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The British Materials Handling Board is a non-profi t organisation devoted to users of all forms of Bulk Material. This month’s Solid Advice from the BMHB looks at optimum hopper discharge conditions by screw feeder.
Screw feeders are widely used for extracting bulk materials
from hoppers for their fl ow benefi ts given by plane fl ow, to control the discharge rate and to secure higher storage capacity or save headroom. In many cases it is only necessary to provide reliable fl ow, but mass fl ow installations demands that extraction takes place across the total area of the hopper outlet, demanding variable screw geometry. This pattern can be obtained by varying the screw pitch by small increments. However, this is far from ideal as small pitches raise the danger of the material ‘logging’ in the narrow gap between fl ights, with the initial pitch limited to one third the screw diameter. An overall slot length of slot equal to eight time the screw diameter is common and, because short pitches are more effi cient in axial transfer than long pitches, the fi rst twenty-fourth length of slot extracts up to half the total output! It is commonly assumed
36 March 2014 Solids and Bulk Handling
that the axial transfer rate of a screw is directly proportional to the swept volume of the screw, but it actually depends on the combination of fl ight helix and contact friction angles. The helix angle of a screw fl ight equal in pitch to its diameter, with a centre tube one quarter of the o/d, varies from tip to root at the centre tube from 17 degrees to 38 degrees and frictional values from less than 15 to more than 30 degrees, so the performance is highly dependent on geometry and bulk material properties as the material moves forward in a spiral manner and that near the centre shaft tends to rotate with the screw. Apart from the danger of not generating extraction in some regions, a wide variation is extraction rate is extremely unfavourable for duties that demand the most uniform fl ow across the hopper cross section. Differential fl ow rates aggravate the condition of materials that segregate during the fi lling of the hopper. Powders prone to ‘fl ooding’ require the maximum residence time in storage to de-aerate and the minimum counter-fl ow velocity against air that is rising to escape, a
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double ‘wammy’ with variable fl ow rates. In fact, because powder in a fl uid condition develops hydrostatic pressure it will preferentially displace more settled material in a fl ow route so the material must settle to a stable fl ow condition in a mass fl ow hopper before entering the converging hopper section where there is inevitably differential fl ow velocity across the cross section. Even fl ow is also required for process operations, such as heating and cooling in storage, and for consistent density at discharge, as variable fl ow velocities generate variable bulk densities. Power and particle attrition may also be issues, as dragging a column of bulk material under a static bed of product causes much greater shear stresses than cutting across a ‘live’ fl ow channel. For more details of screw feeders design see ‘Guide to the Design, Selection and Application of Screw Feeders’, published by The British Materials Handling Board. Contact lynfl
ow@btinternet.com for further information.
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