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Whilepneumaticconveyingisacommonmethodoftransferringpowders, granules, andotherdrybulkmaterials,understandingthisconveyingtechnologycanbeachallenge.Thisarticleaddressespneumaticconveying questions frequently asked by people indrybulkmaterialsprocessingandhandlingplants.

Whether your plant currently uses pneumatic con-veyingornot,chancesare thatyouhavequestionsabout this transfer technology. Here are answers

tosomecommonquestionsaboutpneumaticconveying.

1What’s a pneumatic conveying system?

A pneumatic conveying system transfers powders, gran-ules, and other dry bulk materials through an enclosedpipeline. The motive force for this transfer comes from acombination of pressure differential and the flow of a gas,such as air or nitrogen. (For simplicity, we’ll call the gas“air” in this article.) The system’s basic elements includeamotive air source (also called an air mover, such as a fanor blower), a feeder or similar material-introduction de-vice, a conveying line, a terminationvessel (such as an air-material receiver), and adust collection system.

While some of themost commonmaterials transferred bypneumatic conveying systems are flour, cement, sand, andplastic pellets, any of hundreds of chemicals, food prod-ucts, andminerals can be pneumatically conveyed. Thesematerials are usually fairly dry;materials in slurryor pasteform typically aren’t suited to pneumatic conveying.

2 Howdoes apneumatic conveying systemwork?

Doing work requires energy. To turn a bolt, for instance,you need to provide energy in the form of torque. Suchmechanical torque is usually expressed in foot-pounds,and both elements— feet and pounds—are needed to dothework.Energy is also required tomovematerial througha pneumatic conveying system, but in this case the energyis supplied by pressure differential (in pounds per squareinch) and airflow (in cubic feet perminute).

Inapneumaticconveyingsystem, theairpressure in thecon-veyingline ischangedbythesystem’sairmover,whichgen-eratespressureorvacuum.Where theairmover is located inthe systemdetermineswhether it generates oneor the other:When located at the system’s start, the air mover pushes airthrough the system and the systemoperates under pressure.When located at the system’s end, the air mover pulls airthrough the system and the system runs under vacuum. Bycontrolling the pressure or vacuum and airflow inside thesystem, thesystemcansuccessfully transfermaterials.

3Whatare theadvantagesofapneumaticconveyingsystemoveramechanicalconveyingsystem?

Let’s start by looking atmechanical conveying systems.Aconventional mechanical conveying system runs in astraight line, with minimal directional changes, and eachdirectional change typically requires its own motor anddrive. The mechanical conveying system may be openrather than enclosed, potentially generating dust. It alsohas a relatively large number of moving parts, which usu-ally require frequent maintenance. The system also tendsto take up a lot of valuable real estate in a plant.

Answers to eight common questionsabout pneumatic conveying

MikeWeyandt Nol-TecSystems Inc.

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Ontheotherhand,apneumaticconveyingsystemusesasim-ple,small-diameterpipelinetotransfermaterial.Thepipelinecanbe arrangedwith bends to fit around existing equipment,giving thesystemmore layout flexibility, and thesystemalsohasarelativelysmall footprint.Thesystemistotallyenclosedandtypicallyhasfewmovingparts.

4What are thedisadvantages of a pneumaticconveying system in comparison to amechanicalconveying system?

Apneumatic conveying system typically requires far morehorsepower to operate than an equivalently sizedmechani-cal conveying system. The reason is that changing the airpressure to achieve pneumatic conveying consumes a largeamount of power and is inherently less efficient than ame-chanical conveying system’smechanical transfer. In fact, inapplications with the same transfer rate over the same con-veyingdistance, a pneumatic conveying systemcan require10 times thehorsepowerofamechanicalconveyingsystem.

A pneumatic conveying system also requires a larger dustcollectionsystemthanamechanicalconveyingsystem.Thisis because the pneumatic system has to separate the con-veyedmaterial fromtheconveyingairat thesystem’send.

Somematerialshavecharacteristics thatmake themdifficultto convey in a pneumatic system. Examples are a materialwith a large particle size and high bulk density, such asgravel or rocks, andanextremely stickymaterial, suchas ti-taniumdioxide,which tends tobuildacoatingonanymate-

rial-contact surface. Inapneumaticconveyingsystem,suchbuildup often leads to total pipeline blockage. These diffi-cultmaterials can be easier to transfer in amechanical con-veyingsystemthat’sbeencarefullychosen tohandle them.

5What are the types of pneumatic conveyingsystems?

