METERING SCREW FEEDING

Phil Black - PII Editor 22/07/2011

145 8 minutes read

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This article, by Dan Liptrott of Rospen Industries, examines the process of controlling the flow of powders into a process at a prescribed rate.

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 are fairly universal and can be summarized as follows:-

Diameter of Screw – This is the nominal outside diameter of the screw not the diameter of the tube in which it runs.

Pitch of Screw – The dimension from leading face to the next leading face of the screw.

Box Loading – The level of material in the tube. Usually quoted as a fraction when referring to the screw pitch, i.e.1/2, 1/3 etc. and usually quoted as a % when referring to the material loading within the screw tube/casing as 50%, 30% etc.

Volumetric Feeding – Strictly based on a screw of a known diameter and pitch, metering powder at a specified speed. Any variation in the powders bulk density will cause a direct effect on the 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. The main benefit is that any changes in bulk density are detected and the screw speed varied to maintain the set rate.

PRINCIPLES OF METERING SCREW FEEDING

These can be summed up as follows:-

1. Proper filling of the screw

Otherwise known as entrainment. With this goes correct discharge from the hopper above the screw to ensure starvation does not occur.

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.

Usually this is determined by trials with the conclusion that the higher speeds will cause more problems to the extent that some capacities will tail off and not produce straight line graphs or accuracy will suffer due to bad entrainment.
Rospen feeders have a particular characteristic in usually being linear in that increases in screw speed are generally followed by corresponding identical increases in output.

Material flow from the hopper is assisted by two four blade or spiral agitators with intermittent or continuous running along with vibrator motors for extreme cases.

In the more difficult materials the standard taper hopper shape is replaced with vertical sided hoppers to ensure consistent flow to the screw.

With larger hoppers above 150 litres and up to 0.5 m3, consider the use of a vibrating discharge cone along with air evassers arranged asymmetrically about the hoppers sides to promote an unstable material bridge in the hopper.

In the case of assisted flow by either the above or larger vibrating cone bin activators fitted to larger hoppers, care should be taken not to mis-match the output from the discharger and the screw by too big a factor. Compaction between the two can result causing material which cannot get away to compact and fill the feeder with the result the screw bores or tunnels a hole without conveying smoothly.

Otherwise 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.

2. Screw Geometry and Metering Zone

Once the screw size and pitch have been decided 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 drive shaft of the screw and also the consumed power taken to drive it. An added benefit is to alleviate compression of the powder if this is a problem material

a) Screw Design – Feeders are supplied in single and twin screw configuration. The majority of applications use single screw with variations on pitch and diameter to suit the application.

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 flow properly.

b) Solid and Wire Screws – Solid screws take the form of a continuous Archimedean spiral with a shaft running throughout the screw. The vast majority of cases use this type of screw. 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.

On occasions wire screws are 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.

These also have a downside in that the structure is weaker than a solid screw and more prone to breakage.

A shaft can be placed inside a wire screw to help strengthen it the wire being supported by small pegs from the shaft.

c) Extended Length Screws – 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 dia – 1000 mm
40 mm dia – 1500 mm
50 mm dia – 2500 mm
75 and 100 mm dia – 3500 mm

Bear in mind, however, these are only a guide and will depend on how difficult the product is. It will also be a requirement to increase the motor power to provide additional torque.

When considering long screws try 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.

The use of an oversize tube will also 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.

It should be noted that whilst we can easily machine solid screws to any diameter, pitch or length from solid bar using a fourth axis milling machine, wire screws, because of the inherent method of manufacture, cannot be so easily changed.

3. Consistent Bulk Density

A necessity for volumetric feeding but one which client’s take for granted assuming the same chemical from different suppliers will have the same characteristics.

4. Sizing of Continuous Loss-in-Weight Feeder Hopper

Take maximum feed rate kg/hour and divide by the material bulk density in kg/cu.metre.

This will give cubic metres/hour throughput.

Divide by 6 (ideally a 10 minute run in gravimetric mode).

Multiply above by 1.3 to allow for the ullage above the nett material level.

Select nearest size of standard hopper above this .

Example – Size the hopper for 500 kg/hour continuous duty with material at 800 kg/cu.m.

500 = 0.625m3/hour
800

0.625 = 0.104m3 nett refill required every 10 minutes
6

0.104 x 1.3 = 0.135m3

Select 150 litre standard hopper.

