As knowledge of the concentrations of harmful substances in the atmosphere and of their impact has grown, air filters have been introduced in many industries. Air filters are used in ventilation units to clean supply air. Supply air in many industrial processes, engine combustion air, supply and extract air in laboratories and hospitals as well as extract air in nuclear plants are examples of applications where air is nowadays cleaned. Advances in clean room technology have brought along filters that are technologically more and more advanced.

 

Filters are classified into coarse filters, fine filters, HEPA filters and ULPA filters. This classification is based on the materials used in filters and the resulting differences in separation efficiency. In addition, there are adsorption filters used to remove gases and smells as well as electrostatic filters, though their importance is small because of safety concerns and high manufacturing costs.

 

 

Coarse filters are manufactured from organic fibres, synthetic fibres, glass fibre or metal wire. Coarse dust is separated from these filter media mainly in accordance with the impaction principle. Dust particles do not follow air molecules flowing around the filter fibres but, due to their slowness, collide against the fibres. The fibres may have been treated with dust binding agents, or they have special characteristics to make particles stop on the surface of the fibres.

In case of finer impurities coarse filters are fairly inefficient, and this raises the importance of fine filters.

 

 

In these filters the filter medium used mainly consists of very fine glass fibres, in which fibre diameter varies between 0.1 and 1 mm. Flow velocity through this filter medium usually ranges from 2 to 12 cm/s. The collection efficiency of these filters is mainly based on binding and diffusion.

 

Because of their low mass small particles follow the flows around the filter fibre. They then touch the fibres and are attracted by them, after which the adhesive force fastens the particles to the fibre. This is called binding.

 

Brownian molecular motion captures small particles with a size of less than 1 mm and makes them vibrate. Because of this particles end up against the filter fibre, separating from the air. This phenomenon is called diffusion.

 

Both in binding and diffusion the dust particles finally get stuck in the fibre because of the Van der Waals force. In this case no dust binding agents are needed.

 

 

Because of differences in collection efficiency and application areas different testing methods have been developed for different filter classes. Coarse and fine filters are tested using a method based on DIN 24185. Coarse filters of classes G1 to G4 are tested by exposing them to synthetic dust. The test measures gravimetric weight arrestance.

For fine filters of classes F5 to F9 dust spot efficiency is tested by using natural aerosols, i.e. untreated outdoor air. A filter's dust spot efficiency is determined by taking similar-size air volumes from both upstream and downstream of the filter and running them through the test filter paper. Dust spot efficiency is determined by comparing air volumes needed to achieve the same discoloration rate in the test filter paper. A test for HEPA filters determines the filter's efficiency in terms of particles with a diameter of approximately 0.3 micrometers. Collection of particles is usually the most difficult in this area. Collecting both larger and smaller particles is normally easier.

 

 

The capacities determined in complex filter tests are only achieved if the given filter area data are followed. Especially in coarse filters flow velocity has a large impact on collection efficiency and thereby dust removal efficiency and finally also filter life. This is because filtering capacity depends on a certain velocity with which air runs through the filter material. In fine filters and HEPA filters collection efficiency does not vary significantly even if there were fairly large differences in rated loads.