Many brewers employ powder filters for single pass filtration and are able to produce beers that are suitable for packaging. In a powder filter, filtration is achieved with the use of filter aids that form a filter bed on stainless wire mesh inside a pressurized vessel; or, on cellulose sheets in a plate and frame filter; or, in the form of small holes in a candle filter. While the screens and/or cellulose do not filter at all, but act as septum for the filter aids, it is the applied "cake" of filter aid which does the filtration. The mechanics are very similar to those of sheet filters.
Types of Powder Filters
There are several types of powder filters, namely: (a) plate and frame filters in which the support is a cloth, (b) horizontal leaf or vertical leaf filters in which the support screen is a wire mesh, and (c) candle filters which support the filter aid on a long thin perforated rod.
Plate and Frame Filters
The plate and frame filter, as shown in Figure 15.1, has been the workhorse in breweries around the world for many decades. It is robust and reliable, consistently filtering beer to the specified standards. Plate and frame filters consist of a series of chambers enclosed within a metal frame. Between adjacent frames is a double-sided porous filter plate covered with a cellulose filter sheet folded at the top of the plate so that both sides of the plate are covered.
Leaf filters have a series of stainless steel leaves that are arranged either vertically or horizontally inside a filter body. The filter leaf consists of a stainless steel mesh septum attached to a stainless steel support plate. Screen septums are made with openings ranging from 45 to 70 microns. Unfiltered beer enters the pressure vessel, passes through the filter cake established on the leaf, and exits through the hollow shaft connecting the leaves.
Candle filters utilize a series of candles, containing a number of rings that are hung from a rigid horizontal plate. The candles can be of porous ceramic, but they are usually perforated or fluted stainless tubes covered or surrounded by a stainless steel support of various types.
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