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Despite their apparent simplicity, the complexity of these conveying systems demands proper design and an understanding of the underlying principles
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Screw conveyors play a major role in a wide variety of industrial operations that involve the handling of bulk solids. While their use is generally limited to relatively short conveying distances, their versatility in being able to provide reliable operation for conveying over a wide range of elevation angles from the horizontal to the vertical is well established. Despite their apparent simplicity, their mode of operation is far from simple. For this reason, it is important that conveyor designers, manufacturers and users have some understanding of the basic mechanics governing their performance. This discussion of screw conveyor fundamentals draws upon background research that is presented in Refs. 1-7.
Background
The earliest form of the screw conveyor dates back some 2200 years to the time of Archimedes who developed the helical screw conveyor, mainly for pumping water for irrigation purposes. The Archimedean screw conveyor consists of a helical screw flight that is attached to a cylindrical casing, so they rotate together. Its conveying action by, positive displacement, could only be achieved at low elevation angles that restricted its use mainly to fluids or free-flowing granular materials.
Modern screw conveyors overcome this limitation, allowing bulk materials to be conveyed over a range of elevation angles from horizontal to vertical, with a design that forces relative motion to occur between the rotating helical flight and the casing. In most cases, this is achieved by the casing being fixed and the screw rotating within the casing, as shown in Figure 1.
Practical limitations require a liberal clearance between the flight and the casing, and this has been shown to be beneficial rather than detrimental to performance. The conveyor may be gravity fed, as shown in Figure 1, and in this case, the screw flight projects beyond the casing at the bwer or intake end (this projection is referred to as the choke). The screw must be immersed into the feed material at least to the level of the lower end of the casing, otherwise the conveyor will not elevate the bulk material. Better performance can be achieved by employing an efficient forced-feeding system.