Conveying
decades, there is still an art to selecting the best overall system and engineering it to meet individual needs. The final analy- sis requires an intimate understanding of the material including bulk density, flow properties, temperature, moisture content, inherent hazards and allowable degree of degradation, the actual process and limitations of each conveyor technology. Both pneumatic and flexible screw convey- ors will handle a wide range of products, from fine powders to large particles. Both can be designed to move materials that are friable or fragile, cohesive or have a tendency to pack, as well as temperature- sensitive materials. Consult with a special- ist who does not have a vested interest in selling only one type of equipment, an expert will weigh each parameter and recommend the best solution for you.
CONVEYOR CHOICE
Flexible screw conveyors, also known as a spiral conveyor, helical conveyor or centreless auger conveyor comprise a flexible screw contained within a flexible or rigid tube and driven by an electric mo- tor. Make-up of the system is appropriate to the application - the ‘screw’ fabricated in spring steel or stainless steel whilst the outer tube produced in plastic or steel. It is a relatively simple design, and generally the most economical choice, with efficient performance, high reliability,
and low
capital and operating costs. When properly engineered and tested, it will provide ex- cellent performance across a broad range of applications. There are also systems spe- cifically designed to convey difficult-to-handle materials that tend to pack, cake, smear or fluidise, as well as fragile or brittle materials prone to breakage or crumbling. Pneumatic conveyors are mostly custom-engineered and tend to be specified where longer trans-
fer distances are required. During transfer the material is suspended in a gas stream (most often air, but sometimes an inert gas) introduced by either a positive pressure blower upstream of material intake points, or by a vacuum pump downstream of material discharge points. At the end of the trans- fer line product is separated from the gas stream by filter receivers or cyclone separa- tors, or sent directly into process vessels. Although the pneumatic system is more complex in design and more costly it can be integrated directly into process or production lines. Positive pressure pneumatic conveying
is generally used to convey materials from a single source to one or multiple destinations, over relatively longer distances and with greater capacity than vacuum systems with similar size conveying lines. 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.
MATERIAL CHARACTERISTICS
One of the most important factors to consider is the properties of the material to be
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Flexible Screw Conveyor
www.solidsandbulk.co.uk
Solids and Bulk Handling June 2012 17
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