Cost-efficient screw conveying: Understanding the factors that impact the cost of screw conveyors 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.

Figure 1: A schematic showing the key components of a screw conveyor

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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.

Throughput

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.

Material properties

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.

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Conveyor length

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.

Integrated mixing

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.

Figure 2: Different flighting configurations are used to tailor the design of screw conveyors to meet

specific applications  

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

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turns of the screw (see fig 1) 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.

Contamination elimination

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.

In conclusion

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.