Courtesy of Freudenberg Filtration Technologies
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 1822:2009 for filtration efficiency and zero leakage. HEPA and ULPA filters are also subject to individual tests, with testing based on the latest particle counting methods. In all, the EN 1822:2009 standard is made up of five parts, which we will briefly introduce.
EN 1822-1:2009: classification, performance testing and labelling
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).
EN 1822-2:2009: aerosol production, measuring equipment and particle-count statistics
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.
EN 1822-3:2009: testing flat sheet filter media
Part 3 of the standard 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.
EN 1822-4:2009: scan method (Determining leakage of filter elements)
Part 4 of the standard 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) 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 and H 14), 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.
EN 1822-5:2009: particle separation testing of the filter element
This section of the standard 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.