Conveying
Plant diagram showing pneumatic conveyor line routing
Plant
diagram showing flex-
ible screw conveyor configura- tions
conveyed, including bulk density, flow properties, temperature, moisture content, inherent hazards and allowable degree of degradation. Individual parameters or a combination of requirements can swing the advantage to one conveyor or other. Pneumatic conveying systems are best suited for dry, free-flowing to semi-free-flowing bulk products whilst 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, as well as other pigments and products such as cake mix that have a high fat or oil content. Where it is important to maintain tem- perature and moisture content, exposure to large volumes of air can rule out pneu- matics. While it is possible to condition pneumatic-conveyor air for temperature and moisture, this adversely impacts the
economics, adding considerably to the costs of installing and running the system. These factors may cause a flexible screw conveyor to be a more desirable choice. 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. The scale tilts toward pneumatic convey- ing 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 con- veyor. Where a completely airtight system
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18 June 2012 Solids and Bulk Handling
www.solidsandbulk.co.uk
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