Filtration is the process of passing a fluid through a porous material, causing
particles that contaminate the fluid to become trapped in the material. This results in a
cleaner fluid. Industrial fluids are filtered to remove metallic particles created by the
manufacturing process. Removing the particles will extend the useful life of the fluid,
and reduce the cost of the manufacturing process.
What is surface vs. depth filtration?
Most filter media can be classified as either a surface filter or a depth filter.
Most filter papers can be considered surface filters. They capture the contaminant on the
upstream surface of the paper. This causes the pores that make up the surface of the
paper to become clogged with contaminant, forcing the filter to readily go to
Depth Filters are bulkier and have larger pores than surface filters. They rely on
causing the fluid to travel a longer, more tortuous path than with a surface filter. This
deposits the contaminant deep within the filter medium. The great depth of such a media
structure allows the material to have a very high dirt holding capacity, and longer
What are the important characteristics of filter media?
The important characteristics of filter media are:
- strength - it must be strong enough to withstand the demands of the filter
- pore size - the pores must be small enough to capture target particulate.
- capacity - the filter medium must refrain from blinding prematurely,
shortening filter life, and increasing cost.
- chemical compatibility - The composition must be compatible with the
chemistry of the system. Filters must not leak chemicals into the systems,
nor weaken because of chemical attack.
Details on the key characteristics of filter media:
Any material used to filter metal working fluids must be strong enough for the process.
The best filter membrane in the world is useless if it falls apart. Two tests of fabric
strength are important measurements of its suitability for the application. They are the
Mullen and Tensile strength tests.
The Mullen Test measures the force necessary for a blunt object to burst a sample of
filter media. Expressed in pounds per square inch, the test may be run on either wet or
dry samples. The greater the strength, the less likely the medium will burst while
subjected to the stresses of the filtering process.
The Tensile Test measures the longitudinal strength of a sample of the filter media.
Grab Tensile measures this strength when a sudden pulling force is exerted in the plane
of the filter media. The results are expressed as the pounds of force needed to cause the
sample's web to break.
The Mullen Test predicts the ability of the Web to refrain from bursting during the
filtration process, while the Tensile Test predicts the ease with which the dirty filter
paper will be extracted from the filter equipment.
Other factors that affect the strength of the filter media's web:
- Synthetic fibers are stronger than cellulose fibers.
- A web with long fibers will be stronger than one with short fibers.
- A heavier web, with greater mass, will be stronger than a lighter weight web.
- Spunbound webs with thermally bonded fibers are stronger than wet laid or
carded webs that have chemically bonded fibers.
The pore size of a filter reflects the average pore through which the fluid must pass.
The smaller the pore, the more efficient the filter media will be, enabling it to remove
smaller particles. This is sometimes called the nominal micron rating.
The nominal micron rating is not a useful measurement of pore size. It is an arbitrary
number listing the size particle, in microns, which a filter media is supposed to be
efficient at removing from the fluid. The number has no basis in reality. But it is
often accepted as gospel. Only by testing a filter media, under specific conditions, can
it be determined if it is the correct media for that application.
Factors that effect Pore Size are:
- Fiber Size - the most efficient filter media are made with varying fiber
- Web Formation - webs with uniform fiber distribution will have smaller pores than
webs with widely random distribution.
- Mass - increasing the amount of fibers in a non-woven will reduce the average
pore size and improve efficiency.
- Density - increasing the density of a media's web tends to compress fibers and
reduce the average pore size.
- Fiber Chemistry - polypropylene fibers are highly attractive to oil. In dense
webs this tends to cause the media to be rapidly coated with oil, preventing the
passage of coolant and shortening filter life. But in more porous structures the
fibers can be coated with oil without the media surface blinding. These oil
coated fibers act like fly paper. The particulate then clings better to the