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42 www.aiche.org/cep/ April 2001 CEP

Herman Purutyan,

Thomas G. Troxel,

and Francisco Cabrejos,

Jenike & Johanson, Inc.

PNEUMATIC

CONVEYINGSYSTEM TO

YOUR

HIGHER

MOVING BULK GRANULAR SOLIDS and powdersaround chemical process industries (CPI) plants isalways challenging. While fluids can be transport-ed practically anywhere in a plant through a pipe,granular solids are usually moved by mechanicalequipment such as belt, screw, aeromechanical, ordrag conveyors. These methods impose severe lim-itations on layout and routing, provide (in mostcases) only limited containment of material, andexpose the product to direct contact with movingmechanical parts. Pneumatic and hydraulic con-veying offers the containment and flexibility of

pipeline transport for bulk solids. In principle,pneumatic conveying is simple: disperse a powderor granular solid into a moving fluid stream, send itthrough a pipe to the desired destination, and thenremove the solid from the fluid.

Benefits and drawbacks

Both pneumatic and hydraulic conveying havebeen used to transport bulk solids for many years,but pneumatic conveying is far more common andis found in nearly every industry where bulk solidsare handled. One of the earliest recorded uses wasfor unloading wheat from barges to flour mills at

the end of 19th century in London (1). Grains, aswell as other cargo, such as alumina, cement, andplastic resins are still unloaded using the samebasic methods. Other common applications includeunloading trucks, railcars, and barges, transferringmaterials to and from storage vessels, injectingsolids into reactors and combustion chambers, andcollecting fugitive dust by vacuum.

In addition to the primary benefit offlexibilityin routing, pneumatic conveying also offers theseimportant advantages:

1. Cleanliness and containment properlyconstructed and maintained pneumatic systems canbe virtually dust-free. Vacuum systems offer theadvantage that leakage is into the pipeline, so that

PROPEL

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CEP April 2001 www.aiche.org/cep/ 43

Pneumatic Conveying

For new and existing installations,

there are many system and component

choices available. Heres how to narrowdown the options to make the appropriate

selection for optimal performance.

EFFICIENCY

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Pneumatic Conveying

even damaged or leaky systems contain all of the product.

2. Low contamination Sealed systems can preventmost forms of contamination, and contact with moving me-chanical components is minimal. Pressure systems preventinward leakage and can use dry, inert gas for conveying toexclude oxygen and moisture.

However, pneumatic conveying has limitations and isnot suitable for every application. One of the primarydrawbacks is high power consumption. Taken on the basisof cost per unit weight per unit distance conveyed, pneu-matic conveying is by far the most expensive method oftransporting materials. But, in many cases, the higher costis justified, because the alternatives are not practical due tolayout limitations, containment or cleanliness issues, as

well as the low maintenance needed for these setups.Pneumatic conveying systems are also limited in overall

conveying distance and conveying capacity. The largestcommon ones are generally restricted to about 300 tons/hand 3,000 ft (not simultaneously). A few systems withlonger conveying distances andhigher capacities have beenbuilt. On the other hand, hy-draulic conveying can be usedto transport solids over muchgreater distances and has beenused for long-distance coal(slurry) transportation. Recently,

hydraulic conveying has beenused for long-distance transportof plastic pellets and copperconcentrate.

Pneumatic conveying is alsohampered by the potential forsevere wear of equipment andattrition or degradation of thebulk solid particles, if the system is not properly designedand operated.

Despite these limitations, pneumatic conveying is animportant and practical form of transport with applicationsin almost every part of the CPI. The range of materials that

can be transported is nearly unlimited. Powders and granu-lar materials of nearly every type can be conveyed: fin-ished products as diverse as bathroom tissue, candy, andmetal components; stringy materials such as choppedfibers, as well as metal particles from grinding operations.

The limitations on what can be conveyed depend moreupon the physical nature of the material than on its genericclassification. Particle size, hardness, resistance to damage,and cohesive properties are key parameters in determiningwhether a material is suitable for pneumatic conveying.Not surprisingly, larger particles of heavy material requirea higher gas velocity to become entrained in an air stream.The practical limits of simply increasing the air velocity toconvey ever larger and heavier particles extend to particlesof about 15 in. for most granular materials. Much larger

particles have been conveyed in straight runs. In some un-

derground mining applications, vertical lifts have beenused to pneumatically convey ore containing 23 in. parti-cles hundreds of feet to the surface.

Cohesive or sticky materials are often difficult to con-vey pneumatically. Moist substances that are wet enough tostick to the walls of the pipeline usually cannot be handledsuccessfully. Materials with high oil or fat contents canalso cause severe buildup in pipelines such that conveyingis not practical, although this can sometimes be overcomewith temperature control or flexible pipelines.

Understanding pneumatic conveying systems requires aknowledge of the properties of the bulk solid to be con-veyed, flow of compressible gas in a pipeline, and dynam-

ics of solid particles in a gas stream.

Material characteristics

Particle characteristics, as well as bulk properties of thesolid, are important variables in the design of a pneumatic

conveying system. Among thekey particle characteristics areparticle size and distribution,shape, density, hardness, andfriability. Bulk properties thatare important include bulk den-sity and compressibility, per-meability, cohesive strength,

segregation tendency, explosi-bility, toxicity, reactivity, andelectrostatic effects.

Particle characteristicsParticle size, distribution, andshape are known to be amongthe most significant variables

affecting pneumatic conveying. For example, uniformlysized round and smooth particles are easier to convey thanangular, rough ones having a wide size distribution. Anoth-er rule of thumb is that, to prevent mechanical plugging ofconveying lines, particularly when conveying materials

containing large particles, the pipe diameter must be atleast 35 times the maximum particle dimension.Much work has been done to characterize key convey-

ing parameters, such as minimum conveying velocity, as afunction of particle size. While there are at least a dozenmodels that include the effect of particle size on the mini-mum conveying velocity for a suspension of solids in agas stream, there is no general agreement about whichprovides the most reliable prediction (2). Since many ofthese models produce significantly different results for thesame initial data, experience is often required in usingthem. One of the challenges is to express particle size in away that can be used in a predictive model. For a mono-sized material, it is easy to define the particle size. How-ever, for a material with a wide range of sizes, defining a

Pneumatic conveying is simple in principle.

