Filter principles

All types of separators that filter out aerosols or undesirable airborne particles such as germs, pollen, dust or gas from the air are described as air filters. We distinguish between filters for solid particles or for gaseous particles.

a. Gaseous particles are filtered by adsorption, or gravitational forces, as they prevail between planets. The activated carbon in these filters is able to filter gaseous particles from the air. Different kinds of coal can be used depending upon use.

b. There are traditionally four ways of collecting solid particles: sieve effect, mass inertia, absorption and diffusion. Filter grade, particle size and filter design determine the level of efficiency.

1. Sieve filter

The sieve effect is used most often in air filters, its principle being very simple:  the absorbed particle is larger than the space between each filtering fiber and is thus retained. 

2. Inertial air filters

This concept is used if particles cannot be carried off by airflow due to their large mass. The particles arrive at great spread and hit the filtering fibers rather than being carried onwards by airflow. 

3. Diffusion filters

Tiny particles often evidence irregular proper motion (molecular motion). The path these particles follow may deviate from the direction of airflow and it is thus very likely that particles will encounter filtering fibers.  

4. Interception filters

This concept bases on the fact that particles are attracted to each other. In this case, larger filtering fibers attract the comparatively smaller dust particles. Once particles are intercepted, they remain stuck to filtering fibers.

There are further developed, modern techniques to clear air of fine dust particles aside from conventional concepts such as dry filters, cyclone air filter, liquid and bag filters. For instance: electrostatic air purifiers (electrostatic filters) or wet filtration (dexwet technology).

Electrostatic filters

Electrostatic air purifiers lead air through an electrostatic high-tension field where particles are charged. The air then reaches the actual filter where polarized aluminum plates and grounded plates are alternately arranged. Plates charged with the same electrical polarity as the particles distract the particles from their path. At the same time, they are attracted by the grounded plates as if by magnet and settle there.

Wet filtration

Dexwet Filter technology is based on plastic filter rods/plates coated in oil where dust particles are caught (or stuck). 95% of the air is thus cleaned. The oil-coated filter rods can then be washed off and cleansed of dust or dry sludge and, once recoated with oil, once again become part of the filter cycle. >> Please refer to wet filtration.