The influence of screw feeders on bin flow patterns

Screw feeders are often used in bulk solid handling systems to control the feed from mass-flow bins with wedge or transition shaped hoppers. This requires good geometric design of the outlet-feeder combination in order for the bulk solids to be entrained over the full length of the outlet. Otherwise, unnecessary problems are likely to occur. Achieving an even flow pattern in the bin or hopper can minimize these problems. There are several methods to increase screw capacity in the direction of feed, e.g. stepped pitch, tapered shaft, tapered screw diameter, or a combination of these methods. In this paper a theoretical model for achieving a uniform flow pattern is proposed based on the pitch characteristic of screws. The limitations of some methods for increasing the screw capacity are discussed. Experimental studies on the flow pattern in a wedge hopper with a screw feeder are also included. Four screws of different configurations were investigated on a test rig. All the screws had increased screw capacity in the flow direction. In the experiments a dividing grid was fitted above the screw to form a central division over the axis of the screw to isolate each side into a number of divisions which matched the different pitches of the screw. The results from the experiments are presented and compared with the theoretical predictions.