A FILTER PRESS is essentially an alternate assembly
of FILTER PLATES and FILTER MEDIA which are compressed in a
framework called the FILTER CHASSIS. The plates are constructed in
such a way that the compressive force is applied only to the
perimeter of the filter media and the highly compressed edges then
act as gaskets and prevent leakage of liquid from the interior of
the assembly There are of course many variations on this theme but
only those which relate to the filters described in this brochure
are detailed below.
The filter plates are a very Important part of a
filter press because they provide the essential mechanical support
for the filter media as well as the means whereby the liquid is
distributed to, and collected from, those media. They are designed
to achieve these objectives with the minimum of pressure
differential when the filter is in operation.
Our plates feature one or other of two basic types
of support surface. The first consists of a regular pattern of
raised ribs and this form of construction is found in plates which
are produced by casting or mouldings. With this type of plate the
filter media are supported by the tops of the ribs while the
liquid moves freely along the channels between them.
The second type of support surface consists of flat
sheets having a large number of perforations and this is found
mostly in plates which are of fabricated construction Such plates
usually consist of an open framework to which are attached two
perforated sheets separated by spacing devices. With this type of
plate the filter media lie flush against the flat sheets while the
liquid moves freely between them.
The filter media used in our filter presses are
usually either cellulose mats or filter papers but, in certain
instances, woven fabrics, porous plastics, etc may be used. Filter
papers act mainly as fine sieves or as supports for beds of filter
aid powder which may need to be added to the filtering liquid to
improve Its filterability Cellulose mats can provide a much finer
degree of filtration than filter papers due to depth filtration
whereas filter papers offer economy.
The choice of a suitable filter medium is determined by various
factors such as the size, nature and clogging characteristics of
the insolubles to be retained, the chemical properties and
viscosity of the liquid. the economics of the operation, etc. Some
of these factors cannot be predicted and this is why our
laboratory facilities and extensive experience of assessing
filtration problems are a valuable resource.
It is a common belief that, in a filter press, the filtering
liquid passes through the first sheet of filter medium, then
through the second one and so on, but this is a fallacy. All the
media operate in parallel and thus the more of them that are used
the higher is the rate of flow through the filter and the larger
is the total volume of liquid that can be filtered before the
media are clogged.