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Page 1: Design of Exhaust Ventilation for Solid Materials Handling

2442 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 11

temperatures may range from below 0’ F. to substantially above 1000 O F. without impairing the efficiency of sonic agglomeration. At extreme temperatures, of course, special consideration must be given to the materials of which the collection system is con- structed.

Thirdly, sonic agglomeration of fine particles appears to be independent of the electrical characteristics of the particles. Thus, inert particles and those characterized as insulators may effectively be collected, whereas such properties may exclude their collection by electrostatic means. A fourth consideration lies in the absence within a sonic collection system of any fire hazard. Thus flammable liquid or solid materials as well as combustible gases may be treated without the fire or explosion hazard existing in some other collection systems in which electri- cal discharges, arcs, or short circuits may occur. The gas passed through the sound generator may be air, steam, any other inert gas, or the process gas itself which is being treated; the latter may be tapped off downstream from t,he sonic collector and at that point is compressed for operating the sound generator.

A further consideration is that there must be a sufficient num- ber of particles in each cubic foot of gas so that, as the particles are vibrated in the acoustic field, there may result the proper number of collisions between the particles. If the particles are too widely separated in the gas, an adequate degree of agglomera- tion may not be achieved. The required particle weight per cu- bic foot will vary with the average particle size. As a rough approximation 1 grain per cubic foot is sufficient when the parti-

cle sizes range from 1 to 10 microns, but the grain loading figure may decrease somewhat for aerosols in which the average particle is smaller than 1 micron, as a larger number of collision targets is available. When the normal grain loading is too small, various techniques may be employed to increase it-for example, water 01 other liquid may be sprayed (or condensed) into the aerosol or a second aerosol may be mingled with the first. In some cases the grain loading may be effectively increased by cooling the gas to diminish its volume.

I n conclusion, there is a broad range of recovery problems where sonic collection may advantageously be used. This area appears to include the recovery of many valuable materials throughout the smelting, petroleum, chemical, steel, carbon black, cement, lime and rock products, sulfur, paper, and other process indus- tries. Moreover, many industries are faced with serious nuisance abatement problems arising from their random discharge of ob- noxious fumes, dusts, and smog. Sonic collection techniquw offer an wonomical solution to many such problems.

LITERATURE CITED

(1) Bergmann, L., “Ultrasonics,” New York, John Wiley & Sons, 1944. (2) Brandt, O., Freund, H., and Hiedemann, E., KoZloid Z., 77, 103

(1936); Trans. Faraday Soe., 32, 1101 (1936). (3) Hiedemann, E., KoZZoid-Z., 34, 494 (1933). (4) St. Clair, H. W., Spendlove, M. C., and Potter, E. V., U. 8. Bur.

Mines, Rept. Invest. 4218 (March 1948). (5) Ultrasonic Corporation, patents applied for (1945--49). RECEIVED March 7, 1949.

Design of Exhaust Ventilation for Soli Materials Handling

FUNDAMENTAL CONSIDERATIONS

R. T. PRING, J. F. KNUDSEN, AND RICHARD DENNIS Kennecott Copper Corporation, Garfield, Utah

Solid materials handling includes dumping, storing, discharging, feeding, conveying, elevating, screening, mix- ing, loading, and filling of solids; in these operations no change of state of the material occurs. Dust is the atmos- pheric contaminant and is dispersed into the workroom primarily by air currents set up by the movement of the solid particles. Exhaust capacity requirements to elimi- nate dust dispersion have heretofore been determined experimentally or by the use of arbitrary standards having little relationship to actual induced air volumes. Labora- tory tests under controlled conditions have shown that the air volume, Qa, entrained by falling droplets of water may be expressed by

QA = K U‘x (V, - VI) N where A, = the cross-sectional area of the pattern, V, = the maximum velocity attained by the falling droplets, VI = the velocity intercept a t QA = 0, N = the number of particles in a unit system of single successive drops falling in line, A, = the projected area per particle, L = the length, and W the width of the pattern cross section. Under the conditions of the tests, K = 3.21 X 10-2 and VI = 642

feet per minute. Practical application of this expression requires redefinition of A, and N and the evaluation of factors for particle shape and surface roughness, relative enclosure, and crowding of particles, all under field con- ditions. Details of proper hooding and enclosure of ma- terial transfer points, to reduce the amount of solids car- ried into the ventilating system, are illustrated.

MOSPIIERIC contamination within industrial plants is the result of the dispersion of solid or liquid particulate

matter (dusts, fumes, mists), gases, or vapors from an infinite variety of operations or machines. Such dispersion is ordinarily effected by air currents set up by convection, cross drafts, the motion of machines, or the movement of materials. The control of these air currents thus becomes the important factor in the pre- vention of air contamination and is readily accomplished by properly designed local exhaust ventilation.

