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Selecting the Best Conveyor for Your Application
by Michel Podevyn

Suck, blow, entrain or screw? That’s the decision faced each time a customer approaches a supplier such as Spiroflow Systems with a conveying application requirement. The most important task of any conveyor manufacturer is to make sure we provide the customer with the optimum solution for their application. We have to assure that the conveyor we jointly select and supply is the most efficient and most reliable available at the best possible price.

Although our task sounds simple, it is usually somewhat complicated. A thorough knowledge of the suitability and benefits of each type of conveyor is required before we can recommend a specific system for a specific application. Lastly, we need to consider that although suppliers like Spiroflow sell equipment each day, end user engineers and buyers may be purchasing their first conveyor!

Therefore, it's important that you share as much information with the supplier as possible from the outset, and significantly reduce the risk of any problems you might have later on. Problems often occur when customers withhold critical information, sometimes without realizing it.

Ultimately, a product feasibility test may be advisable to determine the most suitable conveyor for the material concerned, the distance involved and the throughput required. Most conveying manufacturers have a database available of conveyor test results and of actual working conveyors, detailing which products perform well on a particular type of conveyor. Other manufacturers will have so much experience on conveying certain products that they can offer performance guarantees without the need to do any type of feasibility test.

Playing 20 Questions

For a successful selection, do your homework and make sure you can provide the answers to these 20 questions. Keep in mind that this list is a minimum, and though some seem obvious, every one should be thought through to ensure you've covered all the angles:

1.What product(s) or material(s) need to be conveyed?

2.What is its bulk density?

3.What is the condition of the product or material in terms of moisture content, average particle size, temperature and other such criteria?

4.Is the product likely to change in any way in the future?

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

6.To what is the product being conveyed (such as a mixer, sifter, mill, reactor)?

7.If it’s a reactor or processor of any type, is there any steam, gas or solvent given off that might enter the conveyor?

8.What is the horizontal conveying distance?

9.What is the vertical height to which the product has to be conveyed?

10.What is the route of the conveyor (such as inside, outside or number of bends required)?

11.What is the conveying rate in pounds per hour or the batch size over a given time?

12.Will a pre-weighted batch be conveyed or will the conveyor be transferring material to a receiver of a given size, on load cells or is it a continuous process?

13.How often/for how long will the conveyor operate each day?

14.Is it important to deliver the material to the receiver in a homogeneous manner - such as when flakes are added to a liquid to make a lump-free paste?

15.If the product is a mixture, is it essential that the integrity of the mix be maintained?

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

17.What accessories are required (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 explosion-proof motors and other such equipment be required)?

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

19.Will the conveyor be readily accessible for maintenance purposes?

20.How long is the conveyor expected to run between services?

Selecting the right basic conveyor type

It's crucial that the basic method of your conveyor's operation fit the intended use. What follows are the main parameters, benefits and disadvantages of the types of conveyor manufactured by Spiroflow Systems and other manufacturers.

1.Flexible Screw Conveyors

These are often the simplest and lowest costing solution for transferring a variety of materials from at rates of up to 40 tons an hour over distances of up to 65 feet. If greater conveying distances are required, multiple systems can be linked together.
Flexible screw conveyors consist of a special heat treated and tempered carbon or stainless steel spiral that rotates within an Ultra High Molecular Weight Polyethylene (UHMWPE) food grade tube. This type of conveyor is well suited for powdered, granular or flaked materials with a bulk density up to 150lb/ft3.

The term ‘flexible’ means that this type of conveyor can be curved to some extent, depending on its diameter. This provides the user with the flexibility to route the conveyor around obstacles anywhere between the inlet and outlet. Normally, using only one continuous curve is recommended.

For most applications, the spiral has a round cross section but a flat or profiled version can also be used for fine, cohesive or easily smeared materials.
This type of conveyor is designed to operate when full of material; running empty will lead to excessive noise and wear. Having a head of material in the feed hopper is desirable since it helps with the elevation of material upon start-up of the conveyor.

