kiln dry the treated boards as soon as pos-sible at initial dry-bulb temperatures above 130 °F toprevent the internal growth of fungi that have pene-trated deep enough to escape the fungicide. Internalblue stain in the core of chemically treated sapwood isillustrated in figure 8-17.

Precautions must be taken with untreated lumbereven when kiln dried within 1 day of sawing the logs.Sapwood-stain fungi will not grow at temperatureslower than 35 °F, and chemical treating is often cur-tailed, for economic reasons, during winter months innorthern locations. Blue stain will develop on the sur-face of boards during drying at low temperatures andhigh humidities if the surface is not soon dried below20 percent moisture content (fig. 8-18). This has oc-curred with dehumidification drying of untreated soft-woods. Blue stain was found to develop under stickersin untreated southern pine sapwood that was kiln driedat 140 °F to avoid chemical brown stain.

Bacterial.— Bauch et al. (1984) in West Germanyhave associated the formation of dark discolorations inthe sapwood of light-colored tropical woods with con-tamination of the logs and lumber by aerobic bacteria.They found that these bacteria grow on certain chem-ical components in the sapwood extractives, and themetabolic byproducts will discolor during kiln drying,

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especially on surface areas in contact with the stickers.These discolorations were controlled by spraying thegreen lumber with aqueous solutions of weak organicacids, such as propionic acid, before drying. The acidsolution inactivates the discoloration reaction, whichrequires alkaline conditions.

Sticker stains and sticker marks.-Sticker stainsand marks are both discolorations resulting from con-tact of the sticker with the wood surface. Stickerstains are imprints of the sticker that are darker thanthe wood between stickers (fig. 8-19). Sticker marksare lighter than the exposed surface of the board be-tween stickers (fig. 8-14). Although these sticker discol-orations can occur in heartwood, they are much moreprevalent and troublesome in sapwood. The causes ofthese discolorations can be chemical, microbial, or acombination of these.

Figure 8-19—Residual sticker stain in sapwood of kiln-dried sugar maple after planing surface. (M 138660)

Sticker stains probably occur because the wood underthe sticker remains moist longer or because it is rewet-ted. Sticker stains can be fungal sapwood stains thatdeveloped from stickers that were either too wet or con-taminated with dirt and microoganisms. Even whenusing dry stickers, fungal sapwood stain can developunder stickers when drying conditions are poor and thewood is not chemically treated.

Sticker marks are chemical in nature-the exposed sur-face of a board oxidizes more readily than the surfaceunder the sticker where oxidation is restricted. The in-tensity of oxidation staining is influenced by the chem-ical nature of the wood extractives and the presenceof warm, moist drying conditions. High-temperaturedrying can also initiate sticker marking by degrad-ing chemical extractives in the exposed surface ofthe board. Sapwood from a species such as red alderwith highly oxidizable extractives is always subject tosome degree of sticker marking depending on dryingconditions.

Sticker discolorations are almost inevitable, but theycan generally be eliminated with light surfacing of thedried wood. Control measures should be concentratedon drying procedures that will lessen the intensity anddepth of the discolorations. These include using dry,narrow stickers or grooved stickers to reduce the con-tact area and starting the drying of green lumber assoon as possible. Dry-bulb temperatures should bemoderate, and wet-bulb depressions should be suffi-cient for fast drying to avoid checking. There shouldbe good air circulation of at least 200 ft/min acrossthe load. Green boards should be chemically treatedwhen sapwood-stain fungi or bacteria are contributingfactors.

The photodegradation of extractives in green wood thatis briefly exposed to bright sunlight can result in oxida-tive sticker stains and sticker marks during kiln drying(Booth 1964).

Heartwood Discolorations

Discoloration during the drying of heartwood will usu-ally be chemical in nature and not as frequently en-countered as when drying sapwood. Fungal discol-orations will never develop under satisfactory dryingconditions if the green heartwood is sound. Bacteriaare not a problem when drying heartwood, but they docontribute to drying discolorations in wetwood, whichis considered an abnormal type of heartwood and is dis-cussed in the next section.

Chemical.— Heartwood of most species will darkenuniformly during drying, and the intensity of the dis-coloration will depend upon the chemical nature ofthe extractives and the drying temperatures. In greenheartwood, darkening intensifies with increasing dryingtemperatures. An example of unwanted, nonuniformdarkening is the coffee-colored or oily-looking blotchesthat develop during the kiln drying of teak (fig. 8-20).These blotches develop just under the surface of theboard and are chemically similar to the extractives thatcontribute to the normal warm, brown color of teak.Teak dried at kiln temperatures as low as 110 °F will

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Figure 8-20—Chemical discoloration (coffee-coloredblotches) that develop in heartwood of teak during kilndrying. (M88 0159)

Figure 8-22—“Ghost” discolorations in white oakheartwood: middle zone, unfinished; outer zones, fin-ished with stain. (M88 0163)

develop blotches, but they are much darker in wooddried at higher temperatures. The fundamental causeof teak blotching is not known; blotching occurs in lum-ber from trees grown in one region and not in another.The blotches can be lightened somewhat by exposingthe dried wood to bright sunlight.

Fungal.—Most mold-type fungi, such as those caus-ing sapwood blue stain, cannot grow on the chemicalconstituents in heartwood. There is one exception-the mold-type fungus Paecilomyces varioti. This funguscan feed on the tannins and organic acids found in theheartwood of species such as oak. It forms a tan moldon the surface of oak heartwood (not sapwood) under

warm, humid conditions, particularly in predryers anddehumidification dryers with poor air circulation. Theresulting discoloration is usually superficial and canbe planed off, but it will penetrate more deeply intothe board if the surface is not dried below 20 percentmoisture content within the first week or two of dry-ing (fig. 8-21). Control requires using the proper kilnschedule with adequate air circulation across the load.

Streaks of light-brown discoloration that run across thegrain are sometimes found in white oak boards afterdrying and planing (fig. 8-22). These discolorationsresemble sticker stains but they penetrate the entirethickness of the board and cannot be eliminated with

Figure 8-21—penetration of dark chemical discoloration into heartwood of white oak from surface growth of themold-type fungus Paecilomyces varioti. (M86 0283)

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Figure 8-23—Kiln-dried sugar pine shows chemicalbrown stain in center board that contained wetwoodbut not in boards with sapwood (left) or normal heart-

planing. Clear finishes will intensify the streaks, andthey may not be noticed until the last stages of produc-tion, which can be costly to the manufacturing opera-tion. This discoloration is caused by a fungal infectionin the heartwood of the living tree.

Discolorations in Wood Containing Wetwood

Wetwood is an abnormal, water-soaked type of heart-wood; it is initiated by pathological rather than nor-mal physiological changes in the living tree. Anaerobicbacteria are involved in the formation of wetwood, andthey contribute to chemical changes in the extractives,which may later result in drying discolorations. Not allwetwood darkens during drying because of differencesin tree species and bacteria associated with wetwoodformation. For example, wetwood in white pine may or

wood (right). End sections (top) were crosscut fromrough, dry boards, and 1/16 in was planed from boardsurfaces (bottom). (MC88 9039)

may not develop coffee-brown drying stains dependingon the type of wetwood bacteria that infected the livingtree.

Dark discolorations that develop in lumber with wet-wood result from an oxidative or a metallic-tannate re-action. In both situations, wood extractives are chemi-cally degraded by the bacteria (usually under anaerobicconditions in the tree), which results in the productionof compounds that darken when heated under oxidativeconditions or when placed in contact with metals suchas iron.

The familiar coffee-brown stains that develop duringthe kiln drying of wetwood in white pine, sugar pine,and ponderosa pine, and to a lesser extent in aspen,cottonwood, and western hemlock, are the oxidativeenzymatic type (fig. 8-23). Two methods have beenused to control coffee-brown stains in softwood lum-

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ber: chemical treatment and special drying schedules.Treatment of the green wood with antioxidant chemi-cals such as sodium azide and sodium bisulfite is quiteeffective.. Untreated wetwood in high-risk species suchas the white pines must be dried at low dry-bulb tem-peratures with large wet-bulb depressions (see anti-brown-stain schedules in ch. 7). Treated lumber cansometimes be kiln dried at higher temperatures withgood results, but caution and pretesting are advised.

Organic acids are produced by bacterial growth in wet-wood that catalyzes chemical reactions of tannins inthe wetwood with iron, steel, and zinc, resulting indark discolorations. Wetwood in oak, redwood, westernredcedar, and western hemlock is very susceptible tothese metallic stains when the outer shell of the boardis green or wet. Galvanizing or coating steel strapswill not always prevent these stains from forming inpackages of green lumber with wetwood boards (fig. 8-24). Iron stains can generally be removed from woodby treatment with an aqueous solution of oxalic acid ifsurface penetration of the stain is not too deep.

Figure 8-24—Dark discolorations in two green west-ern hemlock boards (upper and middle boards) result-ing from an acid chemical reaction of wetwood extrac-tives with steel band that had an epoxy-powdered zinccoating. Lower green sapwood board did not react.(M88 0161)

Metallic and Alkaline Stains

Metallic discolorations can also develop in normal woodwith high amounts of tannins and related compounds(polyphenols) but not as readily as in wetwood wherehigher amounts of organic acids are present to speedup the reaction. Metallic discolorations are mostlyiron-tannate stains and are likely to develop in oak,chestnut, and walnut, and, to a lesser degree, in otherspecies during kiln drying from steam condensates andwater dripping from steel pipes, beams, and other kilncomponents.

Dark alkaline stains are caused by the chemical reactionof wood extractives with potassium and calcium hy-droxides that leach out from concrete and mortar struc-tures in contact with the wood. These stains might de-velop when lumber is dried in concrete or brick kilnsthat are not kept in good repair. They can also de-velop from contact of wood with solutions containingammonia.

Removal of Discoloration From Dried Wood

Although preventative measures are advocated here, itmay sometimes be economically necessary to removediscolorations that cannot be surfaced off on the planer.Some stains may be removed with a bleaching agent,but some trial and error method is often required tofind the most effective agent for a particular stain.Bleaching operations can be costly in terms of handlingand redrying the wood. To be effective, the bleachingtreatment may have to be so severe that an objection-able amount of natural color is also eliminated. Ofcourse, the bleached wood cannot be resurfaced with-out exposing interior discolorations.

If the stain is not too deep, it can often be removed orreduced in intensity with hydrogen peroxide. A concen-trated aqueous solution of oxalic acid will bleach outchemical sapwood stains but not sapwood stains causedby mold fungi. A laundry bleach of 5 percent sodiumhypochlorite solution can sometimes be used effectively(Forest Products Laboratory 1967, Downs 1956).

Drying Defects of Major Concern inCommercial Woods

All woods are subject to drying defects, but somespecies are more likely to develop certain defects thanothers. Refractory hardwoods such as oak and hick-ory will check more readily than basswood and yellow-poplar, which have less dense and more even-texturedwood. Drying defects will develop more frequentlyin wetwood or sinker stock than in sapwood or nor-mal heartwood. Wetwood occurs quite frequently insome tree species and rarely or not at all in other

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species. Common defects that occur during kiln dry-ing are noted in tables 8-2 to 8-4 for U.S. softwoodspecies, U.S. hardwood species, and imported species,respectively.

Relationships Between DryingDefects and Machining

Lumber can be damaged during machining if it con-tains certain drying defects. Planer splits, brokenknots; knotholes, chipped and torn grain, raised grain,and warp can all occur as a result of improper drying.Precautions taken during drying can minimize or avoidthese defects.

Planer Splits

A long split often develops when cupped lumber is flat-tened as it passes through the planer. End splits-al-ready present aggravate planer splitting. This type ofsplit, also called roller split, lowers the grade and valueof lumber and causes waste. Not only does the amountof cupping increase as the moisture content of the woodis lowered but the wood becomes stiffer and is morelikely to split when flattened.

Splitting on the planer can be reduced by taking stepsto minimize cupping and end splitting through goodstacking practices. Ensuring that lumber is not over-dried will also reduce splitting. For example, softwoodconstruction lumber is frequently dried to a targetmoisture content of about 15 percent. Drying belowthat target will increase the chance that planer splitswill develop. The upper grades of both hardwood andsoftwood lumber are dried to 10 percent moisture con-tent or less to meet end-use requirements. If the lum-ber becomes cupped in the process, splitting cannot beeasily avoided during planing. Planer splitting can bereduced somewhat by relieving drying stresses and rais-ing the moisture content of the surface of the lumber.

Broken Knots and Knotholes

In most grades of lumber, the knots in the surfacedboards should be smooth, intact, and unbroken. Knotscheck and loosen as drying proceeds, and they becomemore brittle as the moisture content of the wood de-creases. While the lumber is in the planer, the knotsare severely hammered as well as cut by the knives.The hammering breaks checked knots and knocks outloose ones, and it can thus lower the grade of thelumber.

In much of the softwood lumber industry, knotty gradesof construction lumber are dried to a final moisturecontent of about 15 percent to permit better machin-ing of the knots. At this moisture content the sound

Figure 8-25—Chipped and torn grain in hard maple.(M 114737)

knots are not severely checked, and the encased knotsare fairly tight: Therefore, common grades of softwoodlumber are usually separated from upper grades anddried by different schedules; the upper grades are driedto 8 to 10 percent moisture content.

In some species, encased knots are held in place largelyby pitch between the knot and the board. If the pitchis removed, the knots fall out of the board. Dryingtemperatures of 160 to 180 °F soften the pitch; it runsout from around the encased knot and the knot fallsout. In these cases, knots can be prevented from drop-ping out by reducing the drying temperature.

Chipped and Torn Grain

When dry lumber is machined, wood may be chippedand torn from some areas on the surface (fig. 8-25).The occurrence of chipped and torn grain is influencedlargely by the operating conditions of the machine, thesharpness and setting of the knives, the feed rate intothe machine, and the slope of grain, including grainvariations. To some extent, however, the susceptibil-ity of lumber to chipping and tearing is affected bythe moisture content of the wood layer being removed.Lumber of extremely low surface moisture content–5 percent and less-chips and tears more during ma-chining than if the surface moisture content is 8 percentor higher. Consequently, kiln operators can prevent thisproblem to some extent by avoiding overdrying and byincreasing surface moisture content with a conditioningtreatment.

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Figure 8-26—Raised grain in Douglas-fir. (M 97880)

Raised Grain

Raised grain (fig. 8-26) occurs primarily when lumberis not uniformly dried to a low enough moisture con-tent at the time of machining. Generally, raised graindoes not develop in wood that is machined while at12 percent or leas moisture content. When wood is ma-chined at a higher moisture content, the action of theknives forces the latewood bands into the softer early-wood bands on the flat grain surface. Subsequently, thecompressed earlywood recovers and lifts the bands oflatewood above the surface. The uneven surface usu-ally reduces the grade and usefulness of the finishedproduct.

Raised grain can occur in all species, but it is most pro-nounced in softwoods like Douglas-fir and southern pinethat have distinct bands of earlywood and latewoodthat are different in density.

Residual Drying Stresses

Whether or not residual drying stresses (caseharden-ing) are considered a defect depends on how the lum-ber is subsequently sawed or otherwise machined. Themost common problems that occur in the use of case-hardened lumber are end checking, planer splitting, andwarping.

End Checks

End checks will frequently develop in the core of afreshly crosscut casehardened board that is exposedto low atmospheric relative humidity, even though theaverage moisture content of the board is fairly low. Thetensile stresses present in the core, coupled with addi-tional stresses brought on by end drying, exceed thestrength of the wood. A check then develops, which canfurther extend into a split.

Planer Splits

Splits can occur in relatively flat casehardened boardsthat are being surfaced. The splits are caused by theinternal drying stresses in the boards, coupled with theforces applied by the machine knives. A conditioningtreatment will reduce planer splits from this cause.

Warp

If transverse or longitudinal stresses become unbalancedduring sawing or any other machining operation on acasehardened board, the board will distort in an effortto rebalance the stresses. Resawing may cause cupping

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Literature Cited

Figure 8-27—Distortion caused by unrelieved dryingstresses: resawed board (left), lumber heavily machinedon one face (center), and grooved lumber (right).(M 111992)

or bowing. The concave faces will be oriented towardsthe saw (fig. 8-27). Ripping may result in crook, inwhich the concave edges usually follow along the sawcut. In planing, the depth of cut is not likely to be thesame on both faces; if the board is casehardened, it willcup with the concave face toward the most heavily cutsurface. When casehardened lumber is edgegrooved,the lips of the groove may pinch inwards (fig. 8-27). Atongue or spline inserted into such a groove may breakthe lips. Cupping usually results when casehardenedlumber is machined into patterns, as in the manufac-ture of molding and trim, or when unequal cuts aretaken from the faces and edges of the lumber in routingand carving operations. Any warping of casehardenedlumber that is due to sawing or machining is a sourceof trouble in further processing.

The relief of drying stresses by a conditioning treat-ment is strongly recommended for lumber that is tobe used in furniture, architectural woodwork, sash anddoor stock, and other products that may require sawingor other machining that may unbalance residual dry-ing stresses. It should also be used when the end useis unknown but could be one of the above-mentionedproducts.

Sources of Additional Information

Amburgey, T. L.; Forsyth, P. 1987. Prevention andcontrol of gray stain in southern red oak sapwood. In:Proceedings, 15th Annual Hardwood Symposium of theHardwood Research Council; 1987 May 10-12. Hard-wood Research Council, P.O. Box 34518, Memphis, TN381840518: 92-99.

Bois, P. J. 1970. Gray-brown chemical stain in south-ern hardwoods. Forest Prod. Util. Tech. Rep. No. 1.Madison, WI: U.S. Department of Agriculture, ForestService, Forest Products Laboratory. 4 p.

200

Bramhall, G.; Wellwood, R. W. 1976. Kiln drying ofwestern Canadian lumber. Information Report VP-X-159. Western Forest Prod. Lab., Vancouver, BC V6T1X2. 112 p.

Brown, W. H. 1965a. Stains in timber-the causes andtheir control. Woodworking Industry. 22(1): 31-32.

Brown, W. H. 1965b. The causes of chemical stains intimber. Woodworking Industry. 22(7): 27-28.

Cech, M. Y.; Pfaff, F. 1977. Kiln operators manual foreastern Canada. Report OPX 192 E. Eastern ForestProd. Lab., Ottawa, ON K2G 325. 189 p.

Gerhards, C. C. ; McMillen, J. M., compilers. 1976.High-temperature drying effects on mechanical proper-ties of softwood lumber: Proceedings of a research con-ference; 1976 February 25-26; Madison, WI: U.S. De-partment of Agriculture, Forest Service, Forest Prod-ucts Laboratory. 161 p.

Hildebrand, R. 1970. Kiln drying of sawn lumber.Maschinen Bau GmbH. 744 Nuertingen, West Germany.199 p.

Knight, E. 1970. Kiln drying western softwoods.Bull. 7004. Portland, OR: Moore Dry Kiln Co. 77 p.(Out of print.)

MCMillen, J. M. 1975. Physical characteristics ofseasoning discolorations in sugar maple sapwood.Res. Pap. FPL-248. Madison, WI: U.S. Depart-ment of Agriculture, Forest Service, Forest ProductsLaboratory. 31 p.

McMillen, J. M. 1976. Control of reddish-brown col-oration in drying maple sapwood. Res. Note FPL-0231.Madison, WI: U.S. Department of Agriculture, ForestService; Forest Products Laboratory. 8 p.

Pratt, G. H. 1974. Timber drying manual. Build-ing Research Establishment Rep. Princes RisboroughLaboratory, Princes Risborough, Aylesbury, Bucking-hamshire, U.K. HP 17 9PX. 152 p.

Reitz, R. C.; Page, R. H. 1971. Air drying of Lumber.Agric. Handb. No. 402. Washington, DC: U.S. Depart-ment of Agriculture.

Scheffer, T. C.; Lindgren, R. M. 1940. Stains ofsapwood and sapwood products and their control.Tech. Bull. No. 714. Washington, DC: U.S. Departmentof Agriculture. 124 p.

Schink, B.; Ward, J. 1984. Microaerobic and anaer-obic bacterial activities involved in formation of wet-wood and discoloured wood. IAWA Bulletin n.s. 5(2):105-109.

Table 8-1-Effect of high-temperature drying on modulus ofelasticity and modulus of rupture of certain species1

Reduction in property (percent) caused byhigh-temperature drying (225-240 °F)

Species Modulus of elasticity Modulus of rupture

Douglas-fir

Western hemlock

Western white spruce

Western redcedar

Southern pine

Eastern spruce

Balsam fir

Jack pine

Trembling aspen

Balsam poplar

1Data derived from study by Gerhards and McMillen (1976).See Literature Cited.

2Compared to lumber dried by conventional temperatures below 180 °F.3Conventional temperature in this study was 200 °F.4Conventional temperature in these studies was 185 to 190 °F.

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Table8-2—Common drying defects in U.S. softwood lumber species

Species Drying defect Contributing factor

BaldcypressOld growthYoung growth

CedarAlaskan yellowEastern redcedarIncense cedar

Heavy stockPort-OrfordWestern redcedar

Heavy stock

Douglas-firCoastal

FirBalsamCalifornia redGrandPacific silver

White Wetwood

SubalpineNoble

H e m l o c kEasternWestern

LarchWestern

PineEastern whiteWestern whiteSugarPonderosa

Young growthLodgepoleLoblollyLongleafShortleafSlashVirginiaPond

RedwoodHeavy stock

SpruceWhite

SitkaYoung growth

End checks, water pocketsChemical brown stain

Resin exudateKnot checks, excessive loss of aromatic oils

Water pockets, collapseResin exudate

Uneven moisture content, collapse, honeycomb,chemical stains, iron stains, resin exudate

Red-brown chemical stainsGray sapwood stainsRing failure, honeycomb

Uneven moisture contentUneven moisture content, shake, splits, warpUneven moisture content, shake, splitsUneven moisture content, shake, splits,

chemical brown stainsUneven moisture content, shake, splits,

chemical brown stainsUneven moisture content, shake, splitsWarp, splits

Uneven moisture content, warp, ring shakeUneven moisture content, warp, chemical

stains, shake, iron stains

Shake (ring failure, checks, resin exudate)

Brown stain, ring failureBrown stainBrown stainBrown stain

WarpWarpBrown sapwood stain, checks, splitsBrown sapwood stain, checks, splitsBrown sapwood stain, checks, splitsBrown sapwood stain, checks, splitsBrown sapwood stain, checks, splitsWater pockets, dark chemical stains, honeycomb

Uneven moisture content, collapse, honeycomb,chemical stains, iron stains

Water pockets, collapse, ring failure

Checks, splits, raised grain

Refractory wood, extractivesWood extractives, poor air circulation

ExtractivesExcessive drying temperatures

Wetwood, excessive drying temperaturesExtractives

Wetwood (sinker stock), extractives

Wood extractivesSapwood extractive, slow dryingWetwood (infrequent occurrence)

WetwoodWetwood, compression woodWetwoodWetwood

Wetwood, compression woodWetwood, compression wood

Wetwood, compression woodWetwood

Wetwood

WetwoodWetwoodWetwoodWetwood (less common in ponderosa pine

than in the soft pines)Juvenile wood, compression woodCompression woodExcessive drying temperaturesExcessive drying temperaturesExcessive drying temperaturesExcessive drying temperaturesExcessive drying temperaturesWetwood (infrequent occurrence)

Wetwood (usually in old growth)

Wetwood (rare occurrence in northern andsouthern limits of botanical range)

Fast growth juvenile wood

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Table 8-3—Common drying defects in U.S. hardwood lumbar species

Species Drying defect Contributing factor

Alder, red Chemical oxidation stains (sticker marks)

AshBlackWhite

Ring failureGray-brown sapwood stain (sticker marks,

stains)Surface checks

Aspen

Basswood, American

Water pockets, honeycomb, collapse

Brownish chemical stain

Beech, American End and surface checksDiscoloration, honeycomb

BirchPaperYellow birch

Brownish chemical stainEnd and surface checksCollapse, honeycomb

Blackgum

Cherry, black

Water pockets, collapse

Ring shake, honeycomb

Chestnut

Cottonwood

Cucumber tree

Iron stains

Water pockets, honeycomb, collapse

Sapwood discoloration

DogwoodEastern and

Pacific

ElmAmerican

SlipperyRock

Hackberry andSugarberry

Hickory

Oxidative sapwood stains

Ring failureWarpRing failureBoxed-heart splits

Sapwood discolorations

Chemical sapwood stains,ring failure, honeycomb

Holly

Laurel, California

LocustBlack andHoney

Madrone

Sapwood stains

End checks

End and surface checks

End and surface checksCollapse

Chemical wood extractives

Wetwood, drying temperaturesTrees from wet sites, drying too slow,

poor air circulation6/4 and thicker stock

Wetwood, drying temperatures

Sapwood from certain areas, dryingtoo slow

Normal wood is refractoryWetwood (occasional)

Extractives in wood from certain sitesRefractory heartwoodWetwood (heartwood), mineral streaks

Wetwood

Wetwood (not common)

Extractives

Wetwood

Poor air circulation

Sapwood extractives, drying temperature

WetwoodGrain orientationWetwoodGrowth stresses

Slow drying with poor air circulation

Slow drying with poor air circulation,wetwood

Extractives, poor air circulation

Refractory wood from old-growth trees

Refractory wood

Refractory woodWetwood

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Table Common drying defects in U.S. hardwood lumber species—concluded

Species Drying defect Contributing factor

Maplesoft

Red andSilver

HardSugar and black

Myrtle, Oregon (seeCalifornia laurel)

Oak, westernCalifornia blackOregon white

OakRed upland

Red lowland

Southern redWhite upland

White lowland

PecanWater

Persimmon

Sapgum

Sweetgum

Sycamore (heartwood)

Tanoak

Tupelo gum

Walnut, black

Willow, black

Yellow-poplar

Sapwood discoloration, ring failure,honeycomb in heartwood

Wetwood, poor air circulation

Sapwood discoloration Extractives, poor air circulationCollapse, honeycomb in heartwood Mineral streaks, wetwood

Honeycomb, collapse, ring shakeHoneycomb, collapse, ring shake

WetwoodWetwood

Ring failureHoneycombIron stains

Collapse, ring failureHoneycombIron stains

Gray sapwood stainsEnd and surface checksIron stainsRing failure, collapseGray sapwood stainsEnd and surface checksIron stainsHoneycomb, collapse, ring failureGray sapwood stains

Severe wetwoodSevere drying of normal heartwood or

wetwood with mild dryingExtractivesWetwoodSevere drying of normal heartwood or

wetwood with mild dryingExtractivesPoor air circulationSevere dryingExtractivesWetwoodPoor air circulationSevere dryingExtractivesWetwoodPoor air circulation

Honeycomb, ring failure

End and surface checksChemical sapwood stains

Sapwood discoloration

Surface and end checksHoneycomb, collapse, water pockets

Honeycomb, ring failure, water pockets

End and surface checksHoneycomb

End checksHoneycomb, collapse, water pockets

Wetwood

Severe dryingSlow drying at low temperature

Poor air circulation

Severe dryingWetwood

Wetwood

Severe dryingWetwood

Severe dryingWetwood

End checksIron stainsHoneycomb, collapse, ring failure

Severe dryingExtractivesWetwood

Honeycomb, collapse, water pockets,failure

Wetwood

Mold, sapwocd stains

Honeycomb, water pockets (rare)

Slow and poor drying, moderate kilnschedule

Wetwood

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Table 8-4—Common drying defects of hard-to-dry Importedspecies1

Species Drying defect

AlbarcoAndiroba

collapseAngelique Moderate tendency to check, slight warpApitong Slow drying with considerable tendency to

Avodirecheck, collapse, and warp

End checks

BalataBalsa (heavy)Banak

Severe checks and warpWater pockets, collapse, splits, honeycombStrong tendency to check, collapse,

BengeBubinga

honeycomb, and warpMild checks and warpSlow drying with tendency to warp and check

Caribbean pineCativoCuangareDegameDeterma

End splits in thick lumberOccasional collapse in dark streaks in heartwoodBrownheart or wet streaks, collapseSome tendency to warp, surface and end checkSome tendency to warp and check

Ebony, East IndianEbony, AfricanGoncalo alvesGreenheart

Very prone to checksSlight tendency to checkSome tendency to warp and checkSlow drying and quite prone to check and end

Hurasplit

Warp

llomba Fast drying, but prone to collapse, warp, andsplit

lmbuiaJarrahKapurKarri

Thick lumber may honeycomb and collapseProne to checks and collapseMild warp and shakePronounced tendency to check

KempasKeruing

Mild tendency to warp and checkSlow drying with considerable tendency to

Mahogany, AfricanManniMora

check, collapse, and warpSevere warp if tension wood presentModerate warp and checksSome tendency to warp

ObecheOpepeParana pinePeroba rosaRamin

Slight tendency to checkSlow drying with tendency to split, check, and

Slight tendency to warpConsiderable checks and warpDark-colored material prone to split and warpSlight tendency to warpMarked tendency to end split and surface check

Severe checks, warp, and collapseOnly minor checks and warpDries readily with only minor defects

Roble (Quercus)Roble (Tabebuia)Rosewood (Indian)Rosewood (Brazilian) Prone to checkRubberwood Severe warp, prone to blue stain and borer attack

Sande Warp if tension wood presentSanta Maria Tendency to warp and slight surface checkSapele Severe warpSepitur End splitsSucupira Considerable checks and warpWallaba Marked tendency to check, split, and warp:

honeycomb in thick lumber

1Species listed in table 1-2 of chapter 1, but not listed in this table, tend to dryeasily with few drying defects.