In this paper, discussion is confined to exhaust ventilation of dusty processes or, more specifically, of solid materials-handling operations. No special problems in duct design or equipment selection are involved, whatever the source of dust; however, certain fundamental principles, peculiar to materials-handling

Page 2: Design of Exhaust Ventilation for Solid Materials Handling

.

November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 2443

terials-handling operations is at transfer points where solid materials drop from one level to another.

Falling material is, in a sense, a low- efficiency fan, drawing in air a t the top of the circuit and forcing i t out a t the bottom. The relationship of static pressure to air volume can be plotted and assumes the shape of a fan characteristic curve. Two phenomena contribute to the movement of air with the material :

1. The compact solids enter the drop, expand through the falling distance, then con- glomerate upon impact a t the bottom of the system. This is a bellows effect, in which air is caused to fill the voids between falling particles only to be forced out by the com- pacting of material a t the bottom of the drop.

2. The force exerted by the falling parti- cles creates a negative pressure within and about the periphery of the column of ma- terial as it drops. The differential between the zone of normal pressure outside the column and that of reduced pressure induces a flow of air toward and along with the falling solids. This may be termed the ejector effect and is, by far, the more important.

Figure 1. Diagram of Test Apparatus

processes, must be recognized by the ventilation engineer if effec- tive dust control is t o be attained. The basis for a successful in- stallation comprises the careful estimation of adequate but not excessive exhaust capacity and the design of enclosures and loca- tion of exhaust hoods to ensure the entrainment of a minimum of solids.

Although much has been written on local exhaust ventilation of operations and machines which lend themselves to a greater or lesser degree of standardization (grinding wheels, degreasing tanks, woodworking machines, spray booths, foundry operations, etc.), the literature contains few authentic, fundamental data on the application of local exhaust ventilation to solid materials- handling systems. Because the flow of materials in each plant is arranged to suit the individual requirements of the process, standardization of equipment and flowsheets is rarely encountered from one industry to the next. For this reason the approach to exhaust ventilation for materials-handling operations has, of necessity, been largely empirical and each iwtallation has been based on trial-and-error methods or on the personal experience of the designer.

For the purpose of this discussion, solid materials handling may be defined as dumping, storing, discharging, feeding, conveying, elevating, screening, mixing, charging, loading, filling, and pack- aging solid materials, in which operations no change of state, physical or chemical, of the materials is effected. The atmos- pheric contaminant dispersed by these operations is, of course, dust. Aside from relatively minor dust generation through frac- ture of materials by impact during handling, the dust dispersed in these operations is present as primary fines in the parent material as i t enters the circuit. Thus, materials-handling operations are dust-dispersing rather than dust-producing processes. Free or hindered fall of material under the force of gravity occurs at some point in the circuit in transferring solids from one place to another. The force exerted by the falling material is, in part, ex- pended in setting up air currents which disperse the fines present in the materials throughout the work space as dust. Although the generation of air currents by the motion of materials- handling machinery, particularly elevators, screens, and con- veyors, cannot be ignored, the chief source of dustiness in ma-

ESTIMATION OF AIR VOLUME

Many attempts have been made to de- . velop reliable methods for estimating air vol-

umes displaced by falling solids. In an earlier paper Pring ( 4 ) pointed out that measurement of air volume es- caping through geometric openings in an enclosure a t the bottom of a gravity. materials-handling system to determine exhaust capacity requirements was subject to considerable error due to the

L

z

0- 2 4 6

Dl5 TAN€ OF fd‘L -FE€T

Figure 2. Theoretical Relationship of Velocity, V,, to Distance of Fall, s

Raindrops 116. 8/89 , and ‘ /e inch in diameter. from

Calculated

Page 3: Design of Exhaust Ventilation for Solid Materials Handling

2444 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 11

r, 2

c

Figure 3. Relationship of Induced Air Volume, QA4, to Velocity of Fall, V,, Determined

Experimentally Droplets lis, 3/82, and I Is inch in relative diameter

resistance of the enclosure and its openings to the outward flow of entrained air. A case was cited in which estimated air volumes determined by anemometer were compared to the volume ex- hausted from the enclosure through an experimental fan so as to produce a condition of no outlT-ard or inward air motion through an opening in the enclosure. The ratio of estimated to actual volumes ranged from 0.36 to 0.89, depending on the area of the openings.

Hatch and Walpole (3) developed a method for the determina- tion of air displacement by falling material in which carbon dioxide is introduced at a known rate into the top of the system and its dilution determined by air sampling a t the bottom. Pro- vided efficient mixing of the gas and the entrained air was effected, the actual volume of the latter was readily obtained by the relationship

where &A = air displaced, liters per minute, X o = nig. per minute of carbon dioxide fed in, X t = mg. of carbon dioxide per liter of air leaving system, and X u = mg. of carbon dioxide per liter of ambient air.