The main benefit of the flexible screw type of conveyor is its inherent simplicity. This results in low initial cost, quick installation time and low maintenance. Many models can be safely stripped down in minutes for thorough cleaning. Moreover, wear is only a problem with abrasive products; life with other materials is almost indefinite. Tubes and spirals can be easily replaced.

Since this type of conveyor should always operate while full of products, it is not recommended for transferring pre-weighed batches to a receiver. These types of conveyors are best used to deliver product from storage or a bag tip station to a weigh hopper or a vessel with a high level switch. For example, it is ideal for maintaining a constant head of material in packing machine hoppers by gently filling to the high level control rather than dumping pulsed batches. Because the in-flight product is constant, flexible spiral conveyors will give very accurate, highly repeatable batches if controlled by a simple time switch.

Although flexible spiral conveyors need to run full of product, they can be emptied at the end of a batch operation or at the end of a shift by removing an end bung and running the conveyor in reverse at a reduced speed if necessary.

2. Aero-Mechanical Conveyors

The second type of conveyor, the aero-mechanical conveyor (AMC), is ideal for total transfer of products at distances from 10 to 85 feet at rates of up to 120 tons per hour.

An alternative and more descriptive name of the AMC is a ‘rope and disk’ conveyor. This is because the AMC consists of several evenly spaced polyurethane disks attached to a wire rope. The rope and disks travel in a continuous loop fashion at a consistently high speed within parallel steel tubes. At each end, there are enclosed housings. Within these housings, the rope assembly runs from one tube to the other around specially designed sprockets. One of these sprockets drives the rope and disks while the other sprocket provides tension to the rope. The high speed of the disks produces an air stream that fluidizes and entrains the product in airflow until it is centrifugally ejected at the outlet. This method of conveying facilitates large capacities with low energy requirements, minimal product degradation and virtually no separation of mixtures.

AMC’s are effectively ‘mechanical vacuum conveying’ and should not be confused with Drag-Link type conveyors. Drag-Link conveyors are slow-moving, heavy duty devices in which cast iron disks are often linked with rods or chains and where the product is scraped along inside the tube.
Over the years, the aero-mechanical conveyor has proven to be a cost efficient method of conveying materials, dust-free and without the need for filtration. The AMC offers total batch transfer, operation at any angle (including vertical) without any loss of capacity and contaminant free delivery. For easy cleaning, they can also be supplied with access panels.

Besides straight-line operation, AMC’s are available in a multitude of ‘round the corner’ configurations. Other than free flowing powders such as acrylics, flour and carbon black, AMC’s can also convey difficult materials such as titanium dioxide. They also do not have any problem conveying granules, flakes or chips.

A major benefit of this type of conveyor is that degradation to the material is almost negligible. This is because an AMC creates a moving current of air in which the material is carried similar to the effect of a vacuum or pneumatic system. However, unlike vacuum or pneumatic systems, the Aero-Mechanical Conveyor has a very important advantage in that it does not need a cyclone or filter to separate the product from the air. This not only saves in capital cost but also reduces maintenance and eliminates environmental issues since the air carrying the material is recycled and not released at the outlet. The material is separated from the air that carries it and the unloaded air current is directed down into the return section of the tube where it is retained in the tube circuit.
An AMC should always be started empty and fed at a controlled rate. With free flowing products, a simple slide gate may be all that is required. In other cases, a controlled feed device, such as a rotary valve or flexible screw conveyor, should be used.

One disadvantage of an AMC is that maintenance can range from moderate to high. The rope tension needs to be adjusted regularly during the all-important start-up period and then checked periodically. Rope life depends upon on the length of the conveyor, the number of starts and stops, solids loading and whether routine inspection and tensioning is properly performed.

Despite this drawback, properly maintained rope and disk assemblies on arduous duties have been known to last 14 years and more.