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particle size to use as a design parameter is more difficult.

First, particle-size data are highly dependent upon themeasurement method. Data obtained by sieve analysis, byfar the most common technique, are certain to differ fromthose generated using laser diffraction. Even measure-ments made by different machines, using the samemethod, can give different results.

Particle size is often expressed as a mean diameter,which does not capture the range of sizes. The nature ofthe size distribution may vary depending on the processused to produce the particles. For example, most materialscreated by crushing or grinding have a log-normal distribu-tion, whereas condensation processes produce Gaussiandistributions, again making a standard definition of particle

size difficult.Particle density affects the minimum conveying veloc-

ity and pressure drop required for transport. Particleshape is another parameter that is difficult to define.Shapes vary from near-spheri-cal to angular, irregular, plate-like, or fibrous. The study ofparticle properties is a sciencein itself and one of the waysparticle shape has been incor-porated into analytical modelsis by assigning a sphericityvalue to particles to account

for their shape relative tospherical particles.

While virtually all of the an-alytical and empirical modelsused to predict the behavior ofpneumatic conveying systemsinclude particle size and density, it is still difficult to cap-ture all of the pertinent particle properties in a generalmodel.

Particle friability

Attrition of particles in a pneumatic conveying systemcan affect the material being conveyed in several ways,

usually with undesirable results. Attrition may alter prod-uct performance. For example, coarse and fine particlesmay have different dissolution rates. If the product con-veyed is to be fed to a reactor, excessive fines could be aproblem. Fines generation can also impair product quality.Most products are sold with a set of specifications that in-cludes a particle-size range. If the size specification isachieved by screening, then any attrition that occurs duringtransfer from screening to packaging may make it difficultor impossible to meet the products specification.

In some cases, attrition can be both a blessing and acurse. For example, at a soda ash plant, some of the prod-uct was sold and some was used internally to make anotherproduct. Pneumatic conveying was used to transfer the ma-terial to both the packaging area and to the reactor for cap-

tive use. Operators found that when they were forced to

use unconveyed material in the reactor, the time required todissolve the soda ash increased dramatically and limitedproduction. In this case, the attrition that occurred in theconveying line was beneficial and essential in meeting pro-duction schedules. On the other hand, the conveyed prod-uct was more difficult to handle and perceived as lowerquality by customers who purchased it.

Fines generation can also affect the flow propertiesof the material. Materials typically become more diffi-cult to handle as the particle size decreases. This can re-sult in flow problems in downstream bins and silos. At-trition can increase the propensity of some materials tocake. For example, attrition of sugar crystals can expose

fresh surfaces, which are more prone to caking. Lastly,excessive fines result in a dustier material that can in-crease the risk of a dust explosion, or, for toxic materi-als, the exposure risk for workers.

Particle friability cannot bedefined easily by an index orvalue derived from tests. Workhas been done to classify mate-rials for comminution purposes(e.g., the Hardgrove grindabili-ty index for coal), but these in-dices apply to a particularmechanism of comminution

and, to our knowledge, thistype of classification has neversuccessfully transferred to par-ticle attrition in pneumaticconveying systems. Overall,attrition in a pneumatic con-

veying system can only be accurately assessed by conduct-ing a series of attrition tests that simulate conditions in thelines, as well as other handling steps in the system, such asfree-fall and flow through a silo.

Particle hardness

One of the disadvantages of pneumatic conveying is the

potential for accelerated line wear, which is a direct func-tion of the hardness of the particles conveyed, as well asthe conveying velocity. One common way of classifyingparticle hardness is the Mohs scale, ranging from 1 (talc)to 10 (diamond).

Bulk characteristicsBulk properties, such as compressibility, which describe

the bulk density as a function of consolidation and gas per-meability, affect how bulk solids behave in pneumaticlines, particularly in systems operating at high-solids load-ings and low conveying velocities. The compressibility andpermeability of a bulk solid determine how readily the ma-terial will deaerate, and how gas flowing through a bed orplug of material will affect the bulk solid.

The compressibility

and permeability

of a bulk solid determine

how readily thematerial will deaerate

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The direct effect of the cohesiveness of a bulk solid on

pneumatic conveying can be buildup in the lines. Indi-rectly, however, cohesive solids cause other serious prob-lems. The challenge is to be able to feed the conveyingline uniformly. Flow stoppages, or erratic flow throughequipment upstream of a conveying line, such as in a bin,feeder or a chute, can be a roadblock to achieving the de-sired transfer rates.

Other bulk properties that must be considered duringdesign include explosivity and toxicity. Materials thatmay contain residual hydrocarbons, such as newly re-acted polyethylene or polypropylene powders, mayhave to be conveyed using nitrogen to limit exposure tooxygen. Static electricity may be a source of ignition

for materials prone to explosion, in which case thecharge must be either dissipated by proper grounding,or neutralized. Materials that are toxic or need strictcontainment may require a vacuum system, in whichany leak will be into the line, rather than out to the en-vironment. Double-walled pipelines under positivepressure have been used to convey materials such ascontaminated soils.

Since particle size affects the pneumatic conveyingcharacteristics of a material, segregation that occurs in ahandling system, which separates the material into coarseand fine fractions, can impact pneumatic conveying. If thesystem is designed for a mixture offines and coarse parti-

cles, conveying only one of them can be a problem. Thesegregation potential of a given bulk solid can be deter-mined by conducting tests (3).

Single-phase flow

Flow of gas in a pipeline is well understood. The con-veying gas obeys the ideal gas law, and its density is afunction of pressure and temperature, as given by:

= P/RT (1)

where = gas density, P = absolute pressure,R = gas con-stant, and T= absolute temperature.

Mean gas velocity in a pipeline is a function ofmass-flow rate of the gas, as well as the density and theflow area:

V=M/A (2)

where V= mean gas velocity, M= gas mass-flow rate, =gas density, andA= flow area.