I n practice, some difficulty has been experienced in obtaining proper mixing of the heavy gas in the air stream entering with the solids.

The only other method in common use for determining exhaust capacity requirements in materials-handling systems is the use of an experimental exhauster connected to a previously constructed enclosure. Although reliable, this approach entails a certain amount of expense and is justified only for larger systems.

Certain state industrial ventilation codes and many technical articles list exhaust volume requirements for screening, conveying, elevating, and storing of solid materials in terms of some physical aspect of the machine or process. Typical examples are:

Screens, Vibrating, Flat Deck. 200 feet er minute through hood openings, but not less than 50 cubic g e t per minute per square foot of screen area (6)

Bucket Elevators. 100 cubic feet per minute per square foot of elevator casing cross section, but not less than 200 feet per minute through all openings (6)

Belt Conveyers, Hoods at Transfer Points. Belt speeds less than 200 feet per minute, 350 cubic feet per minute per foot of belt width but not less than 150 feet per minute through open area; belt speeds over 200 feet per minute, 500 cubic feet per minute per foot of belt width but not less than 200 feet pel minute through open area (1)

Bins, Closed Bin Top. 150 to 200 feet per minute through open area at feed points, but not less than 0.5 cubic foot per minute per cubic foot of bin capacity (1)

Advancement of such criteria can be justified only by the fact that some guide to exhaust capacities is essential, particularly in smaller installations, and no fundamental relationship between exhaust volume requirements and the factors contributing to the volume of effluent air that must be removed has yet been worked out. The danger of implicit confidence in them may be illus- trated by the following example, involving a 54-inch conveyer moving a t 350 feet per minute.

Cu. Feet/bIin Exhaust volume, based on width and speed of belt 2260 Exhaust volume, determined experimentally 9000

The reason for the discrepancy is, of course, that width and speed of conveyer have only indirect bearing on tonnage of ma- terial handled and no relationship to the height of fall onto the belt. Similar error is possible in the reference of exhaust volume! to the area of openings in bins and enclosures.

It has been generally conceded that for a given substance the tonnage and height of fall of material control exhaust capacity re- quirements in materials-handling systems. Many unsuccessfut attempts have been made to evaluate these factors under plant conditions, mostly by the experimental use of fans but also by means of the carbon dioxide dilution method. Believing that the understanding of basic factors entering into the production of air currents by falling materials would eventually lead to an evalua- tion of other practical influences, the authors conducted a rather elementary series of laboratory tests under controlled conditions in order to determine the influence of tonnage and height of fall on induced air volumes.

INFLUENCE OF TONNAGE AND HEIGHT OF FALL Study of these factors required that other variables, such as

particle size and shape, tonnage surges in feeding, variations ir, the level of material in the bin, and degree of enclosure of the

,

0 i D R O P L f J VELOC/TY, EL".

Figure 4. Relationship of Induced Air Volume to Droplet Velocity

Effect of pattern variations Droplet diameter 3/52 inch

Page 4: Design of Exhaust Ventilation for Solid Materials Handling

November 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 2445

r- I I I

40

30

20

/ O / 0 2 4 6

Figure 5. Relationship of Induced Air Flow to Pattern Area for Varying Heights of Fall

Based on a l a i inch diameter droplet. Under test conditions A , was also equivalent to number of holes, n, and water

flow, Q w

system be eliminated in so far as possible. The obvious choice of material was water falling in droplets of relatively uniform size. It was felt that shape, particle size, and density factors could eventually be developed in practice and used in applying data based on water droplets t o falling rock and other solids. No other substance could be utilized on a laboratory scale without considerable auxiliary equipment for storing, feeding, and re- moving the material.

Accordingly, a galvanized metal tank was constructed, 2 X 2 X 2 feet high, set on legs and equipped with a trap, A , for drainage (Figure I). By means of a sheet metal baffle, B, set vertically through one centerline, the tank was divided into two chambers 2 X 1 X 2 feet high. One of these was covered at the top to form a plenum chamber connecting through a 2 foot X 3 inch orifice, C, a t the bottom of the baffle with the other compartment which, being open a t the top, served as a receiving hopper for the falling droplets. To the plenum chamber was attached a 5-inch diameter duct, D, 8 feet 6 inches long, leading to a small centrifugal ex- hauster, E, the discharge from which was carried down a corridor away from the test laboratory. At the mid-point of the duct was located a plate orifice, F , with radius taps, by which the air volume drawn from the receiving hopper through the plenum chamber was measured. Exhaust capacity was regulated by a slide damDer located near the fan inlet.

ing the receiving tank as droplets was determined by measuring the overflow from the trap during timed intervals. As expected, fluctuation in flow did not exceed 1 or 2%.

