By Richard Atkinson, Guttridge Sales Proposal Engineer
Screw conveyors are a popular choice for bulk materials handling and can be used for a wide range of products, from food and pharmaceuticals through to grains and animal feed, chemicals and minerals. These machines have a simple, robust design and present an easy-to-maintain, cost-effective and versatile option for material transport. Additional benefits include the ease with which they can be made dust tight, the flexibility to have multiple outlets and the ability to enable mixing within the conveyor.
This introductory article looks at how screw conveyors work, focusing on the factors that impact design and, consequently, cost. The aim is to provide engineers with the background knowledge needed to use screw conveyors in a cost-efficient way within the constraints imposed by an existing or evolving plant design.
How do screw conveyors work?
In a screw conveyor, material is transported through the rotational action of a screw or flighting mounted on a central tube. The screw sits securely within a containing trough or outer tube, and close tolerances between the flighting and the tube/trough wall ensure efficient transport. Hanger bearings support the central tube, preventing bowing and keeping the flighting in position. A geared motor and chain at one end of the machine rotates the screw at the speed required to achieve a specified throughput.
Specifications of a screw conveyor that are varied to meet the demands of an individual application include:
- Diameter of the tube/trough and associated screw
- Type of flighting and flighting pitch
- Hanger bearing design
- Material of construction
- Welding requirements
- Number and type of inlets/outlets
Understanding the factors that affect these specifications helps ensure the cost-efficient use of screw conveyors within a new or evolving plant design.
The throughput of a screw conveyor is dictated by its diameter and running speed, and the loading levels at which the machine is operated. The throughput required therefore has a major impact on conveyor specifications. A 100% loading level indicates an essentially complete fill, but is unlikely to be practical and/or energy efficient.
Physical properties of the stream that have a bearing on the design specification include abrasivity and flow characteristics. Lower loadings and slower running speeds reduce erosion associated with the transport of abrasive materials, but increase the diameter of conveyor required to meet a given throughput. A machine specified for abrasive materials is therefore likely to be larger than one handling an analogous flow rate of a material that is finer or less aggressive.
Turning to flow properties, materials that are free-flowing are more prone to fall back during transport, decreasing conveying efficiency. Tubular conveyors tend to produce less fall back than trough designs, as a result of eliminating ‘corner gaps’ created by the trough conveyor’s flat lid. A tubular design may therefore be preferable for free-flowing materials, especially when conveying up a relatively steep incline of >25o.
Other aspects of design become more challenging for sticky materials or those containing long strands. For example, if the material being transported is likely to build up on, or get caught around, a hanger bearing then such bearings should be avoided. Similarly the central tube used to mount the flighting may be eliminated since this too presents a site for material hold-up. Centreless screws are widely used to transport ill-characterised, hard-to-handle streams such as sewage sludge and mixed municipal waste.
The length of a screw conveyor is usually dictated by plant layout. However, it is more difficult to achieve mechanical stability in longer conveyors, which increases the need to include hanger bearings. As discussed, hanger bearings are to be avoided for certain streams, in which case mechanical stability is enhanced by increasing the diameter of the centre tube. If tube or trough dimensions are kept constant, such designs reduce the width of the flight, and consequently also reduce conveyor throughput. The need for a thicker centre tube is therefore likely to be associated with an increase in the diameter, and hence increase the overall cost of the machine.
One of the attractions of screw conveyors is that they can be used to transport and mix process streams simultaneously. A mixing conveyor will typically incorporate multiple inlets and a specially designed flight (see figure 2). Ribbon flighting, paddle blades, and cut and folded flighting are all options for applications that require mixing and/or aeration, or where break up of caked or agglomerated material is required. Non-standard flightings such as these may add significantly to the cost of conveyor but offer the benefits of essentially merging two process steps.
Flighting design may also be modified to ensure optimum flow beneath a flood feed inlet. In many instances screw conveyors sit directly beneath a feed hopper with no control over material flow rate. The inlet is simply flooded. Where this is the case, the pitch of a flighting can be varied to achieve efficient transport. The pitch of the flighting is the distance between the same points on subsequent turns of the screw (see fig 1 – main image above) and is usually equal to the diameter of the conveyor. When the screw conveyor is used under flood feed conditions, the pitch of the flighting is normally supplied at a reduced or variable pitch to control the loading and discharge from the machine. For example, a doubling of pitch reduces loading from 100% beneath the flooded inlet to 50% in the rest of the machine.
In certain applications, such as those relating to the transport of food and pharmaceuticals, there is an absolute requirement to avoid contamination. This requirement can influence the material of construction of the conveyor, triggering an upgrade to stainless steel, for example, and also dictates the welding specifications. The joint that attaches the flight to the centre tube has the potential to be a crevice running through the length of the machine, presenting a site for the accumulation of material, so machines such as these will often be double welded, on either side of the flighting, to create a crack and crevice free finish. Clearly this can have a direct impact on cost but has the added benefit of giving the flighting additional rigidity.
Achieving an optimal screw conveyor design for a specific application relies on understanding the factors that affect performance. Moving away from a standard design may be essential in order to engineer a robust solution and is likely to involve additional upfront costs. However, over the long term a well-specified machine will give years of trouble-free service, delivering ongoing economic benefits that will more than offset an incrementally higher initial investment.