Filter testing

Filter testing in accordance with EN 779

Classification and comparative filter testing of coarse-dust and fine-dust filters are conducted in accordance with EN 779. Distinction is necessary here between coarse-dust filters (Filter Grades G1 to G4, with testing up to 250 Pa maximum permissible pressure drop) and fine-dust filters (Filter Grades F5 to F9, with testing up to 450 Pa maximum permissible pressure drop). Filtration efficiency is determined by a gravimetric process (see Fig. 1), with use of ASHRAE dust as test dust.

Fig. 1

For fine-dust filters, additional measurement of the efficiency takes place by introduction of a test aerosol (DEHS) and by determination of the fractional filtration efficiency (see Fig. 2). Here, testing is based on assumption of a particle size of 0.4 µm for purposes of classification. Further determination is made for fine- and coarse-dust filters of the following: initial pressure drop, dust-storage capacity, and the plot of pressure drop as a function of dust collection.

Fig. 2

It is planned to publish a revision of EN 779 before the end of 2011. This revision will stipulate that, for fine-dust filters in Filter Grades F7 to F9, achievement of a minimum efficiency must be reached, in addition to the previous requirements (see assignment to filter grade). Here, all individual measurements are taken into account, with the fraction efficiency for the filter in new condition, and the dust collection in discharged state (after treatment with isopropanol). The new version changes the classification of filters with medium efficiency: from F5 to M5 and from F6 to M6.

Filter testing in accordance with EN 1822

Classification of HEPA filters take place in accordance with 1822. The efficiency is measured by particle counters; distinction is drawn among Filter Grades E10 to E12, H13, and H14, as well as U15 to U17. Analogous to the filter grades in accordance with EN 779, the test procedure changes in conjunction with the group designations. Initially, the fractional efficiency and the most-penetrating particle size (MPPS) are determined with a flat filter medium. The most-penetrating particle size is the size of the particles for which the filter medium demonstrates the poorest degree of separation. This separation minimum lies as a rule in the range between 0.1 and 0.3 µm. In a second step, with use of the complete filter element, the integral efficiency and the local efficiency (the latter established by a scanning technique) are determined within the range of the MPPS. This is done to be able to guarantee freedom from leaks for Filter Grades H13 upward.

The degree of penetration P is calculated from the measured particle concentrations as follows:

P = c (clean) / c (unclean)

The filtration efficiency A is calculated by:

A = 1 – P

HEPA filters (for example, in filter grade H14) achieve for MPPS an efficiency of > 99.995 %. For larger or smaller particles, the efficiency is even better. For filters in grade E, there is no 100 % monitoring, in contrast to HEPA and ULPA filters. Evidence of the efficiency can be provided by visual monitoring and statistical methods.