As the droplets fell into the tank, it was observed that air moved into the column outlined by the falling particles and was carried downward under conditions of extreme turbulence. In measuring the flow of air thus entrained, the blast gate in the ex- haust duct was carefully adjusted until air currents in the tank, as traced by titanium tetrachloride smoke clouds, moved very slowly downward and no smoke escaped the enclosure. Air volumes were obtained by noting differential pressures across the orifice and referring to the calibration chart. Dry-bulb and wet- bulb temperatures and barometric pressures were recorded during each test and all data were corrected to an air density of 0.063 pound per cubic foot.

In every case a blank correction was made, comprising the air flow necessary to approximate the conditions of the arbitrary end point with no water entering the system. Blank air volumes varied from day to day according to the magnitude of convection currents in the room.

End points were determined independently by three observers, with good agreement. Reasonable precision of measurements was obtained, as evidenced by the generally satisfactory duplication of results in repeat tests.

The first variable investigated was the height of fall of the droplets. With constant water flow, number of holes, hole diameter, and spacing and pattern shape, the height of the per- forated pan was varied a t 6-inch intervals from 2.0 to 6.0 feet above the bottom of the tank. Subsequently, with the pan at fixed positions, the number and size of holes and the shape of the pattern were varied separately, but in no case was the spacing of holes changed. It was not necessary to use the absolute values for droplet size, volume, or surface area in evaluating test data because these variables could be expressed relatively in terms of the radii of the holes in the pan.

The following assumptions were applied to all tests:

That evaporation from the surface of the falling particles was

That the distance from the pan to the point of drop formation negligible.

was constant and negligible in all tests.

This t&nk or receiving hopper represented the simplest type of bin as found in industry-Le., rectangular in plan, free from obstructions inside and out, and with no enclosing a t or above the top. The use of water coupled with the installation of a water- sealed drain at the bottom eliminated the change in height of fall , due to variations in level of material in the bin. Unequal dis- tribution of exhaust capacity was avoided by the use of the plenum chamber and narrow orifice a t the bottom.

An adjustable framework, G, of Fisher Flexaframe supported above the open tank top a constant-level pan, H, 1 foot X 6 inches X 2 inches deep, with an overflow connection, J, 1 inch from the bottom. Holes were drilled in the bottom of the pan according to

Water

A valve was provided for regulation of flow. During the tests, water volume was governed by the 1-inch hydrostatic head in the constant level pan and the number and area of holes, the head Figure 6. Pattern Shape Factor, (L/W)O.1*9, from being held constant a t all times. The rate of flow of water enter-

3wT-- the pattern, spacing, and relative size of droplets desired, was su plied to the pan through 0.75-inch tubing, K , from a 30,-

/ /o // 12 000-gaEon storage tank which was independent of line pressure.

(LL)”””

/

Ratio of Length to Width

f

Page 5: Design of Exhaust Ventilation for Solid Materials Handling

2446 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol, 41, No. 11

Figure 7.

Solid lines show satisfactory design, dotted lines poor design.

Conveyer Transfer Point with Ore Pocket to Break Material Fall onto Belt

Exhaust connection is rarely required at top of enclosure

The initial velocity of the water stream leaving the pan under a hydrostatic head of 1 inch was inconsequential with respect to the velocity-distance relationship.

Correlation of induced air volume to height of fall was not satisfactory and it became necessary to compute actual velocities of the droplets falling against the resistance

The relationship of observed air volume to maximum velocity of fall as calculated from Equation 3 for specified heights is shown in Figure 3 for relative drop di- ameters of I/IB, 3 / ~ 2 , and l/g inch. The relationship of velocity to air volume i k

a linear function within the size-range of drops investigated and may be expressed as

For any given velocity and all rela- tive drop sizes investigated the volume of air moved was constant. Therefore, the mass flow of water alone is not a factor in determining induced air flow, as for any given droplet velocity the entrained air volumes were equal, whereas the m,ater flow rates varied in the ratio of-4, 9, and 16.

I t appeared that any energy transfei from the droplets to the air must be through the medium of the air resistance to the falling body as defined by Equs- tion 1,

Ed = KldarZV,2

where R is the air resistance force acting against the falling body a t any velocity. Also, the velocity attained by the entrained air may be expressed as

where PI - Pz represents the pressure differential producing the air movement,

For a single droplet acting on a unit croswerl ional area, A,,

Iz P, - Pz = - nA 0

CJ- - - of air according to Kewton’e law, a con- dition where considerable turbulence or eddying occurs.

