Blockage in vertical slots: Experimental measurement of minimum slot width for a variety of granular materials

The design of some processing equipment requires knowledge of the safe minimum dimensions of slots, channels or conduits for flowing granular materials. This work was prompted by the need to ensure that when narrow slots were used in equipment of this type, they were capable of providing a continuous and uninterrupted flow. There are some data in the literature for blockage of openings in the horizontal plane, but there is very little published information of any sort on vertical slots. The materials tested in this investigation included agricultural seeds (amaranth, panicum, millet, rape, wheat, barley), various plastic granules used in extrusion processes (polyethylene pellets, high impact plastic in the form of cut rod), and a sample of urea. These materials all had narrow size distributions and had maximum dimensions in the range 1 mm to 8 mm, and bulk densities in the range 600 to 800 kg/m3. Widths at blockage were obtained for vertical slots, horizontal slots and also horizontal orifices. Conservative guidelines are given for selection of the minimum size of openings for reliable flow of non-cohesive grains; for vertical slots, the ratio of minimum slot width to characteristic particle dimension is 4.6, for horizontal slots the ratio is 3.3 and for horizontal orifices, the ratio of orifice diameter to characteristic particle dimension is 6.4.