Details on the key characteristics of filter media
The capacity of a filter medium reflects the amount of contaminant which the medium will
capture within its pores before the flow is restricted sufficiently to cause the filter
to index. One can determine the amount of dirt removed by a filter, by measuring capacity
of a filter media, and knowing how many yards of media are used in a day.
The filter media with the greatest capacity will exhibit lower usage than a product
of equivalent efficiency but lower capacity. With growing concerns regarding the disposal
of spent filter media, increasing filter media capacity is critical.
Factors that are important when designing a high capacity filter:
- Caliper - the caliper or bulk of a filter web is measured in thousandths of
an inch. A thicker membrane will have greater capacity than a thin filter
medium. A thick web provides greater opportunity for vertical as well as
horizontal flow through the filter allowing particulate to be captured
without restricting fluid flow.
- Fiber Size - smaller fibers will create more fiber layers and greater
capacity for horizontal as well as vertical fluid flow.
It is important not to confuse filter capacity with filter life. A filter medium with
high capacity will often exhibit longer filter cycles, but filter cycles alone are not a
measure of capacity. The capacity of a filter measures the amount of containment retained
by the media and is expressed in grams per area. Filter Life is only a measurement of the
length of time between filter cycles.
A filter medium must withstand the chemical condition of the fluid being filtered. Under
severe operating conditions the filter medium can be destroyed by the chemistry of the
Important considerations for the media designer include:
- Binders - most webs with short staple fibers require chemical binders to
bond the fibers together. The wrong binder can be dissolved by the
coolant, leaving a slurry of fibers floating in coolant.
- Fiber selection - the most commonly used filter media fibers are polyester,
cellulose and polyolefins (including polyproplylene and polyethylene).
Polyester has the broadest application. It is available for all manufacturing
processes, has good strength characteristics and is suitable for a wide range of
Cellulose fibers are short, require chemical binders and are not suitable when very
high strength is required.
Polyolefin fibers are used in all but the wet laid (paper) forming process. The fiber
can be extruded to very fine diameters, has excellent chemical resistance to wide range
of acidic and caustic fluids, and has very good strength characteristics.
High strength and chemical resistance have been easily attainable for a long time but
the combination of both high filter efficiency and high dirt holding capacity has eluded
filter media designers for years. Most webs were either efficient at removing smaller
particles or had very long filter cycles while removing only the larger contaminant. The
best of both worlds consists of laminating a highly efficient filter paper to a paper
with high capacity