Control & Instrumentation
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 = 800
0.625 = 6
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).
36 Solids & Bulk Handling • August 2010
www.solidsandbulk.co.uk 0.625m- /hour 0.104m3 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.
nett refill required every 10 minutes
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