1 , I

I t In equation form (2 ) <-

(1) ,/ 4- R = Kld,r2YZ 7 I

<

The unbalanced force acting upon the body a t any point may be expressed as mg - Kld,r2V2 and the equation of motion in terms of velocity and acceleration (8) becomes ,

(2)

Equation 2 may be conveniently ex- pressed in terms of velocity and distance (5) as

. nLg - &d,,r2V2 = ma

VT s = In ( ) (3)

Terminal settling velocities for raindrops were obtained from the literature (2) and the relationship of V and s was plotted for droplets ~ / I B , 3/32, and l / ~ inch in di- ameter (Figure 2). The error in using the hole diameter as the drop diameter was Figure 8. Conveyer Transfer Point with Material Falling Directly to Belt

9 dv*2 - v2

and s‘lbRequently shown to be Two exhaust connections are required along conveyer, none at top. Arrows indicate negligible. air movement

Page 6: Design of Exhaust Ventilation for Solid Materials Handling

November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 244'2

a c

Figure 9. Screen Tower, Illustrating Inward Air Currents Due to Long Drop

1. Feed bin 3. Screens 2. Roll feeders 4. Oversize conveyer

5. Undersize chute Arrows indicate air movement

I Y '1

Figure 10. Screen Tower, Illustrating Outward Air Currents Due to Short Drop

1. Feed bin 2. Roll feeders 3. Screens 4. Oversize conveyer 5. Undersize conveyers . Arrows indicate air movement

where n is unity. Therefore

For N droplets in a unit system of single, successive drops fall- ing in line

and, as air flow = VA, then

which, by substituting Kld,rVwZ for R and combining constants, becomes

Because nA, = A,, then, for any value of n,

The air velocities encountered were in the range of turbulent flow, where inertia forces acting on the projected or frontal area of the body predominated over shearing forces. Consequently, the projected area of the droplets may be expressed as

A, = f ( r?

and r in Equation 5 may be replaced with dx, giving the rela- tionship

This expression confirms the linear correlation of &A to (V, - VI) in E uation 4. If NdK and a, are accounted for in K2, their proluct must have been constant under test conditions. In-

asmuch as A,, the cross-sectional area of the pattern, was not varied during thetests forming the basis for Equation 4, it fol- lows that N d A . must be constant for the three drop sizes studied. That this was the case is shown by the following relative values, for constant hydrostatic head in pan:

Hole Diameter $'le inah 8/82 inch 1/8 inch

-Relative Values---- Drop radius, r Total water flow, Qw Drop volume, T I No. of drops per hole, N Projected are& of drops, As NdZ

2 4 8 6 4

12

3 9

27 4 9

12

4 16 64

3 16 12

Figure 4 shows a series of QA us. V , curves covering different, square hole patterns containing from 9 to 36 holes and, for com- parison, the curve from Figure 3 which was based on a rectangular 9 X 4 hole pattern. These data show that for any given pattern shape the linear relationship of QA to V , holds at any fixed value of A,. Figure 5 shows the effect on Q A of variations in A , for three elevations, confirming the theoretical square root relation- ship. Inasmuch as in these tests the hole spacing in the pan was not changed and the hole size was held constant at a/s-inch diameter, A , was also proportional to n and Qw. The correlation of Qa to dx and, consequently, to dG and dE appears to re- sult from the increase in resistance to air entering the column of falling particles as A , or n increases.

Referring again to Figure 4, a variation in Qd is apparent as the shape of the hole pattern is changed a t constant number of holes. A series of tests in which only the ratio of length to width of the hole pattern was varied indicated that the influence of shape could be empirically expressed as

and became unity for any square pattern, increasing to the ex-

Page 7: Design of Exhaust Ventilation for Solid Materials Handling

2448 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 11

Test S O .

15 16 17

101 18 19

102 20 21

PO3 22 23 74 75 76 91 77 78 89 79 80 90 37 24 25 26 27 82 81 28 29

92 93 94 9 5 96 97 62 56 63 57 s 8 59 6 0 61 98 99

100 59 78

66 ti6 87

a8

Hole No. of Diameter, Holes, ri Inch

36 . . . . . . . . . . . . . . . . . . . . I .

36 . . . . I . . . I . . . . . . . I . . . 36 . . . . I . . . . . . . . I

4 9

16 25 36 49 4 9 9

16 25 36 49 64

9 16 25 36 . . . . 24 . . . .

1/16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . .

3/12 . . . . . . . . . . . . . . . . . . . . * . . . . . ~ . . . !/a . . . . . . . . . . . . . . . . . I

. . . 3/32 . . I

. . .

. . . . . . . . ( 8/32 . . . , . . . . . . . . . . . . . I . . . 8 / 8 2 . . . . . . a/32 . . . . . . 1/32 ... ...

