Doyouneed to convey a dry bulkmaterial that’s fluidizable, non-free-flowing, abrasive, hygro-scopic, toxic, or explosive? Findadvice here on choosing a screwconveyorthatcanhandle it.

Ascrew conveyor is one of themost versatile and cost-effec-tivemechanical conveyors for

handlingdrybulk solids.The conveyorprovides enclosed transfer whilemov-ingmaterialshorizontally, vertically, orat an incline and can deliver materialsfrom one ormore inlet points to one ormore discharge points. The screw con-veyor’swide rangeof available config-urations,components,andconstructionmaterialsmakestheequipmentsuitablefor transferringeventhemostchalleng-ingdrybulkmaterials.

A screw conveyor consists of a screwmounted in an enclosed U-shapedtrough (or tubular housing). Thescrew can have one or several sec-tions, and each section consists offlighting mounted on a pipe. At oneend, the screw is connected to a rotat-ingdrive shaft, and theopposite end isconnected to an end shaft. The screwis supported by bearings at the troughends. If the screw conveyor is longenough to require more support,hanger bearings can be suspendedfrom the top of the trough to support

the screw at points between the screwsections.

Selecting a screw conveyor to handleyour challenging material is really amatter of choosing the right combina-tion of components and constructionmaterials. While you’ll need to workcloselywith your conveyor supplier’sengineering department for technicaladvice during this process, here aresome basics for making the rightchoices for handling your tough ma-terial in a screwconveyor.

Fluidizablematerials

Fluidizablematerials— that is,mate-rials that tend to flow like liquidswhen aerated or mechanically agi-tated — can flow uncontrollablythrough a screw conveyor. Examplesinclude alumina, Portland cement,powdered cocoa, fly ash, limestonedust, and talc. To handle this kind ofmaterial, select a larger-diameterscrew with a short pitch (that is, ashort distance between flights), suchas a short-pitch or half-pitch single-flight screw, as shown in Figure 1.The short pitches in these screws willslow material flow through the con-veyor. Running the screw conveyor ata slower speedwill also reduce flood-ing. If your material will discharge toa downstream weighing device, youcan use a double-flight short-pitchscrew (which has two rows of flight-ing around the pipe) to better controlthe material’s flow and minimizesurging, thus providingmoreuniformdischarge to theweighingdevice.

Non-free-flowingmaterials

Non-free-flowing materials includeviscous or sticky materials, materialsthat tend tomat or interlock, and thosethat tend to pack. Examples of vis-cous or stickymaterials include greenmalt, sewage sludge, and wet beetsugar. Such a material can collect inthe joint between the screw flight andpipe and build up on the flight andpipe surfaces, reducing the conveyingcapacity.Tohandle aviscousor stickymaterial, you can choose a single-flight ribbon screw, which has anopen space between the flight andpipe to eliminate buildup on thescrew, or a shaftless screw, which hasa helical screw without a pipe at the

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Tips:

Choosing a screw conveyorto handle your tough materialBillMecke KWSManufacturingCo.

Tips

Figure 1

Screws for handlingfluidizable materials

a. Short-pitch single-flight screw

b. Half-pitch single-flight screw

As appeared in PBEJuly 2009

trough, screw, bearings, and othercomponents made of abrasion-resis-tant materials, such as hardened orhard-surfaced steel. Conveying yourmaterial at a nominal depth of about15 percent of the conveyor’s total vol-ume and running the conveyor at lowspeedwill also reduce abrasivewear.

Hygroscopicmaterials

Hygroscopic materials, such as sali-cylic acid, dried milk powder, soapgranules, and salt, readily absorbmoisture and can build up on screwflight surfaces and collect at the jointbetween the flight and pipe. If yourmaterial is hygroscopic, select avapor- and dust-tight screw conveyorwith shaft seals and other seals thatwill preventmoist air fromcontactingthematerial.Youcan also select a sin-gle-flight ribbon screw or shaftlessscrew (Figure 2) to prevent materialbuildup. If your hygroscopicmaterialis hot, such as a powder that’s dis-charged from a dryer, you need tokeep the screwconveyorwarm topre-vent thematerial fromcontacting coldair and producing condensation thatcan moisten it. You can do this bychoosing a conveyor with a heatjacket that circulates a heatingmedium around the trough, heat trac-ing (electricwiring) that transfers heatthrough the trough walls, or insula-tion. In some cases, you may alsoneed to add a purging system to theconveyor to continuously drawmois-ture off thematerial.

Toxic or explosivematerials

Toxic and explosive materials arehazardous and must be handled in ascrew conveyor that’s carefully cho-sen to contain them. Toxic materials,such as asbestos, arsenic, benzenehexachloride, caustic soda, and leadoxide, giveoff toxic fumesordust thatcan escape the screw conveyor andpose a health hazard to your workers.To handle a toxic material, choose ascrew conveyor with a vapor- anddust-tight housing. The conveyorshould have extremely tight shaftseals, such as gas-purged seals orflange-type gland seals with multiplerings of packing material. Also use adust control system that captures and

center, eliminating any area formate-rial to collect. These screws areshown inFigure 2.

