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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 pressure.

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 filter cycles.

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 system.
  • 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:

Strength
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.
 

Pore size
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 diameter fibers.
  • 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 fibers.

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