Combining Eq. 1 and 2, it becomes evident that themean gas velocity is a function of gas pressure:

V=M R T/P A (3)

Assuming that the gas mass-flow rate and the flow areaare constant, as well as the gas temperature, then velocity

at any two points in the line becomes proportional to the

absolute gas pressure:

V2/V1 = P1/P2 (4)

where P1 and P2 are absolute pressures.Example In a line with an 8 psig pressure difference

between the feed and end points, the gas velocity at the endof the line will be about 1.5 times the velocity at the begin-ning of the line:

V2/V1 = P1/P2 = (8 + 14.7)/14.7 = ~1.5 (5)

Similarly, in a line with a 22 psig total pressure drop,

the gas velocity at the end of the line will be about 2.5times the beginning velocity.

The relationship between gas velocity and pressure dropin a straight pipe is found by a simple calculation:

P =f(L/D) (g U2)/2 (6)

where f = friction factor, a function of Reynolds numberand pipe roughness, given in Moodys chart and others forturbulent flow,L = pipe length,D = pipe dia., g = gas den-sity, and U= gas velocity.

As Eq. 6 indicates, the pressure drop in a pipe is ap-proximately proportional to the square of the gas velocity.

Even in systems with a modest pressure drop of 8 psi, theincrease in velocity from one end of the line to the otherresults in a difference in pressure drop per unit length ofmore than 2. This illustrates the significance of densitychanges in the gas as flow progresses from the beginningto the end of the conveying line. In addition to the pressuredrop, changes in the gas velocity also affect the suspensionof solids in the gas stream. At low velocities, particles maybe sliding on the bottom of the pipe, while at higher ones,they will be fully suspended in the gas.

Two-phase flowWhile single-phase flow in a pipe is well understood,

adding solids into the moving gas stream complicates mat-ters immensely. As solid particles are introduced into a mov-ing stream of gas, the pressure drop in the line begins to in-crease, as momentum is transferred to the particles to accel-erate them to the conveying velocity. However, a number ofinvestigators have found that, in fact, the pressure drop of aflowing suspension actually decreases slightly at low con-centrations with small-sized particle materials. The totalpressure drop consists of two components: the pressure dropdue to gas flow alone, and that required for transporting theparticles. In addition to the gas velocity, the pressure drop isalso a function of a number of other parameters including:

The amount of solids in the pipeline, typically, themass ratio of solids to gas, which is known as the solidsloading ratio or phase density; and

Pneumatic Conveying


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The velocity of the solids relative to the gasRelevant particle properties are the size distribution,

density, shape, and frictional interaction with the pipe wall.

Conveying velocitiesThe moving gas stream applies drag and lift to the parti-

cles. These forces are a function of gas velocity. Terms

such as pickup velocity, saltation velocity, and minimumconveying velocity are used to describe the correlation ofgas velocity to the behavior of solid particles in a line.

Consider a layer of particles lying at the bottom of ahorizontal pipe, with the gas flow slowly increasing. At acertain velocity, there will be enough lift on some particlesto pick them up out of the bed, entrain them in the movinggas, and blow them downstream. The velocity at which thisoccurs is referred to as pickup velocity.

A number of investigators have shown that the mini-mum pickup velocity is a function of the density of the gasand the solids, as well as particle and pipeline diameter.Empirical correlations have been developed based on tests

conducted with a limited number of materials (4).The velocity below which entrained solid particlesbegin falling out of suspension and start settling at the bot-tom of a horizontal pipe is the saltation velocity. This isalso a function of particle and gas density, as well as parti-cle and line diameters (4). In addition, there is a direct rela-tionship between the saltation velocity and solids loadingratio (SLR). In general, saltation occurs at higher velocitiesat higher solids loading.

Perhaps, the most important velocity for the designerof a pneumatic conveying system is the minimum con-veying velocity. This is, the lowest velocity that mustexist in a given system for a given material to preventplugging the line. Some investigators have suggestedusing the saltation velocity with a factor of safety, while

others have developed empirical correlations; however,these correlations often predict widely differing velocitiesfor the same set of conditions.

A study done by Peter Wypych at the University of Wol-longong in Australia (2) provides useful insight into thestate of currently available correlations. Figure 1 shows thepredicted minimum conveying velocities for wheat calcu-

lated using nine different correlations. Each line representscalculations of a particular investigator, as is so for Figure2. Individual investigators are not noted; only the trend isimportant here. The minimum conveying velocity is calcu-lated as a function of pipe diameter. As can be seen, theminimum calculated values can vary by as much as a fac-tor of two. Note that all of the models predict the samegeneral trend that increasing the pipe size requires an in-crease in conveying velocity.

Figure 2 shows minimum velocity calculations as afunction of particle size, using the same set of correlations.The range of calculated velocities is much wider here, but,what is most apparent is that some of the models predict an

increase in the minimum velocity if the particle size is de-creased, while others predict a decrease, and still point atvirtually no change over a wide range of particle sizes.This clearly illustrates that use of these correlations re-quires a thorough understanding of the basis on which theywere developed, and experience in applying them to realapplications.

One reliable method of determining minimum convey-ing velocities is to obtain data from an existing systemconveying the same material. However, this does present achicken or the egg dilemma when such a system is notavailable. In these cases, the best solution is to find theminimum conveying velocities by conducting tests in apilot conveying loop and then scaling up the results. Sincescale-up is not without its challenges, ensure that the test


0

0

50

40

30

20

10

50 100

Internal Diameter of Horizontal Pipe, mm

MinimumVelocity,m/s

150 200

Figure 1. Different correlations do not agree on predicting the minimumconveying velocity.

0.01

0

50

40

30

20

10

0.1 1

Particle Diameter, mm

MinimumVelocity,m/s

10

Figure 2. Predicting minimum conveying velocity based on particle sizegives conflicting results

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loop and the full-scale system are not too different, andthat test conditions in the pilot system cover an adequaterange of velocities and solids loadings.

Conveying pressures

In addition to the minimum velocities required to con-vey a specified amount of bulk solids in a given system,the designer must also know the required pressure drop.This requirement is closely coupled with the rate ofsolids and gas flow. In fact, the same amount of solids

can be transported in a line using a number of velocityand pressure drop combinations. This is best illustratedgraphically in a general state diagram, which is a plot ofpressure per unit length of pipe as a function of convey-ing gas velocity, with constant solids flow rate as a pa-rameter (Figure 3).