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Chapter 9Operating a Dry Kiln

Kiln samples 207Selecting a drying schedule 208

Schedules for homogeneous charges 208Schedules for mixed charges 208

Starting the kiln 209Prestart checks 209Steam-heated kilns 209Direct-fired kilns 210Dehumidification kilns 210

Warmup period 210Spray during warmup 210Time needed for warmup 210

Operating a kiln after warmup 211Reducing heat 211Controlling dry-bulb temperature 211Controlling wet-bulb temperature 211Part-time kiln operation 211

Drying process 212Operation on a moisture content scheduleOperation on a time schedule 212Intermediate moisture content checks 213

Equalizing and conditioning treatments 213Equilibrium moisture content table 213General considerations 213Conditioning temperature 214Conditioning time 214Stress relief at high equilibrium

moisture content 214Moisture content and stress tests 214

Method of testing 214Evaluation of casehardening tests for stress

Modifying kiln schedules 215Cooling a charge after drying 215Operating precautions for safety 215Fire prevention in kilns 216Sources of additional information 216Tables 217

212

A dry kiln, no matter how well equipped with controls,is only as efficient as the operator who runs it. Despiteadvances in control technology that give the operatormore information than was possible in the past, it isstill largely the operator’s judgment that determineswhether a charge of lumber will go through the kiln ina minimum time, emerge uniformly dried to the desiredmoisture content, and be free of undesired drying de-fects and stresses. The operator determines what kilnschedule to use, and whether it should be a time ormoisture content schedule. If kiln samples are to beused, whether manually or automatically weighed ormonitored with probes, the kiln operator is still respon-sible for selecting representative kiln samples. The kilnoperator must monitor the progress of drying, whethermanually or with the assistance of computer readouts,and apply judgment in deciding if adjustments are nec-essary during drying. Also, the operator must applyjudgment in determining when the lumber has reachedfinal moisture content with a minimum of moisture con-tent variation and apply any necessary equalizing orconditioning treatments.

Most of these basic techniques are discussed in otherchapters of this manual. The purpose of this chapter isto summarize and present various aspects of kiln opera-tion to guide the operator in exercising good judgmentin reaching decisions required before and during drying.

215Kiln Samples

When kiln samples are used to control a drying sched-ule, their selection, preparation, and use are importantin kiln operation. Poor procedures here can result inerroneous estimates of moisture content that may in-crease drying defects and drying time. The proceduresdescribed in chapter 6 should be followed as closely aspossible.

Chapter 9 was revised by William T. Simpson,Supervisory Research Forest Products Technologist.

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Selecting a Drying Schedule

One of the first decisions in selecting a dry kiln sched-ule is whether to use a schedule based on moisturecontent or time. This is usually a routine decision be-cause softwood-drying technology has developed timeschedules and hardwood-drying technology has de-veloped moisture content schedules. However, thereare exceptions to this general division of schedules. Ifdrying problems or customer complaints occur withhigh-quality softwood lumber dried by a time schedule,consideration should be given to changing to a mois-ture content schedule. Conversely, repeated experiencewith drying a hardwood species of constant thickness inthe same kiln, particularly one of the easier drying low-density hardwoods, may well lead to the developmentof a time schedule.

Chapter 6 provides guidelines for selecting a kiln sched-ule. They include species, thickness, moisture con-tent, heartwood or sapwood, and grain (quartersawnor flatsawn). Selection of a drying schedule is simplifiedwhen the charge consists of one species, one thickness,a uniform moisture content, all heartwood or all sap-wood, and all quartersawn or flatsawn. The uniformityof variable factors should be maintained as much aspossible for high drying uniformity and quality.

Schedules for Homogeneous Charges

For charges that consist entirely of one class of lumber,a schedule for that class as recommended in chapter 7should be used as a start. After experience is gainedwith that particular class, the schedule can be modifiedas discussed in chapter 7.

Schedules for Mixed Charges

Sometimes it is necessary to dry lumber in mixedcharges, even though this practice is not generally rec-ommended. It reduces the production rate throughkilns and increases the likelihood of variability in thedried lumber. Most mixing is caused by lack of enoughkilns or improper kiln sizing to accommodate differentclasses of drying sorts (groups of sorted lumber). Somemixing is necessary to avoid undue delay in drying sortsthat accumulate slowly and will degrade or stain if leftin green storage too long. In selecting a drying schedulefor a mixed charge of lumber, the drying characteristicsof all the lumber to be included should be considered.Mixed charges can be dried according to moisture con-tent or time schedules. The following examples andsuggestions should be helpful guidelines in selecting kilnschedules for mixed charges.

Example 1: If a charge of lumber is composed of thesame species and moisture content but of varying thick-ness, use the schedule recommended for the thickestlumber. For example, if the kiln charge is both 6/4and 8/4 sugar maple, follow the drying schedule for the8/4 lumber, T5-C2, rather than for the 6/4, T8-C3. Ifthe charge is of 4/4, 5/4, and 6/4 sugar maple, sched-ule T8-C3 could be used. In both cases, the changesin drying conditions would be based on the kiln sam-ples with the highest moisture content, which almostsurely will be the thicker samples. Kiln samples fromthe faster-drying thinner lumber must also be used inorder to equalize properly.

Example 2: If two or more species of the same thick-ness and moisture content are dried together, use theschedule recommended for the species that is the mostdifficult to dry, that is, the slowest or most susceptibleto surface or internal checking. Make every effort tomix species that require much the same drying scheduleand about the same drying time (ch. 7, table 7-33). Forexample, both 4/4 white ash and 4/4 black cherry callfor the same drying schedule, T8-B4. Several specieshave approximately the same drying characteristics.These include 4/4 yellow birch, schedule T8-C4; 4/4black cherry, T8-B4; and 4/4 sugar maple, T8-C3.These species have the sane temperature schedule, T8,but their wet-bulb depression schedules are different.Since the mildest drying condition is recommended, usethe C3 wet-bulb depression schedule.

Example 3: Another example is two species or moreof the same thickness but of varying moisture content,such as a mixture of green 4/4 black cherry and air-dried 4/4 sugar maple with an average moisture con-tent of 25 percent. Green black cherry calls for sched-ule T8-B4 and green sugar maple for schedule T8-C3.The air-dried sugar maple with a moisture content of25 percent calls for initial drying conditions of 150 °Fdry-bulb temperature (step 3 of T8 schedule) and awet-bulb depression of 35 °F (step 5 of C3 schedule),while the T8-B4 schedule for green black cherry callsfor an initial dry-bulb temperature of 130 °F and awet-bulb depression of 7 °F. To avoid damage to thegreen cherry, use the milder T8-B4 schedule.

Example 4: Sapwood of ponderosa pine of Commongrades can be mixed with white fir dimension lumber,provided the white fir does not contain wetwood. Atypical sapwood Common-grade schedule of moderatetemperature and wet-bulb depression should be usedto protect the pine and to equalize the final moisturecontent between the pine and white fir.

Example 5: Heartwood of Common-grade ponderosapine can be mixed with Douglas-fir dimension lumber.

The initial temperature should be 160 °F with a maxi-mum wet-bulb depression of 10 to 15 °F.

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Example 6: 4/4 sugar pine wetwood can be dried with6/4 or 8/4 ponderosa pine Shop or Select. Use as largea wet-bulb depression as the ponderosa will toleratewithout surface checking, but keep the starting temper-ature low enough to prevent brown stain in the sugarpine wetwood.

Example 7: Mill run mixtures of white fir, Englemannspruce, and lodgepole pine can be dried together. How-ever, a moderate schedule should be used to reduce thevariation in final moisture content. Care should beexercised to prevent overdrying.

Example 8: Green Douglas-fir and larch clear lumberdoes not always store well while enough wood to fill akiln charge is being accumulated. Water spray is some-times used to prevent checking during this storage. An-other alternative is to mix the clear lumber with othersorts to avoid long storage while green.

In general, species that have a wide variation in sched-ule requirements should not be mixed for drying.The main concern here is stain and surface checking.Kiln conditions that are humid enough to avoid sur-face checking in some species may cause brown stain,blue stain, or mold to occur in others. For example,Douglas-fir may surface check at the low humidityneeded to prevent brown stain in sugar pine.

These examples serve to illustrate that mixed chargescan be dried successfully, but that caution should beused and that often there is a penalty, such as excessivedrying time, nonuniform final moisture content, and thedanger of drying defects.

Starting the Kiln

The danger of drying defects and excessive drying timecan be reduced if prestart checks and proper startingprocedures are followed. These checks and proceduresvary somewhat with the type of kiln, but all are aimedat ensuring that the equipment is operated properly.

Prestart Checks

Several checks should be made before starting a kiln.

1. Calibration of the wet- and dry-bulb thermometersis not necessary for every charge, but the state of thecalibration check should be kept in mind. If a longtime has passed since the last calibration check or ifprevious performance suggests the possibility thatthe kiln is out of calibration, plans should be madefor a check.

2. A check should be made for adequate steam pres-sure. Is more than one kiln starting up at once?If so, the demand on the steam system may be toogreat to attain desired initial conditions.

3. The air pressure to the recorder-controller should bechecked and any water drained from the air line.

4. Water delivery to the wet-bulb reservoir must be fastenough to keep pace with evaporation. Experiencewill dictate the necessary rate of flow.

5. The wet-bulb wick should be changed if it is dirty orencrusted with mineral scale from the water. Ensurethat the wick does wet.

6. Check that nothing is obstructing airflow over thewet bulb.

7. Consider sir velocity through the lumber. Havechecks in the past indicated a sufficient and uni-form flow? Is there anything different in the currentcharge that could change the flow? Are necessaryend, bottom, or top baffles in place?

8. The vents should be inspected and checked for oper-ation. See that all vents open and close completely.

9. Check the fan operation. See that all fans are turn-ing correctly, no motors are malfunctioning, fansare not spinning on their shafts, and belts are notslipping.

10. Check that doors are in good repair and closetightly.

Steam-Heated Kilns

The following are the general startup procedures forsteam-heated kilns.

1. Set the dry- and wet-bulb controls at the initialtemperatures called for in the schedule.

2. Keep the hand valve on the steam spray line closedduring warmup to avoid excessive steam consump-tion and condensation on the lumber. If there is nohand valve on the steam spray feedline, set the wet-bulb temperature to the lowest temperature possibleto prevent opening the spray line valve. This proce-dure should only be used if it is possible to preventthe vents from opening. Au alternative procedurefor adjusting the wet-bulb control is described initem 11.

3. Implosion can occur in cold climates for up to oneor more hours after startup. To prevent implosion,open the small inspection door or leave the maindoor slightly open before starting the fans. After thefans have operated for several minutes, the door canbe closed.

4. Open the hand valve on the main steam supply line.

5. Open the hand valves on the feedlines to all heatingcoils.

6. Open the hand valves between all the heating coilsand steam traps and in the return lines from thesteam traps to the boiler.

209

7. Open the main air supply valves to the control in-strument and to the air-operated valves on the heatand spray lines. If the control system is electricallyoperated, turn on the power switches.

8. Blow all steam traps to the atmosphere for a shorttime to remove scale and dirt from them.

9. Just before the dry-bulb temperature reaches setpoint, open the hand valve on the steam spray lineor reset the wet-bulb temperature to the recom-mended wet-bulb temperature.

10. If the kiln is equipped with auxiliary vents, keepthem closed during warmup until the wet-bulbtemperature reaches set point.

11. Sometimes when warming up a kiln charge of greenlumber susceptible to surface checking, the wet-bulbtemperature can be brought up gradually ratherthan according to the procedures in items 2 and 9.For example, if the initial drying conditions call for awet-bulb depression of 4 °F, this temperature can beapproximated during warmup by opening the handvalve on the steam spray line for short periods orby gradually raising the wet-bulb indicator if it wasinitially set at a low value. This procedure requiresfrequent monitoring of conditions during warmup.

Direct-Fired Kilns

Direct-fired kilns vary in type of burner and air deliv-ery system, and many do not have a source of steamfor humidification. The starting procedures do not dif-fer much from those of a steam-heated kiln and can besummarized as follows:

1.

2.

3.

4.

5.

Set the dry-bulb temperature, and wet-bulb temper-ature if the kiln is equipped with spray lines, at theinitial set point or points called for in the schedule.

To prevent implosion, open the small inspection dooror leave the main door slightly open before turningon the fans. After the fans have operated for a fewminutes, the doors should be closed.

Start the burner system according to the manufac-turer’s procedures.

If the kiln is equipped with auxiliary vents, keepthem closed during warmup.

If the kiln is equipped with steam or water spray,keep the spray shut off until the dry-bulb tempera-ture has almost reached set point. In warming up acharge of lumber susceptible to checking, the proce-dure outlined for steam-heated kilns can be followed.

Dehumidification Kilns

Startup procedures for dehumidification kilns may de-pend on the particular manufacturer’s recommenda-tions. Certainly, the status of the heating and refriger-ation systems should be checked for proper operation.Auxiliary heat is often added in a dehumidification kilnto reach set point. After that, it is turned off and theheat from the compressor is sufficient to maintain dry-ing temperature in many cases.

Warmup Period

Spray During Warmup

Both heat and steam spray are sometimes used in thewarmup period. This procedure will reduce to someextent the time required for warmup, but the potentialproblems may more than offset the gain in time.

When both the heating and spray systems are on dur-ing warmup, a large quantity of steam is used. Thesteam consumption may exceed the boiler capacityand thereby affect the drying conditions in other kilnsalready in operation.

When the steam spray is on, moisture condensing onthe cold lumber, cold kiln walls, ceiling, and othermetal parts will have several effects. Condensation doesnot allow much drying during warmup, and in fact thelumber will usually pick up moisture. Condensationcan cause water stain on the lumber and contribute tocorrosion of kiln parts. Another danger is the effect onpartially dried lumber that may contain some surfacechecking. Rewetting the surface will usually widen anddeepen surface checks.

Time Needed for Warmup

The time required for warmup depends on many factorsand can vary from 1 to 24 h. Warmup time is length-ened if (1) lumber and kiln structure temperaturesare low, (2) lumber is frozen, (3) temperature of theoutside air is low, (4) initial moisture content of thelumber is high, (5) lumber is thick, (6) density of thespecies is high, (7) heat losses through the kiln wallsand roof are high, (8) seals around closed vents anddoors are poor, (9) some heating coils are inactive, and(10) boiler output is too low.

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Operating a Kiln After Warmup

Reducing Heat

About 1 h after the kiln has reached set point, theheating system can be cut back. In direct-fired kilns,the rate of firing can be reduced. In steam-heated kilns,the amount of heat-transfer surface area, steam pres-sure, or both can be reduced. Surface area is decreasedby closing valves in the feed and drain lines of someheating coils, and steam pressure is reduced by ad-justing the steam pressure regulator. The usual pro-cedure in reducing radiation surface area is to cut offthe larger heating coils first and gradually work downto the smallest coil that will maintain the desired dry-bulb temperature. This procedure should be followedunless past experience has shown how much radiationis required to maintain the desired temperature. Expe-rience will establish the best combinations of coils andsteam pressure for given situations, and these can benoted for future reference.

Controlling Dry-Bulb Temperature

Variations in dry-bulb temperature on the entering-airside of the loads are a major source of poor control ofdrying conditions. These variations are sometimes asso-ciated with faulty kiln design or poor trap maintenance(chs. 2 and 4). The most common cause of tempera-ture variation is excessive heat-transfer area, that is,too many active coils. Excessive coil heat-transfer areacan result in large temperature cycles and waterloggingor air binding of the active heating coils, and these inturn can cause excessive temperature variations alongthe length of coils. To reduce these effects, the smallestamount of radiation and the lowest steam pressure nec-essary to maintain the desired dry-bulb temperature atany stage of drying should be used.

If the division of coils is not fine enough to have thecorrect amount of heat transfer area and the steampressure cannot be regulated, the kiln may have to beoperated at a dry-bulb temperature slightly lower thandesired to obtain a nearly constant flow of steam. Inthat event, the wet-bulb temperature will also have tobe adjusted downward to obtain the desired wet-bulbdepression.

Controlling Wet-Bulb Temperature

Poor control of wet-bulb temperature is usually as-sociated with inadequate kiln maintenance (ch. 4).Quite often, however, the use of a high-pressure steamspray causes wide variations in both the dry- and wet-bulb temperatures. The use of wet, low-pressure steamshould overcome this difficulty. If the reduction in pres-sure does not have the desired effect, desuperheaters

may have to be installed on the steam spray line. Theflow of water should not be excessive and may be con-trolled by a needle valve. To make this possible, thewater pressure must be greater than the steam pres-sure. Ordinarily, water is used to saturate the steamspray only during equalizing and conditioning, or dur-ing the early stages of drying a species that requires alow initial dry-bulb temperature with a small wet-bulbdepression.

Proper venting is also required to obtain good controlof wet-bulb temperature. Such control is attained bygood maintenance and operation of the vent system(ch. 4). Excessive venting will add steam consump-tion and favor the development of drying defects. Onthe other hand, operating the kiln for extended periodswith insufficient venting and at wet-bulb temperaturesabove those called for in the schedule will prolong dry-ing time and favor the development of stain.

Direct-fired kilns, particularly as employed in soft-wood drying, often have no steam spray lines, and theonly means of controlling the wet-bulb temperature isthrough venting. Wet-bulb temperature control oftenis not as critical here as in hardwood drying, and formany species the drying rate is fast enough that ventscan adequately control wet-bulb temperature. Also, attemperatures very much above the boiling point, vent-ing often is not necessary because equilibrium moisturecontent is low at these high temperatures. The mainproblem occurs if equalizing or conditioning is desiredbecause direct-fired kilns are not capable of raisingthe wet-bulb temperature to high levels at the end ofdrying when very little water is evaporating from thelumber.

Some hardwood schedules call for low wet-bulb temper-atures at certain stages in the schedule. Au exampleof this is the schedule for 4/4 red oak (T4-D2). Whenthe lumber reaches 30 percent moisture content, therecommended wet-bulb temperature is 90 °F. In thesoutheastern part of the United States, the wet-bulbtemperature of the outside air may be near that tem-perature, and either excessive or continuous ventingwill occur as the control system attempts to reach thewet-bulb temperature. When this occurs, the wet-bulbset point must be raised to a level that the control sys-tem can achieve. The dry-bulb temperature should notbe raised above that called for in the schedule.

Part-Time Kiln Operation

Kilns are usually operated full time in industrial prac-tice; drying is uninterrupted from the start to the finishof the process. However, some plants, particularly sec-ondary producers, operate kilns part time. In part-timeoperation, the kiln may be shut down during certainhours in order to take advantage of savings in labor,power, or fuel costs.

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Most species, particularly when air dried, can be driedin a part-time kiln successfully. However, equalizingand conditioning treatments usually require full-timeoperation to be effective. For green hardwoods, red-wood, and cedars that are susceptible to surface check-ing, part-time operation during the initial stages mayresult in checking because of the more rapid drop in thewet-bulb compared to dry-bulb temperature during theoff period. Therefore, for these species, operate the kilnon a full-time basis until the danger of surface checkingis past. The vents should be kept closed during the offperiod to reduce heat losses.

Drying Process

After the kiln has been started, the lumber is driedaccording to the schedule selected from chapter 7.Chapter 7 deals mainly with the mechanics of selectingschedules, and the purpose of this section is to discussthe operational aspects of kiln schedules.

Operation on a Moisture Content Schedule

A moisture content schedule requires changes in dryingconditions based on the average moisture content of thecontrolling kiln samples (ch. 6). Operation on a mois-ture content schedule is best illustrated by examples.

Example 1: A charge of 4/4 sugar maple is to be kilndried from green moisture content. In the example wewill use six kiln samples, and the average moisture con-tent of the three wettest samples will be used to controlkiln conditions. The drying schedule is given in table9-1, and the schedule is applied in the following way.

Because the lumber is green, the initial moisture con-tent will be above 40 percent. Therefore, the initialdrying conditions will be those of step 1 for moisturecontent above 40 percent.

Subsequent changes in drying conditions are madewhen the average moisture content of the controllingsamples reaches the value given in the schedule. For ex-ample, when the average moisture content of the threewettest samples is less than 30 percent but more than25 percent, the dry-bulb temperature is 140 °F and thewet-bulb temperature is 121 °F. Because the dryingrate of the kiln samples may vary from day to day, thesame three samples may not be the wettest during allstages of drying. Therefore, the moisture content of allthe samples in the charge should be determined eachtime they are weighed or sensed with a probe. The laststep in the schedule is maintained until the desired finalmoisture content is reached.

In manual control, the controlling kiln samples occa-sionally lose more moisture between weighings than theinterval given in the schedule. When this occurs, a stepin the schedule can be skipped. For example, if the kilnis operating at 130 and 119 °F dry- and wet-bulb tem-peratures, respectively, and the next weighing indicatesthat the average moisture content of the controllingkiln samples is 24 percent, the drying conditions shouldbe set at 150 and 115 °F dry- and wet-bulb tempera-tures, respectively, rather than at 140 and 121 °F. Insome instances, even two steps can be skipped.

As soon as the final moisture content is reached, thekiln is shut off unless equalizing and conditioning treat-ments are required.

Example 2: A charge of partially air-dried 4/4 sugarmaple is to be kiln dried. Eight kiln samples are used.Therefore, drying conditions will be governed by theaverage moisture content of the four wettest samples.The drying schedule will be the same as that used inexample 1, and the procedure is as follows:

If the initial moisture content of the four wettest sam-ples averages more than 40 percent, the initial dryingconditions will be those listed for this moisture content.Subsequent drying procedures will be the same as forexample 1.

If the average moisture content of the four wettest sam-ples is 34 percent, the initial drying conditions will be130 and 119 °F dry- and wet-bulb temperatures, re-spectively. Subsequent drying conditions will be asgiven in the schedule.

If, however, the lumber has regained moisture just be-fore entering the kiln, modify the drying procedure toconform to the recommendations given for air-driedhardwoods in chapter 7.

Operation on a Time Schedule

In a time schedule, drying conditions are changed atpredetermined times. No kiln samples are used, and thetimed changes are based on experience.

Example 1: A time schedule for 8/4 white fir dimen-sion lumber is shown in table 9-2. The kiln is startedat 180 and 170 °F dry- and wet-bulb temperatures, re-spectively; after 12 h the change to step 2 of 180 and165 °F dry- and wet-bulb temperatures, respectively, ismade. After step 2, the change to step 3 is made after36 h, and to step 4 after 60 h. Step 4 is held until a to-tal time of 96 h has elapsed since the start of drying.At this time, the lumber is expected to be approaching

2 1 2

the target moisture content of approximately 15 per-cent for softwood dimension lumber. The decision toterminate drying should be based on whatever criteriaare being used and whether or not the lumber is readyto be removed from the kiln.

Example 2: A time schedule for lower grade 4/4 whitefir is shown in table 9-3. It differs from the schedule intable 9-2 only in the expected time required to reacha final moisture content of approximately 15 percent.Thus, the check for final moisture content should bemade at 84 h rather than 96 h.

Example 3: A time schedule for upper grade 4/4 whitefir is shown in table 9-4. This schedule is milder thanthe schedule for lower grade lumber given in table 9-3;that is, the initial dry-bulb temperature and the wet-bulb depression are lower. The time intervals are alsodifferent, and more total time is allowed before the finalmoisture content is determined. This not only reflectsthe milder drying schedule but also the likelihood thatthe upper grade lumber may have a lower target mois-ture content than the lower grade lumber.

Final moisture contents are often estimated by moisturemeter readings taken inside the kiln. Any required tem-perature correction factors should be applied. Alterna-tively, kiln samples may be used, especially with uppergrade softwood lumber, to help establish the durationof the final step in the schedule.

Intermediate Moisture Content Checks

Near the final stage of drying high-quality lumber, par-ticularly hardwoods, the moisture content should beknown within fairly close limits. Otherwise, the actualfinal moisture content will not be the same as the de-sired final moisture content. Furthermore, equalizingand conditioning treatments will not be effective if themoisture content of the kiln samples is not an accu-rate estimate of the moisture content of the lumber.Intermediate moisture content checks are discussed inchapter 6, and they are often used in the final stages ofdrying to correct the moisture content estimates. Inter-mediate checks are sometimes made on a routine basis,but if they are not, certain danger signals indicate thatsuch tests should be made. If the moisture content ofone or more kiln samples is suspiciously different frommost or if the rate of change of moisture content duringdrying seems quite different, intermediate checks shouldbe made.

Equalizing and ConditioningTreatments

Good moisture uniformity and stress-free lumber canbe obtained by equalizing and conditioning treatmentsdescribed in chapter 7. The following discussion ex-pands on that of chapter 7 and will be helpful in apply-ing the treatments.

Equilibrium Moisture Content Table

To apply the equalizing and conditioning procedures,an operator must know how to determine the wet-bulb depression needed to give the required equilibriummoisture content (EMC) condition. Equilibrium mois-ture content values are given in chapter 1, table 1-6.In the example presented here, however, the use of thistable is the reverse of the explanation given in chapter1. Assume, for example, that a dry kiln is operatingat a dry-bulb temperature of 170 °F, and the operatorwants to know the wet-bulb temperature required toobtain an EMC of 6 percent. The dry-bulb tempera-ture of 170 °F is found in the left column of table 1-6.In the row to the right of this temperature, the EMC of6 percent is found in the column indicating a wet-bulbdepression of 29 °F. Therefore, to obtain an EMC of6 percent at a dry-bulb temperature of 170 °F, a wet-bulb temperature of 170 °F minus 29 °F, or 141 °F,would be used.

General Considerations

1. The recommended procedures for equalizing andconditioning a charge of lumber will produce goodresults in a kiln that is performing satisfactorily, butit is important that the control instruments are incalibration. If poor calibration causes the wet-bulbdepression in the kiln to be different than the rec-ommended setting, the EMC condition in the kilnwill not be correct, and the treatments may not beentirely effective.

2. An equalizing treatment is not necessary if the driestand wettest kiln samples at the end of the dryingprocess have moisture contents within an acceptablerange.

3. Some operators prefer drying the driest samples inthe kiln to a moisture content 1 percent below thevalue recommended in table 7-30 (ch. 7) before start-ing equalization. This may reduce equalizing timeand might even eliminate the need for equalizing.

