This article takes an in depth look at screw conveyors.
Read further to learn more about topics such as:
- What are screw conveyors?
- Components of screw conveyors
- Screw conveyor configurations
- Screw conveyor considerations
- And much more…
Chapter 1: What are Screw Conveyors?
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 the 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 first screw conveyor was the Archimedes screw, which was developed around 250 B.C. An Archimedes screw is a form of positive displacement pump that was used to lift water from low-lying lands to the elevated irrigation ditches. The construction and driving system of Archimedes screws were improved throughout the years to increase their efficiency, durability, and speed while adapting the original mechanism.
Chapter 2: Components of Screw Conveyors
The components of screw conveyors and their types (based on design aspects) are as follows:
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.
The following are the types of screw conveyors based on their flight and pitch design:
Standard Pitch Screw Conveyors
In standard pitch screw conveyors, the pitch is equivalent to the screw diameter. The screw diameter is the radial distance between two flight crests. They are commonly used in horizontal screw conveyors and inclined screw conveyors with below 100 inclination. They are suitable for handling most materials.
Variable Pitch Screw Conveyors
In variable pitch screw conveyors, the pitches increase variably across the screw flight. The pitch is longer at the inlet side to provide additional space for the fed materials as they are being discharged from hoppers, bins, or silos. Variable pitch screw conveyors are used in screw feeders for a controlled volumetric flow rate of materials.
Short Pitch Screw Conveyors
In short pitch screw conveyors, the pitch is equivalent to two-thirds of the screw diameter. They are commonly used in inclined and vertical screw conveyors and in transporting free-flowing materials to prevent flushing.
Half Pitch Screw Conveyors
In half pitch screw conveyors, the pitch is equivalent to half of the screw diameter. Half-pitch screw conveyors are also used in inclined and vertical screw conveyors.
Long Pitch Screw Conveyors
In long-pitch screw conveyors, the pitch is equivalent to one and a half times the screw diameter. They are used for transporting liquids and other free-flowing materials and for rapid material conveying processes. They can also assist in the agitation of the transported materials.
Double Flight Screw Conveyors
In double flight screw conveyors, there are two sets of flights running helically around the length of the conveyor screw; this accelerates the conveying process. The starts of the flights are 1800 apart. Like the standard pitch screw conveyors, the pitch of double flight screw conveyors is equal to the screw diameter. They ensure a uniform flow rate of material and smoothness during transportation.
Tapered Flight Screw Conveyors
In tapered flight screw conveyors, the pitch remains constant, and the flight width gradually increases from two-thirds of the screw diameter on the inlet side to full diameter on the discharge side. Tapered flight screw conveyors offer additional space for the fed material at the inlet to draw off material evenly. They are an economical alternative to variable pitch screw conveyors and are suitable for lumpy solids.
Mass Flow Screw Conveyors
Mass flow screw conveyors have a conical conveyor screw with constant flight pitches adjacent to the feed inlet. Their narrower end is continuously attached to a variable pitch conveyor screw that progresses toward the feed discharge. They are used to provide uniform material withdrawal from hoppers, bins, or silos. They help material flow evenly. The material volume swept incrementally increases with every flight.
The following are the types of screw conveyors based on the flight construction:
Helicoid Flight Screw Conveyors
In helicoid flight screw conveyors, the flights are constructed from a flat steel bar or strip that is cold rolled to form a helix. The stock metal strip is cold-rolled to produce a work-hardened, strengthened, and smooth flighting material. The material of the helicoid flights has a greater thickness at its base and gradually becomes thinner at the outer edge. The helicoid flight conveyor screw is then assembled by mounting and welding the short, individual flights to the center shaft. The flights running across the length of the shaft are connected and fastened by intermittent welds at every end of each flight.
Helicoid flight screw conveyors have high strength and load capacity and are cost-effective. They are used in handling light to moderately abrasive materials such as limestone, cement, and fertilizer.
Sectional Flight Screw Conveyors
In sectional flight screw conveyors, the flights are constructed from flat steel discs with uniform inside and outside diameters. The discs are cut to extend its length using a plasma, water jet, or laser cutter and pressed to form a helix or the individual flight which corresponds to one revolution. The sectional flight conveyor screw is assembled in the same manner as the helicoid conveyor screw.
Sectional flights have constant material thickness. Thicker sectional flights are also available. Hence, sectional flight screw conveyors are ideal for conveying extremely abrasive materials such as glass cullet and alumina.
There are special designs of conveyor screw flights that have different geometrical designs and serve other functions in addition to material transportation:
Cut Flight Screw Conveyors
In cut flight screw conveyors, the outer edge of the screw flight is deeply notched at regular intervals. These notches help in mixing and agitating the bulk material being conveyed. Cut flight screw conveyors are suitable in handling materials that tend to pack, such as hydrated lime, dried milk, and talc.