At high gas velocities, solids par-ticles are generally suspended in air.Under these conditions, the solidsloading ratio is relatively low, typi-cally below 15. This is dilute-phaseconveying. If the gas velocity isslowly decreased, the pressure re-

quired to convey a constant amountof solids also drops. After reaching aminimum, a further reduction in gasvelocity results in an increase inpressure, as particles begin to fallout of suspension and interparticlecollisions increase. This region,where the solids loading ratio is typ-ically higher than 15 and the veloci-ty is below the saltation velocity, isthat of dense-phase conveying. Withmany materials, there is no distinctboundary that separates the dilute-from the dense-phase regions, andconveying can occur over a continu-

ous range from fully suspended to a slow moving bed.

With other materials, very distinct boundaries define re-gions of stable and unstable conveying. Typically, veryfine powders, such as cement, lime, and fly ash, fit into thiscategory, which can cover a wide range of conveying con-ditions. Coarser materials, such as perlite, sugar, and plas-tic pellets, fit into the other category that has a distinct re-gion where conveying is unstable or impossible.

Theoretically, the most efficient conveying can beachieved at the velocities that result in the lowest pres-sure drop, or at the pressure minimum points. However,as can be seen in the general state diagram for coarseparticles, it is not always possible to get stable flow atthe theoretical pressure minimums. For some materials,

flow in this region becomes extremely erratic with se-vere pressure fluctuations, as the solid particles continu-ously fall out of suspension and get reentrained by theconveying gas. Generally, the designer must decidewhether the system will operate in the low-velocity orhigh-velocity region. The advantages and disadvantagesof such systems are discussed below.

It is relatively simple to calculate the pressure dropwhen there is only gas flow in a line, but including the ef-fect of the conveyed solids is considerably more compli-cated. A large amount of work has been done to predictthe pressure drop analytically, but there is still significantdisparity among various methods. The most reliable

method of determining the pressure drop requirement isto use experimental data derived from test loops with theactual material to be conveyed, followed by scaling up ofthe data. There are a number of references that outlinevarious calculation methods in detail (5, 6).

Pneumatic Conveying


Feed Hopper

MultipleDelivery Points

Storage Silo

Blower

InletFilter

Airlock

Feeder

Figure 4. Typical positive-pressure system.

NoFlow

StableDense-Phase Unstable

Dense-PhaseDilute-Phase

IncreasingSolids Flow

Mean Gas Velocity, U

PressureDrop,

P/L

Lines of Constant Solids Flow Rate

Figure 3. General state diagram for solids flow in a pipe.

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In general, these correlations provide

more reliable results when applied tofully suspended dilute-phase systemsand are subject to the same caveats asthe correlations described above forminimum conveying velocity. In allcases, use of these correlations requiresan iterative calculation procedure thatmust be performed in steps along thepipeline to account for the change invelocity, making it impractical toachieve useful results without employ-ing a computer calculation procedure.

Types of systemsSystems can be configured and clas-

sified in a number of ways dependingon their function, operating pressure(positive, negative, combined, orclosed-loop), and magnitude of operat-ing pressure. The most common andoften misunderstood categories are di-lute- and dense-phase. Conveying canoccur over a wide range of conditionsbounded on one end by gas alone withno entrained solids, and at the other ex-treme by a completely full pipe where

the solids are essentially extrudedthrough the line.

Dust collectors are an example ofconveying systems that operate at verylow solids loading, where the perfor-mance is governed almost entirely bythe gas flow. Most industrial conveyingsystems run somewhere in betweenthese two extremes and are rankedbroadly as either dilute- or dense-phase,depending upon the relative solids load-ing and velocity of the system.

Positive-pressure systemsThese are above atmospheric pres-sure and ideal for a single feed pointand multiple delivery points (Figure4). Positive-pressure arrangementsmay be low-pressure, dilute-phase, orhigh-pressure dense-phase. They canhave higher capacities and longer con-veying distances than negative pres-sure systems.

Negative-pressure systems

Negative pressure or vacuum systemsare ideal when the product must bepicked up from a number of different lo-


MultipleDelivery PointsMultiple

Feed Points

Storage Silos

Storage Silos

Filter

Exhauster/Blower

Filter

Figure 6. Combined systems often use vacuum for feeding and positive pressure for conveying

over long distances.

MultipleFeed Points

StorageSilos

Filter

Exhauster

Filter

VacuumReceiver

InletFilter

AirlockFeeder

Figure 5. Vacuum systems are ideal for hazardous solids.

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cations and conveyed to a single destination (Figure 5).

Also, since the pressure in the conveying line is lower thanatmospheric, and leakage is into the line, these schemes arebetter suited for toxic or hazardous materials. For the samereason, negative-pressure systems are used as dust collectorsfor fugitive dusts.

Vacuum systems are usually limited to shorter distancesthan positive ones and typically operate with a dilute, low-solids loading. However, it is possible to achieve high-solids loading and low velocity conveying over short dis-tances (less than 200 ft). The required conveying velocityis often higher in a vacuum system than in a positive one,because the gas density is lower in a vacuum system.

Combined systemsBy combining both types in the same setup, the advan-

tages of each can be exploited (Figure 6). These arrange-ments consist of two sections. A typical one is a pull/pushsystem with a negative-pressure front end, followed by apositive-pressure loop. The benefit is that they capitalizeon the ease of feeding into a vacuum and combine this withthe higher capacity and longer conveying distance whenusing positive pressure. One common example is vacuumunloading of rail cars with transfer to a receiver. From thereceiver, the material is transported to the destination usingpositive pressure. Combined systems can be eitherpull/push or push/pull.

Closed-loop systems

When the conveying gas is other than air, such as nitro-gen, carbon dioxide, argon, or steam, there may be good rea-son for recirculating the gas (Figure 7). In these cases, aclosed loop is used. Since inevitably some leakage willoccur, provisions are needed for makeup gas. Also, since thegas will heat up as it is compressed, heat buildup requiresheat exchangers to prevent overheating.