Table I. S u m m a r y of Test Data

Pattern Shape, L X W 9 x 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 x 4 . . . . . . . . . . . . . I . . . _ . . . . . . . . . . . . . . . . . . . . . . 9 x 4 . I . .

. . . . . I . .

. . . .

. . . .

. . . .

. , . . 2 x 2 3 x 3 4 x 4 5 x 5 6 x 6 7 x 7 2 x 2 3 x 3 3 x 3 4 x 4 5 x 5 6 x 6 7 x 7 8 x 8 3 x 3 4 x 4

6 x 6 9 x 4

18 x 2

6 x 4 8 x 3

12 x 2

. . . .

Drop Height Velocit of Fall, F e e t r ' Feet, 5 Min., 8,

3 . 0 744 . . . . . . . . ~~ . . . ~. ... ~. . . ~ ~o

4 . 0 834 . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . I . . . . . 5 . 0 912 . . . . . . . . .~ 4 . 0 834 . . . . . . . . . . 4 . 0 834 . . j . . . . . . .

Relative Water S e t Air Pro- Flow Volume jected Shape

Cu. Fdot/ Cu. Feci/ Sur- Factor hlin., Qu, Min., Q A face (L/W)o.laP

0.082 0.082 0.082 0.080 0.083 0.079 0.078 0,083 0.082 0.081 0.083 0.082 0.176 0.171 0,173 0.165 0.165 0.175 0.166 0.174 0.173 0.170 0.178 0.328 0.331 0.331 0.330 0,292 0.289 0,332 0,328 0.021 0,048 0.084

0.183 0.254 0.020 0 048 0.044 0.083 0.132 0.171 0.242 0.316 0.046 0 084 0.128

0.171 0,175 0.164 0.118 0.118 0.109

n 128

. . 6

17 12 22 27 26 34 38 35 46 49

(-1 8

17 15 27 31 32 39 45 45 62

7 11 20 37 37 49 66 64

6 7

10 14 16 19 10

13 17 23 27 32 36 18 24 31 27 31 36 23 26 20

10

4 . I . . . . . . . . .. . . . . . . . . . . 9 . . . . . . . . . . . . . . . . . . . .

16 . . . . . . I~ . . . . . . 9

~) . . . . . . . . 9

, . . . , . . . , I ~. . . 9 . .

I .

9 . . .. 9 . . .~

1.12 . . . . I . .. . . . . . . .. . . . , . .

1.12 . . . . . . . . I . . . , . ~~ . . . .

1.12 . . . . . . . - . 1

. * . . 1 .oo

I . . . . . * . . . 1.00 . . . . . . . . . . . I . .

1.00 . . . .

1 . 0 0 1.12 1.36 1.06 1.16 1.28

treme for a long narrow pattern in accordance with Equation 8. Figure 6 evaluates the exponential term.

Combining all variables and introducing a new proportionality constant, the following equation is obtained which applies to all test data:

(9) Qa = K d&, (V, - I'1) -'V dx (L/W)"*'3'

The numerical value of VI, the velocity intercept in Figure 3, is 642 feet per minute. The constant K becomes 3.21 X 10-2 when the observed values for the variables in Equation 9 are substi- tuted. ,4 summary of test data iy given in Table I.

DISCUSSION OF TEST DATA The tests described were undertaken with the object of deter-

mining certain theoretical relationships between air flow and fall- ing bodies. It was hoped that the understanding of these basic factors would facilitate analysis of other variables encountered in the field. It has been found from these laboratory experiments that certain fundamental factors must be considered in any prac- tical study:

The rate of induced air flow is a direct function of the velocity attained by the falling bodies. In the larger particle-size range, the square root of the height of fall may be used.

The rate of air flow does not depend directly on the weight but

rather on the total projected area of the falling material. This in turn depends on the projected area per particle, the number of particles per unit of area, and the total cross- sectional area of the pattern. For a system involving a single column of falling particles

&A = f ( N dX) and for a system of several columns of particles within the same pattern

an expression which takes into account the interference among adjoining particles. This inter- related effect produces an ex- ponential rather than a linear relationship. Because in these tests A , n, the area concept rather than the number of holes was used to permit broader application.

The influence on Qa of the pattern shape assumed by the falling material can be defined in terms of L/W. Q i was smallest when L/W = 1 and increased as the pattern T T ~ $ elongated.

Although N has been defined in terms restricted to fluid flow. it may be expected, in practice, to become the number of parti- cles per unit of material enter- ing the system.

K , the general constant in Equation 9, is known to contain coefficients of particle shape and roughness and degree of enclosure or restriction t o the ingress of air; all three may be considered equal to unity under the conditions investi-

,

gated. These factors must be evaluated in the plant. It is also recognized that IC must

contain variables for hole spacing (particle crowding) and hydro- static head in the perforated pan, which are currently being in- vestigated in this laboratory.