4. If the recommended EMC value for conditioning ata specific temperature cannot be found in table 1-6,use the next highest value given in the table for thattemperature. For example, conditioning a charge oflumber at 170 °F with an EMC condition of 11 per-cent is required. Referring to table 1-6, no wet-bulb

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depression is given for an EMC condition of 11 per-cent at a temperature of 170 °F. Use the next high-est value—11.3 percent. The wet-bulb depression forthe 11.3 percent EMC condition is 10 °F.

Conditioning Temperature

The higher the dry-bulb temperature used in condi-tioning, the faster the relief of casehardening. Gener-ally, the required conditioning EMC can be obtainedat a dry-bulb temperature of about 180 °F in mostwell-maintained kilns operated on low steam pressureor equipped with a desuperheater on the spray lineor auxiliary water sprays. Sometimes it is impossible,however, to obtain the required high EMC conditionsat a temperature as high as desired.

If the required EMC cannot be obtained at a dry-bulbtemperature of about 180 °F, the temperature will haveto be reduced. In such instances, lower the setting onthe control instrument 12 to 24 h before conditioningis started. For example, assume the kiln is operatingat a dry-bulb temperature of 180 °F, and the temper-ature must be reduced to 170 °F to obtain the desiredEMC for conditioning. Twelve to twenty-four hours be-fore conditioning is started, the dry-bulb temperatureshould be reset to 170 °F.

Conditioning Time

High dry-bulb temperatures coupled with high EMCconditions hasten deterioration of dry kiln buildingsand metal in the kiln. Also, large amounts of steamare required for conditioning. Therefore, conditioningshould not be extended any longer than is necessaryto relieve drying stresses (chs. 6 and 7). Condition-ing time depends on the degree of stress in the lumber;lumber species, thickness, and moisture content; andkiln performance. It may vary from 4 h for 1-in-thicksoftwoods to 48 h or more for thick, high-density hard-woods, The minimum time required is determined bymaking casehardening (prong) tests at times when itis believed that stresses are nearly relieved. Recordedresults of these tests will establish good estimates forrequired times in future charges. The casehardeningtest is described in chapter 6.

When air-dried lumber is kiln dried, the conditioningtime varies from charge to charge because the degreeof drying stress in the air-dried lumber varies. Casehardening tests made on air-dried lumber at the timekiln samples are prepared will give an estimate of theamount of stress present and thus a rough indication ofthe amount of conditioning required.

Stress Relief at High EquilibriumMoisture Content

To reduce the time required for conditioning, some kilnoperators use an EMC higher than that recommended.This approach may be satisfactory if conditioning is notcontinued for too long. If it is, reverse casehardening,which is as serious as casehardening, will result. Nosatisfactory method of relieving reverse casehardeninghas been established. In many instances the use of veryhigh EMC values during conditioning gives only super-ficial relief of drying stresses. Therefore, to obtain goodconditioning without incurring risk of reverse casehard-ening, conditioning should be done at the recommendedconditions (ch. 7).

Moisture Content and Stress Tests

Kiln samples are generally used for final moisture con-tent and casehardening tests to make sure that thelumber is at the desired final moisture content andis free of drying stresses. Other boards from the kilncharge can also be used, provided they are representa-tive of the charge.

Method of Testing

To properly interpret the reaction of stress test sec-tions, certain information about the final moisture con-tent and moisture gradient is required. The methodof cutting sections for such tests is given in chapter 6.One section should be weighed immediately after cut-ting, ovendried, reweighed, and the moisture contentcalculated. This calculation will give the average mois-ture content of the kiln sample or board from which itwas cut. If this test is made immediately after the con-ditioning treatment, the moisture content obtained willbe about 1 to 1-1/2 percent higher than before condi-tioning because the surface will have regained moistureduring conditioning. If, however, the test is made afterthe lumber has cooled for about 24 h, in most cases theregained moisture will have evaporated.

A second section should be cut as shown in chapter 6,figure 6-3, to obtain two outer shells, each with a thick-ness of about one-fourth the total thickness of the lum-ber and a core of about one-half the total thickness.The core and shell are weighed separately as quickly aspossible after cutting, ovendried, reweighed, and theirmoisture contents calculated. A third section should becut into prongs, as described in chapter 6, for the stresstest.

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Evaluation of Casehardening Tests for Stress

If the prongs of the stress section turn out to only aslight degree immediately after sawing, the lumber canbe considered stress free, and conditioning can be ter-minated. If, however, the prongs remain straight orpinch in, continue conditioning. The amount of addi-tional time required depends on the amount of move-ment of the prongs, and some experience is necessarybefore judgments can be made. If the prongs move inonly slightly, only a few more hours of conditioningmay be required. If they move so far as to cross, theremainder of the day or overnight may be required forconditioning.

Whether or not to continue conditioning must be de-cided fairly soon after the casehardening test is made;however, all the information required to make that de-cision is not always immediately available. The prongsmay continue to react over a period of time after cut-ting, and unless a microwave oven is available andthe exact drying procedure worked out, the averagemoisture content and moisture content of the shelland core are not known for 24 h. Observation of theprong movement after 24 h as well as the moisture con-tent values at this time will provide useful informationfor future charges. The following conditions may beobserved after 24 h:

1. If the prongs do not move in significantly imme-diately after cutting, if they do not move any fur-ther after 24 h, and if the moisture content valuesof the shell and core are within about 1 percent ofeach other, then the lumber is well equalized andconditioned.

2. If the prongs do not move in immediately after cut-ting but do after a period of time, the core moisturecontent is probably greater than the shell moisturecontent. Longer equalization is then required.

3. If the prongs do not move in immediately after cut-ting and do so after standing but the shell and coretest does not indicate that the core is at a highermoisture content than the shell, then the condition-ing period should be lengthened.

Modifying Kiln Schedules

A major cause of excessive drying defects or dryingtime is to blindly follow a recommended kiln schedulethat has not been proven for specific circumstances. Noschedule will produce the best drying results on a spe-cific item or species in all types of kilns under all typesof conditions. The schedules recommended in chap-ter 7 are generally conservative, and they are meantto be a starting point for modification to an optimumlevel. Before modifications can be made, the recom-mended schedule should be tried. Information for jus-

tifying schedules can be obtained by observing (1) thetype and severity of drying defects, their time of oc-currence, and their effect on degrade or volume loss,(2) the drying time required, and (3) the final moisturecontent. Systematic procedures for modifying schedulesare given in chapter 7.

Cooling a Charge After Drying

After lumber has been kiln dried, it is usually cooledbefore machining. Sometimes, if kiln demand is low,the charge is cooled in the kiln. More often, the chargeis removed from the kiln and held in a protected cool-ing area at the dry end of the kiln. Lumber dried to alow moisture content should not be stored outdoors orexposed to high humidity for extended periods becauseit will regain moisture (ch. 10). Sometimes crackingnoises are heard as lumber cools. This is usually causedby movement between the stickers and the lumber asthermal contraction occurs and is not the result of thelumber actually cracking or splitting.

Operating Precautions for Safety

Working in or around dry kilns is not hazardous if or-dinary precautions are taken. As is the case with mostmachinery and equipment, carelessness is the majorcause of accidents, which can be serious or fatal in andaround kilns. Care should always be exercised aroundhigh-temperature burners, steam plants and steamlines, and operating fans, belts, and shafts. The follow-ing rules and precautions will help prevent accidents.

1. Do not touch the outside surfaces of kilns operatingat high temperature because they can be danger-ously hot-particularly any part that allows continu-ous metal conduction from inside to outside.

2. Shut off the heat, spray, and fans before entering anoperating kiln. If the kiln has been operating at hightemperatures, it should be cooled to a safe level byopening doors and vents before entering.

3. Never enter a kiln in use without the knowledge of acoworker-preferably someone who remains close tothe kiln in case assistance is needed.

4. When an access or main door is opened, stand awayfrom the door. Hot and often humid air rushes outand can be an uncomfortable or dangerous shock,particularly if breathed.

5. Occasionally it may be necessary to enter a kilnwhen the heat, spray, or fans are operating in orderto check their operation. In this case, another personshould be stationed immediately outside the kiln toensure the safety of the person inside.

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6. Never enter a kiln when the wet-bulb temperatureis 120 °F or more without wearing protective cloth-ing that covers the head and body. This tempera-ture limit applies to people in good health. Anyonewith heart or respiratory problems should not enterkilns where the wet-bulb temperature is 110°F ormore. The critical dry-bulb temperature depends onthe individual. In any case, no one should enter orremain in a kiln if either the temperature or the hu-midity makes the person feel ill or more than mildlyuncomfortable.

7. Equip all small access doors with a latch that can beoperated from both sides. Repair faulty latches im-mediately. Never use props to hold a door closed-there is always the possibility that such proppingcould inadvertently trap someone in the kiln. Setup an emergency signal that can be used if someoneis accidentally trapped inside. A signal rapped onsteam pipes will carry a considerable distance.

8. Provide sufficient lighting in kilns to provide safemovement for anyone who enters the kiln. As anadded precaution, a portable light should be carried.

9. A person should not attempt to open or close kilndoors that are too heavy for a single person.

10. Door carriers should be kept in good repair to guardagainst a door jumping the track.

11. Fans should obviously be shut off when inspectedclosely or lubricated, but precautions should thenbe taken to ensure that the fans are not inadver-tently started at these times. If the fan switch is notequipped with a lock in the off position, a sign “DoNot Start Fans” should be placed at the switch.

12. Fan floors are often oily, and precautions should betaken to prevent slipping.

13. Shafts and pulleys should be adequately guarded.

14. When truckloads of lumber are loaded and unloaded,devise a system for workers to know each other’swhereabouts so that no one gets crushed betweentrucks.

15. When loaded kiln trucks are moved by cables, pro-cedures should be established to ensure that allworkers stay clear of the cables when they are un-der tension.

Fire Prevention in Kilns

Fires in dry kilns we usually caused by carelessness,poor maintenance, or poor housekeeping. Precautionsfor minimizing the possibility of fire are as follows:

1. In direct-fired kilns fueled by wood residue, ensurethat the burner is operated according to manufac-turer’s instructions so that live embers do not enterthe kiln.

2. Do not allow smoking in a kiln.

3. Use care with welding or cutting torches.

4. Keep electrical circuits in good repair.

5. Keep all moving parts well lubricated. A hot bearingcan cause a fire.

6. Do not allow uninsulated steam pipes to contactflammable material.

7. Keep the kiln and surrounding area free of excessdebris.

Kilns should be checked regularly outside of regularworking hours so that if a fire starts, it can be foughtpromptly. A definite procedure should be establishedfor workers to follow if a kiln fire should occur. Thefollowing procedures may extinguish the fire or willreduce the spread of fire in a kiln until a firefightingcrew arrives.

1. Install a water sprinkler system and check its opera-tion regularly.

2. Have fire extinguishers available in the area.

3. Keep all kiln doors closed.

4. Close the ventilators.

5. Shut off the fans.

6. In a steam-heated kiln, saturate the air in the kilnwith steam. If there is a bypass around the steamspray control, open that valve or, if not, set the wet-bulb control point as high as possible.

Sources of Additional Information

Boone, R. S.; Kozlik, C. J.; Bois, P. J.; Wengert, E. M.1988. Dry kiln schedules for commercial woods tem-perate and tropical. Gen. Tech. Rep. FPL-GTR-57.Madison, WI: U.S. Department of Agriculture, ForestService, Forest Products Laboratory. 158 p.

Knight, E. 1970. Kiln drying western softwoods.Moore, OR: Moore Dry Kiln Company of Oregon.77 p. (Out of print.)

McMilIen, J. M.; Wengert, E. M. 1978. Drying east-ern hardwood lumber. Agric. Handb. 528. Washing-ton, DC: U.S. Department of Agriculture. 104 p.

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Table 9-1—Moisture content schedule for 4/4 sugar maple(T8-C3)

step

EquilibriumMoisture Temperature (°F) moisture Relativecontent content humidity

(percent) Dry-bulb Wet-bulb (percent) (percent)

Table 9-2—Time schedule for 8/4 white fir dimension lumber

EquilibriumTemperature (°F) moisture Relative

content humiditystep Time(h) Dry-bulb Wet-bulb (percent) (percent)

Table 9-3—Time schedule for 4/4 white fir, lower grade

EquilibriumTemperature (°F) moisture Relative

content humiditystep Time(h) Dry-bulb Wet-bulb (percent) (percent)

Table 9-4—Time schedule for 4/4 white fir, upper grade

EquilibriumTemperature (°F) moisture Relative

content humiditystep Time (h) Dry-bulb Wet-bulb (percent) (percent)

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Chapter 10Log and Lumber Storage

Log storage 220 Kiln drying is only one step in the harvesting, handling,Dry storage 220 and processing of wood products. The best results can

Logs with bark 221 be obtained in kiln drying, therefore, when adequateDebarked logs 222 attention is paid to related phases of wood processing.Transpiration drying 222 Although a dry kiln operator may have no responsi-

Wet storage 223 bility for these related phases, knowledge of them isPond storage 223 required to understand how they interact with dry-Water sprinkling 224 ing. Problems that occur in drying, or that are erro-

Effects of climate on lumber storage 225 neously blamed on drying, are sometimes related to theRelative humidity 225 methods used to store logs and lumber before dryingTemperature 225 and those used to store kiln-dried lumber and finishedRainfall 225 products.Average equilibrium moisture content

conditions by region and season 225Lumber storage 225

Outdoor storage 225Green lumber 226Partly dried lumber 226Kiln-dried lumber 226Pile covers 228

Open shed storage 228Green lumber 228Partly dried lumber 228Kiln-dried lumber 228

Closed, unheated shed storage 229Green lumber 229Partly dried lumber 229Kiln-dried lumber 229

Closed, heated shed storage 230Green lumber 230Partly dried lumber 230Kiln-dried lumber 230

Conditioned storage sheds 230Treating stored lumber 230

When is chemical treatment needed? 231When and where to apply treatment 232How to apply treatment 232

Treating area and equipment 232Dipping operation 233Treating for insect control 233

Precautions for handling chemicals 233Lumber handling and storage in transit 234

Truck transport 234Rail transport 235Ship transport 235

Literature cited 236Sources of additional information 236Tables 237

Logs and lumber go through various storage and trans-port periods while moving through the processing se-quence. Log storage and transit really begin when thetree is felled and continue until the log is sawed intolumber. Similarly, lumber storage and transit includethe time between sawing and drying and the time be-tween drying and end use. The moisture content oflumber should be controlled in storage and transit.Large increases in moisture content during storage maymake lumber unsuitable or out of specifications formany uses, cause lumber to warp, or cause the devel-opment of stain or decay. Large decreases in moisturecontent may cause checks and warp to occur or makemachining and fastening difficult.

Chapter 10 was revised by William T. Simpson,Supervisory Research Forest Products Technologist,and James C. Ward, Research ForestProducts Technologist.

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Figure 10-1—Splits in black cherry millwork from lumber that was sawn from a wind-damaged tree. (M88 0170)

Log Storage

The source of some lumber drying problems can betraced to changes in the wood that began in the treejust before or during the timber harvesting operation.Logs that have been salvaged from forests that weredamaged by hurricanes or tornadoes may yield lum-ber that is likely to split during drying and subsequentmachining (fig. 10-1). Felling the tree with a clippingor shearing machine can initiate radial splits and ringfailures in the end of the log, which may lengthen con-siderably with lumber drying.

After felling, the main stem of the tree is detached fromthe crown, except when transpiration drying is desired.At most commercial logging operations in North Amer-ica, the main stem of the felled tree is either left fulllength (tree-length log) or cut (bucked) into shorterlogs with lengths that correspond to lengths of the in-tended lumber. Tree-length and standard-length logsshould be sawed into lumber as soon as possible af-ter felling, especially during warm weather. However,prompt sawing of logs is not always possible because

of log transportation difficulties or the economic needto stockpile logs at the sawmill. This section suggestsmethods for reducing drying defects that result fromprolonged storage of logs.

Logs need to be stored under conditions that will min-imize defects associated with shrinkage, mainly endchecking, and attacks by fungi, bacteria, and insects.Defects associated with shrinkage are minimal duringperiods of cloudy, wet weather and low temperatures.Fungi and insects are inactive at temperatures below32 °F or under conditions of wet storage with low levelsof oxygen. On the other hand, many types of bacteriacan grow in wood under wet, anaerobic conditions, butnot at subfreezing temperatures. There are two generalmethods for storing logs: dry storage and wet storage.Precautions must be taken with each storage method toensure defect-free lumber.

Dry Storage

Most sawlogs in North America are stored under dryconditions with the bark intact. Occasionally, kiln

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Figure 10-2—Splits in the end of a red oak log resultingfrom ruptures caused by an imbalance in tree growthstresses after felling. (M88 0169)

operators may encounter logs from diseased or insect-damaged trees where most or all of the bark has fallenoff.

Because lumber from logs subjected to transpirationdrying may show up in the drying operation, this sub-ject will be discussed as a part of dry storage.

Logs With Bark

Most lumber that needs to be kiln dried will be sawedfrom logs that were stored on land with the bark intact.If the logs do not contain wetwood, then any lumberdrying problems will usually be associated with seriousend checking of the logs, insect attack, and sapwoodstains.

End checks can occur in all species of logs and aremore pronounced in the denser hardwoods. Deep endsplits can sometimes occur in the log ends, but theseare the result of residual tree growth stresses that be-come unbalanced after the log is bucked, and they can-not be prevented by measures for reducing end check-ing (fig. 10-2). End checks are minimized by keepingthe log ends in cool, moist, and shaded locations. Ifthe logs are valuable and cannot be sawed into lum-ber within a short time, then the ends should be coatedwith a suitable end-sealing compound (fig. 10-3). Theend coating should be thick enough to cover all woodpores, cracks, and irregularities on the surface, yetviscous enough so that it neither cracks nor “sags”excessively. It is good practice to treat the log endswith chemical fungicide before end coating to preventsapwood staining.

Figure 10-3—Oak logs 8 months after they were cutand the ends treated with preservatives. All but twologs were also end sealed; no end checking developed inthese logs. The preservative treatment of the unsealed

logs (topmost and lower left) was of little value oncethe barrier of the surface-treated wood was ruptured byseasoning checks. (M 81288).

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Figure 10-4—Sweetgum logs with heavy sapwood stainat the ends. Under conditions favorable for staining,end stain may appear within 2 weeks and the discol-oration may penetrate into the log as rapidly as 1 ftper month. (M 38236)

Fungal blue stain will develop in the sapwood ofexposed log ends and debarked surfaces duringwarm weather within 2 weeks after the tree is felled(fig. 10-4). Applying or spraying chemical fungicideson all exposed log surfaces will provide adequate pro-tection if the wood does not check or split. Thesechemically treated areas should then be coated witha log end-sealing compound to prevent checking andthe opening of untreated inner wood to fungal attack.Since wood-boring insects can carry spores and hyphaeof sapwood-staining fungi into the logs, even throughareas with attached bark, logs may need to be sprayedwith a mixture of chemicals that control both insectsand fungi.

Figure 10-5—Fungal blue stain and chemical brownstain in sapwood and wetwood of a kiln-dried easternwhite pine board sawed from a log stored during earlyspring on a log deck in the forest. (M88 0168)

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Chemical changes will occur in moist sapwood duringlog storage that may cause chemical discolorations dur-ing subsequent drying. These discolorations can varyfrom gray, yellow, and pinkish to deep brown. Chem-ical stains are likely to occur in lumber from logs thatwere stored under moist, shaded conditions for the pur-pose of preventing end checking. Brown stain and bluestain will develop together in lumber from logs thatwere stored in the forest or similarly shaded locations(fig. 10-5). Prompt sawing of freshly cut logs is the eas-iest way to control chemical sapwood stains becausetreating the logs with fungicides that prevent blue stainwill not be effective.

The most effective method for controlling chemicalstains is to freeze freshly cut sapwood. This can bedone economically only by sawing winter-cut logs innorthern climates during cold weather and using properkiln-drying conditions. At some northern mills, shortlogs of birch, maple, and pine that are to be sawed intospecialty products are frozen to ensure white color.The short logs from winter-cut timber are placed inground depressions and sprayed with water to form acoating of ice. The frozen log decks are covered withsawdust, wood shavings, or other available insulatingmaterial so that the wood remains frozen well into thesummer months.

Debarked Logs

Most logs intended for lumber are debarked on the dayof sawing; the problems associated with the storing ofdebarked logs are thus not carried over into the dry-ing operation. Logs intended for poles and pulpwoodare debarked soon after felling to reduce losses from in-sect borers and decay and to lower sapwood moisturecontent. The disadvantages of early debarking are ex-tensive surface checking and end splitting. Sapwoodstaining can also be quite substantial.

Transpiration Drying

After a tree is cut or girdled, the main stem will losemore moisture if the crown is left attached than if thestem is bucked into logs. This method of drying iscalled transpiration drying. Teak trees in the forestsof southeast Asia are girdled and left standing for atleast a year before felling so that the logs will be lightenough to float to the sawmills via the river systems.In North America, there is some interest in transpira-tion drying because of its potential application to woodenergy production. If tree-length logs are stored in thewoods for a short time, leaving the crown attached alsoseems to provide some protection from ambrosia beetleattack.

The amount of moisture lost depends upon viable fo-liage in the crown and the amount of sapwood in thestem. Softwoods will undergo transpiration dryingthroughout the year if winter temperatures are notbelow freezing, but deciduous hardwoods can only bedried during the summer when the leaves are present.During transpiration drying, oak logs with narrowrings of sapwood will not lose much more than 10 per-cent moisture content whereas sweetgum and yellow-poplar, species with wide bands of sapwood, can loseover 30 percent moisture content. The maximum mois-ture loss from hardwoods will occur in 1 to 2 weeks,but this period will usually be longer for conifers. Onthe west coast of Washington, Douglas-fir will undergoa maximum moisture loss of 30 to 50 percent in about90 days.

In South Africa and Holland, Visser and Vermaas(1986) found that transpiration drying of both hard-wood and softwood trees resulted in total energy sav-ings because of easier handling of green lumber andreduced kiln-drying times. A mass loss of 30 percentafter 1 month of transpirational drying resulted in anenergy saving of approximately 55 percent in the kiln,while a maas loss of 10 percent after 1 week of dryingresulted in an energy saving of approximately 25 per-cent. These authors also noted that the sudden drop inmoisture content with transpirational drying helps tosuppress the development of blue stain in South Africantimber.

Wet Storage

When logs must be stored for a long time at temper-atures above freezing, it is desirable (when possible)to keep them soaking wet. This prevents drying andchecking and inhibits attacks by insects and sapwood-stain fungi. However, some types of bacteria are notinhibited, and the wood may become predisposed todeveloping chemical stains.

Pond Storage

Pond storage includes logs that are stored in lakes,rivers, and salt water estuaries as well as mill ponds.Although pond storage was once a regular practice, it isnow rare in North American mills. Nevertheless, a drykiln operator may receive lumber from logs that havebeen submerged in water. Considerable volumes of logsare rafted from woods to mills along the coast of thePacific Northwest. Foreign lumber is frequently sawedfrom pond-stored logs, and some lumber is salvagedfrom old submerged logs and timber.

Pond-stored logs are usually banded together to in-crease the log-holding capacity of the pond and to pre-vent wetwood (sinker) logs from sinking to the bottom(fig. 10-6). Some logs in the bundle will be above wa-ter and are subject to insect attack, stain, and decay.Until recent Environmental Protection Agency (EPA)

Figure 10-6—Logs banded together in a log pond in merged while others are entirely out of the water.northern California. Some logs are completely sub- (M88 0167)

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Figure 10-7—Water sprinkling of decked hardwood logs. A fine mist effectively covers log surfaces andends. (M 144876).

prohibitions, these types of damage were controlled forseveral weeks by spraying the exposed parts of the logbundles with insecticides and fungicides. Logs raftedand stored in ocean water are also subject to attack bymarine borers and salt water micro-organisms.

Most damage to submerged logs can be traced togrowth of bacteria in the sapwood. In softwoods, pitmembranes in the sapwood are destroyed so that thewood becomes more permeable, and the wood will drysomewhat faster. However, the lumber will also overab-sorb chemicals used to stabilize and preserve the wood,and finishing can be a problem. Honeycomb, ring fail-ure, and collapse are likely to develop in lumber fromlogs and timber that have been submerged for over ayear. Chemical brown stain has been a frequent prob-lem with the drying of ponderosa pine and sugar pinelumber from pond-stored logs. In rare situations, theiron content of the water is unusually high, and woodsgradually acquire a grayish color because of an iron-tannate reaction.

Water Sprinkling

Where log decking is a preferred manner of storage,sprinkling the decks with water provides an effectivemethod for reducing checking, sapwood stains, and de-cay when temperatures are above freezing (fig. 10-7).Sprinkling will not provide certain protection from in-

sect attack although it tends to be more effective thandry log storage in some localities. Nevertheless, thebeneficial effects of using water sprays during warmweather have been reported for western softwoods andeastern hardwoods, especially in the South.

For sprinkling to be effective, the log ends and ex-posed, debarked wood surfaces must be kept contin-uously wet during the entire period of storage. Thisprevents shrinkage and checking of the exposed wood.Water sprays reduce temperatures in and around thelog decks, but the reduction of oxygen from continuoussoaking of the wood is the major deterrent to sapwood-staining and decay fungi.

Bacteria and slime molds, less common in dry-storedlogs, may develop extensively in sprayed logs. Bac-teria can be responsible for chemical stains and in-creased porosity in lumber from wet-stored logs, butthese problems are greater in pond-stored logs thanin logs stored on sprinkled decks. Bacterial problemswith sprinkling can be prevented by not drawing thewater from stagnant reservoirs where drainage fromthe wetted logs is returned and recycled. Under wa-ter sprays, bacteria from wetwood zones in the log mayextend their growth into the sapwood, which will thendevelop brown stains during drying.

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Water sprinkling requires constant maintenance toguard against clogging of hoses and spray nozzles fromdebris and slime in the water. Adequate drainage mustbe provided in the log yard to prevent handling prob-lems with forklift vehicles.

Effects of ClimateOn Lumber Storage

Relative humidity, air temperature, and rainfall of thestorage region are the main factors that determine therate and amount of moisture content change in thelumber and the procedures necessary to protect lum-ber stored outdoors or in unheated sheds.

Relative Humidity

Relative humidity has a much greater effect on woodequilibrium moisture content (EMC) than does temper-ature (table 1-6). The more humid a region, the moremoisture the lumber will absorb and the more rapid therate of absorption. Seasonal estimates of the averagewood EMC for a region can be helpful when trying tocontrol moisture change in lumber stored outdoors.

Storage methods to retain low moisture content in kiln-dried lumber will differ between humid regions like thegulf coast and dry regions like the Southwest. Likewise,storage requirements may differ from month to monthin regions where average relative humidity varies con-siderably with the season, such as inland California.

Temperature

Air temperature has a minor effect on EMC (table 1-6),but its main effect is on the rate of moisture contentchange. Moisture content changes occur faster at warmtemperatures than at cool temperatures. Therefore, iflumber has to be stored at EMC conditions differentthan the moisture content of the lumber, the tempera-ture should be taken into consideration. Some moistureequalization can be effected in storage; the warmer thetemperature, the faster the rate of equalization.

Warm temperatures also increase the hazard of fungalinfection in stored lumber. All lumber is practicallyimmune to fungal infection below 30 °F. When greenlumber is solid piled, mold, stain, and decay fungi willgrow at temperatures from 40°F to 100 °F with the rateof attack increasing rapidly at higher temperatures inthis range. Dipping or spraying freshly sawed lumberwith an approved fungicide reduces the chance of fungalgrowth.

Rainfall

When lumber is protected while stored outdoors, rain-fall does not greatly affect its moisture content. Solid-piled green lumber is often unprotected while temporar-ily stored outdoors before stacking for air or kiln dry-ing. Some wetting of green lumber is not consideredhazardous. If, however, green lumber has been treatedwith a fungicide for extended green storage or ship-ment, protection from rain is needed to prevent leach-ing of the chemicals.

