Principles of Metering Screw Feeding
Principles of Metering Screw Feeding
will be introduced through the correct filling of the screw and screw design (single screw, twin screw, solid screw, wire screw, oversize product, etc.), and then give a conclusion.
Principles of Metering Screw Feeding 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.
Each powder has an ideal speed range through which it will not adversely react to the process of changing direction and filling the pitches of the screw. Invariably this is determined by trials and typically the conclusion is, that the higher the screw feed the greater the potential for capacity and linear accuracy tail off problems.
A typical hopper design usually incorporates the standard taper shape assisted by two or four blade agitators with intermittent or continuous running. For more difficult materials the tapered sided hopper is replaced with vertical walls to ensure a consistency of material flow to the screw. In extreme cases vibrator motors can be used.
With larger hoppers, above 150 litres and up to 0.5 m3 capacity it may be necessary to promote product flow by creating an unstable material bridge in the hopper by use of a vibrating discharge cone along with air evassers arranged asymmetrically in the hopper walls.
In the case of assisted flow by either the above or larger vibrating cone bin activators, care should be taken not to mis-match the output from the discharger and the screw by too great a factor. This can result in compaction, causing material, which cannot get away to compact in the feeder causing it to bore or tunnel a hole without conveying smoothly. The solution to this process is to scale down the problem by batching from large hopper or silos into the feeder hopper using high and low level probes to control the refills from the silo.
Screw Geometry and Metering Zone - Once the screw size and pitch have been determined it is important to realise that to be effective the pitch of the screw selected must enter the metering zone, namely the discharge tube for the first 3 full pitches of the screw. On extended length screws the pitch is opened out to reduce the box loading. This has two effects, it lowers the stress on the on the drive shaft of the screw and also the consumed power taken to drive it.
Feeders are supplied in single and twin, screw configurations with variations on the screw design such as solid and wire screws
Single Screw - The majority of Rospen applications use single screw with variations on pitch and diameter, this is generally found to be satisfactory due to the unique feeder trough profile and method of agitation on the Rospen style of feeder.
Twin Screw – Twin, screw feeders are generally used where extreme flushing of powders can occur or the complete opposite with cohesive powders like pigments or those with a high resin content, which require a large entrainment area to enhance flow. The one main advantage of twin screws is the fact the faces run together and not edge to edge such that a self cleaning effect is the result. Also the pulsing effect is reduced at the outlet, particularly important with sluggish powders.
Solid Screw – Solid screws take the form of a continuous Archimedean spiral with a shaft running throughout the screw. The vast majority of applications use this design principle. Even with the flavour powders usually associated with the snack industry solid screws are used, the only additional feature given to the screw are breaker bars set on to the screw in the trough area to break up any dead area that may form in the trough.
Wire Screw - On occasions wire screws can be used, which take the form of a helix spring manufactured from square section material. This type can only be employed for very difficult fibrous materials, which need a larger entrainment area in the screw, i.e. minimum blade width and no centre shaft. It should be noted that wire screws have minimum surface area, which does not support serious build up of cohesive powders.
Extended Length Screw – In the event the standard length of feed tube is insufficient and a further extension is required work on the following dimensions as a general rule for the maximum permitted tube length.
25 mm diameter – 1000 mm
40 mm diameter – 1500 mm
50 mm diameter – 2500 mm
75 and 100 mm diameter – 3500 mm
Note! the above table is for guidance only and selection will depend on product characteristics and specific applications. There may also be a requirement to increase the motor power to provide additional torque. When considering long screws it may be advisable to reduce the box loading in the tube by sizing the screw diameter with a reduced pitch at the metering point, i.e. in the trough. Even a 90% pitch at this point, which will give a 10% reduction in the tube can have marked advantages on the consumed power.
Oversize Product - If the powder to be handled has a large particle size up to say 5 mm, then problems will occur in either these particles jamming between the edge of screw and the tube, or if not jamming causing extreme increase in torque requirements preventing successful feeding. In such a case fit an oversize tube and/or reduce the metering pitch to ease the box loading.
Cohesive Powders - The use of an oversize tube will assist in cohesive powders which, when compacted in the normal screw/tube clearance cause extreme increases in power requirement. The increased clearance allows material to run on material in an un-compacted state.
Consistent Bulk Density – A necessity for volumetric feeding is a consistent product bulk density but it can be overlooked if it is assumed the same chemical from different suppliers will have the same characteristics. Note! when obtaining density figures be aware that the specific gravity approach relates to solid powder with no air gaps between the particles of powder. Therefore, if in doubt take a sample and pour loosely into a measured container and weigh it.
Feed Rate Calculations – There are no feed rate tables accompanying this paper for the reason that they can be open to mis-interpretation and it is advisable to approach the manufacturers or competent agents with any project details or specific sizing problem. However, for reference purposes, tabular information on Metering Screw Feeders, Feeder Sizing can be found at our website
Conclusion of Principles of Metering Screw Feeding:
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.