Pneumatic conveying systems are classified by their oper-ating principle into two types: dilute phase and densephase.Either can rununder pressure or vacuum.

Dilute phase.Dilute-phase (also called stream-flow) pneu-matic conveying is best compared to what happens in aparking lotonablusteryday.Whilewalking toyourcarafterwork,you’reblastedwithdustanddirt that’sbeenpickedupoff thegroundbyfast-movingwind.Thefaster thewind, themore material hits you. As the wind speed picks up, largerdirt particles also become entrained in thewind. Picture thedebris carried by a tornado or hurricane: These storms arepneumatically transferring enormous “particles” in dilutephase. Just like thewind picks up the dust, the dilute-phaseconveying system relies on the airstream’s velocity to pickupandentraineachparticle,keeping theparticles insuspen-sion throughout theconveying line.

A typical dilute-phase pneumatic conveying system isshown inFigure 1. Thepickup velocity at the system’s start(that is, the airstream velocity at which material is pickedupandentrainedat thematerial feedpoint) isgenerallycon-sidered the system’smost critical area, because the air is at

Figure 1

Typical dilute-phase pressure pneumatic conveying system

Materialsupplyhopper

Materialfeedpoint(rotaryairlock valve)

Valveairleakagefilter

Ventfilter

Air inlet

Air-materialreceiver

Airmover(blower)

Airoutlet

Conveyingline

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its lowest speed in the entire system at this point. Becausethematerial isdropping fromastatic state into theairstreambelow it, thematerialmust immediatelybecomeentrained.The air speed required to pick up the material depends oneachparticle’s sizeanddensity, but can range from3,000 to8,000 fpm. The air mover must also be able to overcomethe flow resistance caused by the frictional loss of the airandmaterial against the conveying line’s insidewall.

A simple way to think of a dilute-phase conveyingsystem is that it operates at a relatively high velocity ata relatively low pressure differential.

Asimpleway to think of a dilute-phase conveying systemis that it operates at a relatively high velocity at a relativelylow pressure differential. To design a dilute-phase pneu-matic conveying system with the air volume to conveyyour material, you must use mass calculations (that is,pounds of material per pound of air) while consideringyour installation location’s ambient air temperature, hu-midity, and altitude. Then to achieve the propermixture ofair andmaterial in the system, youmustmeter thematerialinto it at a controlled rate.

Dense phase.Dense-phase pneumatic conveying is alsobest described with an analogy: It’s similar to what hap-

pens in making sausage, when you use high pressure toforce ground meat into a casing. An ideal dense-phaseconveying system would extrude material with enoughpressure to transfer it in one long, continuous piecethrough the pipeline’s entire length, just like a continuouslength of ground meat inside a sausage casing. But withdry bulkmaterials like powders and granules, this usuallyisn’t possible because of the material’s high frictional re-sistance against the conveying line’s inside wall. Instead,air and material flows through the line in any of severalpatterns (including various forms of two-phase flow andslug flow). [Editor’s note: For more details on variousdense-phase flow patterns, including illustrations, see“Pneumatic points to ponder...,” Powder and Bulk Engi-neering,March2003, pages 64-68.]

While various dense-phase conveying system types areavailable, allusea relativelyhighpressuredifferentialwitharelatively low air velocity. Themost common dense-phasesystem type, as shown in Figure 2, provides batch transferusinga transporter (alsocalledablowtankorpressure tank).In this system, material from a storage vessel is loaded bygravity into the transporter. After the transporter is full, itsmaterial inlet valve and vent valve are closed and com-pressed air ismetered into the transporter. The compressedair extrudes the material from the transporter into the con-veying line and to the destination. Once the transporter andconveying line are empty, the compressed air is turned offand the transporter is reloaded.Thiscyclecontinuesuntil allthematerial required for theprocesshasbeen transferred.

Figure 2

Typical dense-phase pressure pneumatic conveying system

Slug flow

Airoutlet

Air-materialreceiver

Supplementaryair injectors(optional)

Conveyingline

Transporter

Fluidizingair jets(optional)

Materialinlet

Inletvalve

Ventvalve

Ventfilter

Compressed-airinlet

Plantcompressed-airsupply

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Some dense-phase systems have supplemental air injec-tors (also called air boosters or air assists) located alongthe conveying line (Figure 2). Shown in detail in Figure 3,an air injector works by injecting compressed air (or an-other gas, such as nitrogen, to match the conveying gas)into the conveying line. The added air can clear any plugscaused by low air volume or pressure, eliminating thechore of dismantling the line to remove plugs. Using theinjectors reduces the system’s air volume safety factor byreducing the air volume required for reliably conveyingthematerial. (An air volume safety factor is typically builtinto a pneumatic conveying system’s design to ensure thatthe system has slightly more air volume than the applica-tion requires; however, while this extra air volume helpsthe system reliably convey material without plugging, italso increases the system’s energy consumption.)