Solid-piled dry lumber should be protected from rain,preferably in storage sheds. Redrying solid-piled lum-ber that has been wetted by rain is difficult. Solid-piledlumber that has been thoroughly soaked requires stick-ering before it is redried, and redrying may result indrying losses. Also, if rain increases the moisture con-tent of the lumber to 20 percent or more, fungi maygrow and cause stain and decay.

Average Equilibrium Moisture ContentConditions by Region and Season

Estimated monthly wood EMC conditions at variouslocations throughout the United States are given in ta-ble 10-1. They represent average values from climato-logical data and thus may vary from year to year. Also,EMC conditions are often influenced by microclimateswithin regions, so more localized values can be deter-mined from local weather stations.

The Southwestern States are generally the driest re-gions, and the coastal regions, the wettest. Duringsummer months, the states west of the MississippiRiver are much drier than during the spring months.East of the Mississippi, the summer months are slightlymore humid than the spring. Fall is usually more hu-mid than spring or summer in most of the UnitedStates, and winter is generally even more humid.

Lumber Storage

Lumber storage can be classified into five major types:outdoors, open shed, closed and unheated ‘shed, closedand heated shed, and conditioned shed. The desirabletype of storage depends on the moisture content of thelumber and the weather conditions during storage.

Outdoor Storage

Lumber is often stored outdoors because shed or ware-house facilities are not available. Unprotected outdoorstorage is satisfactory for small timbers and lumber forless exacting end uses, although precautions to preventstain, decay, and insect infestation may be necessary.

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Kiln-dried lumber stored outdoors without protectionwill have a rapid increase in moisture content.

Protection against rain is more important for solid-piled lumber than for stickered lumber because rain-water cannot evaporate readily from solid piles. Fur-thermore, rain that penetrates solid-piled lumber mayin time increase the moisture content enough that stainand decay can grow. Storage areas should be open, welldrained, and kept free of weeds and debris that restrictair movement along the surface of the ground, harborfungi and insects, and create a hazard when dry. Theground, particularly along runways for lumber-handlingequipment, should be surfaced with gravel, crushedrock, asphalt, or concrete. Surfacing or paving per-mits vehicles to operate efficiently in all weather andrestricts weed growth. The method of piling for out-door storage depends on the species involved, its mois-ture content, and the degree of drying desired duringthe storage period.

Green Lumber

Green lumber dries during storage. To reduce dryingdefects and kiln-drying time as much as possible, theprinciples of good air-drying practice should be fol-lowed (Reitz and Page 1971). Briefly, these include(1) stacking the lumber properly with dry stickersspaced correctly so as to minimize warp, (2) provid-ing good pile foundations, (3) laying out the yard withadequate spacing between piles and rows of piles, and(4) providing good pile roofs.

If green lumber must be stored in solid piles for morethan 24 h in warm weather, it should be dipped in anapproved antistain solution. Green lumber properlystacked and protected on a good site will lose moisturerapidly with a minimum of defects and can remain out-doors indefinitely without excessive deterioration.

Figure 10-8—High-grade Douglas-fir stored temporarilyunder water spray while the mill accumulates enoughfor a full kiln load. (M88 0165)

Sometimes high-quality green lumber is stored tem-porarily under water spray (fig. 10-8), while lumber isbeing accumulated for a kiln load.

Partly Dried Lumber

If the moisture content of lumber is above 20 percentor if further drying is desired, the lumber can be storedlike green lumber. Lumber that is below 20 percentmoisture content can be solid piled if no additionaldrying is desired. The piles should be fully protectedagainst infiltration of rainwater. Water that penetratesa solid lumber pile is not readily evaporated and islikely to cause stain or decay. Lumber surfaces thatare alternately wetted and dried are likely to check.

Kiln-Dried Lumber

Lumber kiln dried to a moisture content of 12 percentor less can be stored outdoors in dry weather in stick-ered or solid piles for a short time. Extended storagewill result in excessive moisture regain. Figure 10-9shows the change in moisture content of southern pineduring yard and shed storage in solid piles in inlandLouisiana. If the lumber had been piled on stickers, itsmoisture content would have risen to the maximum ofabout 13-1/2 percent in a much shorter time. Duringthe warm, dry season in areas such as the arid South-west and in parts of Idaho, Montana, Nevada, Oregon,and Washington, the outside storage period can be ex-tended considerably without serious effects if pile coversare used.

Kiln-dried lumber can and often is afforded temporaryprotection, particularly in transit, by wrapping in vari-ous types of coated paper. Such wrap for unit packagesof lumber (fig. 10-10) will adequately protect kiln-driedsoftwood lumber under short-term storage conditionssuch as long-haul transport on flatcars, interim stor-age at distribution centers, and short-term outdoorstorage at construction sites. However, coated paperwrappings should not be considered a substitute forstorage sheds when long-term storage of dried lumber isinvolved. The lumber could deteriorate during storageand is susceptible to tearing during handling. If suchstorage is unavoidable, the protective wrap should beinspected periodically for tears or other deterioration.Water that enters packages through tears in the pro-tective wrapping can collect and cause more regain ofmoisture than if no wrap were used. To avoid trappingwater in torn packages, the bottom is often left open.However, moisture from ground water can enter pack-ages if not enough ground clearance is provided by thepile foundations.

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Figure 10-9—Change in average moisture content ofkiln-dried southern pine 1- by 4-in flooring and 1- by8-in boards during storage in solid piles within sheds

and in a yard with a protective roof over each pile.(ML88 5557)

Figure10-10—Covering packages of lumber with wa- the package bottom, and thus will not be damaged byterproof kraft paper wrap. The wrap does not cover forklift handling nor will it trap rainwater. (M 120954)

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Figure 10-11—Stickered lumber yarded for air drying.The well-braced pile foundations of stringers and cross-beams prevent tipping. Most piles are covered with aprefabricated board and batten roof (M 134963)

Pile Covers

High-grade lumber stored in a yard, whether solidpiled or stickered, green or dried, should be protectedfrom the weather. Lumber surfaces exposed to alter-nate wetting and drying will check, warp, and discolor.Stacks of lumber in storage yards can be provided withpile covers the same as are used in air-drying yards(fig. 10-11).

Open Shed Storage

Open sheds provide excellent protection for green andpartially dried lumber. Lumber that has been kilndried to a low moisture content can also be stored inopen sheds for varying periods, depending on weatherconditions.

An open shed is a roofed lumber storage yard. Lum-ber dried to moisture contents as low as 12 to 14 per-cent can be stored in open sheds without significant re-gain of moisture. The atmospheric conditions within anopen shed are the same as those outdoors except thatlumber is protected from direct contact with rain andsun. A shed may be open on all sides or on one sideonly (fig. 10-12). Often the side facing the prevailingwinds can be closed to keep out driving rain.

The shed should be located on an open, well-drainedarea. It should be large enough to permit rapid han-dling of the lumber and have a floor of gravel, crushedrock, blacktop, or concrete firm enough to supportthe piles of lumber and the weight of lumber-handlingequipment. The roof should overhang far enough be-yond the piles of lumber to protect them from drivingrain and snow.

Figure 16-12—Open storage for packages of dry, sur-faced lumber. (M88 0164)

Green Lumber

Green lumber can be stored for long times in opensheds without danger of serious deterioration, providedit is stickered. Such sheds protect the lumber from thesun, rain, and snow, thereby keeping end and surfacechecks and splits to a minimum. To obtain good airdrying in open sheds, adequate spaces should be pro-vided between the sides and ends of the stacks. By al-lowing this free circulation of outdoor air, lumber willdry to as low a moisture content as it does in the openair. The drying time in an open shed is usually shorterand the lumber brighter than if stored outdoors be-cause rewetting is avoided.

Partly Dried Lumber

Open sheds afford excellent protection to partly driedlumber. If the moisture content is above 20 percent,the lumber should be stacked on dry stickers. If it isbelow 20 percent, it can be solid piled unless furtherdrying is desired, in which case it should be stickered.

Kiln-Dried Lumber

Kiln-dried lumber can be well protected from sun, rain,and melting snow when stored in open sheds. An openshed will not, however, prevent regain of moisture dur-ing periods of high humidity, especially if temperaturesare also high. Therefore, storage time should be lim-ited during warm, humid weather. Lumber piles canbe either solid or stickered. Solid-piled lumber will re-gain moisture more slowly than stickered lumber. In-crease in moisture content will be greatest at the endsand in the outer tiers of a solid pile, as illustrated infigure 10-13. The effect of long-term storage in an openshed on moisture content of solid-piled, kiln-dried lum-ber is also shown in figure 10-9.

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Figure 10-13—Change in average moisture content ofsolid-piled, surfaced 1- by 8-in Doughs-fir boards storedin an open shed. (ML88 5556)

Figure 10-14—Closed, unheated storage shed at a dis-tributing yard. (M88 0166)

Closed, Unheated Shed Storage

Closed, unheated sheds (fig. 10-14) are generally usedfor storing kiln-dried lumber, although they also canbe used for storing green or partly dried lumber. Thistype of shed should be provided with reasonably tight-fitting doors. Ventilators are sometimes provided, andtheir need depends on the moisture content of thestored lumber and the tightness of the building.

Green Lumber

Green lumber is sometimes stored in closed sheds, al-though this type of storage will retard drying. The dry-ing can be retarded enough that the growth of mold be-comes a problem. Some drying capability can be addedto closed-shed storage by exhaust vents and circulationfans. The solar heat that is absorbed through the roofand walls of a shed will provide some energy for drying.Care should be exercised for species that are suscepti-ble to surface checking. If air circulation is inadequate,the temperature near the roof will rise and could causesurface checking.

Partly Dried Lumber

Partly dried lumber that is properly piled can be storedin a closed shed without developing drying defects.Lumber should be stickered if it has a moisture con-tent greater than 20 percent. If below 20 percent andno further drying is desired, lumber can be solid piled.If further drying is desired, the lumber should be stick-ered, and it may be advantageous to add fans to circu-late air through the lumber. High shed temperaturesfrom solar energy generally will not cause checking orsplitting in partly dried lumber because these defectsusually occur when moisture contents are higher.

Kiln-Dried Lumber

The object of storing kiln-dried lumber in closed shedsis to minimize pickup of moisture. Thus, lumber shouldbe solid piled. Although kiln-dried lumber will regainsome moisture during periods of high relative humidity,the percentage regained will be less than if the lumberwere stored outdoors. In dry regions, kiln-dried lumbercan be stored indefinitely during hot, dry weather.

The ultimate moisture content lumber will reach ina closed shed depends on the local weather. If sunnyweather prevails, the roof and walls of the shed will ab-sorb solar radiation and heat the air inside. This lowersthe relative humidity in the shed and thus the EMCconditions. Prolonged periods of sunshine can thusresult in low moisture contents. Conversely, if cloudyweather prevails, moisture contents will not be muchlower than in an open shed.

Lumber dried to a moisture content of 10 percent orless, and items manufactured from it, will regain mois-ture if stored for extended periods under conditions ofhigh relative humidity. Excessive regain of moisture fre-quently results in (1) swelling of whole pieces or of cer-tain parts, such as the ends of the pieces, (2) warpingof items such as glued panels, and (3) wood or gluelinefailures in solid-piled items where the moisture regain isconfined to the ends.

During fabrication and use, lumber and items that haveadsorbed excessive moisture during storage may (1) endcheck and split when the high-moisture-content sur-faces are exposed to low relative humidities in heatedbuildings, (2) shrink excessively, (3) warp, (4) suffer ex-tension of end splits, and (5) open at glue joints.

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Closed, Heated Shed Storage

If air in a shed is heated, the relative humidity andEMC are lowered as long as no additional moisture isadded to the air. Thus, storage in closed, heated shedsprovides excellent protection in preventing kiln-driedlumber from regaining moisture. Lumber for use in fi-nal products such as furniture and millwork that willbe used in a heated environment should be stored inheated sheds. A heated shed should be reasonably tightand can be insulated or uninsulated. Heat can be sup-plied by any convenient means as long as the systemcan maintain up to 30 °F above outside temperatures.Circulation is desirable to maintain uniform tempera-ture. Ventilators are generally not necessary but shouldbe provided if any drying is anticipated. Temperaturecan be controlled by a simple thermostat that regulatesthe heating system.

The shed should be located on a well-drained site. Itsfloor should be of gravel, crushed rock, asphalt, or con-crete, and it should be sufficiently firm to support pilesof lumber.

Green Lumber

Green lumber is not ordinarily stored in heated shedsbecause the higher temperatures within the shed maycause end and surface checks or splits. If drying in aheated shed is considered, predryers should be used, asdescribed in chapter 2.

Partly Dried Lumber

Partly dried lumber can be stored in heated sheds forfurther drying. Stickering and ventilating are necessary.If further drying is not desired, lumber should be storedin open or unheated sheds because it will dry further ina heated shed.

Kiln-Dried Lumber

Closed, heated sheds are ideal for storing lumber kilndried to 12 percent moisture content or less. The de-sired EMC of the lumber can be regulated simply byincreasing the temperature in the shed by a certainamount over the outside temperature. This can bedone with thermostats that measure temperature dif-ferentials. When outside air is heated without addingmoisture, even though the absolute humidity remainsthe same, the relative humidity decreases and thus theEMC decreases. The outside temperature and relativehumidity must be known to determine the amount bywhich the temperature in the shed must be increasedto attain a certain EMC. For example, if the outsideair is at a temperature of 50 °F and is at 80 percentrelative humidity, how much must the temperature inthe shed be raised to attain an EMC of 6 percent? The

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answer can be determined by using figure 10-15. Enterthe graph along the arrows that lead from 50 °F and80 percent relative humidity to the point where they in-tersect. Note that this is at an EMC of about 16.5 per-cent and an absolute humidity of about 0.0625 poundof water per pound of dry air (at a barometric pressureof 29.92 in Hg). Since no moisture is being added to theair in the shed, the absolute humidity will remain thesame as we raise the temperature. Therefore, followthe arrowed line down parallel to the absolute humid-ity lines to the point where it intersects the 6 percentEMC line. From this point drop straight down to thetemperature axis and read the required temperature inthe shed as 80 °F or a 30 °F temperature rise.

An alternative way to control conditions in a heatedshed is to control the heater with a humidistat. Whenthe relative humidity is above the set point of the hu-midistat, the heater will be on until the relative hu-midity falls to set point. For example, we know fromtable 1-6 of chapter 1 and figure 10-15 that to maintainan EMC of 6 percent, the relative humidity should becontrolled at about 30 percent.

Conditioned Storage Sheds

Kiln-dried lumber and finished products can also beheld at any desired moisture content in storage by con-trolling both relative humidity and temperature. Thisis the most costly method of controlling EMC becauseof the equipment involved. However, when it is desir-able or necessary to maintain temperature within cer-tain limits, then it may not be possible to maintain rel-ative humidity simply by manipulating temperature.For example, to attain 6 percent EMC when the out-side air is at a temperature of 85 °F and a relative hu-midity of 80 percent, the temperature must be raisedto 114 °F. This temperature is unreasonable in a workarea where people must spend any length of time. Inthis case, refrigeration equipment is required to attain6 percent EMC at a comfortable temperature.

Treating Stored Lumber

Fungal infection and insect attack both pose serioushazards to stored lumber. Fungal infection was foundto be the principal cause of degrade in a study of gradeloss in l-in southern pine lumber. Insect infestationalso causes serious losses in stored lumber, particularlyin the warmer parts of the United States. For protec-tion from fungi and insects, lumber may require a dipor spray treatment in a chemical solution at the storageinstallation. In some cases, this treatment will supple-ment an earlier dip or spray at the sawmill.

Figure 10-15—Psychrometric chart showing the rela-tionship between temperature, relative humidity, abso-lute humidity, and equilibrium moisture content (EMC)of wood at a barometric pressure of 29.92 in Hg. The

chart and arrowed lines illustrate the temperature riserequired to attain 6 percent EMC by heating outsideair originally at 50 °F and 80 percent relative humidity.(ML88 5558)

To minimize fungal and insect attacks on stored lum-ber, air-drying yards should be kept sanitary and asopen as possible to air circulation. Recommended prac-tice includes locating yards and sheds on well-drainedground. Remove debris, which is a source of infection,and weeds, which reduce air circulation. Piling meth-ods should permit rapid drying of the lumber and alsoprotect against wetting.

Open sheds should be well maintained, with an ampleroof overhang to prevent wetting from rain. In areaswhere termites or water-conducting fungi may be trou-blesome, stock to be held for long periods should beset on foundations high enough to be inspected frombeneath.

When Is Chemical Treatment Needed?

Prompt drying will often protect untreated lumberfrom attack by stain, decay, and some insects. For in-stance, untreated lumber uniformly below 20 percentmoisture content is immune to attack by fungi. Withprotective storage it will keep that immunity. However,dried lumber that regains moisture to a level of morethan 20 percent again becomes susceptible to stain anddecay.

The sapwood of all wood species is more susceptiblethan heartwood to decay, stain, or insects. Therefore,the hazards are highest for woods that usually con-tain a high percentage of sapwood. The heartwood ofsuch species as redwood, the cedars, and some whiteoaks has high natural resistance to fungi and most in-sects. But few products--even from these woods-areof heartwood only.

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Damp weather can increase the damage from stain anddecay fungi. Rainfall and humid conditions increasethe hazard to unprotected wood in both open and solidpiles.

Air temperature is highly important. The stain and de-cay fungi grow most rapidly at 70 to 90 °F, grow nomore than one-fifth as rapidly at 50 to 60 °F, and ceasegrowth at about 32 °F. As a result, wood at about25 to 30 percent moisture content, stored in solid pilesin warm weather, may show evidences of stain withina week and early decay infection within a month. Theinitial infections, which are not visible, probably startedshortly after the wood was sawed. With temperaturesof 50 to 60 °F, similar deterioration requires five ormore times as long. At 32 °F or below, the lumber canremain in solid piles indefinitely without adverse effects.

High humidity favors subterranean termites but doesnot affect drywood termites or powder-post beetles.The influence of temperature on insect activity, how-ever, is pronounced. Insects are inactive at temper-atures of 50 °F or below but increase their activityrapidly as the temperature rises above this level. In-sects will approximately double their activity witheach increase of 10 ° above 50 °F, reaching maximumactivity levels at about 80 °F.

When and Where to Apply Treatment

Stain and decay in lumber are normally controlled atsawmills, collection points, and drying yards by dryingthe wood as rapidly as possible below 20 percent mois-ture content. Lumber to be air dried may be treatedwith fungicidal solution by dip or spray before the dry-ing period begins. Sometimes an insecticide is mixedinto the solution if insects are likely to be a problem.

The layer of wood chemically protected by a dip orspray is only “skin deep” and will not stop fungi or in-sects that have already entered the wood. This is whystock is dipped as soon as possible after it is sawed.To illustrate how quickly the dipping must be accom-plished, the safe times are estimated as follows: 1 dayat temperatures of 80 °F or above; 2 days at 70 °F;1 week at 60 °F; and 1 month at 50 °F. Longer delaysat these temperatures progressively lower the benefitfrom surface treatments.

Generally, dip or spray treatments immediately aftercutting are designed to protect green stock only whenit is drying. If treated green lumber is not air dried tobelow 20 percent moisture content, prolonged storagemay require redipping or respraying of the lumber.

Lumber properly dipped in an antistain solution at thesawmill can be stored in solid piles for up to 1 monthin warm weather if further drying is not required. If

longer bulk storage is anticipated, dip-treated stockshould be redipped. Additional dipping can protectpines and hardwoods from stain and decay for 6 to8 weeks in warm weather and western softwoods otherthan pines for 4 to 6 months.

If the lumber was not dipped at the sawmill, dippingat the storage yard may still protect it from fungi dur-ing bulk storage provided the stock is not already in-fected. Infection would not occur if daytime temper-atures in the interval between sawing and receipt atthe yard did not exceed about 40 °F. If temperatureswere higher, however, fungus infection may have al-ready taken place, and solid piling should be avoided.Instead, lumber may be dipped in a fungicidal solutionand open piled.

Because a number of factors affect safe storage time,all dipped bundles should be labeled with the date onwhich they were treated. Representative bundles shouldbe opened from time to time to determine the condi-tion of the stock. Any lumber that shows signs of beinginadequately protected should be designated for earlyuse, redipped, or stickered for air drying.

How to Apply Treatment

Lumber to be dipped at the storage installation willprobably be in unit packages. Thus, the dipping pro-cedures explained here are for unit packages. Whenlumber is dipped, the amount of solution absorbed willbe about 4 to 8 percent of the wood weight, dependingon type of wood and moisture content at the time oftreatment.

Treating Area and Equipment

Location of the treating plant affects the costs and ef-ficiency of the treating operation. Ready access of theplant to packaging and storage areas-and to railroadspurs or shipping docks-will keep costs to a minimumand ensure an efficient handling operation.

Equipment for treating lumber often includes an elec-tric hoist that runs on a monorail attached to the ridgeof a long, open shed. The treating vat can be installedin or above the ground but should be located in thecenter of the shed. This leaves protected areas in bothends of the shed where untreated packages can bebrought in or the treated packages loaded out. Deador electrically operated rollers are often used in bothends of the shed.

The vat should be sufficiently large to admit the largestunit package to be dipped and should hold sufficient so-lution to treat a number of packages without replenish-ment. Provision also should be made for easily addingand removing the treating solution. A well-designed vat

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is about 1-1/2 times the height and width of the largestpackage to be dipped and about 3 ft longer. A drain-board wide enough to accommodate several packagesshould be provided at the removal side of the vat tofree the hoist for continuous treating.

Some type of hold-down device, such as weights or aheavy iron cradle, is required to keep the packages sub-merged in the solution. Weights should be attached tothe pallet that supports the packages (not to the load)in such a way as to compress the packages against thevat bottom. In fact, the boards should be compressedagainst one another as little as possible to allow thetreating solution to penetrate between them.

The vat should be supplied from a mixing tank ofknown capacity. This tank shall hold extra treatingsolution, which can be prepared without interruptingtreating operations. Steam or electrical heating coilsare a desirable supplement to the mixing tank to ensurethat chemicals dissolve rapidly and completely.

Dipping Operation

Packages of lumber should be submerged in the protec-tive solution for at least 5 min and for up to 15 min iflong storage periods are expected. Packages treatedin a waterborne solution should be turned on edgewith the board faces parallel to the sides of the vat.This can be done as the packages are placed in the vat.Packages treated with an oilborne solution need notbe turned entirely on edge during treatment. However,some means should be provided to tilt the bundles asthey are immersed to let air escape from the voids andto allow solution to flow in.

Packages removed from the treating solution should bedrained for a sufficient time to recover most runoff. Adrainage period at least as long as the treating periodusually will be adequate.

Treating for Insect Control

All insects that cause damage to sound (nondecayed)lumber during storage will be either beetles or termites.Wood-destroying beetles cause annual damage amount-

ing to $50 million in hardwood lumber and secondarymanufactured products such as flooring, furniture, andmillwork. Losses from termites can be much higher, al-though most of this loss occurs in wood in buildings;nevertheless, termites can damage lumber stored forsome time in contact with the soil. Treatment may beneeded to control insect damage in both dry and greenwood, regardless of the wetness or dryness of the stor-age location.

The principal beetles that attack stored wood vary intheir need for moisture. Ambrosia or pinhole beetles in-

vade green or partially dried wood but usually are onlya minor hazard in lumber stored away from forestedareas or sawmills. The destructive golden buprestidbeetle lays its eggs in western softwood trees, preferablyDouglas-fir, but viable eggs and wood-boring larvae canpersist for as long as 15 to 20 years in air-dried lumberthat was not kiln dried.

Among the most troublesome and damaging insects tostored lumber are those belonging to the true powder-post beetle group because they infest wood after it isdry. These insects chiefly attack partially dried sap-wood and are particularly damaging to such large-pored hardwood species as oak, ash, hickory, walnut,and pecan.

The other principal insect that might attack storedlumber is the termite. There are two general types oftermites: subterranean and drywood. Practically allwoods are susceptible to their attack. Subterraneantermites are by far the most prevalent type in theUnited States. They must have contact with somesource of moisture, almost always the ground. Drywoodtermites occur only in limited areas along the gulf andPacific southwest coasts, particularly in Florida andsouthern California. Drywood termites and powder-post beetles are the only insects that primarily attackdry wood.

Properly applied treatments that are commerciallyavailable generally provide protection to stored lum-ber against powder-post beetles and termites. Environ-mentally safe boron compounds such as boric acid andborax are toxic to many wood-destroying insects andhave been successfully used in the wood industries ofAustralia and New Zealand for over 40 years. Lumberis immersed for 1 min in a borate solution and storedunder cover for 7 days. Storage permits the borate tothoroughly diffuse through and penetrate the wood andensures excellent protection from damage by powder-post beetles. There is also considerable protection fromdamage by termites and brown-rot decay fungi.

It is important to realize that the dip treatments de-scribed here apply only to the protection of lumber instorage. Preservation of wood for use requires differenttypes of solutions and methods of application.

For wood that might be treated only because of thedanger of subterranean termites, a more efficientmethod of protecting the lumber is to treat the groundunder the storage piles or sheds.

Precautions for Handling Chemicals

All treating solutions should be so handled that none,or as little as possible, gets on the skin and clothing ofworkers. In particular, contact of the skin with the drychemicals should be avoided.

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When lumber dipped in water solutions is to bepainted, sufficient time must be allowed during stor-age or before painting to allow the wood to dry ade-quately. Only rather short drying periods will be nec-essary to remove excess moisture resulting from treat-ment with waterborne chemicals because dipping orspraying results in only a small increase in moisturecontent. Residual oil should be cleaned from any dryhardwood lumber to be painted.

Lumber Handling andStorage in Transit

If carelessly shipped, dry lumber can regain enoughmoisture to require redrying, and green lumber canstain or decay. Such waste is totally unnecessary. Withproper transport procedures, even kiln-dried lumbercan cross the United States or be shipped to foreignports without any appreciable loss of quality.

Lumber moves from sawmills to locations of end useeither directly or through wholesale and retail lumber-yards. Softwoods are usually shipped as finished lum-ber. Hardwoods more often move from the sawmill tothe woodworking plant as rough lumber, although kilndrying and surfacing may take place in transit. Coastalsawmills ship lumber by ocean-going vessels to domesticand foreign ports.

Present-day lumber shipments are usually unitized formechanical handling. The strapped unit-handling pack-ages are loaded by forklift into wide-door railroad box-cars, onto flatcars, and into trucks. Ocean-going vesselsare loaded by ship gear.

Generally, when l-in dry softwood lumber is shipped intightly closed boxcars, in enclosed trucks, or in pack-ages with complete and intact wrappers, average mois-ture content changes can generally be held to 0.2 per-cent per month or less. In holds of ships, dry materialusually absorbs about 1.5 percent moisture during nor-mal shipping periods. If green material is included inthe cargo, the moisture regain of the dry lumber maybe doubled. (On deck, the moisture regain may be asmuch as 7 percent. However, dried lumber is seldomstowed on deck.)

Precautions are also necessary in shipping green lumberby truck or open flatcar. Air flowing over unprotectedgreen lumber as it moves along a highway or rail causesuncontrolled surface drying that may result in severesurface checks. This is especially likely to occur withoak, maple, or beech. Green lumber of these speciesshould be covered with a tarp or reinforced paper toprevent this uncontrolled surface drying.

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Truck Transport

Considerable quantities of air-dried lumber are shippedby truck from sawmills to factories or custom kiln-drying plants. Tractor-trailer units are usually usedfor this purpose, and in most instances the trailer is aflatbed unit that can be loaded and unloaded by lifttruck. The lumber is anchored to the trailer by chainstightened with load binders.

