Screw elevator

Screw elevators operate at all inclinations, up to the vertical. Intermediate bearings offer a flow obstruction, necessitate a gap in the flighting, and require access for service, so their use in screw elevators must be avoided where possible. Screw elevators lengths are, therefore, normally limited by the stiffness needed for the centre tube, to prevent casing contact by deflection or shaft whirling. For this reason screw elevators in plant use rarely exceed 8 m overall casing length. 

In order for the material to enter the swept volume of the screw at the inlet port, it has to provide a positive pressure at the end of the inlet flow sfream. The supply hopper therefore has to be of good flow design to deliver 

High-speed screw elevators self clear, up to a point. On the main run of casing the dynamic action moves forward all material that boimces 

It is not exceptional for a long, steep elevator conveying a fine powder to experience a cyclic oscillation of behaviour at the end of a batch run. Whereas the machine may operate smoothly, continuously transporting away all material entering the hopper until the infeed ceases, once the hopper empties and the inlet region clears, the material in transit is no longer backed-up by following product. 

Screw elevator design entails considerably more expertise than that required for screw conveyors. The performance is more sensitive to the nature of the product handled, and a host of operating hazards, not relevant to screw conveyors, may be experienced. Screw feeders incur different, but similarly specialized, experience for optimum design exploitation. 

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A screw elevator works in a dynamic mode. Figure 5.3, with material swirling around the screw axis and advancing along the axis in a similar pattern to a screw feeder, except that ... 

Figure 5.3 Dynamic mode of material movement in a screw elevator.

Figure 5.4 Feeding of screw elevator by screw feeder.

Another crude form of screw elevator is the so-called flexible screw elevators. The very limited flexibility these machines possess is permitted by the use of a plastic tubular casing and continuous screw flight that does not have a centre shaft. These units mainly have small diameter coreless screws, i.e. without centre shafts, running at relatively high speeds. 

Bates, L. (October 2005) The static screw elevator. Proceedings of the 8th International Conference on Bulk Materials Storage, Handling and Transportation, University of WoUangong. 

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