Materials that tend tomator interlock,such as aluminum chips, mica flakes,dry bagasse, wood chips, and paperpulp, typically have irregular particleshapes and sizes. Smaller particlesand the smaller ends of irregularlyshaped particles — for instance, awood chip’s thinnest edge— can be-come wedged in the clearance be-tween the flight tip and trough wall.This can jam the conveyor and in-crease the conveyor’s frictional horse-power requirement, potentiallyleading to a drive overload, and cancausebending and torque loadson thescrew that eventually lead to thescrew’s mechanical failure. If yourmaterial tends to interlock, choose aconveyor with heavy-duty construc-tion and components, such as a screwwith an oversized pipe and thickerflights, and feed thematerial at a con-trolled rate to the conveyor rather thanflood-feed it. Your conveyor suppliermayalso recommendeither enlarging

or reducing the flight-tip-to-troughclearance to prevent materials frombecoming trapped in it.

Materials that tend to pack underpressure, such as carbon black, hy-drated lime, driedmilk, talc, gypsum,and wood flour, can cause many ofthe same problems as materials thattend to interlock. These particles tendto compact into amass inside the con-veyor, creating high flow resistancethat can increase the conveyor’s fric-tional horsepower requirement andcausebending and torque loadson thescrew. If your material tends to pack,select a conveyor with heavy-dutyconstruction and heavier componentsthat can withstand greater flow resis-tance. You can also choose a single-cut-flight standard-pitch screw, asshown in Figure 3, which acts like achopper to breakup compactedmate-rial and keep it moving through theconveyor. If you’re conveying a finematerial, such as gypsum or stucco,you can prevent it from packing byusing fluidizing pads or another aera-tion device in the conveyor. If thema-terial has large fibrous particles, use avibratory pan feeder or other vibrat-ing feeder to keep the material fromcompacting as it’s fed to your screwconveyor.

Abrasivematerials

Abrasive materials, such as cementclinker, iron ore, foundry sand,wholesoybeans, and titanium dioxide, canquickly wear the screw conveyor’scomponents. If your material is abra-sive, you can specify a conveyor

Figure 2

Screws for handlingsticky materials

a. Single-flight ribbon screw

b. Shaftless screw

Figure 3

Single-cut-flight standard-pitchscrew for handling materials that

tend to pack

removes toxic fumes and dust at theconveyor discharge.

An explosive material contains explo-sive dust and can produce a dust cloudinside the screw conveyor that leads toa fire or explosion, potentially injuringyourworkers and damaging the screwconveyor, nearby equipment, and theplant.Examplesofexplosivematerialscontaining explosive dust are calciumcarbide, seeds and grains, flour, coal,potassium nitrate, sulfur, and tobacco.For handling such amaterial, choose avapor- and dust-tight screw conveyordesigned to withstand explosion pres-sures. The conveyor’s flight-tip-to-trough clearance should be largeenough topreventmetal-to-metal con-tact during conveying. The conveyorcanalsobeequippedwitha troughandscrew made of dissimilar metals,whichare less likely toproducesparks,or the conveyor can be constructed ofnonsparking metals, such as brass orbronze.Also use a dust control system

to capture and remove dust at the con-veyor discharge. Depending on yourmaterial andoperatingconditions,youmay also purge the conveyor with aninertgas, suchasnitrogen.

Partneringwith your supplier

Expect to work closely with yourscrew conveyor supplier’s engineer-ing department to ensure that the con-veyor you choosewill reliably handleyour material. The supplier has yearsof experience designing conveyors tohandle materials like yours. You’llneed to provide detailed informationabout your material’s characteristicsand your desired conveying rate, ex-pected conveyor usage, operatingconditions, and other factors duringthis process. The supplier can runscrew conveyor tests with your mate-rial to demonstrate that the conveyorcomponents and constructionmateri-als can successfully handle your chal-lenging material and meet yourconveyinggoals. PBE

For further reading

Findmore information on screw con-veyors in articles listed under “Me-chanical conveying” in Powder andBulk Engineering’s comprehensivearticle index (in the December 2008i s sue and a t PBE ’s Web s i t e ,www.powderbulk.com) and in booksavailable on the Web site at the PBEBookstore. You can also purchasecopies of pas t PBE ar t ic les a twww.powderbulk.com.

Bill Mecke, PE, is vice president andowner of KWS Manufacturing Co.,3041 Conveyor Drive, Burleson, TX76028; 817-295-2247 or 800-543-6558, fax 817-447-8528 (bmecke@kwsmfg.com, www.kwsmfg.com). HeholdsaBS inmechanical engineeringfrom Texas A&M University in Col-lege Station and is chairman of theConveyor Equipment ManufacturersAssociation (CEMA) EngineeringCommittee for ScrewConveyors.