Air injectors along the conveying line can also be used togently restart flowwhenmaterial is left in the line after theconveying cycle. This is a particular advantage for a sys-tem handling an abrasive or friable material or a materialblend. When the system is restarted without supplemen-tary air, the higher-speedmaterial flow can cause an abra-sivematerial to produce excessive and prematurewear onthe conveying line and other material-contact compo-nents. This higher-speed flow can also damage a friablematerial, resulting in unacceptably high amounts of parti-cle attrition. It can alsodeblendamixtureofmaterialswithdifferent particle sizes and bulk densities. Using supple-mentary air in the line can not only preventwear, attrition,and deblending problems when a power outage or otherevent abruptly shuts down the conveying systemwhen it’sfull ofmaterial, but can providemore systemdesign flexi-bility for an application where you want to intentionallyleavematerial in the line between cycles.

Air injectorsmust be designed as fail-safe check valves toprevent the conveyed material from intruding into the in-jectors’ compressed-air supply. Such intrusion can occurwhen material slugs (also called pistons) form inside thedense-phase conveying line (Figure 2); because of theslugs’ changing velocity in the line, usually before linebends, the air in thepockets between the slugs canbecomepressurized to a level higher than that of the compressedair injected into the line. If this overpressure condition oc-curs at an air-injection point and the air injector doesn’thave a check valve, some particles can enter the com-pressed-air supply.

6 Is there a standardwayof categorizingwhenapneumatic conveying system is operating indilutephase ordense phase?

Unfortunately, there’s no industry standard formeasuringthese operating phases. So just because a pneumatic con-veying systemhas a rotary airlock valve, it’s not necessar-ily operating in dilute phase, and just because a systemhasa transporter, it’s not necessarily operating in dense phase.

However, you can use these rules of thumb for determin-ing apneumatic conveying system’s operatingphase:

•Most dilute-phasepressure systemsoperate below15psi(typically between 4 and 8 psi), while most dense-phasepressure systems run above15psi.

•Most dilute-phase vacuum systems operate below 12inches mercury (typically between 8 and 12 inches mer-cury), while most dense-phase vacuum systems runabove 12 inches mercury (typically between 12 and 14inchesmercury).

•Depending on the conveyedmaterial, most pressure andvacuum dilute-phase systems have an air velocity be-tween 3,500 and 9,000 fpm and most pressure and vac-uum dense-phase systems have a 3,000-fpm or lower airvelocity.

• In a dilute-phase system, the material velocity is nearlythe same as the air velocity. In a dense-phase system, es-pecially onewith slug flow, the averagematerial velocityismuch slower than the air velocity. In either system, thematerial can’tmove faster than the air.

Onecaution:Whenyou’re talking to adense-phase systemsupplier about selecting a new system, make sure that thematerial velocity numbers the supplier is using are clearlydefined. Some suppliers use air velocity andmaterial ve-locity numbers interchangeably. Make sure you knowwhat numbers the supplier is talking about before you ac-cept the supplier’smaterial velocity claims.

Figure 3

Typical supplementary air injector

Compressed-airsupply

Conveyingline

Supplementaryair injector

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7 Howdo I select a pneumatic conveying systemformyapplication?

Major factors to consider.The first thing to addresswhenselecting a dilute- or dense-phase pneumatic conveyingsystem isyourmaterial’s characteristics.These include thematerial’s particle size and shape, bulk density, moisturecontent, abrasiveness, friability, cohesiveness, hygroscop-icity, static charge, explosivity, toxicity, melt point, andmanyothers.

Next, consider how the system will fit into your locationandplant environment.Howwill the system’splanned linerouting and length fit into your installation area?Are thereany physical constraints in the area, such as other equip-ment or spots with low headroom?What will it take to fitthe system into your available area within these con-straints?Andhowcan the systembeconfigured toprovideeasymaintenance access to systemcomponents?

Anothermajor factor to consider is what material transferrate the systemmust achieve to meet your application re-quirements.

