Capacity of Hammermills

Speed, or velocity, while it is the very life of the hammermill, may also function to limit the amount of feed that the mill will take. Thus, in any given machine, the number of rows of hammers used will affect capacity. Or, to state it a little more clearly, for any combination of speed, feed size, and number of rows of hammers there is a definite limit to the amount of material that the mill will receive.

This is understandable when it is considered, for example, that in a machine running 1500 r p m, with four rows of hammers, the receiving opening is swept by a row of hammers 100 times each second, and there is obviously a limit to the amount of material that can enter the space between two successive hammer rows in this short period of time. We find that for some combinations of feed size and product size, more production can be obtained with only two rows of hammers, rather than three, or more. Radial velocity of the material entering the mill will naturally have a direct bearing upon the amount that will drop in between the rows of hammers. Thus, in a well designed mill the feed spout is always so arranged that the material falls, rather than flows, into the crushing zone.

It is hardly necessary to state that the size of product directly affects the capacity of a hammermill, just as it does any type of crusher. The finer the product the more work the machine must do; furthermore, the grate bars, when any are used, must be spaced closer, which means that the open area of the grate section is reduced. When the grate bars are spaced widely, or dispensed with, and the sizing is done over a closed-circuited screen, product size has the same direct influence upon capacity because, the finer the screen openings, the more return load and, hence, the less original feed that can be handled by the mill.