Ten Key Steps for avoiding Screw Feeder Discharge Problems

by Lyn Bates

There is no difference in operating principle between Screw Feeders used to discharge large storage containers and those used to control the dispensation of material to processes. In practice the main function of the former is to provide reliable and complete discharge of the stored contents, with reasonable feed rate control and, in some cases, control of the zone order of discharge. By contrast, Metering Screw Feeders have the prime function of providing an even and accurate rate of feed, and often the secondary function of providing verification and record of performance. The accuracy required varies according to the application. By nature Screw Feeders are volumetric devices, therefore the amount dispensed at a given screw speed depends upon the condition of density of the product. Good design can usually secure consistent density condition, but cannot provide verification or alarm should allowable tolerances by exceeded. For this reason gravimetric control is frequently adopted by way of feedback control from weighing the Loss-in-weight of the feed system. Comments on Metering Screw Feeders are given separately.

1. Establish the minimum orifice size necessary for reliable flow. If the material has difficult flow properties use a Mass Flow Vee-shaped hopper section, with vertical or slightly diverging end faces and a slot length of at least three times its width.

* Extraction must take place over the whole outlet for a Mass Flow hopper to work

2. For Non-Mass Flow Hoppers and 'Collecting' Screw feeders that may be occasionally covered, (as with dust collectors and hoppers for plate filters ), it can still be useful to incorporate pitch changes along the length of the screw. For example, they reduce starting loads and extract from more than one region of the bed over the outlet slot. However, if the screw has to deliver a regulated amount of material, the section of screw nearest to the outlet end of the hopper must be covered with material for the full discharge rate to be maintained.

3. Check that the flight tip clearance is large enough to avoid particle trapping for wedging or granule degradation. Also ensure that the residue in this clearance does not continue up a side wall slope to prevent slip of the material or provide a 'toe' to support an arch. It is good practice to adopt a 'U' shape of feeder casing, so the vertical sides allow material to pass from the hopper to the screw without interference from static residue in the flight tip clearance.

4. Make sure that the distance from the outlet end of the hopper wall to the start of the outlet is adequate to retain the hopper contents when the screw stops. A length of two screw diameters is usually sufficient, unless the bulk material is in a loose condition. Note, - that a screw will not restrain a fluidised powder. If the product is prone to flush, then fit a valve on the discharge port, interlocked to be open only when the screw is running, and even then provide an emergency override 'close' switch.

5. Always confirm that the outlet cannot block, be obstructed or admit a reverse gas flow. Un-vented rotary valves leading into pneumatic conveyors often pass back air by leakage and via returning vane pockets. With no means of escape this air can restrain flow from the hopper. Allow a gas bye pass route for the material to flow freely from the feeder. Although screws can develop powerful axial forces, bulk solids will not transfer pressure round corners or any distance along confined channels. If feeding damp or 'sticky' material it is good practice to expand the casing at the outlet and slightly project the underside beyond the down-wall of the discharge chute, so that material falls clear of the chute walls . Fit a torque limiter or a level detector to isolate the drive if there is any prospect of back up from subsequent equipment. Screws are unforgiving if frustrated and bent-back flights are difficult to straighten.

6. Remember that the starting torque for a discharge screw may be many times the running torque, because of the initial shear strength of a confined bulk solid, particularly with firm granular materials. For tough duties, where flow is not a problem, include some overpressure protection in the form of inverted Vee inserts along or across the outlet slot. Some types of variable speed drives have poor starting toque characteristics, therefore they have to be sized on this basis for feeders rather that on full running speed duty. Calculations for feeder starting torque are difficult to come by, because it depends upon the shear strength and frictional nature of the bulk material and the effective outlet overpressure. Pressures through the outlet are not proportional to width. They are zero at the critical arching span and increase disproportional with width increase. Do not rely upon figures related to the feeder dimensions only. Unless you have specific experience, refer to measured material property test results and first principles of screw mechanics, ( or a specialist ).

7. Distinguish clearly between average and instantaneous feed rate. Between what limits must each fall? Cyclic variations can arise from angular position of screw flight rotation. Short term fluctuations occur due to end point cohesive instabilities with poor flow products and, even with very free flowing materials, small erratic 'flushes; take place on the slope of discharge repose because of small 'avalanches' at low feed rates.

8. If a damp or cohesive material is to be handled, use ribbon screws to avoid product build up around the corner where the screw flight is joined to the shaft. (Ajax LynFlowTM ribbons have a special form, to eliminate the square edge corner connections of the ribbon stay, for improved non-clog performance).

9. Reversible discharge screws are useful, but special care is needed top ensure that the flight construction is completely uniform in order to avoid compacting material within the screw form during transfer in one of the directions. For this reason it is not good practice to use extended lengths of uniform construction screws acting as feeders unless special care is taken in their construction.

10. Cantilever mounted feeder screws allow enormous design flexibility for easi-clean, direct end outlet configurations, simple direct acting cut-off valves, and for forming 'plug seals' to counter pressure differentials and to offer protection for the screw and the product against high temperature or vapour-laden atmospheres. Consult the experts for special designs. The cost of even simple operational problems usually overwhelm the extra capital cost for custom built equipment, and its good to have the technical support on hand to address any adverse eventualities that may arise, regardless of responsibility.


Further reading - ' Guide to the Design, Selection and Application of Screw Feeders'
published by Professional Engineering Publications .


Too Late? - If discharge is the problem, see 'Ten Key Steps for Reliable Flow'
(https://forum.bulk-online.com/showth...w-from-Hoppers)

If the problem rests with the Screw, and not covered above, contact Ajax.


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For any problem not covered, contact Ajax at:
http://www.ajax.co.uk/


PS: If you have questions, or want to discuss certain issues in more detail, copy/paste/quote the original text and your comment here:
https://forum.bulk-online.com/forumd...-Ask-Lyn-Forum




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