Filter classification

Filtration removes extraneous substances from the compressed air. Dust and fog is filtered out of the compressed air on the suction side of the compressor.

On the pressure side, filters are used to filter out dust, dirt particles, deposits, liquids and mist particles.

Filters achieve separation efficiencies of up to 100% for particle sizes down to about 1 micron. In order not to overload these fine filters with coarse particles, mechanical coarse matter separators are fitted prior to the fine filters.

Depending on the mechanism of separation, one distinguishes between two types of filter:

• Surface filters
• Depth filters

With surface filtration, the screening or sieving effect plays the primary role as separating mechanism. Impurities, (as long as they are larger than the specified pores) are separated out on the surface of the filtering material and form a filter cake in the course of the service life. As a consequence of this, there is the side effect that, to a small extent, smaller particles than the effective pore width can also be separated.

As the impurities are deposited mainly on the surface, a simple regeneration of filter elements of this design is possible.

Depth filtration makes use of fibre beds consisting of a maze of the finest individual fibres. There are, therefore, no specific pore sizes. Such filter materials act not only as a sieve, mainly retaining particles corresponding to the pore size, but also separate impurities which are considerably smaller than the fibre maze structure. A combination of several separating mechanisms is responsible for this:

• Direct impingement
• Electrical charge
• Diffusion
• Sieve or screen effect
• Attachment through van der Waal’s forces.