Mass flow

Fig. 1 Two flow patterns that can occur in a bin: funnel flow and mass flow

In funnel flow, an active flow channel forms above the outlet with nonflowing material at the periphery. As the level of material in the hopper decreases, layers of the nonflowing material may or may not slide into the flowing channel, which can result in the formation of stable ratholes. In addition, funnel flow can cause product caking, provide a first-in-last-out flow sequence, and increase the extent to which sifting segregation impacts the discharging material.

In mass flow, all of the material is in motion whenever any is withdrawn from the hopper. Material from the center as well as the periphery moves toward the outlet. Mass flow hoppers provide a first-in-first-out flow sequence, eliminate stagnant material, reduce sifting segregation, provide a steady discharge with a consistent bulk density and a flow that is uniform and well controlled. Requirements for achieving mass flow include sizing the outlet large enough to prevent arching and ensuring the hopper walls are sufficiently smooth and steep enough to promote flow at the walls.

Useful Bulk Flow Parameters. Armed with information about the bulk properties of the powder, engineers can optimize the selection of storage and handling equipment. These same properties can be used to retrofit existing processes to correct flow problems.

Discussed below are several bulk solids handling properties that are relevant to predicting flow behavior. The direct application of these parameters has been proven over the last 30 years in numerous installations handling the full spectrum of powders used in the P/M industry, including metal powders, fine chemical additives, polymers and waxes, and graphites/carbons (Ref 3, 4).

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