Friday 06 February, 2015
Understanding the factors that affect the cost of screw conveyors is essential when it comes to rationalising and controlling capital expenditure on this popular class of mechanical conveyor. Here, Richard Atkinson, Guttridge Internal Sales Proposal Engineer, highlights the most commonly encountered causes of a high price tag.
1. High throughput
The throughput of a screw conveyor is defined by its size, running speed and loading. Throughput therefore has a dominant impact on cost. Higher throughputs necessitate the use of larger diameter machines and more powerful motors, however the relationship between cost and size is not linear – a machine with double the throughput would be expected to be around 30 – 60% in price.
2. Conveyor length
Conveyor length is usually dictated by plant layout but achieving mechanical stability in longer machines increases the requirement for, for example, hanger bearings. Features such as these add to the cost of longer conveyors. However longer machines, where practical, are still an economic choice compared with the use of shorter conveyors in series.
3. A free-flowing product stream
Free-flowing product streams are more prone to fall back, an issue that reduces the efficiency of screw conveying. Fall back is especially problematic when conveying up a relatively steep incline (>25o). Switching from a trough to a tubular design substantially reduces the potential for fall back by eliminating the ‘corner gaps’ created by the trough lid. Tubular conveyors are, however, more costly than a trough design of comparable specification.
4. An abrasive product stream
Lower loadings and slower running speeds reduce the erosion associated with the transport of abrasive materials but increase the diameter of conveyor needed for a given throughput. A machine specified for abrasive materials is therefore likely to be larger, and more expensive, than one specified for a less aggressive material, all other factors being equal.
5. Eliminating mechanical internals
Ill-characterised streams containing strands of sticky or straw-like material, as exemplified by mixed municipal waste and sewage sludge, have a tendency to accumulate on conveyor internals inhibiting reliable operation. An optimised design for such materials will therefore eliminate internals as far as possible. The use of hanger bearings, which support the central tube and flighting of a conveyor, can be avoided by increasing the diameter of the central tube to give it greater rigidity. This necessitates an increase in conveyor diameter to maintain throughput. Centreless screws are also an option and may be essential for successful operation, but these are more expensive, on a like-for-like basis, than conventional screw designs.
6. Crack and crevice free design
Certain applications call for a crack and crevice free design, most usually to eliminate corrosion and/or the risk of cross-contamination. In screw conveyors, such requirements demand double welding of the flighting to the central tube followed by grinding of the welds and sometimes polishing to achieve a suitable finish, through the length of the machine. This procedure is manually intensive and therefore adds cost but has the added benefit of increasing the mechanical rigidity of the machine.
7. Flooded inlets
Ensuring optimum flow beneath a flooded flow inlet requires the use of non-standard flighting. At a flooded inlet, material flows from a feed hopper into the inlet of a screw conveyor, in an uncontrolled way. Where this is the case, the pitch of a flighting (which is the distance between the same points on subsequent turns of the screw), is specifically modified to control loading. For example, a doubling of the pitch a short distance after a flooded inlet, reduces loading from 100% beneath the hopper to 50% in the rest of the machine.
8. Customised flightings
Customised flightings are not only used to handle flooded inlets (see above), and always increase the cost of a screw conveyor. For example, screw conveyors can be used to integrate mixing and transport in which case ribbon, paddle blades and cut and folded flightings may be specified. Alternatively, where machines are operated at high loadings or fast running speeds, the design of the flighting may be dictated by the need for enhanced mechanical strength. Wherever flighting manufacture becomes a bespoke task there is an associated increase in cost. See diagram below.
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