Mechanical

Flexible-screw conveyor

Outlet

Hopper

Polyethylene tube

Metal screw

Industries such as pharmaceuticals, plastics and rubber, and food, rou-tinely need to move bulk materials. How that material gets moved is vitally important to factory effi-ciency. So when specifying a con-veyor system, engineers designing plants and processes should be familiar with the variety of options they have.

Here are the main parameters, benefits, and disadvantages of four major types of conveyors.

Flexible-screw conveyorsFlexible-screw conveyors are of-

ten the simplest and least expensive for moving a variety of materials at up to 40 tons/hr over distances up to 65 ft. If materials need to travel farther, several such conveyors can be linked together.

These conveyors consist of a stain-less or heat-treated and tempered-carbon steel screw that rotates within an ultrahigh-molecular-weight-poly-ethylene food-grade tube. For most applications, the spiral has a round cross section but a flat or profiled ver-sion can also be used for fine, cohe-sive, and easily smeared materials. The term flexible refers to the fact that these conveyors can be curved to some extent, depending on its diam-eter. This lets users route the conveyor

Selectingthebest

conveyorWhat you should know before speccing your next conveyor.

around obstacles. Normally, using only one continuous curve is recommended. This type of conveyor is well suited for powdered, granular, and flaked materials with a bulk density up to 150 lb/ft3.

Flexible-screw conveyors are designed to operate when full of material. Running empty leads to excessive noise and wear. (Although flexible spiral conveyors should run full, they can be emptied at the end of an operation or shift by removing an end bung and running the conveyor in reverse at a reduced speed if necessary.) It is also not rec-ommended that these conveyors be used to move batches or large slugs of material. Instead, they are best used to move materials from storage or bag-tip station to a weigh hopper or vessel. For example, they are ideal for main-taining a constant head of material in packing-machine hoppers by gently filling rather than dumping in batches. Because material flow is constant, flexible spiral conveyors deliver accurate, highly repeatable batches if controlled by a simple timing switch. And having a head of mate-

Michel PodevynPresident and CEOSpiroflow Systems Inc.Monroe, N.C.

Highlights Flexible-screw conveyors

are often the simplest and least-expensive option.

Aeromechanical conveyors move materials, not boxes or products.

Vacuum conveyors can follow long, complex routes.

Pneumatic conveyors are the most versatile but can be the most costly.

Edited by Stephen J. Mraz stephen.mraz@penton.com

AUGUST 21, 2008 MACHINE Design.com 83

Mechanical

Aeromechanical conveyor

Discharge of solids

Chute draws in air to replace

air that is discharged.

Any air displaced

by solids is vented.

Solids are fluidized and accelerated.

Conveyor is moving 85%

solids, 15% air.

Polyurethane discs

Wire rope

veyor (AMCs) is rope-and-disk conveyor. This is because AMCs consist of several evenly spaced polyurethane disks attached to a wire rope. The rope and disks travel quickly around a continuous loop within parallel steel tubes. Hous-ings enclose both ends of the conveyor, and the rope and disks run from one tube to the other around a pair of sprockets. One sprocket drives the rope and disks while the other tensions the rope. The speed of the disks creates an air stream that fluidizes and entrains material until it is cen-trifugally ejected at the outlet. This lets AMCs move mate-rials from 10 to 85 ft at up to 120 tons/hr with little power,

rial in a feed hopper helps with the elevation of material upon start-up of the conveyor by consistently sending the same amount of material into the conveyor inlet. This lets the conveyor operate more efficiently and makes batches more accurate.

The main benefit of flexible-screw conveyors is their inherent simplicity. This leads to low initial cost, quick in-stallation, and low maintenance. Food and dairy versions, for example, can be safely stripped down in minutes for cleaning.

Wear is only a problem with abrasive products. Life with other materials is almost indefinite. Tubes and screws are easily replaced. One of the latest developments is abra-sion resistant rubber tubes for such applications as aggre-gates, sand, cement, and glass cullet.

Aeromechanical conveyorsA more descriptive name for an aeromechanical con-

What engineers should knoWBefore selecting a conveyor, designers should be familiar with their specific application. answering these questions will help them select the right conveyor: 1. What product or material will be moved?

2. What are its bulk densities?

3. What is its moisture content, average particle size, and temperature?

4. is the material or product likely to change in any way in the future?

5. From what is the product being moved (for example, from a silo, bulk bag, or bag-tip station)?

6. to what is the product being moved (such as a mixer, sifter, mill, or reactor)?

7. if its going to a reactor of any type, is there any steam, gas, or solvents given off that might enter the conveyor?

8. how far will the conveyor travel horizontally?

9. how far will it travel vertically?

10. What route will the conveyor take (such as inside, outside, bends, total number of bends)?

11. Will the conveyor move a constant a stream of material or preweighed batches on load cells?

12. at what rate is material being moved in pounds per hour or batch size over a given time?

13. how long will the conveyor operate each day? how many days per week?

14. should the conveyor deliver material in a homogenous manner, such as adding flakes to a liquid to make a lump-free paste?

15. if the product is a mixture, is it essential to maintain the integrity of the mix?

16. is the material fragile? how important is it to minimize damage during conveying?

17. What other equipment is needed (such as a bag-tip station, bulk-bag discharger. or receiver hopper)?

18. Will the conveyor operate in a dusty or otherwise hazardous area (i.e., will neMa-rated explosionproof motors and other such equipment be required)?

19. is the conveyor manufacturer also supplying the control panel, level sensors, and other accessories?

20. Will the conveyor be readily accessible for maintenance?

21. how long is the conveyor expected to run between services?