Through-the-fan systemsWhen the conveyed solid or the solids loading is rela-

tively light, these systems offer the benefits of a combined

system, but with less equipment (Figure 8). Here, all of theconveyed material travels through the fan. If longer con-veying distances are required, the line can be extended byadding more fans.

Pneumatic conveying system components

A pneumatic conveying system consists of four basiccomponents: the gas mover, the solids feeder, the pipeline,and the separator. While their placement may vary depend-ing on whether the system is in vacuum or pressure, theirbasic functions remain the same.

The gas mover provides the proper flow rate of gas re-quired for the transport at the right velocity and pressure.The solids feeder introduces the solid particles at a con-trolled rate into the pipeline where they are mixed with the

conveying gas. Positive-pressure systems require devices

to feed material from atmospheric pressure into a pressur-ized pipeline, while negative-pressure systems can call forfeeders with a good seal to minimize leakage of gas intothe pipeline. An acceleration zone is required right after thefeed point to speed up the solids to the steady transport ve-locity in the pipeline. The pipeline consists of straight sec-tions, both horizontal and vertical, connected together withbends. In the separator, the solids are decelerated and re-covered from the gas stream and then stored in a silo or fedinto another unit. The gas is typically released into the at-mosphere. Controls, safety equipment, and instrumentationare also required.

A challenge for every designer is to combine the differ-ent types and models of equipment on the market so that thesystem operates efficiently over its specified range. Reliable

Pneumatic Conveying


Heat Exchanger

Blower

MakeupGas

FilterFilter

Airlock

Feeder

StorageSilo

Receiver

Figure 7. Closed-loop systems are employed when the gas must berecirculated.

Radial-BladeOpen-Wheel Fan

Conveyor

Filter

Figure 8. Through-the-fan operation is simple, requiring less equipmentthan a blower-based setup.

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flow from the bin/silo through the solids feeder and into thepipeline is an absolute necessity as a starting point. Unfor-tunately, most bin/silo and feeder suppliers do not considerthe effects of downstream equipment, while many pneumat-ic conveying suppliers often overlook the need to design orspecify a bin or feeder to provide reliable material flow.

Gas movers

By far, the most common device for moving gas in apneumatic conveying system is the Roots-type rotary lobeblower. This blower is prevalent, because it provides an

economical source of gas flow that meets the pressure (orvacuum) and flow requirements of the largest category ofsystems, namely those that operate at less than 15 psig forpressure systems and 6 psig for vacuum systems. A keyfeature is that the Roots blower delivers a nearly constantvolume over its operating pressure range. This is impor-tant, since control of the gas flow in a pneumatic convey-ing system is critical for stable operation.

Fans are used for low-pressure systems, and compres-sors (or plant compressed air or process gas) for higherpressures. Fans are generally limited to high-volume, low-pressure applications (flow > 1,000 cfm; presure < 2 psi),and require careful design because of their inherent operat-

ing characteristics. The most common fans used in convey-ing applications are radial blade machines, which havemaximum pressure ratings of 2040 in. of water (0.72psig). In many cases, they are operated as through-the-fansystems. For applications handling very light material,such as chopped textile fiber, recycled foam, sawdust, andother light nonabrasives, fans offer a simple and effectiveway of transporting material.

Compressed gas can be used for any positive-pressureduty, but it is usually not cost-effective for pressures lessthan 20 psig, unless a source of free process gas is avail-able that is suitable for the conveying requirements. In thecase of fans and rotary blowers, the flow rate is a functionof the speed of the machine and the operating pressure.Most compressed gas systems use a pressure receiver to

store a volume of compressed gas and allow the compres-

sor to operate intermittently. To use compressed gas forconveying requires some means to control both the flowrate and pressure. Flow rate control can be accomplishedby using a feedback flow-control device, or a static flow-control orifice or choked-flow nozzle.

A comparison of the typical relationship between flowrate and operating pressure for fans, blowers, and com-pressed gas systems is shown in Figure 9. Since most con-veying systems experience a range of operating pressuresbetween the extremes of an empty line and a fully loadednetwork, the gas flow through the system will change ac-cording to the characteristics of the supply system.

It is apparent from Figure 9 that a system driven by a

fan may experience a significant change in gas-flow ratefor small changes in pressure, while one supplied withcompressed gas may operate at an essentially constant flowrate over a wide range of pressures. The ability of com-pressed gas and lobe-type machines to produce a nearlyconstant flow of gas over a wide range of pressures offersstability and allows these systems to recover from upsetsthat may cause momentary rises in pressure.

Solids feeders and pressure seals

For proper operation of a pneumatic conveying system,the solids fed into the line must be controlled. Frequently,problems of nonuniform feed into the pneumatic convey-

ing lines are thought to cause difficulties with pneumaticconveying systems, whereas they really originate in the up-stream equipment (7).

Feeding solids into a positive-pressure system requires ameans of sealing against the pressure in the pipeline. De-vices used for this purpose include rotary valves, doubledump-valves, specially designed screws, and eductors.Some of these devices control the rate of solids flow intothe line and, hence, are truly feeders, while others onlyprovide a pressure seal, but do not meter solids.

Rotary valves can be used to provide a seal, as well asmeter solids into a line. As feeders, they are placed at theoutlet of hoppers of silos and bins, and their speed deter-

mines the solids throughput. While they are primarily usedfor systems at less than 15 psig, specially designed rotaryvalves exist that seal up to 100 psi.

While using a rotary valve both as a feeder and a pres-sure seal reduces the amount of equipment needed, it alsorestricts the opening of the hopper feeding the line to thesize of the valve. When handling cohesive solids, this maylead to flow stoppages due to arching and ratholing in thehopper above. This can be avoided by properly designingthe hopper with an appropriate feeder that can provide uni-form flow, and using a rotary valve not as a feeder, but as apressure seal only.

Rotary valves used as feeders can also cause flow prob-lems by nonuniformly drawing material across the outlet ofthe hopper. Solids enter the rotary valve on the side of the


40 50 60

Radial BladeFan

Roots-TypeBlower

CompressedGas/Nozzle

70 80 90 100 110

0

40

60

80

100

20

% of Rated Flow Discharging to Atmosphere

%o

fRatedDischargePressure

Figure 9. How fans, blowers, and compressed gas stack up.

