The influence of granular vortex motion on the volumetric performance of enclosed screw conveyors

This paper is concerned with the volumetric performance of enclosed screw c onveyors with particular reference to the influence of vortex motion. Vorte x motion arises as a result of internal friction, friction between the gran ular material and surface of the helical blade, and the infinitely variable helix angle of the helical flight from the outer periphery of the blade to the shaft. The vortex motion, together with the degree of fill, govern the volumetric efficiency and, hence, the volumetric throughput. An analysis o f the vortex motion in vertical or steeply inclined screw or auger conveyor s is presented. It is shown the vortex motion is characterised by the tange ntial component of the absolute grain velocity being substantially constant with the radial position of a point on the blade. On this basis, an expres sion for the volumetric efficiency is derived. The volumetric throughput ca n then be predicted. The paper also presents results from an experimental s crew conveyor where it is shown that the analytical predictions correlate v ery closely to the measured results. (C) 1999 Elsevier Science S.A. All rig hts reserved.