= (.0721)1/2(3600)1/2(0.33)1/2 = (.269)(60)0.57 = 9.21 = Ab/Ac Filter press area = 921 ft2
The following equation calculates the 4-hr cycle on the filter press:
The following equation calculates the 8-hr cycle on the filter press:
This procedure is only approximate if the pressure varies during the batch, and graphic integration using the pump characteristic curve provides better results.
The results are not valid for batch pressure filters if the cake is appreciably compressible. For either batch or continuous filtration of compressible solids, environmental engineers should conduct tests at various average pressures.
For batch filters, in addition to adequate filtration area the design must provide sufficient cake space to contain the total volume of the wet filter cake deposited during the proposed cycle time; otherwise the cycle can come to a premature end. This specification requires knowledge of the wet cake density and solids content, which should be measured in the laboratory and not assumed.
If Tdb is not negligible compared to Tfb, environmental engineers can include its value (the batch cake discharge time) in the batch equations, and the calculation is more precise. However, high precision should not be expected in filtration calculations. As in the rotary filter case, environmental engineers obtain the best results for a batch filter in laboratory measurements using a mini-filter of the same general design as the full-scale filter.
This example shows that a batch filter requires a larger area than a continuous filter for the same slurry and average filtrate rate. The batch filter, however, has only two electrical drives—the feed pump and the discharge mechanism. The continuous rotary drum filter usually has six drives, including the feed pump, pan mixer, drum drive, belt discharge drive, vacuum pump, and filtrate pump.
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