Screw conveyors, or auger conveyors, are industrial equipment used in transporting bulk quantities of granular solids (e.g., powder, grains, granules), semi-solids, liquids, and even non-flowing materials from one point to another. They uphold high operational efficiency by eliminating the need for workers to manually move around loads. Screw conveyors primarily consist of a rotating screw shaft that is installed within a trough. As the screw shaft rotates, the material moves linearly. They can be designed to provide horizontal, vertical, and inclined travel paths.
Several types of screw conveyors are designated to handle a specific material behavior. Bulk materials may be abrasive, non-free-flowing, fluidizing, hygroscopic, or hazardous. The products handled by screw conveyors are the likes of cereals, fertilizers, animal feeds, ash, gravel, cement, and solid wastes.
The components of screw conveyors and their types (based on design aspects) are as follows:
Conveyor Screw
The conveyor screw is the main component of a screw conveyor; it is responsible for pushing the solids through the length of the trough. It is composed of a shaft with a wide blade running helically around its length. This helical structure is called flight. Conveyor screws work like enormous screws; the material travels one pitch as the conveyor screw rotates in full revolution. The pitch of the conveyor screw is the axial distance between two flight crests. The conveyor screw stays in its position and does not move axially as it rotates to move the material across its length.
Trough
The trough of a screw conveyor houses the conveyor screw and supports the bulk material during transportation. Its length can be supported by saddles or flanged feet. A rigid, removable flat cover is present across the length of the trough to protect the bulk material from contamination, dust, and hazardous elements.
The trough provides access for bulk material feeding and withdrawal. The trough inlet is typically connected to another device, like belt conveyors, rotary valves, and screw feeders that meter or control the volume of the material entering the screw conveyor. These devices are usually connected to hoppers, silos, and bins which serve as a storage vessels for the bulk material. The trough inlet can also be located directly on the discharge side of an upstream process equipment.
Trough Materials
The type of material used to produce the trough varies according to the application, the type of screw conveyor, and the amount of corrosion or abrasion resistance the trough may have to endure.
1. Mild Steel - Mild steel is the most common type of material used to produce screw conveyor troughs and is suitable for every conveying application.
2. Stainless Steel Grades 304 and 316 - Stainless steel screw conveyors are used due to stainless steel's resistance to corrosion, abrasions, and high impact due to it being work hardened.
3. Abrasion Resistant Steel Grades AR235 or AR400 - Abrasion-resistant steel screw conveyor troughs are used for abrasive materials because it is two to four times harder than mild steel.
Electric Motor
Screw conveyors are powered by an electric motor that is located near the trough discharge. This arrangement allows the natural pulling of the bulk material to the drive end and puts tension on the screw flights; this results in less fatigue and wear to the screw conveyor components.
Screw Conveyor Considerations
The nature of the bulk material to be handled must be considered in selecting and designing a screw conveyor.
The material of construction is critical for abrasive materials. Hence, the screw conveyor must be constructed from a hard and abrasion-resistant metal such as hardened steel, carbon steel, or rolled steel. Reducing the angle of inclination below 15° will minimize the abrasive wear induced on the screw conveyor components.
Fluidizing materials tend to "flood" and flow uncontrollably when agitated or aerated. The easiest way to avoid this is to reduce the conveyor speed. Shorter pitch conveyor screws can also be used to slow down the speed of these materials.
For sticky, hygroscopic, and viscous materials, a ribbon-type flight or shaftless screw conveyor must be used to prevent material build-up. The components may be lined with PTFE to keep materials from adhering. The internal components of screw conveyors handling hygroscopic materials must be free from air and vapor to prevent the absorption of moisture.
Other properties of the bulk material such as density and particle size must be known. An estimate of the volumetric and mass feed rate must also be determined. These data are important for determining the structural and strength requirements and screw conveyor sizing.
Lastly, the distance and environment of the path to be traveled by the bulk material must also be considered.
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Several types of screw conveyors are designated to handle a specific material behavior. Bulk materials may be abrasive, non-free-flowing, fluidizing, hygroscopic, or hazardous. The products handled by screw conveyors are the likes of cereals, fertilizers, animal feeds, ash, gravel, cement, and solid wastes.
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