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Screw conveyors are a very popular equipment in solids processing industry to move and / or dose products. Their principle allows a simple and quite robust operation, although the design must be well detailed in order to avoid issues that could lead to damages to the product and to the equipment itself (generation of foreign bodies). Screw feeders are used with product causing no or medium abrasion but should be avoided with products causing high level of abrasion ; the screw rotating in the product, is indeed very sensitive to abrasion phenomena.
A screw fitted directly to a hopper upstream is often referred in the litterature as screw feeder while the equipment alone is often referred as screw conveyor. The screw conveyor takes sometimes the denomination of Auger screw.
A typical screw conveyor design is shown below :
Figure 1 : Screw conveyor principle drawing and key components
2. Position in the process
Screw conveyors are usually placed below hoppers where they can convey horizontally the product to another process operation. When the conveying involves also a weighing, the screw feeder is actually used as a dosing equipment.
Screw conveyors can also introduce solids in a pneumatic conveying system. In most of the case, they are used in vacuum transport since there is no pressure problem, but some designs can also used to feed the product towards a pressure conveying line. For this particular purpose, the screw flight design must allow a compression of the solid so that the plug formed avoids the pressurized air of the conveying line to come back through the screw. This cannot be used for all products, thus, for pressure conveying, airlock rotary valves are usually preferred to screw conveyors for introducing the product.
Screw feeders are found in any industries handling bulk materials, to convey and / or to dose bulk solids :
- Food processing
- Grains / Animal Feed
- Water treatment (sludge processing)
3. Important considerations
Industrial screw conveyors can be very long reaching >5 m, however this kind of design can only be used with non sensitive product and when no cleaning is required. It is indeed not possible to remove the flight of such screws, and if some designs exist to still access the inside (trough design - show example), the efficiency of cleaning is reduced and the time to clean will be long. Besides, the length of the screw makes it prone to bending and to possible metal / metal contacts with the housing, risking to pollute the product with metal chips. As a reference, a screw flight of 4 m can bend up to 7-8 mm from the shaft straight axe, if no proper provision have been taken to allow for such bending, metal / metal contact can happen.
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