How to Eliminate Flow Problems

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Posted in: , on 16. Sep. 2003 - 21:51

Go with the flow - or how to avoid that nasty hammer rash on hoppers:

Eddie McGee, Technical Manager of Ajax Equipment Ltd, Bolton explains how you can eliminate flow problems in hoppers by using novel geometry and inserts.

Stopping bulk materials from arching or rat holing in hoppers is an essential requirement if plant is to work at optimal performance capacity. Unless all the material in a hopper is caused to move during the discharge process some of the static contents will remain in place indefinitely if the hopper is refilled before being completely emptied. The static material can gain in strength so much so that complete discharge becomes no longer possible and the effective volume of the hopper is severely reduced. Other materials behave in a fluid manner when initially filled into a hopper. Freshly loaded contents of this type need to settle to a stable flow condition before reaching the outlet, or they are prone to 'flush' uncontrollably. This effect is prevented when the hopper works in a 'first in - first out' manner and a 'heel' of material is present that will discharge first and allow time for the newly entered powder to settle. Mass flow type hoppers, where the material slips on all the wall contact surfaces and there are no 'dead' regions of storage provide the optimum flow condition of the bulk material. Mass flow hoppers have the additional advantage of securing reliable flow through outlet sizes considerably less than when the material is not slipping on the container walls. The ability of the material to slip at the hopper wall is governed principally by the bulk solid’s friction coefficient against the wall material and the shape of the hopper. Ajax Equipment routinely measure wall friction coefficients with test apparatus of our own design and have the technological base to design reliable flow hoppers based on the results of such tests. This approach is a sound way of avoiding flow problems; gravity is available for free so it makes sense to use it rather than throw extra money at the numerous discharge aids that actually reflect a failure of design.

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Figure legend:

Ajax Two-Stage Hopper and Screw


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