Courtesy of Freudenberg Filtration Technologies
Filter classification standard EN779:2012
In Europe and many other countries in the world, the coarse and fine dust filters used in air-conditioning and ventilation systems are usually selected in accordance with the classification described in the EN 779 standard entitled “Particulate air filters for general ventilation”. The methodology described in this standard is based on a laboratory test procedure, with the aim of achieving reproducible and comparable results. However, since the test aerosols and test dusts used will not usually resemble the air pollutants that an air filter is exposed to in actual operation; the results of the laboratory tests are transferable to actual applications only with very restricted relevance.
The EN 779 has been in force as a European standard since April 2012 in a new, revised version. The most significant change from the previous version is the introduction of minimum efficiencies for Classes F7 to F9 and the renaming of Classes F5 and F6 as M5 and M6. The introduction of these minimum efficiencies is an important step forward in filter standardization, and constitutes a milestone in the filter industry, in its thrust for higher quality standards, and thus for improved protection of both man and machine.
Laboratory test methods
The test is performed on a filter element of standard size (see EN 15805), suitable for installation in a rectangular duct measuring 610 mm x 610 mm – with a test volume flow between 0.24 m3/s (850 m3/h) and 1.5 m3/s (5400 m3/h). Since a filter’s operational behavior is crucially dependent on the volume flow in operation, the filter classes and all other test results from the test performed in accordance with EN 779 always relate only to the test volume flow specified in each case.
The principal results of the test are:
- Gravimetric arrestance efficiency for synthetic dust
- Efficiency: corresponds to the number-referenced fractional collection efficiency for 0.4 μm particles of the synthetic test aerosol
- Pressure drop
- Dust holding capacity for synthetic dust
- The test procedure laid down in DIN EN 779 is shown in diagrammatic form in Figure 1
Classification of coarse dust filters
Coarse dust filters are classified by their average arrestance (Am) in relation to the synthetic ASHRAE dust. A final filter installed on the downstream side of the filter being tested is weighed before and after the dust has been fed in. The increase in mass measured at the final filter corresponds to the mass of dust that has penetrated through the filter being assessed. The difference from the total mass of dust fed corresponds to the mass of dust arrested in the filter being tested. This is put in relation to the mass originally fed in, whereupon the average (gravimetric) arrestance of the test filter is calculated.
Classification of fine dust filters
Fine dust filters are classified in accordance with their average efficiencies (Em). For this purpose, between the individual dust loading stages, the filter element is exposed to a synthetic droplet aerosol, and the particle number concentrations are measured before and after the filter. The efficiency is calculated from the difference between the two concentrations – referenced to the concentration of 0.4 μm particles measured on the upstream side. Following the test, the average efficiency is calculated as an integral mean value of the individual efficiencies determined as a function of the dust loading.