Deciding on the single most efficient method to move bulk material – flakes, powders, granules – from point A to point B is a matter of matching material flow characteristics, process parameters such as flow rates and distances, source and destination, plant configuration and space limitations. Then factor in purchase, maintenance, labour and cleaning costs, and the choice becomes clear!
The most frequently specified conveying technologies for bulk handling are the flexible screw and pneumatic systems.
A flexible screw conveyor, also known as a spiral conveyor, helical conveyor or centreless auger conveyor, consists of a flexible screw contained in a flexible or rigid tube and is driven by an electric motor. Build materials are specified according to the application, the screw fabricated of spring steel or stainless steel, the outer tube of plastic or steel. It is a relatively simple design and generally the most economical choice providing efficient performance, high reliability, low capital and operating costs. Designs are available for handling extremely fine powders to conveying difficult-to-handle materials that tend to pack, cake, smear or fluidize, as well as fragile or brittle materials prone to breakage or crumbling.
Pneumatic conveyors move bulk materials by suspending them in a gas stream (most often air, but sometimes an inert gas) that is introduced by either a positive pressure blower upstream of material intake points or by a vacuum pump downstream of material discharge points. Product is separated from the gas stream at the end of the line by filter receivers or cyclone separators or sent directly into process vessels. Positive pressure pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations over relatively long distances and with greater capacity than vacuum systems of similar size. Vacuum systems allow easy pick-up of materials from open containers using wands, so are better suited to transport material from multiple sources such as storage vessels, process equipment, and rail cars to single or multiple destinations.
Core factors to evaluate when selecting a type of conveying system are:
• Material characteristics
• Material source and destination
• Conveying parameters
• Plant conditions
One of the most important factors to consider is the properties of the material including bulk density, flow properties, temperature, moisture content, inherent hazards, and allowable degree of degradation. Both pneumatic and flexible screw conveyors will handle a wide range of products, from fine powders to large particles and can be designed to move materials that are friable or fragile, as well as temperature-sensitive materials.
Pneumatic conveying systems are best suited for dry, free-flowing to semi-free-flowing bulk products whilst extremely fine (submicron) powders are best conveyed with a flexible screw system because the amount of dust created by the process is minimal and requires little or no air filtration at the discharge point. Fine particles can make it difficult to keep the filters clean in a pneumatic conveying system, requiring greatly increased area of filtration media, which can add cost and require larger space for installation. Flexible screw conveyors are also the technology of choice when dealing with blended materials, because unlike pneumatic conveyors, they prevent the separation of blends throughout the entire length of the conveyor, regardless of differences in the flow characteristics, bulk density or particle size of ingredients.
Specially engineered flexible screw conveyors are available for moving more difficult materials that might cause a pneumatic conveyor to plug and a general-purpose screw conveyor to bind or seize. These uniquely designed conveyors have specially engineered screws, tight tolerances and straight conveyor tubes to efficiently handle a broad variety of non-free-flowing products. Examples include materials that are moist such as brown sugar, materials that tend to cake, stick, or clump like TiO2 and other pigments and products like cake mix that have high fat or oil content.
The scale tilts toward pneumatic conveying when handling hazardous materials that require inert gas blanketing to prevent explosions, oxidation, or other changes in product characteristics, although it is also possible to blanket a flexible screw conveyor. Where a completely airtight system is necessary, pneumatic conveying is the method of choice. Likewise, with very-high-temperature material, a positive pressure pneumatic conveyor has the advantage. A vacuum system cannot be ruled out, but measures must be taken to protect the air mover and filter media from the heat.
If degradation of product during transfer is a concern either system must be properly designed to minimize damage and this is where testing becomes important. Pneumatic conveying has been used to move products as diverse as bran flakes, bottle caps, capsules and tablets without damage. Experience with certain materials demonstrates that pneumatic conveying can significantly alter its bulk density, much more than with a flexible screw conveyor.
Abrasive materials are a unique situation and personnel accustomed to handling abrasives expect to maintain equipment and replace components. With flexible screw conveying, the inner screw may need to be replaced periodically, but downtime is minimal. Dilute phase pneumatic systems can also handle abrasive materials if other parameters favour this conveying method, but will require periodic replacement of elbows, rotary valves and other components.
Material source and destination
When materials are introduced from multiple sources, either sequentially or simultaneously, pneumatic conveyors are the better choice since separate flexible screw conveyors may be required for each source, increasing cost. Since pneumatic conveyors require separate receiving equipment at every destination, such as filter receivers, weighing valves or rotary airlock valves, flexible screw conveyors can be the lower cost alternative when delivering to multiple discharge points which are in close proximity.
For material in bags, drums or boxes, a vacuum conveying system with a pick-up wand can pull material directly from the container. Use of a flexible screw conveyor requires that the containers be dumped or discharged into a hopper fitted with an intake adapter. Either technology is suitable for conveying products being discharged form bulk bags when the bag is properly suspended above a receiving hopper that is equipped with the appropriate intake adapter.
Where short-distance transfer is required either mechanical or vacuum conveying may be suitable but vacuum units tend to be more expensive. As distance increases, you reach the limits of a single flexible screw conveyor and may require several conveyors in series. At some point the multiple mechanical conveyor transfer system becomes more costly and less practical than a comparable pneumatic system.
Cleanability may also drive your conveyor choice, particularly for food processing or pharmaceutical/biotechnology applications. Flexible screw conveyors are easier to clean because they lack internal seals, crevices or joints that can trap particles or breed contamination. Simply reversing the screw rotation will evacuate residual material and allow the smooth interior surfaces to be flushed with air, water, steam or cleaning solution. A pneumatic conveying system must be designed to decrease cleaning difficulty, with conveying lines broken into detachable sections and supports that allow removal of heavy rotors from rotary airlock valves, increasing cost significantly.
If complete conveyance of a batch is essential, positive pressure or vacuum pneumatic conveyors hold the advantage over flexible screw conveyors, which contain residual material after the conveyor has stopped discharging. This is why the lower end cap of a flexible screw conveyor must be removed and the screw reversed to evacuate residual material before the conveyor can be sanitized.
A plant layout – routing requirements and space considerations – are all important when comparing conveying options. Where the conveying route is straight or can be gently curved, a flexible screw conveyor is the preferred option and where there are numerous changes in direction or turns in a short space a pneumatic system has the advantage.
Physical limitations such as floor space, ceiling height also impact the choice of conveyors.
While the characteristics of material and process may clearly dictate one technology over the other, both flexible screw and pneumatic conveyors are suitable for most bulk conveying applications. The decision then comes down to economics, with flexible screw conveyors offering lower capital and operating costs, especially over shorter distances involving lower capacities whilst pneumatics, although higher in initial cost and power consumption, offering greater cost effectiveness when conveying in higher capacities and/or over longer distances.
In the final analysis, an expert, unbiased opinion and full-scale testing should confirm the proper conveyor choice for your needs.
A fully equipped, state-of-the-art testing facility will contain both pneumatic and mechanical bulk handling equipment to produce an objective recommendation based on the actual process material. By verifying performance you can avoid costly misjudgments and delays in getting your system up and running.
By: Alan Walton – Sales Manager – Flexicon Europe
Flexicon (Europe) Ltd, Herne Bay, Kent
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