Selecting the best conveyor for an application.
Suck, blow, entrain or screw? That's the decision we have to make each time a customer approaches conveyor suppliers with a conveying application. The most important task of any conveyor manufacturer is to make sure we provide the customer with the most 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 more complicated than that. To recommend a conveying system for an application, a thorough knowledge of the suitability and benefits of each type of conveyor is required. We also need to know what questions to ask the buyer to make sure that the proper conveyor is selected and installed. Also, we need to consider that although conveyor suppliers sell equipment each day, end user engineers and buyers may be purchasing their first conveyor.
As conveyor specialists, suppliers not only need to have an in-depth knowledge of their own equipment, but also the broader scope since, on rare occasions, we have to be in a position to know when an application is beyond our capabilities and when we have to suggest a solution outside of our scope.
In selecting a conveying system for your application, it is important that you share as much information with the supplier as possible from the outset. By sharing this information, the risk of any problems you might have later is significantly reduced. Problems often occur when manufacturers or customers withhold critical information.
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. Some 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.
Information required from the manufacturer
As part of the selection process, the manufacturer needs to do its homework and provide the conveyor supplier with answers to certain questions. As a minimum, we feel that a manufacturer should provide answers to the supplier to the following 20 questions:
* What product (s) or material (s) need to be conveyed?
* What is its/their bulk density(ies)?
* What is the condition of the product or material in terms of such criteria as moisture content, average particle size and temperature?
* Is the product likely to change in any way in the future?
* From what is the product being conveyed (for example, from a silo, bulk bag or bag tip station)?
* To what is the product being conveyed (such as a mixer, sifter, mill, reactor)?
* If it's a reactor of any type, is there any steam, gas or solvent given off that might enter the conveyor?
* What is the horizontal conveying distance?
* What is the vertical height to which the product has to be conveyed?
* What is the route of the conveyor (such as inside, outside or number of bends required)?
* What is the conveying rate in pounds per hour or the batch size over a given time?
* Will a pre-weighed 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?
* How often/for how long will the conveyor operate each day?
* Is it important to deliver the material to the receiver in a homogenous manner--such as when flakes are added to a liquid to make a lump-flee paste?
* If the product is a mixture, is it essential that the integrity of the mix be maintained?
* Is the material fragile, and how important is it to minimize damage during conveying?
* What accessories are required (such as a bag tip station, bulk bag discharger or receiver hopper)?
* 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)?
* Is the conveyor manufacturer also supplying the control panel, level sensors and other accessories?
* Will the conveyor be readily accessible for maintenance purposes?
* How long is the conveyor expected to run between services?
Besides the questions above, the manufacturer will most likely also require you to supply other information. What kind of information depends on the type of conveyor being considered and the manufacturer himself.
Selecting the right conveyor for your application
What follows are the main parameters, benefits and disadvantages of the types of conveyors manufactured by Spiroflow Systems, as well as other manufacturers.
Flexible screw conveyors
Flexible screw conveyors are often the simplest and lowest costing solution for transferring a variety of materials from point A to point B 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 150 lb./ft.
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. Specific models are available for various applications that can be safely stripped down in minutes for thorough cleaning.
Wear is only a problem with abrasive products; life with other materials is almost indefinite. Tubes and spirals can be easily replaced. One of the latest developments is abrasion resistant rubber tubes for such applications as aggregates, sand, cement and glass cullet.
[FIGURE 1 OMITTED]
Since this type of conveyor should always operate while full of product, 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 mn 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.
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 capacities up to 120 tons per hour with low energy requirements, minimal product degradation and virtually no separation of mixtures.
AMCs are effectively mechanical vacuum conveying, and should not be confused with drag-link type conveyors. Draglink 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, AMCs are available in a multitude of round-the-comer configurations. Other than free flowing powders, such as acrylics, flour and carbon black, AMCs 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 aeromechanical 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 on the length of the conveyor, the number of starts and stops, solids loading, and whether routine inspection and tensioning are 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 aero-mechanical conveyor with an integral automatic rope tension monitoring and adjustment system. This additional option to the aero-mechanical conveyor can, for some facilities, literally turn a nightmare into a dream.
The third conveyor type, the vacuum conveyor, is the obvious choice when 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 products in the event of accidental damage to the conveying tubes, thus minimizing the escape of product to the 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 type 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. These type filters reduce maintenance and minimize product loss.
Pneumatic conveyors are probably the most versatile of all conveying systems, with the main negative aspect being cost considerations. 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 types of conveyors
Beside these four types of conveyors, there are several other types available. These include:
* Rigid screw conveyors--beware of the seals and bearings;
* Bucket elevators--this type of conveyor is ideal for the most delicate products, but generally not for those that are dusty;
* Flat belt conveyors--this type of conveyor is mainly used in quarries and mines;
* Vibratory feeders--vibratory feeders are ideal if only very short conveying distances are required; and
* 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 aeromechanical conveyor with a consistent in-feed of material.
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 the 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. Health, safety and environmental benefits also usually outweigh the costs involved.
Nevertheless, the conveyor type has to be the right one for the job. That's where your conveyor supplier plays a significant, if not a key role.
by Michel Podevyn, Spiroflow Systems
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|Title Annotation:||Process Machinery|
|Date:||Jun 1, 2008|
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