Lesson 2/Learning Event 3
tion. Because the charts used to determine screen capacity are based on empirical data, there may be fluctuations
between these calculated results and actual field performance. Therefore, for planning purposes, it may be
assumed that the top screen will be able to handle the 60 tons per hour and is acceptable.
The capacity of the bottom screen is calculated similarly. (In other screening operations, intermediate screen
capacities are also calculated in this manner.) Remember that the bottom screen must have the capacity to handle
the total by-product, or 34.2 tons per hour in this example. Halfsize for the bottom screen is tonnage of product
up to and including #4.
Therefore:
Q = B x A x D x H x O x W (Values given above.)
= (1.2) x (40) x (0.9) x (0.81) x (0.93) x (1.0) = 32.6 tph
The bottom screen is able to remove only 32.6 tph of the by-product, while the demand is 34.2 tph. Although the
difference between the demand, 34.2 tph and the capacity, 32.6 tph is not great, the worst case must be considered
rather than say, "close enough." Therefore 1.6 tph of by-product (34.2 - 32.6) = 1.6 tph does not get screened out
and is retained in the product stockpile. Therefore, the actual product is 27.4 tph (25.8 + 1.6 = 27.4 tph).
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