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THE FACTORS GOVERNING the determination of capacity for any open circuit crushing stage are much the same as those we outlined in connection with the secondary stage. In a properly designed crushing plant excepting those plants which by-pass a portion of the pit-run around the head end of the flow line and inject this fraction back into the system at one of the reduction stages-the open circuit tonages decrease from stage to stage, because fractions of the flow which are finer than the setting of each stage are scalped off and bypassed around it. Sometimes finished product fractions are taken off immediately after the primary, or secondary, stage and sent directly to the finished-material storage; this bleeding off of finished material may also be carried on at each scalping point in the flow line. These various possibilities in flow diversion, and re-entry, point clearly to the necessity for preparing a complete flow-sheet of the proposed plant before attempting to calculate the required capacity of any crushing stage. Only by doing this can we hope to approximate the requirements for the different stages.
The facts on which we base our flow-sheet must of course be reasonably accurate if the flow-sheet itself is to be of any value. Admittedly, the true facts are sometimes difficult, if not impossible, to compile for a new operation. This is quite likely to be the case in opening up a new gravel deposit, unless a very thorough, and costly, system of development work is conducted before the plant is designed. Another unpredictable factor is that of the market for various sizes of product. It is a factor which has a very direct and marked influence upon the flow-sheet, particularly upon the amount of work to be accomplished in the reduction crushers.
These uncertainties all indicate quite definitely the need for flexibility in the design of the crushing plant, especially those plants designed for the
production of commercial aggregates, and this need is probably more important as regards the reduction crushing stages than anywhere else in the flow-sheet. The attainment of this flexibility does not necessarily entail the installation of an abnormally high amount of excess capacity, over and above the figures indicated by the flow-sheet calculations. If the plant is arranged so that reduction crushing capacity can be added conveniently, as and if required, we have the flexibility we need, and the margin of capacity to be provided in our selection of crushers need not greatly exceed the predicted maximum as determined from the flow-sheet.
We can very seldom pick a crusher, or a battery of them, that will have exactly the rated capacity to match our calculated requirement for any given stage; and if we could do so it would not be sound practice to hew that close to the mark, because it is not practicable to maintain a crushing stage at its full-rated capacity 100 percent of the time. If we have an ample surge storage ahead of the stage to insure continuous feed we need only compensate for decrease in capacity due to mechanical causes. From 10 to 15 percent should take care of this.
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