Making a choice. Assuming that your material can bepneumatically conveyed (that is, it doesn’t have a largeparticle size and high bulk density and it isn’t extremelysticky), and without considering any other factors at thispoint, start by considering the conveying system with thelowest initial investment cost: dilute phase.

Whether the dilute-phase system should operate underpressure or vacuum in your application depends on twomain factors:

•Whether air leaking into or out of the systemwill have anegative effect: For instance, air leaking out of a pressuresystem that conveys a toxic or explosivematerial can cre-ate hazardous conditions inyourplant. In this case, a vac-uumsystemwouldbe a safer choice.

•Whether your material packs tightly (like wood shav-ings), is cohesive and tends to build up on surfaces (liketitanium dioxide), or is fibrous (like some grain hulls):Formaterials like these, a vacuumsystemmaybe thebet-ter choice. This is because the vacuum in the conveyingline tends to pull the particles apart during conveyingrather than push them together or into the line walls aspressure conveyingdoes.

Some factors maymake a dense-phase conveying systembetter suited to yourmaterial. The dilute-phase system re-liesmainlyonproviding ahigh enoughair velocity tokeepmaterial entrained in the airstream.This relativelyhighve-

locity candamagea friablematerial, resulting inunaccept-ably high amounts of particle attrition. It can also deblendamixtureofmaterialswithdifferent particle sizes andbulkdensities. In both cases, the gentler conveying in a dense-phase system—which reliesmore on a high pressure dif-ferential than a high air velocity—is often a better option.The dilute-phase system’s high velocity can also be aproblem for handling an abrasive material, which canquickly wear the conveying line and other components,making adense-phase system the clear choice in this case.

Also consider a dense-phase conveying systemwhenyoursystem will include long conveying line runs. A dense-phase system typically requires a smaller line diameterthan a comparably-sized dilute-phase system.Thismeansthat the dense-phase system not only requires less air vol-ume but, in a long system, the smaller-diameter line sec-tions are much lighter, making their installation andreplacement easier. Thedense-phase system’s smaller andlighter elbows and similar components provide the sameadvantages.

Choosing pressure or vacuum operation for your dense-phase conveying system depends on the same factors asfor a dilute-phase system: whether air leaking into or outof the systemwill have a negative effect andwhether yourmaterial packs tightly, is extremely cohesive and tends tobuild upon surfaces, or is fibrous.

The supplier should conduct the tests with the samematerial the installed system will handle in your plantand simulate your field conditions as closely aspossible.

8 Howcanpilot-plant testinghelpmedesign apneumatic conveying system?

Once you’ve decided on a dilute- or dense-phase pneu-matic conveying system and determined whether it willoperate under pressure or vacuum, have the system sup-plier run pilot-plant tests of the proposed system in thesupplier’s test facility. The supplier should conduct thetests with the samematerial the installed systemwill han-dle in your plant and simulate your field conditions asclosely as possible. This includes configuring the pilot-plant system, if possible, with the same conveying linerouting, length, andnumber of bends that the installed sys-temwill have and running the tests under your plant’s am-bient air temperature andhumidity conditions.

Before the supplier runs the tests, youshoulddefine severaltest criteria. Basic criteria include whether the proposedsystem conveys your material, at what rate it conveys the

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material, andhowmuchair the systemconsumes.Youalsoneed todefinecriteria specific toyourapplication. If, for in-stance, you’re conveying a friable material and are con-cerned about material attrition, you need to define howmuchattrition is acceptable and thenaccuratelyverifyhowmuchattritionoccursduring the tests.

The supplier will use the pilot-plant test results to size theconveying system that’s installed in your plant.Make surethat you understand exactly how the supplier will use thetest results to size your installed system and, in particular,how the supplierwill use the data to calculate the installedsystem’s material conveying rate and air consumption.

PBE

For further reading

Findmore information on pneumatic conveying basics inarticles listed under “Pneumatic conveying” in Powderand Bulk Engineering’s comprehensive article index (inthe December 2008 issue and at PBE’s Web site,www.powderbulk.com)and inbooks available on theWebsite at thePBEBookstore.Youcanalsopurchase copies ofpastPBE articles atwww.powderbulk.com.

Mike Weyandt is corporate sales manager for Nol-Tec Sys-tems Inc., 425 Apollo Drive, Lino Lakes, MN 55014; 651-780-8600, ext. 244, fax 651-780-4400 (mikeweyandt@nol-tec.com, www.nol-tec.com). He has 30 years experi-enceinpneumaticconveyingfordrybulkmaterials.