Few data are available on moisture changes duringtruck shipment. Time in transit is short, seldom ex-ceeding a week even on longer hauls, so little change inthe lumber’s moisture content would be expected fromatmospheric humidity. Many lumber-hauling truckshave flatbeds that are fully enclosed with canvas cov-erings over skeleton frameworks. When kiln-dried lum-ber is transported in these covered trucks during cold,moist weather, the outer boards will gain 3 to 7 percentmoisture content in their outer shell. This moisture up-take can develop within a week, and such boards willgive a casehardened reaction even though they wereproperly conditioned during kiln drying. Trucked lum-ber can also be wet by rain or splashed road water.

High-value, air-dried lumber is often protected bycovering the load with canvas tarpaulins (fig. 10-16).Lower grade lumber is seldom protected at all, espe-cially on short hauls. Some protection is recommendedduring truck transport within a wet or moist climatezone during wet periods. Precautions should also betaken when a shipment will cross several climate or ele-vation zones.

Figure 10-16—Packages of kiln-dried hardwood lum-ber on a truck trailer are covered with a tarpaulin.(M 142893)

Rail Transport

Some years ago, the Forest Products Laboratory stud-ied the changes in moisture content of softwood lum-ber shipped in tight railroad boxcars from West Coastsawmills to midwestern U.S. markets. These stud-ies involved five boxcar loads of l-in clear Douglas-firshipped from a West Coast sawmill to the Chicago,IL, area during late winter and spring. The time inrail transit averaged 18.5 days; the shortest periodwas 14 days and the longest, 22 days. Average mois-ture content of the five carloads of kiln-dried boardsat the time of loading was 8 percent, and the aver-age gain in moisture content was 0.2 percent. Thesevalues were baaed on an average of 18 teat boards dis-tributed throughout the boxcar load in each shipment.In another study, test boards in a carload of Douglas-firquarter-round and crown molding, which were at 8 per-cent moisture content when loaded, regained 0.8 per-cent in moisture in a 20-day transit period from theWest Coast to the vicinity of Chicago. Thus, no signif-icant change in moisture content of dry lumber need beexpected during the usual haul in tight boxcars.

A study of moisture changes in rail shipments of kiln-dried hardwood lumber was conducted by the ForestProducts Laboratory. These shipments were of kiln-dried pecan lumber, transported in wide-door boxcarsfrom midsouth Mississippi to a furniture company inNorth Carolina, a distance of about 900 mi. Each loadof unitized lumber packages contained four test boardsfor moisture analysis. Test shipments were made fromJune through November, and the increase in moisturecontent was less than 0.5 percent moisture content.Conventional flatcars have become widely used for thetransport of dried lumber because they can (1) savehandling time and shipping cost, (2) hold twice theload of conventional boxcars, and (3) be loaded by lifttrucks to save handling time. Improvements in unitizedpackage wrapping have made it possible to obtain theseadvantages without much increase in moisture content,even on long hauls.

Unitized packages on flatcars are usually protected, ei-ther partially by tarpaulins or entirely by flexible, wa-terproof packaging that completely “tailor-wraps” eachpackage. One common type of waterproof packaginguses composite kraft paper that is reinforced with glassfiber coated with polymer. The packaging is frequentlysupplied with additional reinforcement at stress pointssuch as edges and corners. Improvements in packagingmaterials have made possible the shipment of kiln-driedlumber with little change in moisture content and agood retention of brightness.

Wrapping for unitized packages of lumber should befree from rips to be effective. Rain that enters throughrips is held by the sheeting, and the package may act asa humidifier. If so, moisture regain may be higher thanif the lumber were unprotected.

Ship Transport

Lumber is often transported overseas in ships while itis either green, partly dried, or kiln dried. A study con-ducted in Canada, which involved 33 shipments of l-inlumber from the Canadian west coast to five differentports, concluded that seasoned lumber stored belowdecks, either by itself or together with green lumber,will not undergo moisture regain of serious propor-tions (table 10-2). This study also indicates that well-dried lumber may undergo significant moisture regain ifstored on deck, although it is not commonly stored inthis way.

Similar tests were made with 2-in Douglas-fir lumber.The kiln-dried lumber had a moisture content of 9 to10 percent when stowed. The overall average moisturegains for the seasoned 2-in lumber were as follows:

Lumber stowed below deckswith dry lumber . . . . . . . 1.3 percent

Lumber stowed below deckswith green lumber . . . . . . 2.4 percent

Lumber stowed on deck withgreen lumber . . . . . . . . . 4.2 percent

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Literature Cited

Nielson, R. W.; MacKay, J. F. K. 1985. Sorting ofdry and green lodgepole pine before kiln drying. In:Proceedings, joint meeting Western Dry Kiln Clubs;1985 May 8-10; Vancouver, BC. Corvallis, OR 97331:Oregon State University School of Forestry: 63-69.

Reitz, Raymond C.; Page, Rufus H. 1971. Air dry-ing of lumber: A guide to industry practice. Agric.Handb. 402. Washington, DC: U.S. Department ofAgriculture. 110 p.

Visser, J. J.; Vermaas, H. F. 1986. Biological drying ofPinus radiata and Eucalyptus cladocalyx trees. Journalof the Institute of Wood Science. 10(5): 197-201.

Sources of Additional Information

Cech, M. Y.; Pfaff, F. 1977. Kiln operators manual foreastern Canada. Report OPX 192E. Ottawa, ON: East-ern Forest Products Laboratory. 189 p.

Ellwood, E. L.; Ecklund, B. A. 1959. Bacterial attackof pine logs in pond storage. Forest Products Journal.9(9): 283-292.

Findlay, W. P. K. 1967. Timber pests and diseases.Pergamon Series of Monographs on Furniture andTimber, Vol. 5. Pergamon Press. 280 p.

Johnson, N. E.; Zingg, J. G. 1969. Transpirational dry-ing of Douglas-fir: Effect on log moisture content andinsect attack. Journal of Forestry. 67(11): 816-819.

McMillen, J. M. 1956. Coatings for the prevention ofend checks in logs and lumber. Forest Products Lab-oratory Report No. 1435. Madison, WI: U.S. Depart-ment of Agriculture, Forest Service, Forest ProductsLaboratory. (Out of print)

McMinn, J. W. 1986. Transpirational drying of redoaks, sweetgum, and yellow-poplar in the upper Pied-mont of Georgia. Forest Products Journal. 36(3):25-27.

Reitz, Raymond C. 1978. Storage of lumber. Agric.Handb. 531. Washington, DC: U.S. Department ofAgriculture. 63 p.

Salamon, M. 1973. Drying of lodgepole pine and sprucestuds cut from flooded timber: A progress report. In:Proceedings, 24th annual meeting Western Dry KilnClubs, Oregon State University, Corvallis; 51-56.

Scheffer, T. C. 1958. Control of decay and sap stain inlogs and green lumber. Forest Products Laboratory Re-port No. 2107. Madison, WI: U.S. Department of Agri-culture, Forest Service, Forest Products Laboratory.13 p. (Out of print)

Scheffer, T. C. 1961. Protecting stored logs and pulp-wood in North America. Material und Organismen.4(3): 167-199.

Wagner, F. J. Jr. 1978. Preventing degrade in storedsouthern logs. Forest Products Utilization Bulletin.Atlanta, GA 30309: U.S. Department of Agriculture,Forest Service, State and Private Forestry. 4 p.

Williams, L. H.; Mauldin, J. K. 1985. Laboratory dipdiffusion treatment of unseasoned banak (Virola spp.)lumber with boron compounds. Res. Note SO-313.New Orleans, LA: U.S. Department of Agriculture,Forest Service, Southern Station. 8 p.

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Table 10-1—Equilibrium moisture content of wood, exposed to outdoor atmosphere, in the United States

Equilibrium moisture content in different months (percent)1

Location Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Portland, MEConcord, NHBoston, MAProvidence, RI

Bridgeport, CTNew York, NYNewark, NJWilmington, DE

Philadelphia, PABaltimore, MDNorfolk, VAWilmington, NC

Charleston, SCSavannah, GAKey West, FLBurlington, VT

Cleveland, OHSouth Bend, INCharleston, WVLouisville, KY

Nashville, TNMobile, ALJackson, MSDetroit, Ml

Milwaukee, WIChicago, ILDes Moines, IAKansas City, MO

Little Rock, AKNew Orleans, LADuluth, MNBismark, ND

Huron, SDOmaha, NEWichita, KSTulsa, OK

Galveston, TXMissoula, MTCasper, WYDenver, CO

Salt Lake City, UTAlbuquerque, NMTuscon, AZBoise, ID

Reno, NVSeattle-Tacoma, WAPortland, ORSan Francisco, CA

Juneau, AKSan Juan, PRHonolulu, HI

1The values were calculated by means of average monthly temperatures and relative humidities given in Climatological Data monthly reports of the WeatherBureau and the wood equilibrium moisture content to relative humidity relationship.

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Table 10-2—Average gain in lumber moisture content during ocean shipment1

Lumber moisture content increase(percent)

Number ofshipments

Shipmentdestination

Time intransit(days)

Stowed withdry lumber

below decks

Stowed withgreen lumberbelow decks

Stowed ondeck with

green lumber

11

10

6

3

3

(Average)

England

Australia

South Africa

EasternCanada

Trinidad

1Lumber used was 1-in kiln-dried Douglas-fir.

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Chapter 11Energy in Kiln Drying

Energy consumption in drying systems 239Definition of terms 239

Units of thermal energy 239Latent heat of evaporation 239Heat capacity 240Heat of adsorption 240Thermal conductivity 240Overall heat transfer coefficient 241

Heat transfer concepts 242Identification of energy consumed in

wood drying 242Latent heat of evaporation 243Heat loss from dryer 243Heat loss associated with vent air 244Sensible heat demand of wood and kiln

structures 245Electrical energy for air movement 245Steam generations and delivery loss 246

Energy demand in various wood drying systems 246Forced-air drying 246

Latent heat of evaporation 246Heat loss 246Vent air loss 246Sensible heat 246Electrical energy for air movement 247Energy source and delivery system 248

Air drying followed by kiln drying 249Predrying followed by kiln drying 249Dehumidification drying 249Solar drying 249Vacuum drying 250Platen pressdrying 250

Practical applications 250Energy partition in a typical forced-air kiln 250Fuel costs and delivery systems 250Maintaining high energy efficiency in

existing forced-air kilns 251Heat recovery from vent air 252List of symbols 252Literature cited 253Sources of additional information 253Tables 254

Chapter 11 was written by John L. Tschernitz,Chemical Engineer.

Drying of materials in general and of wood in par-ticular is energy intensive, primarily because a highamount of energy is required to evaporate water (liquidto gas). Depending upon the type of equipment usedto dry wood, the efficiency level of the operation mayrequire one and one-half to four times the energy actu-ally needed to evaporate the water. In addition, greenwood to be dried may contain, by weight, as much astwo-thirds water. Wood can be successfully dried indifferent types of dryers. However, even when optimallyoperated, the dryers may have different levels of effi-ciency as an inherent property of their physical designand the materials of construction, and their efficiencymay also be affected by environmental factors. Certainpractices or maintenance procedures may further re-duce dryer efficiency. In this chapter, we discuss energydemand as related to various methods of drying, typesof environmental and geographical factors, fuel, andequipment misuse.

Energy Consumption inDrying Systems

Definition of Terms

A list of symbols is provided at the end of this chapter.

Units of Thermal Energy

In English notation, the unit of energy is the Britishthermal unit (Btu), which is defined as the amountof energy required to heat 1 lb of liquid water 1 °Fat 40 °F. Because the quantity of energy used in anyone process is such a large number, the unit therm(100,000 Btu) is often substituted for Btu. Very of-ten, energy is quoted as cost per million Btu (106 Btu).Some economists use an even larger unit, the quad(1015 Btu).

Latent Heat of Evaporation

The energy consumed at constant temperature forphase change from solid to liquid (heat of fusion) andliquid to gas (heat of evaporation) is called latent heat.For drying, the latent heat of vaporization (liquid togas) of water is about 1,000 Btu/lb, a value that atlow temperatures is a slightly decreasing function of

239

temperature (for example, 1,054 at 70 °F to 970.3 at212 °F). For water, the latent heat of fusion (ice toliquid) is considerably lower, 144 Btu/lb.

Heat Capacity

The heat capacity (or specific heat relative to water)of solids, liquids, and gases is by definition the amountof energy (Btu) required to heat 1 lb of material 1 °F.The actual value will differ with the physical or chem-ical composition of the material and again is a func-tion of temperature. For the materials associated withdrying of wood, values for heat capacity are given intable 11-1.

Heat of Adsorption

At any moisture content greater than 30 percent (fibersaturation point), the water content exists in twostates: (1) water as liquid in the cellular structure ofthe wood and (2) adsorbed water within the woodsubstance--so-called hygroscopic water. The latterstate represents a molecular invasion of the complexwood polymer structure. The energy associated withremoving this water in drying (liquid to gas) is nowgreater than the latent heat of vaporization. For levelsof moisture less than 20 percent, the heat of adsorptionincreases exponentially as the moisture content dropsfrom 20 to 0 percent. Table 11-2 shows some valuesthat must be added to the latent heat of vaporization.These values are sometimes referred to as the heat ofwetting, so called after an experimental technique usedfor measurement. The values in table 11-2 are derivedfrom an equation that approximates experimental data(Weichert 1963):

(1)

where

Ah, is the differential heat of wetting (Btu/lb) and

Mi is intermediate moisture content (percent).

Thermal Conductivity

One property of matter is that energy flows (is trans-ferred) from a higher to a lower temperature. For thesame difference in temperature (identical areas andthicknesses), various substances will transfer energyat different rates. This variation in heat transfer rateis characterized by a thermal conductivity coefficientdefined by means of the following equation:

where

(2)

Q is energy transferred (Btu/h),

At the temperature difference between hot and cold(°F),

A surface area (ft2), and

thickness of substance (ft).

The units of are then Btu/h/ft/°F. Some typical val-.ues of thermal conductivity are shown in table 11-1.

Sometimes the units of are expressed differently.Equation (2) can be rearranged as

putting the dimensions for as

If and A are given in inches, then is given in

Therefore,

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Figure 11-1—Diagram of panel construction. (ML885612)

Overall Heat Transfer Coefficient

Thermal conductivities defined by equation (2) are notdirectly useful in considering heat losses from dryerssince in dry kiln construction, multiple layers of dif-ferent materials exist in series; for example, gas, alu-minum, urethane foam, aluminum, and gas. (Note: Toa lesser degree, parallel layering may be used.) Themore useful coefficient is the overall heat transfercoefficient U, defined by

The coefficient U is very often expressed as an R value,where R = 1/ U. The reason for expressing heat lossin this form is that when the characteristics of a givenconstruction are known, the heat loss can be calculatedsimply by knowing the area and the difference in high(inside) and low (outside) temperature.

When manufacturer specifications ( U or R values) formultilayer walls are not known, it is possible to esti-mate U from known thermal conductance (table 11-1)and heat transfer coefficients. In a steady-stateoperation (fig. 11-1),

(3)

where

U is the overall heat transfer coefficient (Btu/h/ft2/°F),

th air temperature within the dryer, and

tc air temperature outside the dryer.

241

The heat flow through each layer can be expressed inthe form of equation (2):

or

and

or

where hc is heat transfer coefficient of the surface airlayer outside the kiln, hh is the heat transfer coefficientof the surface air layer inside the kiln, and

Substituting for temperature drops in each layer,

or

(4)

For the panel shown in figure 11-1, which is made ofaluminum, urethane foam, and aluminum, an R valuefor the composite can be estimated with the proper-ties given in table 11-1. It is important that consistentunits for and be used. An example of the methodfor calculating an R value using equation (4) is shownin table 11-3. It should be cautioned that this calcu-lated R value may be too high when the panel hasmetal ends and joints (parallel layering). Under thesecircumstances, the walls and roofs of an assembled kilnmay have greater heat losses than predicted.

242

Heat Transfer Concepts

Heat is transferred from one body to another by con-duction, convection, and radiation.

Conduction is the energy transfer from a high to a lowtemperature through a medium (solid, liquid, or gas,alone or in combination).

Convection is a complex combination of heat conduc-tion and mass flow; it is the most important form ofheat transfer between solid surfaces and liquids or gases(Kreith 1965). Convection can be subclassified as freeor natural convection (the physical displacement of en-ergy by movement of material (gas and liquid) inducedby density differences) or forced convection (displace-ment and mixing induced by fans and pumps). Freeand forced convection can take place independently orin combination.

Radiation is energy transfer across transparent spacesby electromagnetic means such as infrared wavelengths.The amount of energy transferred will be controlledin conduction by temperature difference (t2 - t1); inconvection within a medium by density differences orforced means, or both; and in radiation by the dif-ference in the fourth power of absolute temperature

In any system exhibiting energy transfer, any of thesemechanisms may occur singly or in combination. Forpractical considerations of energy losses in dry kiln op-eration, the overall heat transfer coefficient U is suffi-cient for describing heat losses. Inside the kiln, radia-tion may be a factor in operating performance. Naturalcirculation (convection) kilns are no longer of commer-cial importance.

Identification of Energy Consumedin Wood Drying

Even though each individual dryer will consume differ-ent combinations and quantities of energy per unit ofwater evaporated, it is useful to consider energy con-sumption for the general case, which will help one tounderstand the limitations and advantages of differentdrying systems.

In general, all the possible elements of energy consump-tion and supply in wood drying that appear in variouscombinations in specific drying systems can be listed asfollows:

1. Latent heat required to evaporate water (also heat ofadsorption and possibly heat of fusion)

2. Heat loss from dryer structures by conduction fromthe high-temperature interior through the walls, ceil-ing, and floor to lower temperature regions outside

3. Heat loss associated with vent air used to removewater vapor from the dryer (and air loss from leakydryer structures in excess of necessary venting)

4. Sensible heat (heat capacity) required to heat thelumber and building structure to drying temperature

5. Electrical energy needed for air movement

6. Energy source and delivery system

Each of these items is discussed in the followingsections.

Latent Heat of Evaporation

Latent heat is directly and invariably determined bythe wood volume, specific gravity, and expected per-centage of moisture change (expressed on a dry basis).

where

(5)

qa is total heat (Btu) required to evaporate water fromwood substance,

λ latent heat of vaporization (Btu/lb H2O) (moisturecontent >20 percent),

Mo original moisture content (percent), and

Mf final moisture content (percent).

Let

where

qf is energy (Btu) needed to evaporate free water perdrying run and

qb is energy (Btu) needed to evaporate bound waterper drying run.

For final moisture contents greater than 20 percent,

For final moisture contents less than 20 percent, qb canbe calculated from equation (5), substituting

where volume is total green volume charged to thekiln (ft3), specific gravity is based on ovendry–greenvolume of wood (lb/ft3), ρH2O is the density of water(62.4 lb/ft3), and

(6)

(see table 11-1). See table 11-2 for definition of Thevariable λ is a function of temperature that can be ex-pressed by the following equation (Keenan et al. 1969):

where evaporation occurs at temperature t (°F)

Heat Loss From Dryer

The magnitude of the quantity of heat loss from thedryer will depend upon the difference in temperaturebetween the inside and the outside of the dryer, thearea of the dryer surfaces, the materials of construc-tion, and the time of dryer operation for any batchrun. Heat loss can be expressed as the sum of heat lossthrough various kiln surfaces at different times in thedrying schedule:

(7)

where

q is heat loss through walls (Btu),

Ui overall heat transfer coefficient of individual dryerstructural components (Btu/h/ft2/°F),

Ai surface area of walls, ceiling, floors, and doors (ft2),

t2 dry-bulb temperature (°F),

t1 exterior or ambient temperature (°F), and

θ i drying time (h).

Since the temperature of the dryer will vary with time,as by schedule, the time θ will be broken into timesteps: θ 1, θ2, . . ., θ i. Also, the U values of walls, ceil-ing, and floors will differ: U1, U2, . . ., Ui. The outsidesurface temperature t1 will vary night to day and theground (floor) temperature will be higher than the out-side air, as will the third wall common in tandem in-stallations. The value t1 will have seasonal variation forany one location and will vary according to the localclimate. Wind may be a factor.

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Heat Loss Associated With Vent Air

The kiln most used in wood drying is a forced convec-tion dryer wherein air is the means used to supply heatto evaporate water as well as to remove water from thedryer as water vapor-air mixture. For operations be-low 212 °F, air will always be needed to vent the dryer.The energy required to heat this vent air from an am-bient temperature to dryer exhaust temperature repre-sents a big part of the energy required to dry wood; itis the reason why more than 1,000 Btu are needed perpound of water evaporated in convective dryer opera-tion. Because air will hold more water vapor at highertemperatures, less vent air is needed at higher oper-ating temperatures (for equal relative humidity in thevent exhaust). It is an easy task to calculate necessaryventing under various conditions of operation. This ismore clearly understood by looking at the psychromet-ric chart in chapter 1, appendix 1-A, figure 1-A-1. Forany given temperature (dry bulb) and relative humid-ity (wet bulb or wet-bulb depression), one can definea very useful quantity, the absolute humidity H (alsocalled the humidity ratio). The units of this term arepounds water per pound dry air. The vent air needs(volume of vent air per pound of water evaporated)can be calculated as follows:

Then

(8)

where

V is vented moisture air volume at STP (ft3),

Vair vented dry air volume at STP (ft3), and

VH2 O vented H2O vapor volume at STP (ft3).

(STP is standard temperature and pressure: 32 °F,1 atm.)

For the following derivation, let the basis be 1 lb ofevaporated water, where the following definitionsapply:

H2 is pounds of H2O per pound dry air in vent air,

H1 is pounds of H2O per pound dry air in ambient air,

ma, is pounds of air needed to vent 1 lb of evaporatedwater, and

where is the density of water vapor (18/359 lb/ft3

at STP). So

Heat loss in vent air qv (Btu per pound H2O evapo-rated) is calculated as

where Cp is heat capacity (Btu/°F/lb) and qH 2O is sen-sible heat of the vapor component.

It is interesting to look at venting rates and energy con-sumption as a function of dryer temperature and rela-tive humidity. Using equations (8) and (9), we can nowcalculate V and qv for a dryer operated at two relativehumidities, 20 and 80 percent, and different temper-ature levels (dry bulb), assuming a constant ambientcondition of 80 °F, 65 percent relative humidity. Thevalues of H2 and H1 can be found in the psychrometricchart (ch. 1, app. 1-A, fig. 1-A-1). The resulting valuesof V and qv are given in table 11-4.

The following example uses values V and qv from table11-4. Assume the following:

1. Ambient conditions, 80 °F, 65 percent relative hu-midity, 50,000 fbm red oak kiln

2. Schedule step 1,100 °F, 80 percent relative humidity(6 °F wet-bulb depression)

3. Dry wood weight (WOD) of 145,000 lb

4. Moisture content per day (DR) of 4 percent

Calculate the following:

1. Vent rate (VR) (ft3/min, STP)

2. Energy to heat vent air (QR) (Btu/min)

3. Drying rate per minute (DW) (lb H2O per min)

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Example: w i t h

where

From table 11-4,is average heat capacity of ovendry wood(Cpw = 0.327 Btu/°F/lb dry wood),

heat capacity of liquid water

MCi percent moisture content at any step change.

Thus, From this equation, one observes that the heat capacityof wet wood is far greater than that of dry wood; forexample, at 100 percent moisture content, Cp = 1.327,while at 10 percent, Cp = 0.427 Btu/lb/°F.

For the kiln structure, the sensible heat is

Sensible Heat Demand of Woodand Kiln Structures

whereBy definition, sensible heat is that energy required toraise the temperature of either solids, liquids, or gaseswithout phase-change or chemical reaction. In the caseof wood drying, the sensible heat is consumed by rais-ing the wood from ambient temperature to the finaldischarge temperature as it leaves the kiln. The kilnstructure and furnishings must be heated from somelow level (ambient and completely cooled) or from anintermediate temperature if little time has elapsed be-tween dryer ‘batch operations. Therefore, the sensibleheat demand can be stated as

t2 is kiln temperature,

t1 is beginning kiln temperature, and

represents the product of the heat capacity andweight of individual kiln components other than dryingwood.

where qs is total sensible heat.

For wood being dried, the sensible heat is

(10)Electrical Energy for Air Movement

where

CpM is heat capacity of the combined wood and waterat moisture content step MCi and

At, is temperature change between steps in dryingschedule,

Electric power is needed for air circulation in mostdryer types. For any given kiln, the actual power de-mand for circulating the air will vary with air velocity,package width, board roughness, and sticker thickness.An increase in velocity and package width and a de-crease in sticker thickness all will increase the powerdemand. For one sticker thickness and package width,a maximum attainable velocity exists that correspondsto the maximum power load. A 1-hp motor at maxi-mum load would dissipate 2,547 Btu/h, or 0.746 kW/h.Electrical energy is converted to thermal energy withinthe dryer in two ways. If the motors are external tothe dryer, then only the work done in air movement isconverted to heat by friction (air and bearing friction)minus the work of venting. The heat generated withinthe motor is lost to the external environment (approx-imately 10 percent of power input). If the motors arewithin the dryer compartment, then all the electricalconsumption appears as heat.

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Steam Generations and Delivery Loss

For indirect fire, steam is the most frequent heat trans-fer medium from fuel to dryer for heating. (Oil, water,and rarely air are other possible heat transfer media.)The closed system of fuel burner, boiler tubing, andsteam delivery to finned tube heating coils (conden-sate return to the boiler) is the drying system most fre-quently encountered. The net heat as delivered to thekiln represents a fraction of the total energy availablein the fuel (heating value) charged to the boiler. De-livery losses may be incomplete combustion, high fuelmoisture content, high stack gas temperatures, steamsupply line losses, and boiler heat losses. The deliveredenergy may be of the order of only 75 percent of theheating value of the fuel used.

Energy Demand in VariousWood Drying Systems

Six distinct energy-consuming factors in drying systemswere identified in the previous section. Each of thesemay be present in different dryers to varying relativeand absolute degrees. The various drying systems tobe considered, in decreasing frequency of use, are asfollows:

1. Forced-air convective drying-most common dryingmethod

2. Air drying followed by kiln drying

3. Predrying followed by kiln drying

4. Dehumidification drying

5. Solar drying (alternately with supplemental energy)

6. Vacuum drying (platen, radiofrequency, and forcedair)

7. Platen pressdrying

Forced-Air Drying

Latent Heat of Evaporation

Latent heat demand is unalterably dependent onlyupon the amount of water evaporated. Thus, the mag-nitude of the quantity of latent heat is only a functionof the initial and final moisture content, species, den-sity, temperature of evaporation, and total volume ofwood in the dryer. To reduce fuel use for this purpose,one would have to lower initial moisture content by airdrying. The quantity of latent heat may represent 20 to60 percent of the energy consumed within the dryingchamber.

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Heat Loss

One can reduce heat loss through the kiln walls byselecting equipment with good insulating proper-ties(high R values), which remain so under the harsh con-ditions found in most kilns. Special care must be con-sidered in roof construction and maintenance. Heat lossthrough the floor can be substantial, and insulation ap-plied during construction will be worth the extra cost.From equation (7) one can see that for given walls,roof, and floor construction, the important factors thatincrease heat loss are area, temperature difference (in-side compared to outside), and time. To a lesser ex-tent, increased air velocity within and outside the dryerwill also increase heat loss. Small dryers have a largerheat loss per board foot because of the greater surfacearea per unit volume (related to wood capacity). Thegreater the temperature difference, the greater the heatloss; the longer the drying time at a given tempera-ture, the greater the heat loss. While high dryer tem-peratures will increase the heat loss per unit time, theshorter drying time may actually reduce the total loss.

