Screw Feeder Extraction for Uniform Flow
by Lyn Bates, AJAX Equipment Co., U.K.
Despite the wide use of screw feeders for discharging hoppers, with a notable exception, (1), there is little published describing the flow patterns they generate. There can be some difficulty of compensating for the initial flight transfer capacity and basing the extraction on swept volume takes no account of the inefficiency of long screw pitches. Users are therefore rarely aware of the potential improvements in performance that they are missing. The benefits of Mass Flow are well known, but less widely understood are the significant difference between mass flow and uniform flow and the radical difference in extraction pattern needed when extracting from a rectangular or a circular hopper. Uneven drawdown can have serious structural and performance consequences, ranging from complete physical collapse to large density variations, ‘flushing’, segregated discharge and extended residence times that raise flow or product quality problems. However, whilst uniform flow demands uniform extraction in plane flow silos, extracting from the more common circular form requires a progressive reduction to the centre and then a progressive increase for the remaining exposed length.
1. Entrainment patterns of screw hopper dischargers. ASME Jrn. of Eng. For Industry. May 1969 ps. 295 - 302
© 2018 by Lyn Bates, U.K.
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