ENCLOSURE DESIGN AND HOOD LOCATION

Industrial dust-producing operations for which local exhaust ventilation is the most commonly employed control measure may be classified into two broad categories:

1. Those operations in which the dust produced is detrimental or of no recovery value with respect to the principal products of the process. Because of its nuisance value, its complete removal by exhaust ventilation or other means is necessary. Grinding, buffing, sanding, etc.. belong in this classification.

Those operations in which the dust produced or dissemi- nated is in no way different from the original material in usefulness and value with respect to the products of the operation. I ts re- moval from the process is to be avoidcd where possible. Mate- rials-handling operations are included in this category.

2.

A vital consideration in the ventilation of materials-handling operations is the necessity of minimizing the amount of material carried into the exhaust ducts in order to reduce duct, abrasion and to avoid loss of or inconvenience in recovering the portion of the product removed. As previously stated, air enters with the solids at the top of the circuit and, consequently, must be re- moved a t the bottom by properly located exhaust connections.

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November 1949 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y d

2449

Figure 11. Isometric Sketch of Tripper Conveyer Loading Ore Bin 1. Ore chute 2. Plow with vertical 3. Return belt

idlers

T o permit the settling out of coarser particles before they are carried into the exhaust system, the volume of effluent air must be kept a t a minimum by restricting the ingress of air, reducing the height of fall where possible, and effectively enclosing the impact zone. Frequently the drop of material can be broken up into stages by stepped chutes, so that part of the entrained air will be separated from the solids and, rising, oppose the influx of ad- ditional air a t the chute entrance.

Exhaust connections must be located so as to remove the con- taminated air near its point of dislodgement from the solids, and for this reason, they are seldom required at the top of the circuit. To keep all but the finest particles from entering the veptilation system, the air is preferably removed in a direction normal to the path of the solids.

Figure 7 illustrates the application of these principles to a con- veyer transfer point where the fall of material is broken by a pocket in the chute. Solid lines illustrate good design; broken lines, poor design of chute and hooding. The back of the chute is carried well behind the head pulley to catch spill and to encom- pass a belt-cleaning device. By providing a generous opening be- tween chute and hood and locating the single exhaust connection well above the belt, air velocities sufficient to carry coarse particles are avoided. No exhaust is required a t the head pully level, but restricting enclosure is essential. Where material falls directly to the conveyer, two exhaust connections are provided, as in Figure 8, for air dislodged behind the impact zone cannot be captured by a single hood at the front without transporting coarse particles in the air stream. I n this case, a self-closing cleanup chute is provided over the tail pully to facilitate the return of spill to the belt.

Figure 9 shows a screening plant in which the undersize ma- terial falling through a relatively great distance creates inward air

4. Self-closin cover made

5. Bin structure from 013 belting

motion about screens and feeders. An exhaust connection a t the bottom of the undersize chute provides adequate dust control in the operating area, but the great falling distance introduces complications a t that point with respect to the air volume re- quirement and the amount of coarse material carried into the duct.

Where the screen undersize drops through a short distance onto conveyers, as in Figure 10, outward air currents may be expected in the operating zone. In this case, the drop of undersize ma- terial is insufficient to provide inward air motion around the screens and feeders.

Figure 11 illustrates a satisfactory means of enclosing a bin where a tripper conveyer discharges material through parallel slots running the length of the structure. Mounted a t front and rear of each enclosed tripper discharge chute are plows with rollers attached, which force apart the self-sealing closure as the tripper moves along the bin. The closure comprises old belting resting one side on the other a t the apex of an inverted V and hinged along opposite edges to the sides of the slots in the bin top. The combination of this cover with a small exhaust system connected to the bin top provides efficient dust control.

ACKNOWLEDGMENT

The authors are particularly indebted to C. K. Hanson for his verification of mathematical relationships expressed in this paper. The assistance of J. G. Wickens in the preparation of illustrations and H. W. McCullough in typing the manuscript is gratefully acknowledged.

NOMENCLATURE

a = acceleration of particle a t any point, feet per minute per

A, =: pattern area, square feet = nA, minute

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I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 11 2450

/lo = 4. = d, = d, = ( I = K = I; = m =

n = i\; = P =

Q, = r = R =

v = v, =

&A

$ =

relative cross-sectional area acted upon by single drop relative projected surface per particle, square feet air density, pounds per cubic foot particle density, pounds per cubic foot gravity constant, feet per minute per minute constant applying to all test data length of pattern, feet particle mass corrected for buoyancy effect,

number of holes in pan relative number of drops per hole pressure net air flow, cubic feet per minute water flow, cubic feet per minute relative particle radius = hole radius, feet air resistance to falling particle, pound feet per minute pep.

m y e )

V A = air velocity, feet per minute VT = terminal settling velocity of particle, feet per minute Vw = maximum velocity of water droplet, feet per minute W = width of pattern, feet

LITERATURE CITED (1) American Standards Association, unpublished data. (2) Drinker, P., and Hatch, T. F., “Industrial Dust,” New York.