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outlet where the empty pockets are first exposed to the

bulk solid. In addition, internal convergence of some rotaryvalves can reduce the active hopper opening, again result-ing in nonuniform discharge from the hopper. These prob-lems can destroy mass flow in the hopper, even if the hop-per itself is properly designed.

In positive-pressure systems handling fine materials,pressure sealing and venting at the feed point are critical.Fine powders often flow out of hoppers at rates muchlower than do coarse granular materials. When designingvessels that handle fine powders, it is essential that themaximum discharge rate from an unrestricted outlet becalculated to ensure that the opening is large enough tofeed the downstream process. This calculation is a strong

function of the permeability and compressibility of thepowder (8). If the hopper opening is too small, then flowrate limitation, independent of how fast a feeder is run,will occur.

This problem is exacerbated by gas leakage up from thepneumatic conveying line into thehopper. The amount of leakagethrough a rotary valve is a functionof the total pressure drop across thevalve, and the size of the valve, aswell as the clearance between therotor tips and the valve body.

For example, a 12in. rotary

valve operating at a 10 psi differ-ential pressure could have as muchas a 100 cfm leakage. This amountof gas going into a conical hopperwith a 12 in. dia. results in a gasvelocity of approximately 2 ft/s atthe hopper outlet. This flow canimpose a significant body force onthe particles trying to flow down, and can retard flow.

Leakage through a rotary valve can be reduced by en-suring that the valve is properly vented and that the vent isnot blocked; the pressure drop across the valve is not toohigh for the type and size of valve used; and that the valve

is properly maintained such that a tightfi

t is kept betweenthe rotor tips and the valve body.In high-pressure systems, blow tanks or transporters

are often used to introduce the solids into the line. A cer-tain amount of solids is transferred into the transporter,which is then sealed and pressurized. The entire contentsof the blow tank are fed into the line, then the pressure isvented and another batch of solids is transferred in.While this results in a batch operation, using two blowtanks and alternating between them can provide nearcontinuous feed.

Pipeline components

Perhaps the single most debated question regarding thepipeline is what type of elbow is best. The answer, of

course, depends upon the application. However, the

amount of debate is often out of proportion to the signifi-cance of the difference between the various types of bends.In many systems, the type of bend makes relatively littledifference in the performance and operation. For systemshandling nonabrasives that do not degrade during convey-ing, the type of elbow is not a critical decision and shouldprobably be made based on cost.

The key factors to consider in selecting pipe bends are: Abrasive wear Product degradation Product buildup Pressure lossAbrasive wear is by far the most critical element in

selecting an elbow. Since virtually all of the wear occursat the bends, abrasives will quickly wear through anelbow. Pipeline wear is a strong function of the convey-ing velocity. Since the velocities in a line are typicallyhigher near the discharge, most wear is found near the

end of the conveying lines.Many investigators have shownwear to be proportional to ve-locity raised to a power be-tween 2 and 4.

Wear is also a function of rel-ative hardness of the particlesbeing conveyed, as well as the

impact angle. In general, wear insteel and aluminum elbows is ata maximum for impact anglesaround 20 deg., which occur atthe entrance of long-radius el-bows. Wear can be reduced byusing short-radius elbows, orblind tees, but these options

both increase the pressure drop, and perhaps more impor-tantly, particle attrition. For mildly abrasive materials, theuse of hard, wear resistant elbows may provide reasonablewear life, but for highly abrasive solids, even these typesof elbows can be worn out quickly.

The most effective means of reducing wear is to de-crease velocity. Abrasive wear is one of the primary rea-sons for using a low-velocity dense-phase system. Withlow velocity, the problem can be reduced to the pointwhere expensive wear-resistant components are notneeded and standard bends can be used without exces-sive damage.

Product degradation also occurs primarily at bends in thepipeline (Figure 10). As with wear, decreasing the velocityis the most effective way to minimize attrition of particles.This can be done by minimizing and controlling the veloci-ty in dilute, fully suspended conveying systems, or by usinga low-velocity dense-phase scheme. In many cases veryfragile materials can only be successfully conveyed in lowvelocity systems. Typically, smooth radius bends produce

Pneumatic Conveying


Figure 10. Product attrition (left photo) takes place mainly atpipeline bends.

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lower attrition than blind tees or mitered bends.Certain types of buildup also can occur primarily at el-

bows (Figure 11). When handling plastic pellets, a prob-lem is formation of thin strips, often referred to asstringers or snakeskins. These cause difficulties in down-stream handling equipment. The strips build up as pelletsdeposit thin molten layers of plastic on the pipes (primari-ly at the elbows), which periodically break loose. This isdue to localized melting. The sources of heat can be resid-ual heat from the extrusion process, hot conveying air,ambient conditions, or particle friction. Using pipe with a

rough interior surface that causes the particles to tumble,rather than slide, can reduce the amount of heat generated.The rough surface can be achieved by sand blasting orshot peening, or by using specially made grooved pipecomponents. Since these surfaces are likely to becomesmooth again over time, periodic surface treatment is usu-ally necessary to ensure roughness.

Pressure loss in pipeline bends is generally greater inblind tees and short radius bends than in smooth, long ra-dius bends. Most investigators have found that increasingthe bend radius much beyond 46 pipe dias. provides di-minishing returns, because the added length of pipe neces-sary to make the bend offsets any benefit of the more grad-

ual turn. Also, it has generally been found that the differ-ence in pressure loss between a long-radius bend and ablind tee is relatively small when compared to the totalpressure drop in a system. This, of course, depends uponthe system layout. In a short system with a large number ofbends (for example, a 200 ft line with 9 bends), the contri-bution of each bend will be much more significant that in along pipeline with only a few bends (say, a 1,200 ft linewith 4 bends). In the latter case, changing the bends fromblind tees to long radius sweeps may not make a significantdifference in the operating conditions.