Cut and Folded Flight Screw Conveyors
In cut and folded flight screw conveyors, the outer edge of the screw flight has both notches and folds. The folds are raised from the surface of the flight. The notches and folds partially retard the flow and assist in the thorough mixing of the material being conveyed. They also help in the aeration of light substances, as well as in achieving uniform temperature during heating and cooling.
Ribbon Screw Conveyors
Ribbon screw conveyors have an open gap between the inner edges of the flight and the shaft. Ribbon screw conveyors can be constructed to have single and double flighting:
Single Flight Ribbon Screw Conveyors
Usually are ideal in transporting sticky and viscous materials that tend to build up at the conveyor screw shaft.
Double Flight Ribbon Screw Conveyors are also suitable for conveying sticky materials and provide more uniform discharge. They can also be designed for mixing dry materials; the double-flight ribbon conveyor screw has an outer ribbon flight and an inner ribbon flight with a smaller diameter. Both flights are opposite-handed. This design moves the material back-and-forth, ensuring a thorough mixing action.
Standard Pitch Single Flight Screw Conveyors with Paddles
This type of screw conveyor gently and thoroughly mixes the bulk material by positioning paddles in between screw flights.
Standard Pitch Screw Conveyors with Paddles
This type of screw conveyor only consists of adjustable paddles positioned helically across the length of the conveyor screw shaft. A pitch may contain up to four paddles to aggressively mix the bulk material and control its flow. This type provides the greatest mixing action among all types, but it has the least efficiency.
Screw Conveyor Handedness
The handedness of a screw conveyor determines the direction of the material flow with respect to the direction in which the conveyor screw is rotated. A conveyor screw can be right-handed or left-handed. A right-hand screw conveyor pulls the bulk material toward the end of its conveyor screw when it is rotated clockwise. A left-hand screw conveyor pulls the bulk material toward the end of its conveyor screw when it is rotated counterclockwise. The direction of material flow is reversed as the conveyor screw is rotated in the opposite direction. However, it is not advisable to operate screw conveyors in this manner because the bulk material should always be handled on the carrying side of the flight.
Screw conveyor handedness is determined by viewing the end of the conveyor screw. The flighting is wrapped around the shaft in a clockwise direction for right-hand screw conveyors and in a counterclockwise direction for left-hand screw conveyors. Right-hand screw conveyors are more common than left-hand screw conveyors.
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 vessel for the bulk material. The trough inlet can also be located directly on the discharge side of an upstream process equipment.
The following are the types of screw conveyor troughs:
U-shaped Trough. The U-shaped trough is the most common and versatile type of screw conveyor trough. They are cost-effective and have a simple construction.
- Angle Flange Trough
- Formed Flange Trough
- Double Formed Flanged Trough
- Formed Channel Trough
- Drop Bottom Trough
- Flared Trough
Rectangular troughs can withstand abrasive bulk materials during transportation. The gap between the conveyor screw and the trough walls is filled with bulk material. The static layer of the bulk material protects the trough walls from wear and fatigue.
- Angle Flange Trough
- Formed Flange Trough
- Angle Flange – Top and Bottom Trough
- Formed Flange – Top and Bottom Trough
Tubular troughs can accommodate a larger screw diameter and tightly contain and protect the bulk material from weathering, dust, and contaminants. They can operate at higher speeds and have greater capacity per revolution. They are typically used in inclined screw conveyors. The types of tubular troughs based on their construction method are the non-split and split tubular troughs, which have single- and double-piece constructions respectively. The halves of the split tubular troughs are joined by forming flanges.
A jacketed trough is a special type of screw conveyor trough that assists in heating and cooling the material during its transit from one point of the process to another. They also help in maintaining the temperature of the bulk material. Jacketed troughs are usually made from concentric troughs where the utility fluid flows between them.
Hanger bearings support the multiple conveyor screw sections and provide a bearing surface for them. They consist of a plain linear bearing, which is suspended and housed in a frame that fits on the conveyor screw trough.
Couplings facilitate the transmission of torque between multiple screw sections by compensating for the misalignment between the screw shafts. They increase the efficiency of the screw conveyor. Couplings and hanger bearings are both critical components when extension of the length of the screw conveyor is desired.
The internal collar is a seamless tubing that is installed at the end of the screw shaft. It is used to reduce the inside diameter of the shaft to be compatible with standard-sized fittings.
End lugs are installed on the non-carrying side of the first and last flights of the conveyor screw. They prevent flow obstruction by preventing material build-up at the ends of the conveyor screw. They also provide additional support to the screw.
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.
Author: John Hamlin
Company: IQS Directory
Original article can be found here