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Mechanical

Vacuum conveying

Filter-cleaning assembly

Head

Filters

Filter receiver

Filtered air

Vacuum-breaker valve open

Pump intake filter

Pressure-relief valve

Air pump (blower) silencer

Air pumped outside

Motor

Material-input point

Throttle-valveintake filter

Flow

Discharge valve (open)

Destination of material

Compressed air in (60

to 80 psig)

rate materials from the air. This saves money and reduces maintenance and environmental issues because the air carrying the material is recycled.

AMCs should always be started empty, then fed or loaded gradually. With free-flowing materials, this can be done with a simple slide gate. In other cases, a controlled feed device, such as a rotary valve or flexible screw conveyor, should do the trick.

One disadvantage of AMCs is that maintenance costs can range from moderate to high. Rope tension needs regular adjustment during all-important start-up periods and then checked periodically. Rope life de-pends upon on conveyor length, num-ber of starts and stops, loading, and whether routine inspection and ten-sioning is properly performed. Regu-lar maintenance can be minimized by selecting an AMC with rope-tension monitoring and adjustment.

Despite this maintenance draw-back, properly maintained AMCs have lasted 14 years and more despite arduous duty.

Vacuum conveyorsVacuum conveyors are the first

choice when material must be moved over long distances and torturous routes. They are usually limited to throughputs of around 10 tons/hr at distances up to 330 ft.

Vacuum conveyors use air to move

minimal product degradation, and virtually no separation of mixtures.

AMCs are effectively mechanical vacuum conveying and should not be confused with drag-link convey-ors which are slow-moving, heavy-duty devices in which cast-iron disks linked by rods or chains scrape mate-rial along inside a tube.

Over the years, the AMC has proven to be a cost-efficient and dust-free way to move materials, which eliminates the need for filtration. AMCs can move batches and operate at any angle (including vertical) with-out sacrificing capacity or contami-nant-free delivery. They can also be supplied with access panels to make cleaning easier.

Besides straight-line operation, AMCs are available in several around-the-corner configurations. In addi-tion to free-flowing powders such as acrylics, flour, and carbon black, AMCs can move difficult materials such as titanium dioxide. They also have no problems moving granules, flakes, or chips.

A major benefit of this type of con-veyor is that degradation to the mate-rial is almost negligible. This is be-cause AMCs create a moving current of air in which the material is carried, similar to the effect of a vacuum or pneumatic system. However, unlike vacuums or pneumatics, AMCs do not need a cyclone or filter to sepa-

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emptying road and rail tankers into silos or moving material from silos to large-scale production processes. Capacities of up to 100 tons/hr are not unusual.

The two main disadvantages of pneumatic conveyors are a relatively high initial installation cost and the amount of filtration required. The high cost is due to the need for an expensive blower or compressor, as well as larger diameter (up to 6 in.), pressure-tested pipe, supporting the larger pipes, and filtering the large volume of air used. As with vacuum conveyors, self-cleaning reverse-jet filters are a big help in reducing main-tenance. Maintenance is needed to ensure these systems are leak-free for best efficiency and, above all, to avoid associated health and environmental issues leaks cause.

Other conveyorsThere are several other types of

conveyors, including:Rigid-screw conveyors. Beware

of the seals and bearings.Bucket elevators. This type of

conveyor is ideal for most delicate products, but generally not for those that are dusty.

Flat-belt conveyors. These are mainly used in quarries and mines.

Vibratory feeders. Good for dis-tances of 6 ft or less.

Air slides. Well suited for moving dense materials downhill.

In some applications, a mix of conveyor types is appropriate. For example, short, easy-to-clean flexi-ble-screw conveyors are often used to consistently feed material to a long-distance AMC.

In selecting a conveyor, the key is to find a supplier you have confidence in and are comfortable with. The sup-plier should provide a performance guarantee for the material you will be moving. After that, it comes down to the usual commercial considerations of price and delivery. Normally, in-stalling a conveyor is part of a larger project. But heath, safety, and envi-ronmental benefits usually outweigh monetary issues. Nevertheless, the conveyor has to be the right one for the job. Md

materials through a pipeline. They are easy to route, have few moving parts, are dusttight in operation, and leave minimum residue in emptied stor-age containers and transport tubes. Because they suck in air, vacuum con-veyors are often used to move toxic and otherwise hazardous materials because the tubes contain all materi-als and minimize release of them to the atmosphere.

Either an exhauster or fan at the receiving end of the system provides the motive force. For low-capacity motion, air-powered Venturi systems offer low capital costs and are not as expensive to operate as many have been led to believe. And reverse-jet self-cleaning filters clean the air and return it to the atmosphere after use. The filters also reduce maintenance and minimize product loss.

Vacuum systems are normally the only choice when its necessary to suck material out of bags or other open-top containers, such as kegs and drums. These conveyors are also ideal for pulling materials from sev-eral sources.

Pneumatic conveyorsPneumatic conveyors are probably

the most versatile of all, but they are also the most expensive. They have virtually no limit on capacity, product type, distance, or routing. Lean-phase versions, in which the ratio of material to air is low, can move mountains of material. Lean-phase conveyors typi-cally move 1 lb of air for every pound of materiel. Pressure drops across such conveyors range between 1.5 to 20 psig and air velocity is between 60 and 75 fpm with the entrained ma-terial traveling at about 70% of that speed due to the slip factor.

Dense-phase or plug-flow versions can move slugs of material at lower speeds with minimal degradation. They move about 100 lb of material for every pound of air, but air pres-sure can be as high as 100 psig. Slugs move between 10 and 30 fps.

Positive-pressure models are gen-erally used to move materials from one place to one or several differ-ent destinations. They are normally used for major tasks, such as quickly

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