Disengagers, filter receivers

Separating the solids from the gas can be accomplishedin a number of ways ranging from inertial separation,

where the solids settle by gravity, to pulse-jet cleaned fab-

ric filters. In most circumstances, it is nearly impossible todischarge gas from a conveying system without using ahigh-efficiency fabric filter to meet environmental regula-tions. Usually, separation actually occurs by a combinationof inertia and filtration. Since many of the materials con-veyed encompass a wide range of particle sizes, the largerparticles separate as the gas stream enters the receivingvessel and the smaller ones at the filter surface. When ma-terial is delivered to several receiving bins, it is not uncom-mon to use an inertial or cyclone separator at each deliverypoint and direct all of the conveying gas to a single fabricfilter. This is more economical than providing a fabric filterat each delivery point, but it does allow for cross-contami-

nation between the bins.The selection of a gas-solids separator should be

based on the material characteristics degree of separa-tion required, environmental regulations, the concentra-tion of solids and cost. The efficiency of various typesof separators for various size particles is given in thetable below.

Selecting a pneumatic conveying system

Selecting the best system for your application dependson the process requirements and the characteristics of thematerial to be conveyed. Answers to the following ques-tions could be used in determining which type of convey-

ing system is appropriate: Can the material be conveyed pneumatically?While most materials can be conveyed in a dilute-phase

system, not all are suitable for dense-phase conveying. Inaddition to determining the required conveying velocitiesand pressures, conducting lab-scale tests can answer ques-tions such as whether particle attrition and line buildupwill be problems. Analysis based on the permeability andcompressibility of the solids can be used as a first-pass de-termination of whether the material can be conveyed indense phase.

What are the layout restrictions?Although pneumatic conveying allows more flexibility

in routing than do mechanical conveyors, it is best to limitthe number of bends and avoid placing them close togeth-er. If the layout requires a large number of turns, and close-ly placed elbows, then the pneumatic conveying line willbe susceptible to plugging due to loss of solids velocity.


Efficiency of separator, % 50 micron 5 micron 1 micron

Inertial collector 95 16 3Medium-efficiency cyclone 94 27 8High-efficiency cyclone 98 42 13Shaker-type fabric filter >99 >99 99Reverse-jet fabric filter 100 >99 99

The efficiency of different types of separatorsvaries with particle size.

Figure 11. Elbows are a place for product buildup.

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Also, reducing the number of elbows will reduce system

pressure, hence, velocities, which can reduce line wear, aswell as particle attrition.

What is the maximum transfer rate required?The maximum vacuum in most conveying lines is 57

psig, whereas pressures of up to an order of magnitudehigher than this can be found in positive-pressure sys-tems. Because of the availability of higher pressuredrops, positive-pressure systems can provide higher con-veying rates than do vacuum arrangements. As a generalrule, there is no upper limit on the transfer rate for a vac-uum conveying system, but the conveying distance be-comes much shorter and the line size much larger at hightransfer rates, because of the limited pressure differential

available. In contrast, rates of up to several hundred tonsper hour can be achieved with positive-pressure systems.

What is the required conveying distance?

The range of operating pressures for vacuum systemsnot only limits the conveying rate, but also the convey-ing distance. Vacuum systems are limited in general toless than 300 ft. Longer distances are possible with com-bined systems.

How many feed and discharge points are required?As mentioned above, vacuum systems have the advan-

tage of accommodating multiple feed points, since the gasmover is located at the discharge end. Therefore, a vacuumsystem may be appropriate when feeding from multiplesilos or bins to a day bin or a receiving hopper. If the pro-cess requires the product to be delivered to multiple loca-tions, for example, a single silo feeding several receiving

bins, then a pressure system may be advantageous, sincethe air mover is located upstream of the solids feed point.

Both pressure and vacuum systems can unload materialreceived in trucks, rail cars, barges, or ships. Vacuum hasthe advantage of requiring little or no conveying compo-nents built into the transportation devices. However, it islimited to relatively free flowing materials that can be easi-ly fed into the conveying line, as well as transportingacross only short distances. To transfer materials overlonger distances, a combination, pull/push, or push/pullsystem can be used.

Is a conveying gas other than air required?If the material reacts with oxygen, then an inert gas,

such as nitrogen, may be used as the conveying gas. Toreduce the cost, a closed-loop system can be used. In suchsystems it is important to ensure that there is a supply ofmakeup gas to prevent a drop in pressure, since some gaswill inevitably be lost. Also, a heat exchanger is generallyneeded to prevent the buildup of heat in the line. If an in-erting environment is needed, then a pressure system ispreferred to keep oxygen from entering the line.

Is the material hazardous?If exposure to the material or release to the environment

is a concern, then a vacuum system should be considered,since any leaks that occur will be into the line.

Is the material friable? Is particle attrition a concern?

If so, then a low-velocity system may be appropriate,since attrition is a strong function of velocity. If you oryour supplier do not have experience conveying this mate-rial, then it may be necessary to run pilot tests to determinethe level of attrition expected.

Is the material abrasive?Wear, similar to attrition, is a strong function of veloc-

ity; therefore, if the material is abrasive, use a low-veloc-ity system. Wear is often difficult to assess in pilot tests,because it occurs slowly and it is often not practical to ac-cumulate enough run time in a lab to determine wear life.However, as a rule of thumb, if your material is greaterthan a 5 on the Mohs hardness scale, it is likely thatabrasive wear will be significant in fully suspended di-lute-phase systems.

Pneumatic Conveying


Two Related AIChE Courses

April 2327, 2001 Houston

# 032 Flow of Solids in Bins, Hoppers, Chutes,

and Feeders

For those who want to keep all types of bulk solids

flowing smoothly and reliably throughout the plant

with minimum downtime and maximum quality con-

trol. Intended for design, project, and research engi-neers, as well as any plant operations personnel who

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lems or purchasing solids handling equipment.

Experts youll hear from: Author Herman Purutyan,

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#033 Pneumatic Conveying of Bulk Solids

For those experiencing problems with existing pneu-

matic conveying systems or plan to design or speci-

fy a new system. You will learn how individual com-

ponents join together to form a system that operateseffectively. Attendees will gain a better understand-

ing of common problems, be able to identify prob-

lems in their plants and apply various solutions,

evaluate your in-plant systems and determine their

effectiveness, understand system layout based on

proper operation, and learn how to choose system

components.

Expert youll hear from: Eric Maynard.

