Single Vs. Multiple Stage Crushing
There are no set rules for determining whether the secondary stage should consist of one crusher, 01' of two 01' more machines operating in parallel. The decision must be made upon the merits of each problem.
Obviously if the required receiving opening necessitates the selection of a crusher whose capacity equals or exceeds that of the primary crusher there is no object in going to a two-stage arrangement. Frequently this will be the case where a primary jaw crusher is to be followed by a gyratory machine. On the other band, if the primary is a large gyratory, and the full output of this crusher is to gauge the output of the plant, it will be necessary to install a battery of two 01' more gyratories for the secondary reduction. The number and size of these will depend upon the size of the primary, its setting the type of secondary to be used, and its setting. It is desirable that the multiple set-ups be selected in even numbers. Rather than odd; that is either two or four units rather than three. The difficulties in achieving an equitable distribution of feed into three units has been amply demonstrated in a number of plants.
From the standpoint of flexibility there is something to be said in favor of the multiple secondary stages, although the advantages are not so pronounced here as they are in the reduction crushing stages, unless of course the secondary stage is likewise a finishing stage. When the secondary stage is simply an open-circuit step in the over-all reduction flow line, the advantage of the multiple unit rests in the fact that single machines may be taken out of service for repairs with~ out total interruption of the plant operation, although the feed rate must be reduced unless enough excess capacity is installed in this stage to permit cutting one machine out of service without affecting the average flow-rate through the plant. It is questionable if this much excess capacity is ever justified in a two-unit secondary stage; on the other hand, if the stage is to comprise four 01' more units, it is sound engineering to provide such extra capacity. The answer must in any case be predicated upon the relative importance of uninterrupted full-capacity operation.
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