Recommended Construction Features for hopper discharge screws
➢ Apart from small hoppers and feeders that are less than around 100mm diameter, it is good practice to make the feeder as a separate, bolt-on unit to the hopper outlet flange. This facilitates removal for repairs, replacement, modifications and access through the hopper outlet should any ever be required.
➢ Progressive extraction along the total hopper outlet length is essential for mass flow, minimising powder needs and attrition. Uniform pitch construction feed screws should only be used where: - the length of outlet is less than one and a half time the screw diameter, or the zone order of discharge of the hopper contents is unimportant and power requirements are not significant.
➢ The inlet width to the screw must exceed the ‘critical arching span’ for a gravity feed type machine. Excess width, especially liable with multi-screw feeders, will increase the shear force, and hence power needs, of extraction
➢ Do not use any screw section with a pitch greater than the screw diameter.
➢ Ensure that there is a geometric increase in transfer capacity at axial distances, but increases are no greater than the diameter of the screw for incremental extraction.
➢ Screws varying in pitch should not have a pitch less than three quarters of the flight face depth or greater than twice the flight face depth. Increments should be at least 25% of the previous pitch and the overall exposed length to the hopper contents not exceed six times the screw diameter.
➢ Screws with taper or stepped shafts should have pitch to flight face depths within the same range as above, may extract for up to eight times the screw diameter.
➢ Screws with a combination of taper or stepped shaft and variations in pitch may have an exposed length under the hopper outlet of up to ten times the screw diameter.
➢ The casing, with a working clearance around the screw, should be of a ‘U’ form to avoid the residue forming a support ‘toe’ to oppose slip on the hopper wall. The width of the feeder outlet should at least equal the screw diameter.
➢ For feeder screws between 100 and 600mm diameter, the maximum speed should be limited to (100 – screw diameter in mm) rpm max. and 5 rpm min. (Below 20 rpm it may be prudent to fit inclined ‘attenuating strips’ to the shaft at 90 degree spacing, immediately prior to the outlet port of the casing to reduce the cyclic discharge effect of the slowly rotating flight.
➢ On no account use intermediate bearings on a full screw.
➢ Use a shroud over the screw immediately beyond the hopper exit to resist ‘flushing’ and prevent material ‘over-carrying’ by riding on top of the emerging screw contents.
➢ Ensure that the working face of the screw flights are smooth and free of weld runs and minor ‘steps’ between the individual flight sections.
➢ Continue the screw at least two screw diameters beyond the hopper outlet before allowing material to discharge through an outlet port and do not extend the flight more than a nominal distance beyond the start of the outlet port to avoid the danger of material packing at the discharge end.
➢ Small, multiple-screw feeders may be used to provide a more even discharge than a single screw. For example, a triple screw feeder with screws of similar proportions but half the diameter of a single screw, will run at nearly three times the speed for a given output and, with the screw flights ending at equal angular spacing, tend to ‘pulse’ with only 10% of the instantaneous variation of the single screw. The width of the hopper interface will also be 50% wider, to offer improved screw filling characteristics.
➢ Techniques are available to mitigate ‘pulsing’ of the discharge due to the screw pitch geometry. Contact a specialist manufacture of custom built screw equipment.
➢ An access, inspection port is often useful above the final outlet. A detector or trip switch fitted to a hinged plate above the outlet should be used if there is any prospect of the outlet becoming blocked.
➢ Natural shaft deflection should not exceed 75% of the flight tip clearance for when running empty. Note that when conveying material with a full screw there is a tendency for the screw to lift, rather than sag. Fine damp materials and products that ‘cake’ can form a firm layer under the screw and exert a high bending stress, best countered by a thin flight edge.
➢ Feeder screws can be inclined, but generally should not be angled at more than 15 degrees upwards or downwards to the horizontal.
➢ Note that the transfer efficiency of a screw is related to the combination of its helix angle and the flight face friction, so is material dependent, particularly so on coarse flight helixes.
Feeder screws can be used for compaction and forming ‘plug seals’, but this usually involves specialised design as the resistance of wall friction in an enclosed tube has an exponential form and can ‘lock up’ to damage the screw if not carefully designed.
Gravimetric Screw Feeders
The commonest form of gravimetric screw feeder is the Loss-in-weight Type. The feeder and its immediate supply hopper are all mounted on load cells and the whole mass weighed at very frequent intervals, typically 50 times per second. The change in weight is compared with a pre-set value corresponding to that required by the process and any discrepancy forms the basis of a screw speed adjustment.
An inevitable feature is that the system can only run for a period of time before the hopper needs to be refilled. This should be done quickly, before the initial contents are exhausted, and the screw drive set to a volumetric mode at a value based on the prior adjustments until sufficient material has been entered to re-tare the system and revert to the gravimetric control. Typically, sufficient product is held in the hopper for a two to five minute runs and refill takes 10 to 15 seconds.
When re-fill is called for, the remaining contents should beadequate to supply the screw with product over the volumetric feed period and until the fresh supply has stabilised to a controllable state flow condition. The supply channel should preferably be of mass flow form, otherwise a preferential flow channel might develop that allows the fresh input to pass through to the screw. As this fresh product is likely to be in a more dilate condition than that previously settled in the hopper, the feeder will be dealing with variable density material and present a greater challenge to the control system.
The design of the rapid refill is an integral part of the system. Stimulating the frequent flow and discharge of a batch of bulk material within a short time, without creating a very loose density condition, calls for a well-designed discharge device; particularly when dealing with very fine powders.
Another type of gravimetric screw feeder is the weigh-feeder. This is a short screw mounted on load cells that transfers product from a controlled feed supply and weighs the amount of material in transit. The feed is adjusted to discharge the amount required in the time that corresponds with the transfer time to the receiving point. This method has the virtue of providing a continuous supply with virtually steady state flow conditions in the supply hopper that holds a large stock. Under these conditions, a mass flow supply hopper is recommended to deliver a consistent, speed related mass discharge rate.
Feeder/ conveyors
Screws that are required to transport material over some distance may also need to control the rate of conveying by extracting from a supply hopper. A relatively short conveying distance that can be accommodated in a single span screw offers no great difficulty in using a short pitch section to extract from the supply hopper and a longer pitch section for the conveying portion. The expansion after leaving the flood feed region reduces the work content by eliminating the confinement.
Summary
It will be seen that feed screws perform many feeding functions. A good understanding of the characteristics of the bulk material with which they are to be used and measured values of their flow related properties are necessary for the designer to provide assured performance. As with any equipment supplied for controlling the supply of bulk material, it is necessary to ensure that, not only is the correct feed rate delivered, but that the condition in which it is delivered is suitable for the following equipment and of optimum quality for subsequent use. For advice contact