The effort, worry and cost of this regular maintenance can be avoided by selecting an AMC with an integral automatic rope tension monitoring and adjustment system such as the patented system now offered by Spiroflow Systems. This additional option to the AMC can, for some facilities, turn a nightmare into a dream!

3. Vacuum Conveyors

The third conveyor type, vacuum conveyors, is the obvious choice where products need to be conveyed over longer distances and torturous routes. Vacuum conveying is usually restricted to throughputs of around 10 tons/hour at distances over 330 feet.

A vacuum conveyor uses air to convey materials through an enclosed pipeline. It provides a solution for users requiring a system that is easy to route, has few moving parts, is dust tight in operation and empties a product leaving minimum residue. Since the air is sucked-in, Vacuum Conveyors are the preferred choice for toxic or otherwise hazardous in the event of accidental damage to the conveying tubes thus minimizing the escape of product to atmosphere.

Either an exhauster or a side channel high efficiency fan located at the receiving end of the system, provides the motive force. For low capacity conveying, air powered “Venturi” systems are ideal. Venturi systems offer low capital cost and are not as expensive to operate as many potential customers have been led to believe.

Vacuum systems are normally the only conveying choice for customers that want to suck material out of bags or other open top containers such as kegs and drums. These systems are also ideal for applications with multiple inlets.

Reverse jet self-cleaning filters clean the conveying air and return the air to the atmosphere after use, reducing maintenance and minimizing product loss.

4. Pneumatic Conveyors

Pneumatic Conveyors are probably the most versatile of all conveying systems, with the main negative aspect being cost. There is virtually no limitation on capacity, product type, distance or routing. Lean phase systems (where the ratio of product to air is low) can move mountains of product. Dense phase or plug flow systems move ‘slugs’ of product at lower speeds with minimal degradation.

Positive pressure pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations. Pneumatic conveying systems are normally the preserve of ‘big league’ applications such as the rapid discharging of road and rail tankers into silos and the transfer of product from silos to large-scale production processes. Capacities of up to 100 tons per hour are not unusual.

The two main disadvantages for pneumatic conveyors is the relatively high initial installation cost and the amount of filtration required. As with vacuum conveyors, self-cleaning reverse jet filters are a big help in reducing maintenance. Maintenance is required to make sure these systems are free of leaks to ensure optimum efficiency and, above all, to avoid the associated health and environmental issues that leaks cause.

Other conveyor types

Beside these four types of conveyors, there are several types of conveyors available. These include:

Rigid Screw Conveyors – Beware of the seals and bearings!

Bucket Elevators – Ideal for the most delicate products but generally not for those that are dusty.

Vibratory Feeders – Ideal over very short conveying distances are required.

Air Slides – Air Slides are fine for dense materials that only require downhill conveying.

In some applications, a mix of different conveyor types is appropriate. For example, short easy-to-clean flexible screw conveyors are often used to provide a long distance aero-mechanical conveyor with a consistent in-feed of material.

Conclusion

In selecting a conveyor for your application, the key is to find a conveyor supplier in which you have confidence and are comfortable with in providing you with right solution for your conveying application. The conveyor supplier should be able to provide you with a performance guarantee for the material you will be conveying. After that, it comes down to the usual commercial considerations of price and delivery. Normally, installing a conveyor is part of a larger project that offers commercial savings. Heath, safety and environmental benefits also usually outweigh the costs involved. Getting the right conveyor supplier will be the key to getting the right conveyor for the job.

Michel Podevyn is the President and CEO of Spirolflow Systems of Charlotte, NC, a worldwide supplier of a wide range of both standard and custom powder handling equipment, specializing in Bulk Bag Dischargers and Fillers, Mechanical, Flexible and Pneumatic Conveyors and Bag Packing equipment. For more information about this article, contact Spiroflow Systems, Inc. at 704-291-9595, FAX 704-291-9594 or email This e-mail address is being protected from spam bots, you need JavaScript enabled to view it Spiroflow can also be found on the web at www.spiroflowsystems.com.

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