Vent Air Loss

Vent air loss may represent more than 25 percent ofthe total fuel consumed in the drying system. In equa-tion (9) for heat loss in vent air, note that the impor-tant factors in necessary venting are temperature differ-ence between the dryer and makeup air, and the differ-ence in absolute humidity between inside and makeupair. Thus, the most efficient operation of a given sys-tem is high absolute humidity in the dryer (vent gases),along with low ambient humidity. Examples are shownin table 11-4. It should not be overlooked that equa-tion (9) assumes that the desired humidity is estab-lished by controlled venting-no steam spray humidifi-cation. If the dryer is not tight (air leakage) or is over-vented because of bad control, steam will be introducedto maintain the humidity, resulting in greater energydemand.

Sensible Heat

When the dryer is cold and charged with cold or evenfrozen lumber, energy is consumed in heating the woodand kiln structure, in addition to drying and venting.This places the maximum demand on the heat deliverysystem; this energy demand is shown as a calculatedexample in figure 11-2 for the case of a red oak hard-wood dryer. If the capacity of the heat supply system,such as boiler, is insufficient, the heat-up period willtake longer. The current boiler capacity in forced con-vection kilns is about 30 Btu/fbm/h for hardwoods and225 Btu/fbm/h for high-temperature drying of soft-woods. It is in the nature of the thermodynamics ofthe dryer operation that the energy required to heat

Figure 11-2—Energy partition in kiln drying 4/4 red oak. (ML88 5613)

the wood is actually utilized in evaporating the water(the latent heat of evaporation decreases with increasedtemperature). The actual sensible heat loss is associ-ated with the final temperature of the dry wood andthe kiln structure.

Electrical Energy for Air Movement

Sufficient air movement through a stickered package oflumber is important for optimum drying of wood. Thelevel of airflow (or velocity) needed will depend uponthe rate of drying (high or low temperature), width ofpackage, sticker thickness, and hardwood in contrast tosoftwood operation.

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Particularly in hardwoods and thicker stock, the needfor high air velocity (for effective heat transfer and re-moval of evaporated water) is diminished as the aver-age moisture content of the wood drops below 30 per-cent. Electric power consumption (cost) can be reduced

if one can control the air velocity over wide limits.Three fan laws allow us to better understand this phe-nomenon. These laws are based on constant air densityand fan configuration.

1. The total airflow (volume per unit time) is directlyproportional to the speed of the fan in revolutionsper minute (rpm).

2. The total pressure (head) is proportional to thesquare of the speed.

speed.3. The power (bhp) is proportional to the cube of the

Thus, if fan speed is reduced by 50 percent, the air ve-locity will be reduced 50 percent; the pressure head willdrop by 75 percent, and the shaft power by 87.5 per-cent. In actual performance tests under controlled con-ditions of axial and centrifugal fans of many designs,these three fan laws have been proven correct. Thecombined motor-shaft-drive system may reduce thepower savings as predicted by the third (cubic) fan law.Actually a properly designed electronic speed controlfan-motor unit will come close to obeying the cubiclaw-all other factors being constant. There will al-ways be bearing losses, but for the most part, these aresmall. With adequate speed control of the motor-drivesystem, power savings approaching 87.5 percent can berealized with a 50-percent reduction in fan speed. Theefficiency of a fan-motor installation, expressed as cubicfeet per minute of delivered air per horse power, willvary with actual fan design even though the cubic lawstill applies.

In the past, velocity could be reduced economicallyonly by using two-speed motors or variable mechani-cal drives (dc motor controls were possible, but costly).With the advent of solid-state electronic controls, it isnow possible to vary the speed of ac induction motorsas well as dc motors. For ac induction motors operat-ing at constant voltage, the speed can be changed byvarying the frequency and current. This is generallycalled-a variable frequency drive (VFD) system.

Likewise, for fans with dc drives, the electronic controlscan vary the voltage to change the speed. This sys-tem is called a silicon-controlled rectifier (SCR) system.The advantage of a VFD over a SCR system is that itcan be retrofit into existing drying systems without re-placing the motor. However, the operator should beaware that some fan motors are not immediately com-patible with this modification, and care must therefore be taken to ensure that the fan works properly.Moreover, the state of the art is such that the solid-

state circuits are not as stable at first as they are rep-resented to be. The cost of frequent breakdown andsubsequent repair can negate any power savings. Because of these disadvantages, the SCR has become the“old workhorse” for continuous speed control.

Energy Source and Delivery System

Forced-air convection dry kilns can be heated indi-rectly with steam as supplied to heating coils (indi-rect fire), directly by heated gases (direct fire), or withlow-pressure exhaust steam (to coils) from a turbo-generator assembly (co-generation).

Indirect fire.—The most common source of steamfor drying is a boiler. The capacity of boilers is fre-quently rated as boiler horsepower. In terms of Btu,one boiler horsepower is 33,446 Btu/h. For a 50,000-fbm hardwood package dryer, the supply design isabout 30 Btu/fbm/h, or 45 boiler hp. For high-temperature southern pine drying, 100,000-fbm capac-ity, the boilers are sized to at least 225 Btu/fbm/h or more than 675 boiler hp. The choice and availabilityof fuels for a boiler system affect energy costs. Thecurrent and past trends in fuel costs are shown intable 11-5.

The most desirable furnace boiler system would be ca-pable of using multiple fuel types to take advantage ofchanging fuel markets, but capital costs would be pro-hibitive. Depending upon completeness of combustion,excess combustion air, stack gas temperature, and fuelmoisture content, the net energy delivered to the dryeras steam should be between 70 and 80 percent. Theoperator should always be vigilant that the boiler sys-tem is operating at highest efficiency. For wood-wastefuel, a special concern is the moisture content of thefuel because this affects combustion efficiency and netavailable energy. Figure 11-3 shows the realized heat-ing values of wood-waste fuels as a function of moisturecontent (wet basis).

Although indirect fire (steam) is inherently less efficientthan direct fire, the advantage of a steam source forequalizing and conditioning cannot be ignored.

Direct fire.-As the name implies, direct fire sendsthe products of combustion from a burner assemblyinto the dryer chamber. Direct-fired systems inherentlyuse leas fuel than indirect-fired systems. Natural gasburners are particularly efficient for dry kilns when gassupplies are cheap and available. Wood-waste direct-fired burners are also available. Ash flyover into thekiln can occur with this type of burner; burner designand performance need to be scrutinized. In certainimproper installations, kiln fires have occurred.

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Figure 11-3—Recoverable heat energy, available poten-tial heat, and heat losses for a typical wood fuel perpound of wet fuel at various moisture contents. Thefuel has a higher beating value of 8,500 Btu per pound.The higher heat value considers all the water of com-bustion to be liquid. The combustion heat recovery sys-tem is assumed to be operating with 40 percent excessair and a stack gas temperature of 500 °F, fairly typicalfor an industrial system. A constant conventional heatloss factor of 4 percent and complete combustion arealso assumed (Ince 1979). (ML88 5611)

Co-generation.—A growing trend in U.S. industry isco-generation: high-pressure steam is generated in theboiler system, passed through a turbine generator com-bine for production of electricity, and then exhaustedas low-pressure steam for process heating. This systemis particularly attractive to firms using large quantitiesof thermal energy since it can result in 8 to 20 percentsavings in energy costs. However, the usefulness of co-generation must be evaluated on a case-by-case basis,taking into account local fuel supply and costs, alongwith electric power rates.

Air Drying Followed by Kiln Drying

Standard practice, at least in hardwood drying, hasbeen air drying followed by kiln drying. The merits andlimitations of air drying are discussed by Rietz (1971).Our concern here is energy savings. If one considers ourprevious example of a 50,000-fbm kiln as operated inMadison, WI, which dries red oak from different mois-ture levels, various total energy demands can be calcu-lated for various lumber thicknesses. These calculatedestimates are shown in table 11-6. The energy savingsare self-evident.

Predrying Followed by Kiln Drying

In recent years, utilization of predryers for controlled“air drying” of hardwoods has increased markedly. Thequality of predried wood is high; kiln capacity needsare reduced; and, depending upon the season, kiln lo-cation, and construction, energy needs are somewhatreduced compared to those for drying green lumber.Wall insulation, tightness of the building structure, andcontrolled ventilation are important. The biggest de-sign problem other than structural integrity, however,is air distribution in heated structures holding 1 mil-lion fbm of lumber. These dryers operate in the rangeof 85 to 95 °F. All the elements of increased energy de-mand are present here.

Dehumidiication Drying

Discussions of dehumidification drying appear in chap-ters 2 and 7. Since the only energy source for dryingis electric power, the cost of this method of drying willdepend upon local electric rates, which vary greatly indifferent parts of the United States. With the use of aclosed refrigeration cycle, the net energy to evaporate1 lb of water is much less than 1,000 Btu, but one mustcarefully understand the manner in which fuel savingsare expressed. A system that uses 50 percent less en-ergy is not a bargain, because the energy costs threetimes as much as other types of fuel; electric powercosts are as high as $20 per 106 Btu. Because of theclosed heat cycle, a tight kiln is very important. Mod-ern kilns of this design operate at temperatures as highas 160 °F.

Solar Drying

Analysis has shown that caution should be exercisedin considering solar energy as a means of lowering fuelcosts; it is not a universal solution to energy economyin wood drying. Therefore, no one should leap into in-vesting in such technology without carefully consideringengineering criteria as well as the overall operatingeconomics (Tschernitz 1986).

For a passive solar kiln with supplemental energy, thefollowing observations are made:

1. Supplemental energy is necessary to maintain rapid,consistent drying times for all seasons and alllocations.

2. The solar surfaces (if the kiln is essentially a green-house in design) should be isolated from the dryerduring night hours and during periods of low solarinflux.

3. The proper choice of solar cover material and kilnwall insulation is critical for enhanced fuel savings.

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4. The winter months in the north are not practical forsolar drying on any scale.

5. The supplemental energy could be direct-fired gaswhen available. Wood waste, while cheaper, mightrequire greater capitalization. Electrical energy istoo expensive under most circumstances, althoughcapitalization would be low. Use of electrical en-ergy in conjunction with dehumidification is possible(Chen 1982), but capital costs would be high in thiscase.

6. The choice of collector surfaces in practice is re-stricted to roof and south wall (or sloping roof only).

7. The solar kiln, operating as a scheduled dryer, re-quires that conditioning for stress relief must be pro-vided in some manner at the end of the drying cycle.

8. For noncommercial operators who require only lim-ited quantities of dried wood, a small do-it-yourselfunit (1-fbm capacity) built with low-cost materials(such as discarded glass and plastic) would be use-ful even in the northern tier of states for at least9 months of the year, even without supplementalenergy (Bois 1977, Rice 1987).

Vacuum Drying

For many years, vacuum drying has been promoted pe-riodically. The energy used to evaporate the water canbe conductive heating (platen), radiation (radiofre-quency or infrared), cyclic heating (forced air), or acombination thereof. No vent air is required; some con-ducted heat losses occur. The cost of energy, primarilyelectrical, is high and capital costs are also high. Vac-uum drying has a special place in the family of dryingsystems as a fast method for drying thick lumber.

Platen Pressdrying

In platen pressdrying, heat loss from high-temperaturepresses is high although no vent air is lost. High-pressure steam or oil is required. Energy is not theprincipal concern in the choice and operation of platendrying equipment. The virtue of this method is speedand the possible improved properties of wood products.

Practical Applications

Energy Partition in a Typical Forced-Air Kiln

If the drying rate of a given wood species can be es-tablished as a function of temperature, relative humid-ity, moisture content, board thickness, and air velocity(which is related to sticker thickness), then it is possi-ble to model the energy consumption at any time in thedrying schedule. The results of such computations for ared oak drying system are shown in figure 11-2, wherethe percentage of each day’s energy demands (whichvaries day to day) attributed to evaporation of water,vent losses, building losses, and boiler losses are plot-ted as a function of time. The average moisture contentof the lumber after each day is also shown, along withtotal energy demand per day.

This analysis assumes a 50,000-fbm dryer (35 ft long,29 ft wide, 27 ft high) as operating in Madison, Wis-consin, in the month of April, which approximates theaverage condition for a total year. The example is for4/4 red oak, using a slightly modified T4-D2 schedule(ch. 7).

The highest daily energy demand is the first daywarmup; as the drying rate decreases, the building en-ergy loss fraction tends to increase; as the dryer tem-perature is raised and relative humidity drops, the ventenergy losses are greater. This pattern will shift winterto summer and location to location. Building energylosses will be higher in the winter; vent losses will in-crease with very humid ambient conditions. The sea-sonal change in total energy demand can be illustratedby a series of calculated total energy demands for asmall (1,000 fbm) and a large (30,000 lbm) 4/4 redoak dryer as operated in different regions of the UnitedStates. In addition, the reduced energy consumptionof a large kiln compared to a small one is indicated.These calculated values are shown in table 11-7.

Fuel Costs and Delivery Systems

The economic criterion for choosing fuel for dryingwood is not simply the fuel with the lowest cost. Ifthat were the case, solar energy would be the firstchoice, electricity the last. The values shown in table11-5 are average past and current fuel costs. There ismuch local variation in fuel costs, particularly electric-ity, as well as seasonal variation.

To select the best fuel, the most obvious considerationis the uninterrupted availability of fuel supply. Thenext and perhaps most important consideration is thecapital cost of the equipment needed to convert the fuelinto useful thermal energy. This is best illustrated inconverting solar energy, where collecting the energy

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and its intermittent delivery are currently too costly formost large-scale commercial wood drying operations.

Thirdly, the efficiency of converting fuel to useful en-ergy must be examined. A low-cost conversion appa-ratus may consume much larger quantities of fuel andmay therefore incur a higher net operating expense.Direct fire (wherein the combustion gases enter the dry-ing chamber) should be considered since a steam boilerand heat transfer coils are not needed. Direct fire istherefore 100 percent efficient, requiring less capitalinvestment.

Decreasing drying time may be a way to compensatefor a high-cost fuel such as electricity. For example,rapid drying of the stock in an electrically heated dryermay be economically sound considering both the quan-tity and value of the product and sharply reduced dry-ing time.

Maintaining High Energy Efficiencyin Existing Forced-Air Kilns

The following list (McMillen and Wengert 1978) shouldbe helpful in increasing the efficient use of energyduring kiln drying.

1. Use as much air drying or forced-air drying aspossible--preferably drying to 25 percent moisturecontent or less. While this will reduce the cost ofenergy, lumber degrade costs may offset the energysavings.

2. Do not use steam spray or water spray in the kilnexcept during conditioning. Let the moisture comingout of the wood build up the humidity to the desiredlevel. Steam may have to be used, however, whenvery small wet-bulb depressions are required.

3. Repair and caulk all leaks, cracks, and holes in thekiln structure and doors to prevent unnecessaryventing and loss of heat. Make sure the doors closetightly, especially at the top. Temporarily plug anyleaks around the doors with rags, and order new gas-kets, shimming strips, or hangers if necessary. Ina track kiln, use sawdust-filled burlap bags to plugleaks around tracks. Adjust and repair the vents sothat they close tightly.

4. For brick or cinder block kilns, maintain the mois-ture vapor-resistant kiln coating in the best possiblecondition. This will prevent the walls and the roofsfrom absorbing water. Dry walls conduct less heat tothe outside.

5. For outdoor aluminum kilns only, paint the exteriorwalls and roof a dark color to increase the wall tem-perature by solar heat and reduce heat loss from thekiln. Check to ensure that weep holes are open, notplugged. Painting would be disastrous on permeablewalls like brick or cinder block.

6. In many kilns, more heat is lost through the roofthan through the walls, and much of this loss is dueto wet insulation. To reduce heat loss, consider in-stalling a new roof or repairing an old one. Addmore insulation if necessary. Make sure the interiorvapor barrier or coating is intact (see suggestion 4).

7. Install or repair baffling to obtain high, uniform airvelocity through the lumber and to prevent short cir-cuiting of the airflow. This pays off in saving energy.Reverse air circulation every 6 h only.

8. Research has shown that in the early stages of dry-ing, high air velocities (more than 600 ft/min) canaccelerate drying. In the late stages, low velocities(250 ft/min) are as effective as high velocities anduse less energy. Therefore, adjust fan speeds duringa run if possible.

9. Calibrate and check the recorder-controller for effi-cient operation. Kiln conditions should not oscillatebetween periods of venting and steam spraying, andventing and steaming should not occur at the sametime.

10. Check the remainder of the equipment. Are trapsworking? Do traps eject mostly hot water with little,if any, steam? Do valves close tightly? Are heatingcoils free of debris? Is valve packing tight? Is thereadequate water for the wet bulb?

11. Accurately determine the moisture content of thedrying wood. Do not waste energy by overdrying because the sample boards do not represent the load.Try to plan the loads so that when they are suffi-ciently dry, someone will be available to shut off thekiln (and, if possible, to unload, reload, and begin anew run). Do not allow a kiln load of dry lumber tocontinue to run overnight or through a weekend.

12. Unload and reload the kiln as fast as possible,but avoid doing this until the air temperature haswarmed up from the morning low temperature. Donot cool the kiln unnecessarily.

13. In a battery of adjacent kilns, avoid unloading orloading a kiln while the adjacent kiln is at 180 °F oranother high temperature.

14. During nonuse periods, close all valves tightly andkeep kiln doors closed. Use a small amount of heat,if necessary, to prevent freezing of steamlines andwaterlines.

15. Use accelerated schedules where possible. Checkchapter 7 for schedules for accelerating scheduleswith minimum risk. The higher the drying tempera-ture, the more efficient the energy use.

16. If possible, reduce the length of time used for condi-tioning; some low-density hardwoods can be condi-tioned in 6 h.

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17. Finally, check with the manufacturer of your equipment to determine if steam pressures can be low-ered or gas or oil flow rates reduced during periodsof constant dry-bulb temperature. For top efficiency,check the burner as well.

Heat Recovery From Vent Air

The use of air-to-air heat exchangers-sometimes calledeconomizers-for partial recovery of energy exhaustedfrom dry kilns has been considered ‘for decades. Cost,efficiency, and design problems made practical appli-cation of these units marginal. Rising fuel and boilercosts, along with new designs, again make such recov-ery systems worth considering. In addition to conven-tional air-to-air heat exchangers, “heat pipes” (Perryand Chilton 1973) have recently been incorporated intoa new design for dry kilns with interesting possibilities.

List of SymbolsA surface area (ft2)

Cp heat capacity (Btu/°F/lb)

Cpw heat capacity of ovendry wood (Btu/°F/lb drywood)

DR moisture content per day (percent)

DW rying rate per minute (lb H2O/min);

H1 pounds water per pound dry air in ambient air

H2 pounds water per pound dry air in vent air

hc, hh heat transfer coefficient of surface air layers

thermal conductivity (Btu/h/ft/°F)

substance thickness (ft)

Mf final moisture content (percent)

Mi intermediate moisture content (percent)

Mo original moisture content (percent)

ma pounds air needed to vent 1 lb evaporated water

MCi moisture content at any step change (percent)

Q energy (Btu/h)

q heat loss through walls (Btu)

qa energy needed to evaporate water in wood (Btu)

qb energy needed to evaporate bound water perdrying run (Btu)

qf energy needed to evaporate free water per dryingrun (Btu)

qkiln sensible heat of kiln structure (Btu)

qs total sensible heat (Btu)

qv energy lost in vent air (Btu)

qwood sensible heat of wood (Btu)

QR energy needed to heat vent air (Btu/min)

T absolute temperature (°R)

tc air temperature outside dryer (°F)

th air temperature within dryer (°F)

U overall heat transfer coefficient (Btu/h/ft2/°F)

V vented moist air volume at standard tempera-ture and pressure (ft3)

Vair vented dry air volume at standard temperatureand pressure (ft3)

VH 2 O vented water vapor volume at standard tempera-ture and pressure (ft3)

VR vent rate (ft3/min)

WOD dry wood weight (lb)

∆ H pounds water per pound dry air evaporated(H2 –H1 )

∆ ha differential heat of wetting (Btu/lb)

∆ t temperature difference (°F)

∆ ts temperature change between steps (°F)

θ drying time (h)

λ latent heat of evaporation (Btu/lb)

λ' λ + ∆ha

ρ density (lb/ft3)

ρ' H2O density of water vapor

Literature Cited

Bois, P. 1977. Constructing and operating a smallsolar-heated lumber dryer. FPU Tech. Rep. 7. Madi-son, WI: U.S. Department of Agriculture, Forest Ser-vice, State and Private Forestry, Forest Products Labo-ratory. 12 p.

Chen, P. Y.; Helmer, W. A.; Rosen, H. N.; Barton,D. J. 1982. Experimental solar dehumidification kilnfor lumber drying. Forest Products Journal. 32(9):35-41.

Ince, P. J. 1979. How to estimate recoverable heat en-ergy in wood or bark fuels. Gen. Tech. Rep. FPL-29.Madison, WI: U.S. Department of Agriculture, ForestService, Forest Products Laboratory. 7 p.

Keenan, J. H.; Keyes, F. G.; et al. 1969. Steam tables.John Wiley & Sons. 162 p.

Kreith, F. 1965. Principles of heat transfer, 2d ed.Scranton, PA: International Textbook Co. 620 p.

McMillen, J. M.; Wengert, E. M. 1978. Drying east-ern hardwood lumber. Agric. Handb. 528. Washing-ton, DC: U.S. Department of Agriculture. 104 p.

Perry, R. H.; Chilton, C. H. 1973. Chemical Engineers’Handbook, 4th ed. New York: McGraw-Hill. 1904 p.

Rice, R. W. 1987. Solar kiln: A solar heated lumberdrying kiln is easy to build, operate, and maintain.Workbench, Jan.-Feb. 7 p.

Rietz, R. C.; Page, R. H. 1971. Air drying of lumber: aguide to industry practices. Agric. Handb. 402. Wash-ington, DC: U.S. Department of Agriculture. 110 p.

Tschernitz, J. L. 1986. Solar energy for wood dryingusing direct or indirect collection with supplementalheating and a computer analysis. Res. Pap. FPL-RP-477. Madison, WI: U.S. Department of Agriculture,Forest Service, Forest Products Laboratory. 81 p.

Weichert, L. 1963. Investigations on sorption andswelling of spruce, beech, and compressed beech woodbetween 20 o and 100 °C. Holz als Roh-und Werkstoff.21(8): 290-300.

Sources of Additional Information

Air Movement Equipment

American Society of Heating, Refrigeration, and Air-Conditioning Engineers. 1988. Fans. In: ASHRAEHandbook: Equipment. Atlanta, GA: ASHRAE.Chapter 3.

Jorgensen, Robert, ed. 1983. Fan engineering, 8th ed.Buffalo, NY: Buffalo Forge Company. 823 p. (Con-tact: Air Movement and Control Association (AMCA),Arlington Heights, IL)

Co-Generation

Garrett-Price, B. A.; Fassbender, L. L. 1987. Co-generation right for your plant? Chemical Engineering.April 27: 51-57.

Electrical Energy for Air Movement

Carroll, Hatch, and Associates. 1986. Additionalsawmill electrical energy study. Portland, OR.: Bon-neville Power Administration; final report; contractDE-AC79-85BP23462. 70 p.

Carroll, Hatch, and Associates. 1987. Guidebook toelectrical energy savings at lumber dry kilns throughfan speed reduction. Portland, OR.: Bonneville PowerAdministration; final report; contract DE-AC79-85BP23462. 40 p.

Energy Recovery for Vent Air Streams

Karmous, M.; Callahan, J. 1988. Forced venting heatexchanger reduces energy use in dry kilns. Salem, OR:Oregon Department of Energy. 2 p.

Rosen, H. N. 1979. Potential for energy recovery fromhumid air streams. Res. Pap. NC-170. St. Paul, MN:U.S. Department of Agriculture, Forest Service, NorthCentral Forest Experiment Station. 10 p.

Toennison, R. L. 1985. Heat exchangers for lumberdry kilns. Norris, TN: Division of Land and EconomicResources, Office of Natural Resources and EconomicDevelopment, Tennessee Valley Authority; TechnicalNote B55. 32 p.

WSEO. 1990. Energy tips for industry. 1990. DryKiln Retrofit/Replacement. Olympia, WA: Washing-ton State Energy Office.

253

Forced-Air Kiln Drying Table 11-1—Heat capacity and other properties of selectedmaterials

Breiner, T.; Quarks, S. L.; Huber, D.; Arganbright,D. G. 1987. Steam and electrical consumption in acommercial scale lumber dry kiln. In: Proceedings ofthe Western Dry Kiln Association; 1987 May 20-22;Coeur d’Alene, ID. Corvallis, OR: Kozlik-Vandeventer,Inc: 83-94.

Smith, W. Ramsay. 1984. Energy consumption withlumber drying. In: Proceedings, Western Dry KilnClubs; 1984 May 9-11; Reno, NV. Corvallis, OR:Western Dry Kiln Clubs. p. 112-16.

Humidity Computations

Zimmerman, O. T.; Lavine, Irvin. 1964. Psychromet-ric tables and charts, 2nd ed. Dover, NH: IndustrialResearch Services Inc. p. 172.

Solar Energy

Duffie, J. A.; Beckman, W. A. 1980. Solar engineeringof thermal processes. John Wiley & Sons. 762 p.

Wood and Fuel

Curtis, A. B., Jr.; Foster, B. B.; Darwin, W. N., Jr.1986. A preliminary economic analysis for a woodenergy system: Computer program documentation.U.S. Department of Agriculture, Forest Service,Southern Region. 30 p.

Harris, R. A.; McMinn, J. W.; Payne, F. A. 1986. Cal-culating and reporting changes in net heat of combus-tion of wood fuel. Forest Products Journal. 36(6):57-60.

Saeman, J. F., ed. 1975. Wood residue as an energysource. In: Proceedings, No. P-75-13, Forest ProductsResearch Society, Madison, WI. 118 p.

Wood Combustion Equipment

The Technology Applications Laboratory (GIT). 1984.The industrial wood energy handbook. Van NostrandReinhold Company. p. 240.

Table 11-2—Differential heat of wetting and latent heat ofbound water at 150 °F1

254

Table 11-3—Calculated R value for aluminum panel Table 11-4—Vent rates and associated heat losses per pound ofwater evaporated at various dryer temperatures and relativehumidities1

255

Table 11-5—Fuel costs

Table 11-6—Total energy demand for Table 11-7—Energy demands for a low-temperaturekiln drying air-dried red oak drying schedule for 4/4 red oak in three regions of the United State!

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Glossary

This glossary includes generally accepted definitionsof a limited number of terms currently used in wood-drying literature. It also includes closely related termsthat are not fully defined in their special application topresent-day drying in most dictionaries or glossaries.

The following abbreviations are used throughout thismanual.

Btu British thermal unit

COD wt Calculated ovendry weight

DB Dry bulb

EMC Equilibrium moisture content

fbm Board feet (foot board measure); althoughit is not used in this manual, MBF is a com-monly used term for thousand board feet

FSP Fiber saturation point

MC Moisture content

OD wt Ovendry weight

RH Relative humidity

sp. gr. Specific gravity

WB Wet bulb

WBD Wet-bulb depression

wt Weight

Absorption, liquid—The taking in or imbibing of aliquid.

Adsorbed water—In context of wood drying, ad-sorbed water is held in wood substance by hygroscopicor molecular attraction. (Syn: bound water)

Air, entering—Heated air just as it enters the kilnloads of lumber.

Air, fresh—Air brought into the dryer to replacevented air.

Air, laminar—In kiln drying, airflow across the lum-ber, parallel to the stickers, which is very smooth andlayered, with no eddies or swirls; generally considereda condition in which velocity is too low to produce anoptimum drying effect.

Air, leaving—Air just after it leaves the kiln loads oflumber. It is usually at a lower temperature than theentering air.

Air, short circuiting of—See Short circuiting of air.

Air, turbulent—In kiln drying, airflow across thelumber, parallel to the stickers, which is not layeredand has fluctuations creating definite eddies and swirls;generally considered preferable to laminar flow for opti-mum drying.

Air binding—The presence of air (generally in pock-ets) in steam coils and traps, which interferes with thenormal flow of steam and condensate.

Air drying—See Drying, air.

