23 JULY 2003
steel, plate thickness and additional stay bosses can be used to compensate for their
lower strength. Cast iron and ductile iron plates are the most durable materials because
of their strength and chemical and corrosion resistance. However, they are more costly
and weigh considerably more than the polypropylene plates. Epoxy-coated and rubber-
coated steel plates offer lower initial costs than the iron plates and have moderate
strength, weight, and chemical resistance. However, these plates are susceptible to
corrosion and chemical resistance if the epoxy coating is not maintained or the rubber
2-22.214.171.124 Filter Media. The initial selection of filter media is one of the most important
equipment variables in filter press applications. Durability, ease of cake release,
minimum blinding, and chemical resistance are all important. The designer must
evaluate materials of construction, permeability, and overall construction and weave. If
properly installed and maintained, the life expectancy of a filter media is between 1000
and 4000 cycles (WPCF 1983). In most cases, the initial selection filter media will be
based on manufacturer's experiences with similar applications.
a. Filter media are available in several materials and different permeabilities.
The most commonly used materials are polypropylene, polyester, and nylon, with, poly-
propylene being the most common because of its durability and resistance to ferric
chloride and lime conditioning chemicals and acid solutions used to wash filter media. It
is, however, limited to operations below 90 degrees C (200 degrees F). Polyester is
slightly more durable than other material because of its low stretching ability; however, it
is expensive. Nylon is typically only used where conditioning and media washing (i.e.,
acid wash solution) require no chemicals.
b. The overall construction and weave are also important aspects of the filter
media. The media construction typically consists of either monofilament, multifilament,
or spun fibers. Filter media constructed of multifilament fiber warp, monofilament fiber
weave, or satin design weave are typically used because of their smooth surface
characteristics, which help improve cake release properties and reduce media blinding.
Calendaring is an optional method used to increase the smoothness of the filter media
by heat pressing or ironing them to provide a finish that increases cake release.
c. Permeability is a measurement of the openness of the weave as deter-
mined by air flow through a given area of media at a given pressure drop. The typical
permeabilities range from 1.5 to 2.4 L/s (3 to 5 scfm) as measured on the Frazier Scale,
which measures the amount of air that passes through a wetted cloth at differential
pressure of 1 atmosphere. The permeability of the filter cloth may change through use
because it becomes impregnated with solids, swells, and its weave becomes distorted.
Although permeability affects the initial stage of filter cake formation, once the filter cake
begins to form, the filter cake itself serves as the filter medium and is relatively
independent of the filter media (cloth). Media blinding and cake release are also
important aspects of permeability.
d. In addition to general construction aspects, stay bosses are added and the
plate perimeter is often reinforced to improve wear of the media. This reinforcement