EN 1822 and ISO 29463
Test methods for EPA, HEPA and ULPA filters
When the very highest levels of air purity are required – for example, in cleanrooms for the pharma or food industries, or in hospital operating theatres – EPA, HEPA and ULPA filters are the solution.
In Europe, these filters are subject to classification according to EN 1822 for filtration efficiency and zero leakage. HEPA and ULPA filters are also subject to individual tests. The international standard ISO 29463 is based on European standard EN 1822 and will probably replace this standard in the future. Both standards are based on the latest particle counting methods.
EPA / HEPA / ULPA filter classification is based on values for local particle collection efficiencies (local values) and integral efficiency (integral value). Each is assessed on the basis of the most penetrating particle size (MPPS). For the classification of EPA filters, a leak testing is not possible and not necessary. Therefore, no local values as leak detection limits are given for this group.
Part 2 of the standard includes the definition of measuring instruments and aerosol generators, and sets out the statistical basis for evaluating low-level particle counts.
Part 3 of the standards describes the testing of fractional collection efficiencies and the determination of the particle size for which the efficiency is a minimum (MPPS) of the flat sheet filter medium.
Method: Samples of the filter medium are subjected to a defined air flow, to which a test aerosol is added. Partial flows of the test aerosol are sampled upstream and downstream of the test specimen and the particle number concentrations are measured using particle count methods. From these results, the fractional collection efficiency curve can be plotted and the particle size with the highest penetration can be determined, which is known as the Most Penetrating Particle Size (MPPS). These results are depending on the filter medium and the air velocity, and hence have to be determined product specific.
Part 4 of the standards governs the testing of the leak-proof qualities of the filter elements. Irregularities in filter medium production, in the seal between the filter medium and the frame or any tiny leaks in the material itself can lead to local increases in penetration and corresponding increases in local particle concentrations on the clean-air side. For this reason, HEPA and ULPA filters (classes H 13 to U 17 as well as ISO 35 H to ISO 75 U) are individually leak-tested.
To test the filter element, it is subjected to a constant airflow from an aerosol with an average particle size which corresponds to the hardest particle size to capture (MPPS). Using movable probes, local particle concentrations are measured on the downstream side, which give together with the particle concentrations measured on the upstream side the results for the local penetration and the local collection efficiencies, respectively. If the local penetration values stay within the limits set out in the standard for each individual filter class, the filter can be classified as leak-free. As part of this process, pressure loss is also measured for each filter element.
For HEPA filters (classes H 13 to H 14 as well as ISO 35 H to ISO 50 H), the leak test can also be achieved using an oil thread leak test (see right).
If the test is successful, the test report will certify the filter’s leak-free status and also its integral particle collection efficiencies. The filter will then receive an individual identification number. Unlike HEPA / ULPA filters, EPA filters are not tested individually.
EPA filters are subject to a type test, in which collection efficiencies are determined as an average of certain random tests as part of the quality control.
This section of the standards defines the integral collection efficiency for the filter element as a whole. Typically, this value is calculated by averaging the local results achieved in Part 4. Alternatively, the integral collection efficiency can be determined via the single point measurement of a fixed sampling probe.
Alongside the previously determined leak-proofing test, the integral collection efficiency is used to classify the filter element according to the table of filter classes given in part 1 of the standard. This is certified in the test report.