Air reversal—Changing the airflow to flow in theopposite direction through a load of drying lumber orproducts.

Air travel, length of—The distance between theentering- and leaving-air sides of the kiln charge.

Air velocity—The speed at which air moves, generallymeasured in feet per minute.

Air volume—The total amount of air occupying ormoving through a given space, generally measured incubic feet.

Anemometer— An instrument for measuring airvelocity.

Annual growth ring—The growth layer put on a treeeach year in temperate climates, or each growing seasonin other climates; each ring includes springwood andsummerwood.

The glossary was compiled by R. Sidney Boone,Research Forest Products Technologist.

257

Baffle—A piece of canvas, metal, or wood used for de-flecting, checking, or otherwise regulating the flow ofair.

Load—A rigid or flexible panel placed to minimizethe amount of air short-circuiting over, under, andbetween the kiln loads of lumber or other woodproducts.

Balance—An instrument used for measuring mass orweight and often used in weighing moisture content sec-tions and kiln sample boards.

Bark—Outer layer of it tree, which consists of a thin,living inner part and a dry, dead outer part that is gen-erally resistant to moisture movement.

Bastard sawn—Lumber in which the annual growthrings make angles of 30° to 60° with the surface of thepiece.

Blue stain—See Stain, blue.

Bolster—A piece of wood, generally a nominal 4 in.in cross section, placed between stickered packages oflumber or other wood products to provide space for theentry and exit of the forks of a lift truck.

Bow—The distortion in a board that deviates fromflatness lengthwise but not across its faces.

Boxed heart—The term used when the pith falls en-tirely within the outer faces of a piece of wood any-where in its length. Also called boxed pith.

Bright—A term applied to wood that is free fromdiscolorations.

British thermal unit (Btu)—The amount of heatnecessary to raise 1 lb of water 1 °F in temperature.

Bulb—The temperature-sensitive part of a thermo-static control system.

Control—The sensing part of the controlling system,located in the kiln, which contains a temperature-sensitive liquid, gas, or electronic sensor.

Controlling dry—The bulb that controls the dry-bulbtemperature.

Controlling wet—A bulb, kept completely covered atall times with a clean, water-saturated wick or poroussleeve, which automatically controls the wet-bulbtemperature.

Double-end control—Control bulbs, usually located ineach longitudinal half of the kiln, which control kilntemperatures for their respective zone, independent ofeach other.

Dual control—Two bulbs of a Y-shaped control sys-tem. They are usually located on each kiln wall di-rectly opposite each other and control the tempera-ture of the entering air regardless of the direction ofair movement.

Recorder—The temperature-sensitive part of a sys-tem that records but does not control kiln conditions.

Recorder-controller—A bulb attached by means of acapillary tube to a recording-controlling instrument.

Zone control—A bulb or sensor that controls thetemperature within a zone.

Bypass pipe or duct—A pipe or chamber that per-mits air, steam, or water to be diverted from their reg-ular channels.

Cam—A rotating plate so cut that the edges act as aguide for a pin rolling along the edge. In drying con-trol instruments, the pin is employed to control tem-peratures and/or moisture conditions in the dryingchamber.

Cambium—The one-cell-thick layer of tissue betweenthe bark and wood that repeatedly subdivides to formnew wood and bark cells.

Capacitance, electrical—The property of an electri-cal conductor or configuration of conductors that allowsit to store potential energy in the form of an electricalfield.

Capillary action—The combination of solid-liquid ad-hesion and surface tension by which a liquid is elevatedin a vertical tube or moved through a cellular structure.

Casehardening—A condition of stress and set inwood in which the outer fibers are under compres-sive stress and the inner fibers under tensile stress, thestresses persisting when the wood is uniformly dry.

Casehardening, reverse—A final stress and set con-dition (in lumber and other wood items) in which theouter fibers are under a tensile stress and the innerfibers are under a compressive stress as a result of over-conditioning.

Cell—A general term for the minute units of woodstructure, including wood fibers, vessel segments, andother elements of diverse structure and function, havingdistinct cell walls and cell cavities.

Charge—See Kiln charge.

Chart, recorder—A sheet, usually circular, on whicha graphic record of kiln temperatures is transcribed.

Check—A lengthwise separation of the wood thatusually extends across the rings of annual growth andparallel to the wood rays. Checks result from dryingstresses.

Surface—A check starting on a wide-grain surfaceand extending into the interior of a board.

End—A check starting on an end-grain surface andextending along the length of a board.

Internal—Checks originating in the interior of a pieceof wood or extensions of surface and end checks.

Circulation, air—The movement of air within a kilnby either natural or mechanical means.

Direction of—The direction of movement of airthrough the kiln charge, expressed as longitudinal,transverse, or vertical.

Forced—The movement of air within a kiln by me-chanical means.

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Longitudinal— Air movement through the kiln chargeto be expressed as front to rear or rear to front.

Natural— The movement of air within a kiln by nat-ural means. Reversible. Capable of change in the di-rection of air movement.

Transverse— Air movement through the kiln chargefrom wall to wall to be expressed as right to left orleft to right.

Vertical— Air movement through the kiln charge fromtop to bottom or bottom to top.

Co-generation— The simultaneous generation of elec-tricity and low-pressure steam for on-site use, such asin drying.

Coil header (or manifold)—A pipe fitting to which anumber of pipes are connected on one side.

Coil, intermittent operation of—The alternateopening and closing of the valve that controls steamflow into the coil.

Coil, pipe—One or more runs of pipes, the function ofwhich is to heat the air in the kiln.

Booster— A supplementary coil, usually located be-tween tracks of a multiple-track kiln, used to addheat to air that has already moved across a trackloadof lumber.

Ceiling—A coil placed near the kiln ceiling to warmthe ceiling and roof, thus preventing moisturecondensation.

Double-end—Coils usually extending half the lengthof the kiln from both ends and usually operating asseparate units.

Multiple-return-bend header—A coil usually with thedischarge header located below the supply header,the pipes running back and forth with a 180° elbowat the bends.

Plain header (horizontal or vertical)—A coil consist-ing of a supply and discharge header at opposite endswith the pipes running from one to the other.

Single-return-bend header (horizontal or vertical)—Acoil with the discharge header usually located underor on the side of the supply header, the pipes runningfrom the supply header to a 180° bend and back tothe discharge header.

Coil radiating surface—The entire uninsulated sur-face area of a heating coil.

Collapse— The severe distortion or flattening of singlecells or rows of cells in wood during drying, often evi-denced by a caved-in or corrugated appearance of thesurface of the piece.

Compression failure—Rupture of the wood structureresulting from excessive compression along the grain. Itmay develop as a result of bending in the living tree orduring felling. In surfaced lumber, compression failuresappear as fine wrinkles across the face of the piece.

Compression wood—Abnormal wood formed on thelower side of branches and inclined trunks of softwoodtrees. It has relatively wide, eccentric growth rings withlittle or no demarcation between springwood and sum-merwood and more than normal amounts of summer-wood. Compression wood shrinks more than normalwood longitudinally, causing bow, crook, and twist.

Condensate— Water formed by the cooling of steam.

Conditioning—See Stresses, relief of.

Conditioning treatment—A controlled hightemperature-high relative humidity condition used ina dry kiln after the final stage of drying to bring abouta uniform moisture distribution in the boards and torelieve drying stresses.

Conduction, heat—Transmission of heat through orby means of a conductor.

Controller—An instrument that automatically con-trols kiln temperatures.

Convection, heat—Transfer of heat from heating coilsto lumber by means of air.

Course, lumber—A single layer of lumber.

Crib— A stickered kiln truckload of lumber usuallystacked onto kiln trucks and kiln bunks to make a load6 to 10 ft wide, 10 to 16 ft high, and as long as thelumber being stacked.

Crook—A distortion of a board in which the edges de-viate from a straight line from end to end of the board.

Cup—A distortion of a board in which there is devia-tion from flatness across the width of the board.

Cycle, heating—The time intervening between succes-sive openings of a control valve.

Cycle, temperature—The time between the max-imum and minimum temperatures during a heatingcycle.

Decay—The decomposition of wood substance byfungi.

Advanced (or typical)—The older stage of decay inwhich the destruction is readily recognized becausethe wood has become punky, soft and spongy, stringy,ringshaked, pitted, or crumbly. Decided discolorationor bleaching of the rotted wood is often apparent.

Incipient— The early stage of decay that has not pro-ceeded far enough to soften or otherwise perceptiblyimpair the hardness of the wood. It is usually accom-panied by a slight discoloration or bleaching of thewood.

Defects, drying—Any irregularity occurring in or onwood, as a result of drying, that may lower its strength,durability, or utility value.

Degrade, kiln—A drop in lumber grade that resultsfrom kiln drying.

Dehumidification kiln—See Kiln, dehumidification.

Density—The weight of a body per unit volume.

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Depression, wet-bulb—The difference between thedry- and wet-bulb temperatures.

Desorption—The loss of adsorbed (hygroscopic) mois-ture from wood to the surrounding air.

Desuperheater— A device for removing from steamthe heat in excess of that required for saturation at agiven pressure. In kiln drying, atomized water injectionis often used to eliminate the superheat from the steamemployed for humidification.

Dew point—The temperature at which steam or watervapor begins to condense.

Diamonding—A form of warp in which the cross sec-tion assumes a diamond shape.

Diffuse-porous wood—A hardwood in which thepores tend to be uniform in size and distributionthroughout each annual ring or to decrease in sizeslightly and gradually toward the outer border of thering.

Diffusion— Spontaneous movement of heat, moisture,or gas throughout a body or space. Movement is fromhigh to low points of temperature or concentration.

Direct fired—A method of heating a dry kiln wherethe hot gases produced by burning gas, oil, or woodwaste are discharged directly into the kiln atmosphere.

Dry-bulb temperature—The temperature of the airindicated by any type of thermometer not affected bythe water vapor content or relative humidity of the air.

Drying, air—Process of drying lumber by natural con-ditions in a yard or in an open unheated shed.

Drying, precision kiln—Process of drying wood inwhich controlled procedures are followed in order toobtain a stress-free product that has a desired moisturecontent and has suffered a minimum loss in strength.

Drying in transit—The partial or complete kiln dry-ing of lumber by a drying facility located between theshipping and fabrication points.

Drying rate—The amount of moisture lost from thelumber per unit of time.

Duct, air—A rectangular, square, or circular passage-way to conduct air.

Electrodes—In testing wood for moisture content,devices made of an electrical-conducting material forconnecting wood into the electric circuit of an electricmoisture meter. In high-frequency heating, metal platesused to apply the electric field to the material beingheated.

Insulated—In testing wood for moisture content, spe-cial electrodes for use with resistance-type electricmoisture meters that are coated with an insulatingmaterial to limit or control the point of contact be-tween the electrode and the wood.

End check—See Check, end.

End coating—A coating of moisture-resistant materialapplied to the end-grain surface to retard end drying ofgreen wood or to minimize moisture changes in driedwood.

Equalization— Bringing the pieces of lumber in a kilncharge to nearly uniform moisture content. See Treat-ment, equalization.

Equilibrium moisture content—The moisture con-tent at which wood neither gains nor loses moisturewhen surrounded by air at a given relative humidityand temperature.

Evaporation—The change from the liquid to thevapor form.

Extractives—Substances in wood that are not an in-tegral part of the cellular structure and can be removedby solution in hot or cold water, ether, benzene, orother solvents that do not react chemically with woodsubstance.

Fan

Centrifugal fan—A device for moving air by means ofa rotating wheel or impeller, which gives a centrifugalaction to the air being moved. Frequently used forpressure venting.

Deck fan—Fan mounted with fan impeller horizon-tally oriented in a horizontal panel opening, such as afloor or ceiling opening.

Disk or propeller fan—An axial-flow fan with the airflowing through the impeller parallel to the shaftupon which the impeller is mounted. The impellerblade is designed to deliver about the same volume ofair in either direction of rotation.

Pitch—The angle a fan blade is set with respect tothe axis of the propeller fan shaft.

Shroud—Flanges around the wall opening for a diskfan impeller that give support to the wall and provideprotection for the impeller and personnel.

Fiber, wood—A comparatively long (<1/25 to1/3 in), narrow, tapering hardwood cell closed at bothends.

Fiber saturation point—The stage in the drying orwetting of wood at which the cell walls are saturatedwith water and the cell cavities are free from water.It normally applies to an individual cell or group ofcells, not to whole boards. It is usually taken as ap-proximately 30 percent moisture content, based on theweight of ovendried wood.

Flat-sawed— Lumber sawed in a plane approximatelyperpendicular to a radius of the log. See Grain.

Flitch—A portion of a log sawed on two or more sidesand intended for remanufacture, as into lumber orveneer.

Fluctuation, steam pressure—Variation of steampressure.

260

Flue, vertical—A vertical space, usually 6 in or lessin width and extending the length and height of a kilntruckload or package of lumber.

Grain—The general direction of the fibers in wood orlumber. When used with qualifying adjectives, it hasspecial meanings concerning the direction of the fibersor the direction or size of the growth rings. Specificterms for fiber and growth ring direction are as follows:

Cross grain—Grain deviating from a line parallel tothe sides of the piece.

Diagonal grain—A form of cross grain resulting fromsawing at an angle with the bark of the log.

Interlocked grain—A form of spiral grain in which thefiber direction gradually alternates from right-hand toleft-hand spiral and back again in adjacent groups ofannual rings.

Spiral grain—A form of cross grain that results dur-ing the growth of the tree; the fibers take a spiralcourse about the trunk instead of the normal verticalcourse.

Straight grain—Grain parallel to the sides of thepiece.

Coarse grain—wood in which the growth rings arewide or have major differences in density and colorbetween springwood and summerwood.

Edge grain (or vertical grain)—The grain in lumberproduced by quartersawing so that ‘the edges of thegrowth rings are exposed on the widest faces of thepiece, and the rings form angles of 45° to 90° withthe widest faces.

Fine grain—Wood in which the growth rings arenarrow and inconspicuous.

Flat grain—The grain in lumber produced by flatsawing so that the tangential faces of the growthrings are exposed on the widest faces of the piece andthe rings form angles of less than 45° with the widestfaces.

Green lumber (or grass green)—Lumber cut fromfreshly felled trees.

Growth rate—The rate at which a tree has laid onwood, measured radially in the tree trunk or in the ra-dial direction in lumber. The unit of measure in use isthe number of annual growth rings per inch.

Hardwoods—Woods produced by one of the botanicalgroups of trees that have broad leaves in contrast to theneedles or scalelike leaves of the conifers or softwoods.The term has no reference to the actual hardness of thewood.

Heartwood—The wood extending from the pith tothe sapwood, the cells of which no longer participatein the life processes of the tree. Heartwood may be in-filtrated with gums, resins, and other materials thatusually make it darker and more decay resistant thansapwood.

Heat

Conduction—The transfer of heat within a materialor from one material to another in contact with it.

Consumption—In kiln drying, the total heat requiredto heat the wood and the kiln structure and to evap-orate the water from the wood, as well as heat losses,including venting.

Convection—The transfer of heat to or from a mate-rial by reason of the mass movement of a fluid or gasin contact with it. In kiln drying wood, air is gener-ally used as the medium of exchange.

Exchanger—Normally a device for transferring heatfrom one fluid or gas to another without allowingthem to mix. Examples: fin pipes, radiators.

Latent heat of evaporation—The heat requiredto change water into steam without a tempera-ture change and at constant pressure; for exam-ple, water at 212°F and at atmospheric pressurechanges to steam at the same temperature by adding970.3 Btu/lb.

Loss—The amount of heat lost by transmissionthrough the building walls, roof, doors, floor, andvents.

Radiation—The transfer of heat by waves throughspace by reason of a temperature difference existingbetween two bodies. In common terminology in kilndrying, all forms of heat transfer are often lumpedinto one term--radiation of heat.

Sensible heat—n kiln drying, the amount of heatrequired to raise the kiln and lumber to drying tem-perature. (Syn: enthalpy)

Total heat—The latent heat of the water vapor in theair-water vapor mixture plus the sensible heat of themixture.

Transfer coefficient—An experimentally derived num-ber for a particular system that quantifies the rate ofheat exchange between two zones.

High-temperature drying—Use of dry-bulb temper-atures of 212°F or more.

Honeycombing—Checks, often not visible on the sur-face, that occur in the interior of a piece of wood, usu-ally along the wood rays. See Ring failure.

Humidity, absolute—The weight of water vapor perunit volume of space.

Humidity, relative—Ratio of the amount of watervapor present in the air to that which the air wouldhold at saturation at the same temperature. It is usu-ally considered on the basis of the weight of the vapor,but for accuracy it should be considered on the basis ofvapor pressures.

Hygrometer—An instrument for measuring relativehumidity, often consisting of dry-bulb and wet-bulbthermometers.

Hygroscopicity—The property of a substance thatpermits it to adsorb and retain moisture.

261

Hysteresis— The tendency of wood exposed to anyspecified temperature and relative humidity conditionsto reach equilibrium at a lower moisture content whenabsorbing moisture from a drier condition than whenlosing moisture from a wetter condition.

Implosion— The caving in of kiln doors and/or wallsbecause of a sudden marked decrease in pressure belowatmospheric within the kiln. Normally occurs at kilnstartup with very cold lumber or when restarting afterfan failure early in a run.

Indirect fired—A method of heating a dry kiln wherea hot fluid (steam, water, or oil) flows into the kilnin pipes and gives off its heat to the kiln atmospherethrough a suitable radiating surface.

Infiltration, cold air—The uncontrolled and inadver-tent entry of cold air into the dryer through cracks inthe walls and ceiling, or leaky doors, or openings otherthan the fresh-air intake.

Juvenile wood—The initial wood formed adjacent tothe pith, characterized often by lower specific gravity,lower strength, higher longitudinal shrinkage, and dif-ferent microstructure than mature wood.

Kiln—A heated chamber for drying lumber, veneer,and other wood products in which temperature andrelative humidity are controlled.

Automatically controlled—A dry kiln in whichdrying conditions are controlled by the action ofthermostats.

Compartment—A dry kiln in which the total chargeof lumber is dried as a single unit. At any given time,the temperature and relative humidity are uniformthroughout the kiln.

Conventional-temperature—A kiln for drying lumberand other wood products typically operated in therange of 110 to 180°F.

Dehumidification—A type of kiln in which themoisture removed from the lumber, or other woodproduct, is condensed out of the circulating air,which is reheated instead of being exhausted to theatmosphere.

Elevated-temperature—A kiln for drying lumber andother wood products typically operated in the rangeof 110 to 211°F.

Forced-circulation—A dry kiln in which the air is cir-culated by mechanical means.

High-temperature—A kiln for drying lumber andother wood products operated at temperatures above212°F.

Low-temperature—A kiln for drying lumber or otherwood products typically operated in the range of 85and 120°F.

Manually controlled—A dry kiln in which dryingconditions are controlled by the manual operationof valves and ventilators.

Multiple-track—A dry kiln equipped with two ormore tracks.

Natural-circulation—A dry kiln in which air circula-tion depends on the power of gravity and the varyingdensity of air with changes in its temperature andmoisture content.

Package-loaded—A trackless compartment kiln fordrying packages of stickered lumber or other woodproducts. The dryer usually has large doors that canbe opened so that the kiln charge can be placed in orremoved from the dryer by forklift trucks.

Progressive—A dry kiln in which the total charge oflumber is not dried as a single unit hut as severalunits, such as kiln truckloads, that move progres-sively through the kiln. The temperature is lowerand the relative humidity higher at the entering end(green end) than at the discharge end (dry end).

Reversible-circulation—A dry kiln in which the di-rection of air circulation can be reversed at desiredintervals.

Side-loaded—See Package-loaded.

Single-track—A dry kiln equipped with one track.

Track-loaded—A compartment kiln for drying stick-ered lumber that is stacked on kiln trucks, which arerolled into and out of the kiln on tracks.

Vacuum—A compartment kiln in which lumber isdried at less than atmospheric pressure either contin-uously or intermittently during the drying cycle.

Kiln charge—The total amount of lumber or wooditems in a dry kiln.

Kiln charge, mixed—Same as kiln charge but com-posed of more than one species or thickness of lumberor wood items.

Kiln drying—Process of drying lumber in a dry kiln.

Kiln leakage—The undesirable loss of heat and vaporfrom a kiln through badly fitted doors and ventilatorsor through cracks in the walls and roof.

Kiln run—The term applied to the drying of a singlecharge of lumber.

Kiln sample—A section 30 in or more in length cutfrom a sample board and placed in the kiln charge sothat it can be removed for examination, weighing, andtesting.

Controlling—Some of the wettest samples used tocontrol the drying. The number depends on the totalnumber of samples used and the composition of thekiln charge.

Driest—The kiln sample with the lowest moisturecontent.

Fastest drying—The kiln sample that loses thelargest amount of moisture in a given period.

Pocket—A space provided for the kiln sample in thekiln packages of lumber.

Slowest drying—The kiln sample that loses the leastamount of moisture in a given period.

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Weight, current—The weight of a kiln sample atgiven times during the drying process.

Weight, final—The weight of a kiln sample after thecompletion of drying.

Weight, green (initial, original)—The weight of a kilnsample prior to kiln drying regardless of its moisturecontent.

Wettest—The kiln sample with the greatest amountof moisture.

Knot—That part of a branch that has become over-grown by the body of a tree. The shape of the knotdepends on the angle at which the branch is cut.

Laminar air—See Air, laminar.

Loading, cross-piled—Lumber piled on kiln trucksand placed in a dry kiln with the long axis of the loadperpendicular to the length of the kiln.

Loading, end-piled—Lumber piled on kiln trucks andplaced in a dry kiln with the long axis of the load par-allel to the length of the kiln.

Longitudinal— Generally, the direction along thelength of the grain of wood. A longitudinal sectionmay be a plane either tangential or radial to the growthrings.

Lumber, kiln-dried—Lumber that has been dried ina dry kiln to a specified moisture condition.

Lumber, shipping-dry—Lumber and other woodproducts that have been air or kiln dried to a suffi-ciently low moisture content to prevent stain, mold,and decay in transit; generally taken to be 25 percentmoisture content or less.

Lumber storage room—A room maintained withinspecified equilibrium moisture content limits so thatlumber stored in it will not gain or lose moisture be-yond fixed limits.

Makeup air—Ambient air that replaces vent air usedto exhaust water vapor being released within the dryer.

Meter, moisture—An instrument used for rapid de-termination of the moisture content in wood by electri-cal means.

Mineral streak—An olive to greenish-black or browndiscoloration of undetermined cause in hardwoods, par-ticularly hard maples; commonly associated with birdpecks and other injuries; occurs in streaks usually con-taining accumulations of mineral matter.

Moisture content of wood—Weight of the watercontained in the wood, expressed as a percentage ofthe weight of the ovendried wood.

Average—The percentage of moisture content of asingle piece of wood or the sum of the moisture con-tents of a number of pieces divided by their number.

Core—The moisture content of the inside part of amoisture section remaining after a shell one-fourththe thickness of the section has been removed.

Determination of—The testing of lumber to deter-mine the amount of moisture present. This is usuallyexpressed in terms of percent of the ovendry weight.

Find—The moisture content of the wood at the endof kiln drying.

Green—The moisture content of wood in the livingtree or freshly sawn wood.

Initial—The moisture content of the wood at thestart of kiln drying.

Shell—The moisture content of the outer one-fourthof the thickness of a moisture section.

Moisture distribution—The variation of moisturecontent throughout a piece of wood, usually from faceto face but sometimes from end to end or from edge toedge.

Moisture gradient—A condition during drying inwhich the moisture content uniformly decreases fromthe inside toward the surface of a piece of wood. Also aterm used specifically to denote the slope of the mois-ture content distribution curve.

Moisture gradient, reverse—A condition followingmoisture regain in which the moisture content is higherat the surface than inside the wood.

Moisture meter—See Meter, moisture.

Moisture range—The difference in moisture contentbetween the driest and wettest boards or samples.

Moisture section—A cross section, 1 in. in lengthalong the grain, cut from a kiln or random sample andused to determine moisture content.

Initial weight of—The weight of a moisture sectionimmediately after being cut from a kiln sample orboard.

Ovendry weight of—The weight of a moisture sectionafter being ovendried to a constant weight.

Mold—A fungal growth on lumber taking place mainlyat or near the surface and, therefore, not typically re-sulting in deep discolorations. Growths are usually ashgreen to deep green in color, although black is common.

Old growth—Timber in or from a mature, naturallyestablished forest. When the trees have grown duringmost if not all of their individual lives in active com-petition with their companions for sunlight and mois-ture, the timber is usually straight and relatively free ofknots.

Ovendry—A term applied to wood dried to constantweight in an oven maintained at temperatures of from214 to 221°F.

Pervious wood—A wood through which moisturemoves readily.

Piling

Box—The flat piling of random-length boards on kilntrucks so that the ends of the completed load are invertical alignment. The longest boards are placed onthe outside of the load, and the shorter boards are

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alternately placed with one end even with one end ofthe load or the other.

Edge—Piling lumber so that the broad face of theboard is vertical.

Flat—Piling lumber so that the broad face of theboard is horizontal.

Pipe

Condensate—The pipe on the downstream side ofheating coils and steam traps that carries condensateback to the boiler.

Feed—Usually the pipe conducting steam from thecontrol valve to the heating coils.

Fin—A heating pipe with many finlike projectionsthat increase the radiation surface.

Plain—A heating pipe with a smooth outer surface.

Steam-spray—A pipe with numerous holes or nozzlesthrough which steam is ejected to increase the rela-tive humidity in the kiln.

Pit—A relatively unthickened part of a wood cell wallwhere a thin membrane may permit liquids to readilypass from one cell to another. A “bordered” pit has anoverhanging rim that is not present in a “simple” pit.

Pitch—The mixture of rosin and turpentine or othervolatiles produced in the resin canals of pines and otherconifers. Term also applied to mixtures of nonvolatileliquids or noncrystalline solids and volatile oils in otherspecies.

Pocket—An opening, extending parallel to thegrowth rings, that contains or has contained pitch.

Streak—A well-defined streaky accumulation of pitchin the wood of certain softwoods.

Pith—The small, soft core occurring in the structuralcenter of a tree trunk, branch, twig, or log.

Plainsawed— Another term for flat-sawed or flat-grained lumber.

Platen pressdrying—Contact heating of wood be-tween heated metal plates to affect drying while underrestraint.

Plenum chamber—The space between the lumberstack and kiln wall for air circulation on the pressureside of a fan or blower in which the air is maintainedunder pressure.

Pore— The cross section of a specialized hardwood cellknown as a vessel. See Vessels.

Predryer—A type of low-temperature dryer. Stickeredloads or unit packages of lumber or other wood prod-ucts are placed in a large building provided with fans,heating system, and vents such that air of a given tem-perature and humidity can be circulated through thelumber.

Pressdrying, platen—See Platen pressdrying.

Psychrometer—An instrument with both wet-bulband dry-bulb thermometers for determining the amountof water vapor in the atmosphere.

Psychrometric char—A table or graph used to re-late the absolute humidity, relative humidity, and dry-and wet-bulb temperatures.

Quartersawed— Lumber sawed so the wide faces areapproximately at right angles to the annual growthrings. See Grain.

Radial— Coincident with or generally parallel to a ra-dius of the tree from the pith to the bark. A radialsection is a lengthwise section in a plane that passesthrough the pith.

Radiation—A term often used in kiln drying to de-scribe heat transfer from heating coils to lumber. Inthis common use, it is understood to include both con-vection and radiation heat transfer, although the for-mer is the most important in kilns.

Balanced—Construction and arrangement so as toensure equal radiating surface and uniform tempera-tures throughout the kiln.

Excessive—A greater amount of radiation thanrequired.

Flexible—The arrangement of the heating systeminto small coils equipped with hand valves that, whenopened or closed, permit rapid adjustment of the ra-diating surface to meet the required needs.

