Screw Conveyors
A screw conveyor is the most basic form of conveying, simply moving material from point "a" to point "b". Screw conveyors commonly handle reasonably dry to sludge type materials. All screw conveyors require that the feed into the inlet be controlled. The screw diameter, screw speed, and the housing loading determine the through put capacity of a screw.
Under normal conditions the housing loading is never greater than 45%. If the feed rate into a screw conveyor surges, so will the discharge rate. Depending on screw length and material being conveyed, if the surges are too frequent or they last too long, the screw drive, shafts, bearings, etc. may all be undersized. It is important that a screw conveyor has effective product control at the inlet.
Screw conveyors can range from simple diverters to conveyors moving materials long distances in one direction. Along its length the conveyor may have several discharges. The use of a slide gate enables the option of several discharge points from one conveyor. When a screw conveyor cannot have an intermediate hanger then it is necessary to put the screw on an oversized pipe in order to keep the deflection of the screw to less than " in the center.
Inclined Screw Conveyor
A screw in the horizontal is a conveyor or feeder, but as a screw is inclined it very quickly becomes a lift or pump. All of the charts & formulas for calculating speed and capacity for a screw, in the horizontal are no longer applicable, what now comes into play is product knowledge and experience for sizing and calculating speed and horse power for incline and vertical screws.
SCREW FEEDERS
An effective screw feeder must have the inlet filled at all times. The design of the flight pitches under the inlet will control the product draw down in the inlet and subsequent product loading in the conveying portion of the housing after the inlet. There will always be a section of shroud after the inlet. This creates a round conveying path that the screw draws through. With this round conveying path the screw can only convey the material that has fallen into the flight pocket in the inlet. Material flow into the inlet of the screw feeder is also critical. The screw feeder can not meter material out of the inlet unless the material uniformly fills the flight pockets in the inlet.
On a screw feeder, product degradation, product compaction and equipment wear are all considerations. Particular care needs to be taken with the layout of the progressive pitch flights through the inlet and under the shroud as well as the length and shape of the displacement pipe sleeve or cone in the inlet section.