Determination of integral and local efficiency of Hepa and Ulpa filters by application of an automated scanning technique

There is a series of clean room applications where full performance of the used high efficiency particulate air filters (HEPA) and ultra low penetration air filters (ULPA) is essential. This includes microelectronics, aerospace, pharmaceutical, food and nuclear industry as well as laboratory and hospital applications. To assure a certain clean room class there is a specification of different HEPA and ULPA filter classes given by the EN 1822 standard. Serious subsequent damages potentially arise in case the used filters are underperforming with respect to the nominal filter classification. For that reason a reliable testing is required from filter manufactures by the clean room operators.

To give a technological support to filter manufacturers this paper will present a special technical solution to perform reliable and retraceable performance testing of HEPA and ULPA filter elements regarding integral and local efficiency. For filter testing an aerosol in MPPS range is generated. Long-term stability of aerosol generation is very important especially for local filter efficiency measurements. During the filter scanning according to EN 1822-4 a potential leak is found at a certain x,y-position in case particle counts are exceeding the so called leak signal value. This leak signal value is calculated by a number of test parameters such as nominal filter class, upstream concentration, downstream particle counter flow rate, probe dimension and scanning velocity.

As a second step these potential leak positions are measured again with a fixed probe in order to verify leak status and to determine minimum local filter efficiency. Integral filter efficiency will result from adding particle counts of the total filter scan and referring these counts to the upstream particle concentration. Besides this scanning procedure EN 1822-5 describes determination of minimum local efficiency and integral efficiency by performing measurements with a fixed probe at several arbitrary sampling positions of the filter cross section. But this static efficiency measurement of cause does not give a 100% filter control.

For realizing these test procedures three different instrumental setups will be described in detail. For fully automated testing there is an option including a flexible filter holder for filter dimensions ranging from 203x203 mm up to 1830x1220 mm. In case a limited number of filter dimensions has to be tested a cabinet filter holder uses different adapter plates. In both cases a three-axis computer controlled scanning traverse is applied using precision stepper motors with a spatial resolution of 0,05 mm. Therefore the operator will be able to fix and repair leaks found in a filter very easily after testing. For existing laboratory test benches realizing the so called oil thread test an add-on equipment for manual scanning will be presented.

As a particle counter is used instead of an optical inspection much smaller upstream concentrations will be sufficient which will result in a much smaller filter contamination just from its testing. Furthermore the inspection of 100% of the filter cross section is documented in this case.

Printed on 2018-10-20