Raised grain—A roughened condition of the surfaceof dressed lumber in which the hard summerwood israised above the softer springwood but not torn loosefrom it.

Rays, wood—Strips of cells extending radially withina tree and varying in height from a few cells in somespecies to 4 in or more in oak. The rays primarily serveto store food and transport it horizontally in the tree.

Redry—A process in which material that has beendried but is at a moisture content level higher than de-sired is returned to the dryer.

Refractory—In wood, implies difficulty in processingor manufacturing by ordinary methods, difficulty indrying, resistance to the penetration of preservatives, ordifficulty in machining.

Relative humidity—See Humidity, relative.

Resin canal (or duct)—Intercellular passages thatcontain and transmit resinous materials. They extendvertically or radially in a tree.

Ring, annual growth—See Annual growth ring.

Ring failure (or separation)—A separation of thewood during drying. Occurs along the grain and paral-lel to the annual rings, either within or between rings;called honeycomb and ring check in some localities. SeeShake.

Ring-porous wood—Wood in which the pores of theearlywood (springwood) are distinctly larger than thoseof the latewood (summerwood) and form a well-definedzone or growth ring.

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Sample—See Kiln sample.

Sample board—A board from which one or more kilnsamples will be cut, or a board taken from a kiln truck-load during drying for the purpose of cutting a mois-ture section.

Sap—The moisture in green wood, which contains nu-trients and other extractives in solution.

Sap stain—See Stain, blue.

Sapwood—The layer of wood near the outside of thelog that is actively involved in the life processes of thetree. Usually lighter in color than the heartwood.

Sapwood stain—See Stain, blue.

Seasoning— Removal of moisture from green wood,and in some cases relief of stresses, in order to improveits serviceability. (Syn: drying)

Second growth—Timber that has grown after the re-moval, whether by cutting, fire, wind, or other agency,of all or a large part of the previous stand.

Sensible heat—See Heat, sensible.

Set—A localized semipermanent deformation in woodcaused by internal tensile or compressive stresses.

Compression—Set, occurring during compression,that tends to give the wood a smaller than normaldimension after drying. Usually found in the inte-rior of wood items during the last stage of drying butsometimes in the outer layers after overconditioningor rewetting. Also caused by external restraint duringrewetting of dried wood.

Tension—Set, occurring during tension, that tends togive the wood a larger than normal dimension afterdrying. Usually occurring in the outer layers dur-ing the first stages. Also caused by external restraintduring drying of wet wood.

Shake—A separation along the grain, the greaterpart of which occurs between and within growth rings.Found in stumps and ends of freshly cut logs and greenlumber. See Ring failure.

Short circuiting of air—The movement of airthrough other than desired channels. Usually resultswhen a kiln charge is improperly loaded and/or baffled.

Shrinkage—The contraction of wood caused by dryingthe material below the fiber saturation point.

Longitudinal—Shrinkage along the grain.

Radial—Shrinkage across the grain, in a radialdirection.

Tangential—Shrinkage across the grain, in a tangen-tial direction.

Sinker—A log that sinks or has low buoyancy inwater.

Sinker stock—Green lumber or other green sawmillproducts that will not float in water. Sinker stock maybe sawn from sinker logs that were water-logged duringponding or from freshly cut logs containing wetwood.The green moisture content is abnormally high, and

the lumber tends to dry slowly and is prone to developchecks and honeycomb.

Softwood— Wood produced by one of the botanicalgroups of trees that, in most species, have needle orscalelike leaves.

sorter

Drop—A mechanical lumber-sorting device that sortslumber for thickness, width, and length by droppingthem into separate compartments accordingly.

Edge—A mechanical lumber-sorting device consistingof grooves or slots in which the lumber is placed onedge. Lines of live rolls, arranged under the slots,carry the lumber to the desired bin or compartment.

Tray—A mechanical lumber-sorting device consistingof a series of trays one above the other into which thelumber is ejected by either mechanical or electricalsignaling devices.

Specific gravity—The ratio of the ovendry weight of apiece of wood to the weight of an equal volume of water(39°F). In drying, specific gravity values are usuallybased on the volume of the green wood.

Split—A lengthwise separation of the wood, caused bythe tearing apart of the wood parallel to the wood rays.

Spray line—A plain pipe of varying sizes and lengthsthat is drilled with holes of various sizes and spacingthrough which steam is injected into the kiln.

Springwood (earlywood)—The part of the annualgrowth ring that is formed during the early part of theseason’s growth. It is usually less dense and mechani-cally weaker than summerwood.

Stain—A discoloration in wood that may be causedby such diverse agencies as micro-organisms, metal, orchemicals. The term also applies to materials used toimpart color to wood.

Blue (sap stain, sapwood stain)—A bluish or grayishdiscoloration of the sapwood caused by the growthof certain dark-colored fungi on the surface and inthe interior of the wood, made possible by the sameconditions that favor the growth of other fungi.

Chemical—A general term including all stains thatare due to color changes of the chemicals normallypresent in the wood, such as pinking of hickory andbrowning of some softwoods, particularly the pines.

Chemical, brown—A chemical discoloration of wood,which can occur during the air drying or kiln dryingof several softwood species, caused by the concentra-tion and modification of extractives.

Iron-tannate—A bluish-black surface stain on oakand other tannin-bearing woods following contact ofthe wet wood with iron, or with water in which ironis dissolved.

Mineral—An olive to greenish-black or brown discol-oration in hardwoods, particularly maple, caused bybird peck or other injury and found either in massdiscoloration or mineral streaks. The mineral associ-

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ated with such streaks is frequently calcium oxalate,which has a tendency to dull machining knives.

Sticker—A gray to blue or brown chemical stain oc-curring on and beneath the surface of boards wherethey are in contact with stickers (also fungal sapstain when found only in the sticker area).

Water—A yellowish to blackish surface discolorationcaused by water that dripped onto the wood duringdrying.

Weather—A very thin grayish-brown surface dis-coloration on lumber exposed for a long time to theweather.

Steam—The vapor into which water is converted whenheated.

Exhaust—Steam that has already passed through asteam engine or machine.

Flash—The reevaporation of hot water produced byexcess heat when the water is discharged to a lowerpressure.

Live—Steam obtained directly from the boiler.

Superheated—Steam at a temperature higher thanthe saturation temperature corresponding to thepressure.

Steam binding—The presence of steam in the drainline between the heating coil and trap, which temporar-ily prevents the drainage of condensate and air fromthe coil.

Sticker— A wooden strip placed between the coursesof lumber in a kiln load and at right angles to the longaxis of the boards to permit air circulation.

Alignment—The placing of stickers in a pile, pack-age, or truckload of lumber so that they form verticaltiers.

Spacing—The distance between stickers measuredfrom center to center.

Stress, drying—An internal force, exerted by eitherof two adjacent parts of a piece of wood upon the otherduring drying, caused by uneven drying and shrinkage,and influenced by set.

Tensile—Stress in the outer layers of wood duringthe early stages of drying when the layers are tryingto shrink but are restrained by the still-wet interiorregion; also, the stress in the interior layers later indrying as they try to shrink and are restrained by theset outer shell.

Compressive—Stress found in the interior region ofwood during the early stages of drying, caused by theshrinking of the outer shell; also, stress in the outerlayers later in drying caused by the shrinking of theinterior.

Stress free—Containing no drying stresses.

Stress section—A cross section of a sample that is cutinto prongs of equal thicknesses, from face to face.

Stresses, relief of—The result of a conditioning treat-ment, following the final stage of drying, which causes aredistribution of moisture and a relief of the sets.

Stresses, reversal of—The normal change from ten-sion in the surface and compression in the center tocompression in the surface and tension in the center.

Summerwood (latewood)—The part of the annualgrowth ring that is formed during the latter part of thegrowing season. It is usually denser and mechanicallystronger than springwood.

Surface check—See Check, surface.

Tangential— Coincident with or generally parallel to a.tangent at the circumference of a tree or log, or growthrings. A tangential section is a longitudinal sectionthrough a tree perpendicular to a radius.

Temperature— Degree of hotness or coldness.

Cold zone—The lowest entering-air dry-bulb temper-ature in the kiln.

Drop across the load—The reduction in the dry-bulbtemperature of the air as it flows through the loadand is cooled by evaporating moisture from the loadof lumber.

Dry-bulb—The temperature of the kiln air.

Hot zone—The highest entering-air dry-bulb temper-ature in the kiln.

Longitudinal variation of—The range of entering-airdry-bulb temperatures in a kiln measured along thekiln length.

Wet-bulb—The temperatures indicated by any tem-perature measuring device, the sensitive element ofwhich is covered by a smooth, clean, soft, water-saturated cloth (wet-bulb wick or porous sleeve).

Temperature gradient, longitudinal—A term usedto denote longitudinal temperature differences within adry kiln.

Tension wood—A type of wood found in leaning treesof some hardwood species, characterized by the pres-ence of fibers technically known as “gelatinous” and byexcessive longitudinal shrinkage. Tension wood fiberstend to “pull out” on sawed and planed surfaces, givingso-called fuzzy grain. Tension wood causes crook andbow and may collapse. Because of slower than normaldrying, tension wood zones may remain wet when thesurrounding wood is dry.

Texture—A term referring to the size of wood cells.Thus, “fine-textured” wood has small cells and “coarse-textured,” large cells. Where all the cells of a softwoodor all the pores of a hardwood are approximately thesame size, as seen on the cross section, the wood canbe called “uniform textured.” The term is sometimeserroneously used in combination with soft or hard.

Thermocouple— A temperature-sensing device madeby soldering or fusing two dissimilar metal wires to-gether and connecting the wires to a potentiometer or

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similar device, thereby determining the temperatureat the junction. Copper-constantan thermocouples areusually used in dry kiln work.

Tracheids—The elongated cells that make up thegreater part of the wood of the softwoods; frequentlyreferred to as fibers.

Transverse—The directions in wood at right angles tothe wood fibers or across the grain, including radial andtangential directions. A transverse or cross section is asection through a tree or timber at right angles to thepith. It has an end-grain surface.

Trap, steam—A device that discharges air and con-densate from steam-heating coils but limits the passageof steam.

Treatment, equalization—A controlled temper-ature and relative humidity condition used in a drykiln at the end of drying to stop the drying of the dri-est boards while allowing the wettest boards to con-tinue drying, thus reducing the moisture range betweenboards.

Treatment, steaming—Spraying steam directly intothe kiln to attain a condition at or near saturation inthe initial stages of kiln drying to retard the growthof mold. Also used to increase the rate of heating coldlumber. Sometimes used needlessly during other stagesof drying to restore surface moisture, and often usedwithout proper control to partially relieve stresses atthe end of drying.

Twist—A form of warp caused by the turning or wind-ing of the edges of a board so that the four corners ofany face are no longer in the same plane.

Tyloses—Extensions of parenchyma cells into thepores or vessels of some hardwoods, notably white oakand black locust, prior to or during heartwood form-tion. They tend to prevent or greatly retard moisturemovement through the vessels.

Vacuum kiln—See Kiln, vacuum.

Vapor barrier—A material with a high resistance tovapor movement, such as asphalt-impregnated paper,that is used in combination with insulation to controlcondensation.

Vapor pressure—The pressure exerted by a vaporwhen the rates of condensation and evaporation are inequilibrium between the liquid and vapor state.

Ventilator (or vent)—An opening in the kiln roof orwall, or in the blower duct work, that can be opened orclosed in order to maintain the desired relative humid-ity condition within the kiln.

Automatic control—A ventilator that is opened orclosed by a thermostat.

Linkage—The adjustable, pivoted rods connecting thevent cover to an air valve or to a hand-operated level,which facilitate the opening and closing of the vents.

Manual control—A ventilator that is opened or closedby hand.

Vessels—Wood cells in hardwoods of comparativelylarge diameter that have open ends and are set oneabove the other so as to form continuous tubes. Theopenings of the vessels on the surface of a piece of woodare usually referred to as pores.

Virgin growth—The original growth of mature trees.

Wane—Presence of bark or the lack of wood from anycause on the edge or corner of a piece.

Warp—Any variation from a true or plane surface.Warp includes cup, bow, crook, twist, and diamonding,or any combination thereof.

Water, bound (adsorbed, hygroscopic)—Moisturethat is bound by adsorption forces within the cell wall;that is, the water in wood below the fiber saturationpoint.

Water, free—Moisture that is held in the cell cavitiesof the wood, not bound in the cell wall.

Water box—A water container that is mounted underthe wet bulb and supplies water to the wick.

Water pocket—An area of unusually high moisturecontent in lumber; pockets are of various sizes andshapes. Also called wet pocket.

Waterlogging—The presence of water in steam coils,which interferes with the normal flow of steam and seri-ously affects the heating efficiency of the coil.

Wet-bulb temperature.—See Temperature.

Wetwood—Green wood with an abnormally highmoisture content that generally results from infectionsin living trees by anaerobic bacteria, but may also re-sult from water logging during log ponding. Wetwoodcan occur in both softwoods and hardwoods; the greenlumber is usually difficult to dry without defects. Al-though difficult to recognize, wetwood is often charac-terized by a translucent, water-soaked appearance anda sour or rancid odor.

Wood—The hard material between the pith and thebark in the stems and branches of trees, made up of avariety of organized hollow cells and consisting chemi-cally of cellulose, hemicelluloses, lignin, and extractives.

Wood, reaction—In wood anatomy, wood with moreor less distinctive anatomical characteristics, formedin parts of leaning or crooked stems and in branches.Reaction wood consists of tension wood in hardwoodsand compression wood in softwoods.

Wood, refractory—See Refractory.

Wood, ring-porous—See Ring-porous wood.

Wood substance—The extractive-free solid materialof which the cell walls of ovendried wood are composed.Wood substance has essentially the same specific grav-ity in all species.

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INDEX

Air circulationbaffles, 58

perforated, 115slotted, 115

disk fan, 57external fan, 58forced-air system, 44internal fan, 57

cross shaft, 57lineshaft, 57

kiln fan, 56natural draft, 44plenum chamber, 59plenum width, estimating, 59propeller-type fan, 57

Air movement, equipment for determining, 85Aluminum panels, inspection and maintenance, 88Baffling, 114-116Balance. See also Scale.

electronic top-loading, 75indicating, 78self-calculating, 78triple-beam, 75

Bastard sawn, 4Bending strength. See Modulus of rupture (MOR)Bevel siding, softwood schedules for, 144Biocides, 192Blending chamber. See Mixing chamber.Bourdon tubes, 62Box piling, 110Brown stain, 189, 222

softwood schedules for control, 143Cam controller, 64Capillary tubes, 62Casehardening, 12, 125-127

defects associated with, 199Cedar oil, softwood schedules for retaining, 143Centrifugal blower, 53Color, wood, 5Combustion chamber, 53Compartment kiln, 43-48Compressed air, quality, 91Compression wood, 5, 179Conditioning treatment, 145-147. See also

Kiln operation, conditioning treatment.Construction features, kiln, 50-61

air-circulation system, 56-59control valves. 56heating system, 51-54humidification system, 60

materials, 50steam traps, 54venting system, 60

Construction lumber, 6appearance, 6nonstress-graded, 6stress-graded, 6

Construction materials, kiln, 50aluminum, 50brick, 50concrete block, 50floor, 51foundation, 51plywood, 51poured concrete, 50wood, 51

Convection oven, for drying moisture sections, 79Conventional-temperature kiln, 49Defect, drying, 179

alkaline stain, 197bow, 187boxed-heart splits, 186broken knots, 198checked knots, 186chemical stain, 189, 194, 222chipped grain, 198collapse, 12, 183commercial woods, 1, 6crook, 187cup, 187diamonding, 187discoloration, 189-197

removal, 197end checks, 182heartwood discoloration, 194

chemical, 194fungal, 195

honeycomb, 12, 185loose knots, 186machining, relationship to, 198metallic stain, 197planer splits, 199raised grain, 199residual drying stress, 199ring failure, 186rupture of wood tissue, 180sapwood discoloration, 189

bacterial stain, 193blue stain, 191, 222brown stain. 190

269

chemical, 189fungal stain, 191oxidation, 189-191sticker marks, 194sticker stains, 194

shrinkage, 180splits, 182surface checks, 180torn grain, 198twist, 187uneven moisture content, 188warp, 180, 187water pocket, 188wetwood discoloration, 196

Degrade, drying, 179Dehumidification kiln, 66

advantages, 66disadvantages, 66energy demand, 249

Desuperheater, 61Dry-bulb temperature sensor, 62, 90Drying

benefits, vienergy consumption, 242, 246process in wood, 9rate, factors influencing, 10specialized approaches, 66specialized kiln types, 66-72

Drying conditions, equipment to control, 61-66automatic, 61computerized, 65fully automatic, 64manual, 66semiautomatic, 61zone, 65

Earlywood, 4Economizer, 60Edge grained, 4Electronic servo module, 63Elevated-temperature kiln, 49EMC wafer. See Equilibrium moisture content waferEnergy, 239-255

consumption, 239-246demand, 246-250

air drying followed by kiln drying, 249dehumidification drying, 249forced-air drying, 246-249platen press drying, 250predrying followed by kiln drying, 249solar drying, 249

vacuum drying, 250energy partition, 250fuel costs, 250heat capacity, 240heat of adsorption, 240heat loss, 243-244

dryer, 243steam delivery, 246steam generation, 246

vent air, 244heat transfer, 241latent heat of evaporation , 239, 243, 246overall heat transfer coefficient, 241practical application, 250sensible heat, 245, 246thermal conductivity, 240units, 239

Energy source. See Heating system, kiln.Equalizing treatment, 146-147, 213Equilibrium moisture content. See Moisture

content, equilibrium.Equilibrium moisture content wafer, 62, 69Fans, 56, 96Fiber saturation point, 8Fire prevention, 216Fire retardants, softwood, schedules for lumber

treated with, 144Firsts, 6Flatsawn, 4Flatgrained, 4Fungal stain, 191-193, 195-196

sterilizing treatments, 145Fungicides, 192Grade, lumber

B&BTR, 7C&BTR, 7Factory Select, 7Firsts, 6hardwood, 6KD, 7MC-15, 7S-Dry, 7S-GRN, 7Seconds, 6Select Shop, 7Selects, 6softwood, 6

Grain, 5Growth rings, 4Heartwood, 4Heat exchanger, 60Heating system, kiln, 49, 51-54

direct fire, 49, 53electricity, 50, 71hot oil, 50, 53hot water, 50, 53indirect, 52solar, 50, 69steam, 49, 52. See also Steam heating.

High-temperature kiln, 49Honeycomb, 13, 135, 185Humidity sensor, 60, 62, 66Hygrometer, 84Insects

lumber treatment, 233sterilizing treatment, 146

Instrument, kiln. See Recorder-controller.

270

Inspection, dry kilns and equipment. SeeMaintenance, dry kilns and equipment.

Insulation values, 51, 67, 88Interlocked grain, 179Juvenile wood, 5, 179Kiln classification, 43-50

heating and energy source, 49operational techniques, 43temperature of operation, 48

Kiln loading, 114-116package loading, 116track loading, 115

Kiln operation, 207-217conditioning treatments, 146-147, 213-214

stress relief, 214temperature, 214time, 214

cooling a charge, 215drying process, 212-213equalizing treatments, 146-147, 213fire prevention, 216modifying schedules, 138-139, 215moisture content tests, 214part-time, 211safety precautions, 215starting, 209-210stress tests, 214-215temperatures, 211warmup, 210

Kiln performance, 119Kiln samples, 112, 117-131, 207

bolster space, 113controlling samples, 124cutting, 120determining moisture content, 120-122final test, 125intermediate test, 124-125, 213moisture content test, 214number, 119ovendrying, 121placement, 123pockets, 113recording data, 127schedule changes, 124selecting, 120stress test, 125, 214variability of material, 118-119

grain, 119heartwood, 118moisture content, 118sapwood, 118sinker stock, 118species, 118thickness, 118wetwood, 118

weighing, 121Kiln schedules, 133-178, 208

conditioning treatments, 145-147

dehumidification, 145equalizing treatments, 145-147hardwood, 135-141

air-dried lumber, 137alternative schedule, 140assembling a drying schedule, 136-137general, 135high-temperature, 140imported species, 140material considerations, 135maximum strength, 140modification, 138-139moisture content, 135-140predried lumber, 137special, 140-141steam-heated kiln, 135time, 140

homogeneous charge, 208mixed charge, 208modifying, 215selecting, 208softwood, 141-145

air-dried lumber, 142brown-stain control, 143, 190bundled short-length items, 144conventional-temperature, 143fire-retardant-treated, 144high-temperature, 143knotty pine lumber, 145large timbers, 144maximum strength, 144modifying, 142moisture content, 142poles, 144preservative-treated, 144resawed products, 144retaining cedar oil, 143setting pitch, 143special purpose, 143-145tank stock, 145time schedule, 142-145

sterilizing treatments, 145Knots, 186Latewood, 4Load-cell system, 65Log storage, 220-224

dry, 220-223debarked, 222with bark, 221

end coating, 221pond, 223staining, 221, 224submerged, 223-224transpiration drying, 222water sprinkling, 224wet, 223

Low-temperature kiln, 49

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Lumber storage, 225-234chemical treatment, 230-234closed, unheated shed, 229conditioned storage shed, 230equilibrium moisture content, 225green lumber, 226kiln-dried lumber, 226open shed, 228outdoor, 225partly dried lumber, 226pile covers, 228relative humidity, 225staining, 230temperature, 225transit, 234-235

rail, 235ship, 235truck, 234

treating, 231-234Maintenance, dry kilns and equipment, 87-102

air-circulation system, 95belts, 96fans, 96fan baffles, 96fan motors, 96fan shafts, 96load baffle system, 96oil lines, 96pulleys, 96

checklist, 99-101heating system, 92

direct fire, 94steam, 92

humidification system, 95kiln structure, 87

ceilings, 87doors, 89floors, 89rail supports, 89rails, 89roofs, 87walls, 87

kiln trucks, 96problems, locating, 97protective coating, 97recording-controlling instruments, 90

calibration, 91dry-bulb sensors, 90wet-bulb sensors, 90

steam-heated kiln. See Steam-heated kiln,maintenance.

traps, 54, 94valves, 93venting system, 95

Masonry, inspection and maintenance, 88

Maximum strength uses of lumber, 140, 144aircraft, 140, 144ladders, 140, 144sporting goods, 140, 144

Microwave oven, for drying moisture sections, 79Mineral streaks, 179Mixing chamber, 49, 53Modulus of elasticity (MOE), effect of drying

temperature, 180Modulus of rupture (MOR), effect of drying

temperature, 180MOE. See Modulus of elasticity (MOE)Moisture, 7, 8

bound water, 8free water, 8

Moisture content, 7-8classes, 135determining number of kiln samples, 119equilibrium, 8

and relative humidity, 8equipment for determining, 75

balances and scales, 75distillation equipment, 75drying ovens, 79electric moisture meters, 80saws, 79

test, 214Moisture content schedules, 134

hardwood, 135-141operation on, 212softwood, 141-145

Moisture meter, electric, 80dielectric power loss, 81resistance, 81

Moisture section. See Kiln samples.Mold, sterilizing treatment, 145MOR. See Modulus of rupture (MOR).Oven, drying, 79

electrically heated, 79steam-heated, 80

Package-loaded kiln, 46Pencil stock, softwood schedules for, 144Pine squares, softwood schedules for, 144Pitch, softwood schedules for setting, 143Pitch, fan, 58Pitch pockets, 179Pith, 4Plainsawn, 4Platinum RTD-type bulbs, 63Plenum chamber, 59Predryers, 68

advantages, 68disadvantages, 68

Preservatives, softwood schedules for lumbertreated with, 144

Presorting, to control uneven moisture content, 189Progressive kiln, 48

272

Psychrometer, 84Psychrometric chart, 39Quartersawn, 4Reconditioning, removing collapse-related defects, 184Recorder-controller, 62, 90

calibration, 91on-off mode, 62proportional valves, 62

Resistance, electrical, 13-15Ring shake, 179RTD. See Temperature sensors, resistance

temperature detector (RTD).Safety precautions, 215-216Sample boards, 120Sap, 7Sapwood, 4Scale. See also Balance.

self-calculating, 78Schedules, 133-178Seconds, 6Selects, 6Shingles, softwood schedules for, 144Shrinkage, 11

differential, 12and moisture content, 12

Side-loaded kiln, 44Sinker stock. See Wetwood.Slashgrained, 4Solar kiln, 69

absorber surface, 69collector, 69collector surface, 69energy demand, 249greenhouse type, 69

Solar radiation, 73Sorter, lumber, 106Sorting, lumber, 103-108

grade, 104grain, 104heartwood, 104length, 106moisture content, 104sapwood, 104species, 103thickness, 105wetwood, 104

Specific gravity, 10Specific heat, 15Spiral grain, 179Springwood, 4Stackers, 110-112

automatic, 110semiautomatice, 112

Stacking, lumber, 112clamps, 114restraining devices, 113weights, 113

Stains, 189-197Steam-heated kiln

hardwood schedules, 131-141maintenance, 92

feedline insulation, 92pipes, 92pressure gauges, 93regulators, 93traps, 93unions, 92valves, 93

Steam heating, 49, 52back pressure, 54blowdown valve, 54booster coils, 52check valve, 54condensate, 54condensate header, 52control valves, 56

gate, 56hand, 56

discharge header, 52distribution header pipes, 52finned pipe heating coils, 52reheat coils, 52return-bend header, 52return-bend heating system, 52single-pass coils, 52steam traps, 54strainer, 54traps, 54

gravity, 54impulse, 55inspection and maintenance, 93inverted bucket, 54mechanical, 54open bucket, 54thermodynamic, 55thermostatic, 55

Steam spray systems, maintenance, 95Steel components, maintenance, 88Sterilizing treatments, 145Stickering, 106-110

load supports, 107stain, 107, 194sticker guides, 109

Stickers, 107alignment, 109auxiliary, 109care, 110locations, 108material, 107moisture content, 107pinned, 114serrated, 114size, 107spacing, 108

273

Stiffness. See Modulus of elasticity (MOE).Storage, log and lumber, 219-238. See also

Log storage; Lumber storage.Stress, drying, 12-13

hydrostatic tension, 12Stress tests, 125-127, 214-215Summerwood, 4Superheat, 61Temperature

dry-bulb, controlling, 211equipment for determining, 82-84wet-bulb, controlling, 211

Temperature-limit switches, 53Temperature schedules

code numbers, 135, 139, 140shifting, 139steam-heated kilns, 135

Temperature-sensing bulbs, 61Temperature sensors

gas-filled, 62liquid-vapor, 62proper location, 90resistance temperature detector (RTD), 62

Tension wood, 5, 179Texture, 5Thermal conductivity, 15Thermal expansion, 16Thermocouple wire, Type-T, 66, 83Thermometer

dry-bulb, 84electric digital, 66, 83etched-stem, 66, 84glass-stemmed indicating, 66maximum, 66, 84wet-bulb, 84

Time schedules, 134, 140, 142determining number of kiln samples, 119operation on, 212softwood, 142-143

Track-loaded kiln, 44Traps, steam, 54, 93Unstackers, 112Vacuum kiln, 70

electrically heated conductive blankets, 71energy demand, 250high-frequency electrical energy, 50, 71steam-heated platens, 71

Venetian blinds, softwood schedules for, 144Venting system, 60

powered, 60pressure, 60static, 60

Vertical grained, 4Warehouse dryers, 68Water spray systems, maintenance, 95

Wet-bulb depression schedulesH-type, 138hardwoods, 135shifting, 138softwoods, 142steam-heated kilns, 135

Wet-bulb temperature sensor, 62Wetwood, 196

responsible for uneven moisture content, 189susceptible to collapse, 184

Woodcommercial species, 1electrical properties, 13structural features, 2-6

variations, 5thermal properties, 15

conductivity, 15expansion, 16specific heat, 15

weightcalculated, 10dry, 7wet, 7

Wood rays, 4

274