Environmental engineers must use variations of these deep-bed design methods wherever batch leaf precoat filters and rotary vacuum precoat filters are used. These precoats usually consist of diatomaceous earth or asbestos fibers preap-plied to the filter medium. Since these materials do not have measurable particle diameters, Dp is unknown and the Ergun equation is not applicable. Therefore, test data are required even for initial pressure loss.
On a batch leaf precoat filter, the differential pressure increases continuously to the maximum allowable value. The precoat and impurities are then washed off, and a new precoat is applied. These filters can be operated without a vacuum pump when the filtrate is continuously removed with a centrifugal pump capable of operating at a low NPSH.
On a continuous rotary precoat filter (similar to the one in Figure 7.47.4), a slowly advancing knife continuously shaves off a few thousandths of an inch of the precoat plus
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