5. Problems and Possible Solutions

* Feed rates keep dropping and then suddenly increase back to where it was, usually on a cyclic basis.
Solution – material is building up within the pitch of a solid screw and reducing the carrying volume. It reaches a point where it cannot sustain its own weight and breaks away leaving the pitch clear to fill properly again.

Replace with a wire screw to cut down surface area for build up.

* Cohesive/Sluggish Powders.
Solution – Do not attempt to run at maximum speeds and size the screw and pitch re reflect this. Use either twin screws or a wire design or combine both.

* Friable Granules.
Solution – Obviously slow speed with increased tube diameter with a wire or solid screw will be acceptable.

* Segregation of mixed powder with widely varying bulk densities.
Solution – Try not to use vibration on the hopper and minimise the agitator effect if possible to a 2 blade intermittent action.

* Extreme pulsing effect from the tube outlet.
Solution – Usually this becomes more apparent at reduced screw speeds. Fit a cross wire sleeve to break up the materials extruded structure.

* Flushing – usually caused by excessive air entrained within the product by a previous operation.
Solution – If you cannot provide sufficient time delay within the feeder hopper to condition the material with vibration, consider the use of a large fabricated rotary vane similar to a rotary valve to replace the agitator in the exact same spot over the screw.

Solution – Fit close tolerance discharge tubes and/or increase the number of pitches of the screw by fitting double start blading or half pitch.

* Cannot gain sufficient height to feed into the client’s process from the infeed position.
Solution – Incline the screw. Be careful that combined with this is invariably an increase in feed centres so the motor power rating must be increased to compensate. With volumetric feeders just provide a frame to support the feeder, but in the case of gravimetric feeding ensure the feeder is suspended from a horizontal weigh frame to improve the stability and accuracy of weighing.

6. Loss-in-Weight or Gravimetric Feeding

First thing to remember is that if you cannot handle any powder with reasonable accuracy volumetrically putting a weighed system on it will not solve the problem and could make matters worse.

Continuous Loss-in-Weight

Refill rates are critical and must ideally be 10 times the output rate of the feeder. When a feeder is re-filled it goes into a volumetric mode on the last known signal when in loss-in-weight. Until the weigh platform senses the high level of material in the hopper it will stay in volumetric mode.

After the refill it will, within 3 seconds, re-learn the loss-in-weight signal and react accordingly.

Obviously the smaller the refill rate with the screw still discharging product, the longer the volumetric stage on refill and the benefits of loss-in-weight are reduced.

Any size of feeder up to 150 litre hopper can be mounted to a weigh platform. Larger hoppers will require to be suspended from a weigh frame to provide stability of the system.

As with all weighing systems the main problems are caused by site vibration particularly if it is of a low frequency high amplitude characteristics such as locally sited ball mills or en masse vibrating tray conveyors with heavy reaction bases.

The fitting of anti-vibration mounts can help under the feeder/weigh frame in limiting the effect.

On particular large rate loss-in-weight feeders, where you require to control say 20 Te/hour of product and the refill rate should be 200 Te/hour, this becomes on most installations an impossibility. Even direct discharge from a silo may not keep up with this rate.

The alternative is to run a continuous flowmeter device whether screw or belt with a volumetric pre-feeder usually a screw.

The two are run in closed loop control to maintain the set rate with the added advantage that the pre-feeder and hopper, because it is volumetric, does not require the same high rate of refill as the gravimetric control.

The pre feeder hopper is fitted with high and low level probes to control refills in the conventional sense.

Whilst this solution is invariably more expensive due to the number of machines employed, it provides a stable and sure method for large feed rates.

Batching Loss-in-Weight

Uses the same controller as continuous to measure the loss of material from the feeder but is set to switch off when a pre-determined target weight is reached on a fast/trickle speed setup.

Whilst it is ideal to batch in one operation, if it is impossible to get a large enough hopper in place, multiple runs can be made to make up the batch but refill rates are not so critical as a continuous loss-in-weight system due to the cyclic nature of batching operations.

Rospen Industries
Stonehouse
Gloucestershire

Can be contacted on

Tel: 01453 825212
Fax: 01453 828279
E-mail: enquiries@rospen.com
Web: www.rospen.com