Details on the key characteristics of filter media (continued):

Capacity
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.

Chemical compatibility
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 fluid.

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 chemicals.

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