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The leaf and plate filters are batch filters, and a critical element in their selection is the capacity of the filter for the amount of cake to be treated in a single batch. Flow through the filter will depend upon the nature of the suspended solids and their
concentration, the liquid viscosity, the pressure available, and a number of other variables. The acceptable value for the flow rate through the filter medium will normally be measured in a test rig. Then the filter can be sized from the calculation of
required area:
Filtration area =Required feed rate/Medium filtration rate
A filter with sufficient filtration area to handle the required flow rate or suspension may still fill up with cake too quickly. This may raise the required pressure drop too far or overfill the spaces between neighbouring filter elements. An acceptable
cake thickness must be determined by test, whereupon the required filtration area is found from:
Filtration area =Total cake volume per cycle/Maximum cake thickness
The larger of these two measures of filtration area is then the one to be used.
Attractive features of the leaf and plate filters are their ease or operation and maintenance, the large filtration area to vessel volume ratio, the high filtration rates that can be achieved, and their ability to form homogeneous and compact cakes that
can be efficiently washed with relatively small wash volumes. There is a minimum number of seals (i.e. potential leak points) between feed and filtrate. Most designs enable filtration, washing, drying and discharging in a closed system.