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Metering Screw Feeders Article

Created on Wednesday, 04 January 2012 17:47
Published on Wednesday, 04 January 2012 17:47
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The following points provide a detailed consideration relating to the design of metering feeders used in controlling the flow of powders into a process at a prescribed rate. This perspective is based on the 25 plus years of experience from Rospen Industries a leading UK company, synonymous with metering and loss in weight feeders.

Whatever means are employed to actually control the powder, certain basic fundamentals need to be assessed and understood before a solution can be arrived at.

Firstly, the terminology used to describe aspects of the equipment is universal and can be summarised as follows:

Diameter of Screw – This is the nominal outside diameter of the screw not the diameter of the tube in which it runs, nominal, because a 40 mm screw is in fact 38 mm diameter.; a 20 mm screw is 19 mm diameter etc. These differences only occur because the nearest available stainless steel tube schedule requires selection of these dimensions to give a proper running clearance.

Pitch of Screw – The dimension from leading face to the next leading face of the screw. In metering screw feeder terms the pitch dimension relates to the nominal screw diameter, applicable on screws up to 100 mm diameter. Above this size, the pitch will be smaller than the diameter but is still referred to as full pitch.

Box Loading – This refers to the level of material in the screw tube/casing and is usually quoted in % terms i.e. 50% etc. When box loading is related to the screw pitch then fractional terminology i.e.1/2, 1/3 is invariably used. For reference, Rospen apply 100% loading. However, for long feed applications the screw may require an expanding pitch to reduce the box loading in the tube.

Volumetric Feeding – Strictly based on a screw of a known diameter and pitch, metering powder at a specified speed. It must be noted that that any variation of material bulk density will have a direct effect on the ultimate feed rate.

Gravimetric or Loss-in-Weight – A feeder usually identical in design to the volumetric feeder but mounted to a weigh platform to measure the weight lost from the feeder at intervals of time. (A separate paper is available, which covers the design principles)


PRINCIPLES OF METERING SCREW FEEDING


These can be summed up as follows:

Correct filling of the screw – Otherwise known as entrainment. This process is enabled by the knowledge of the powder and critically, its handling characteristics, correct design of the storage hopper, and both the impact and necessity of flow aids.


Screw Design
– Feeders are supplied in single and twin, screw configurations with variations on the screw design such as solid and wire screws

Conclusion – The use of Metering Screw Feeders can provide accuracies of 1% to 2% depending on material characteristics and consistency of properties as highlighted throughout the above article.

Author - Mike Page
Senior Sales Engineer
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