McGraw-Hill Book Co., 1936. (3) Hatch, T. F., and Walpole, R. H., Jr., Industrial Hygiene Fouri-

dation of America, Pittsburgh, Pa., Preventive Eng. Series Bull. 3, Part 1 (1942).

(4) Pring, R. T., Am. Inst. Mining Met. Engrs., Tech. Pub. 1225 (1940).

( 5 ) Reddick and Miller, “Advanced Mathematios for Engineers,” New York, John Wiley & Sons, 1938.

(6) State of New York, Department of Labor, Board of Standards and Appeals, Albany, N. Y., “Rules Relating to Control of Silica Dust in Stone Crushing Operations,” Industrial Code Rule No. 34, 1942.

RPCEIVED Maroh 7, 1949.

e in the Atmo tion to

MORRIS KATZ Defence Research ChemicaI Laboratories, Ottawa, Canada

Sulfur dioxide from industrial gases in low concentra- tions is widely distributed in the atmosphere. In ex- posures of sufficient duration to concentrations higher than about 0.40 p.p.m. it may be toxic to sensitive plants a t periods during the growing season when physiological activity is high and the conditions for rapid absorption of this gas by the leaves are a t a maximum. However, low concentrations, in the range up to 0.10 to 0.20 p.p.m., have been demonstrated to be without influence on plant life, in the absence of visible markings. There is no effect, in this case, after long-continued exposure on rate of growth, yield of crop, photosynthesis, respiration, or on the daily march of the stomata. The effects may be beneficial if the plants are growing in a sulfur-deficient soil or nutrient. No basis has been found for the theory

ONTAMINATIOK of the atinoephere by sulfur dioxide froin combustion products of industrial and smelting operations

and the resultant effects on vegetation have been the object of intensive investigation for nearly 100 years. Early work on this subject was hampered by the lack of accurate methods for deter- mining concentrations in the air in the vicinity of affected vegeta- tion. Consequently, the observations were more or less qualita- tive in character and generally confined to a study of the symp- toms on the leaves. This was later supplemented by more or less crude experiments in which incredibly high concentrations were applied to experimental plants under cabinets to reproduce the gross symptoms of damage. It is probable that experimentation with reliably determined concentrations of sulfur dioxide, under known conditions, was first introduced into this subject by the Selby Smelter Commission (33) in 1915. However, more accurate and systematic observations were rendered possible after the de- velopment of a continuous, automatic method of analysis by Thomas in 1928. Subsequent modifications and adaptation of this apparatus to carbon dioxide and other gases have introduced a high degree of precision in atmospheric pollution investigations (92-96, 103, 104).

of invisible injury. The literature on the subject has been reviewed and the results of investigations carried out by the writer and his collaborators on various aspects of the sulfur dioxide pollution problem have been pre- sented: occurrence of sulfur dioxide in the atmosphere of industrial areas; sulfur content of vegetation; effect on soils; symptoms and diagnosis of injury from sulfur dioxide and other factors; retardation of diameter in- crement in conifers; experimental studies on the influ- ence of environmental factors on susceptibility, the effects on conifers in natural habitat and transplanted stock, yield of crop plants, stomatal behavior, and photo- synthesis and respiration. I t is hoped that the methods and results described will serve as a guide in investigations of effects of other industrial waste gases on plant life.

One of the most comprehensive investigations ever undertaken on this subject was organieed in 1929 when the National Re- search Council of Canada was requested to study the problem of alleged damage in Stevens County, Wash., by smelter fumes emitted from the stacks of the Consolidated Mining and Smelting Company of Trail, B. C. The investigation was initiated with measurements of the sulfur dioxide concentrations in the atmos- phere at various points in the Columbia River valley by means of Thomas automatic recorders, surveys of the condition of field and forest crops, and the sulfur content of vegetation. But, as time went on, the scope of the work mas enlarged to include a great amount of experimental work, as well, on the effect of sulfur diox- ide on conifers and crop plants under controlled conditions. The work covered a period of about 8 years and the results were pub- lished in book form in 1939 (35).

Previous investigations were concerned mainly with the in- fluence of high concentrations of sulfur dioxide on vegetation-- that is, concentrations in excess of 1 p.p.m. It soon became cvi- dent, however, that the majority of the fumigations in the Columbia valley were of comparatively low intensity but long