For more information:

www.aiche.org/education (800) 242-4363

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Is the material hygroscopic?

Hygroscopic materials exposed to humid air often be-come more difficult to handle. Buildup in the lines maybecome a problem. Perhaps, more importantly, flow prob-lems may develop in the solids handling equipment down-stream of the pneumatic conveying system. If the materialis hygroscopic, then dry air may be needed. Tests can berun to determine how much moisture your material is like-ly to absorb during transit and while in storage.

Is the material temperaturesensitive?Heat is generated as the conveying gas is compressed.

When conveying materials that are sensitive to tempera-ture, the amount of heat generated can cause line buildup,as well as downstream handling problems. If the material

is sensitive to temperature, then it may be necessary to in-clude a heat exchanger in the line. Pilot conveying testscan provide valuable insight into potential problems whenother operating experience is not available.

To sum up

Processes that work as intended from the day of startupdeliver the most economical value to the user. For new in-stallations, being able to produce the target capacity and es-tablishing a market position can mean the long-term viabili-ty of a business. When upgrading systems, extended pro-duction interruptions can mean not only a brief loss in rev-enue, but also an opportunity for the competition to make

inroads. Therefore, careful planning upfront, rather thantroubleshooting in the field, is more important than ever.

Understanding the fundamental operating principles ofpneumatic conveying, asking the right questions at the

early design stages, and making decisions based on process

requirements and the characteristics of the material will in-crease the potential for success. But, the pneumatic con-veying system is only a part of a process, and successfuloperation will require all parts of the process to work effi-ciently. Adopting a systems view, understanding not onlythe operation of the pneumatic conveying system, but alsoits interaction with the solids handling equipment upstreamand downstream, must be carefully considered. CEP


Literature Cited

1. Reed, A. R., and M. S. A. Bradley, Advances in the Design of

Pneumatic Conveying Systems: A United Kingdom Perspective,

Bulk Solids Handling, 11 (1) (Mar. 1991).

2. Wypych, P. W., The Ins and Outs of Pneumatic Conveying, Proc.,

Reliable Flow of Particulate Solids III, Porsgrunn, Norway (Aug.

1113, 1999).

3. Prescott, J. K., et al., Bench-Scale Segregation Tests as a Predictor

of Blend Sampling Error, paper presented at AAPS-PDA 2000.

4. Cabrejos, F. J., and G. E. Klinzing, Minimum Conveying Veloci-

ty in Horizontal Pneumatic Transport and the Pickup and Saltation

Mechanisms of Solids Particles, Bulk Solids Handling, 14 (3), pp.

541550 (July/Sept. 1994).

5. Perry, R. H., and D. Green, Perrys Chemical Engineers Hand-

book, 6th ed., McGraw-Hill, New York pp. 5-46, 5-48 (1984).

6. Klinzing, G. E. et al., Pneumatic Conveying of Solids: A Theoreti-

cal and Practical Approach (Powder Technology Series), 2nd ed.,

Chapman & Hall, Boca Raton, FL (April 1997).

7. Purutyan, H., et al., Solve Solids Handling Problems by

Retrofitting, Chem. Eng. Progress, 94 (4), pp. 2739 (Apr. 1998).

8. Royal, T. A., and J. W. Carson, Fine Powder Flow Phenomena in

Bins, Hoppers and Processing Vessels,Bulk 2000, London, (1991).

To join an online discussion about this article

with the author and other readers, go to the

ProcessCity Discussion Room for CEParticlesat www.processcity.com/cep.

HERMAN PURUTYAN is vice president for Jenike & Johanson, Inc., Westford,

MA ((978) 392-0300; Fax: (978) 392-9980; E-mail: [email protected];

Web: www.jenike.com). Since joining Jenike & Johanson in 1991, he has

designed reliable handling systems for a wide range of materials for the

food, pharmaceutical, and chemical industries. He lectures frequently on

the subject at AIChEs continuing education series, as well as at in-house

courses to individual companies. He has published numerous articles on

the field of bulk solids handling. He is the holder of two patents. Purutyan

received his bachelors and masters of science in mechanical engineering

from Worcester Polytechnic Institute in Worcester, MA, and his MBA from

Babson College in Wellesley, MA.

THOMAS G. TROXEL is vice president for Jenike & Johanson, Inc., San Luis

Obispo, CA ((805) 541-0901; Fax: (805) 541-4680; E-mail:

[email protected]). After graduating from college in 1981, he went to

work at General Dynamics Co., prior to joining Jenike & Johanson. He

helped to open the firms San Luis Obispo facility in 1982. Troxel has been

involved in many aspects of the firms consulting and research activities on

a wide range of projects, including flow properties, testing, modeling,

blending, pneumatic conveying, and fluidization. He has been a major

force behind the firms expansion of services in the areas of mechanical

design engineering and supply of custom built equipment. The latter

includes mass-flow screw feeders, portable antisegregation bins for

pharmaceuticals, BINSERT tumble blenders, and storage bins for a wide

variety of applications. He has published numerous articles and papers in

the field of bulk solids handling, and lectures frequently on the subject

both through professional organizations, such as AIChE, as well as to

individual companies. Troxel is a graduate of California Polytechnic State

University (Cal Poly) in San Luis Obispo, with a BS in engineering science.

He was named the Outstanding Senior in the Engineering Science program.

FRANCISCO CABREJOS is a consulting engineer for Jenike & Johanson Chile in

Via del Mar, Chile ((+56) 32 690 59; Fax: (+56) 32 690 596; E-mail: jenike-

[email protected]). He has been consulting with the firm since 1995, and

has undertaken more than 70 projects, mainly in the mining and materials

handling industries. He is also a part-time professor at the Universidad

Santa Mara, and is author of several technical papers in bulk solids

handling and pneumatic conveying. Cabrejos studied mechanical

engineering at the Universidad Tcnica Federico Santa Mara, Chile, and

holds MS and PhD degrees from the University of Pittsburgh. His doctorate

was awarded for experimental research in gas/solids suspension and

pipeline flow. He received the 1994 Lewis F. Moody Award from ASME for

the best paper in the Fluids Engineering Division.

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2001v97n4p42-55