Sheet (Pad) Filters
Sheet filters (see Figure 15.3) are classified as depth filters made of preformed sheets ranging in thickness from 3 to 4 mm. The sheets are of a simple construction, being made of ordinary cellulose fibers. Sometimes they also contain extra substances to increase their filtration efficiency, such as kieselguhr and/or perlite. Some are impregnated with PVPP or activated carbon too. As a general rule, the more filter aids the better the retention. An inert resin is added to provide wet strength; and-depending on the type of resin-also to create a positive charge or "zeta potential." This positive charge enhances the retention of small negatively charged particles/microorganisms. Sheet filters are available in a large variety of grades with flow rates ranging from 0.8 to 2.0 hl/m2/hr, depending on the retentivity of the sheet.
Three different filtration mechanisms are responsible for separation effects. The first separation mechanism is surface filtration, which filters out particles that are too large to enter the filter sheet. The second filtration effect is mechanical depth filtration, which deals with particles that are small enough to enter the sheet filter but too large to exit. The third filtration mechanism is adsorptive retention filtering.
In craft breweries, both plate and frame and sheet filters are often mounted on the same chassis and separated by a changeover plate. This allows brewers to accomplish two filtration stages-rough and polish filtration-in a single step. Alternatively, some craft brewers use a two-stage filter sheet process, with rough, clarifying sheets in the first stage and tight, sterile sheets in the second stage.
Advantages and Disadvantages
The advantage of sheet filtration is that it requires no secondary filter equipment or materials. The disadvantages are the high labor cost of handling the sheets, the lack of automation, and the cost of sheet filters, which is about double the comparable cost for other filter aids.
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