Quality assurance of EPA, HEPA and ULPA filters
The European filter test standard is the most important basis for the testing and classification of absolute filters. The standard is based on state-of-the-art particle measurement technology and approved methods for determining efficiencies. It consists of five parts. The filter is assigned to the respective filter class on the basis of the results from sections 4 (local discharge) and 5 (integral discharge).
For filters of classes H13 and higher, an individual test report and serial number is generated. Therefore, each filter from H13 can be assigned its own individual test. Individual testing of EPA filters is not required by the standard and is possible with the test procedure described. EPA filters are tested within the framework of random testing, whereby the separation is determined as an average value from individual, random measurements.
Part 1: Classification, performance testing and marking
In the first part of the standard, filters are divided into three groups according to their filtration performance based on the filtration efficiency or pass efficiency:
- Group E: EPA - EPA filter (high efficiency particulate air filter, or Efficient Particulate Air).
- Group H: HEPA - Filter (High Efficiency Particulate Air)
- Group U: ULPA - ULPA Filter (Ultra Low Penetrating Air)
Within these groups, there is a further subdivision that describes the filter more concretely in terms of its performance. This results as follows:
Part 2: Aerosol generation, measuring equipment, particle counting statistics
The following second part of the standard provides important specifications regarding the test aerosol and the subsequent measurement and particle counting. In order to ensure uniform standards for the measurement of filtration performance, only certain test aerosols may be used. MANN+HUMMEL uses DEHS as the test aerosol in accordance with DIN EN 1822-2.
In addition to the type of aerosol, it must be ensured that the particle production rate is sufficiently large so that statistically permissible measurement results can be determined.
To ensure that all the necessary information is available for the test run, variables such as the pressure, the pressure drop across the filter, the temperature, the humidity and, above all, the concentration are recorded for the entire series of measurements.
Ultimately decisive for the filter class, however, is the filtration efficiency of the filter. This is determined during the test via a particle counter.
Part 3: Testing of the planar filter medium
Testing of the planar filter medium is essential for determining the fractional collection efficiency by a particle counting method. From this measured value, in turn, the fractional collection efficiency curve for the tested filter medium can be obtained.
In detail, a sample of the filter medium is clamped flat in the test filter holder and flowed through with the test air stream according to the specified filter medium velocity. Subsequently, the generated test aerosol is added to the test air stream in the inlet so that a homogeneous mixture is formed.
To determine the degree of separation, partial streams are then sampled on the upstream and downstream sides by means of probes, and the concentrations are recorded for various particle sizes. The fractional collection efficiency curve can then be determined from these measurements.
It is immediately noticeable that the curve for a certain particle size shows a minimum with regard to the degree of separation: this critical point is designated MPPS. This stands for Most Penetrating Particle Size - i.e. the point at which most of the particles push through the filter.
While a finished filter in operation will never consist of only one flat layer, the knowledge gained about the point of minimum separation efficiency is relevant for further measurements of a commercial filter.
Part 4: Leak test of the filter element
The leakage test is used to check the filter element for locally unacceptably high flow rates.
In the so-called scan method, the manufactured filter is clamped in the filter holder and the test air and test aerosol flow through it at nominal volume flow. The measurement is performed at the minimum particle size of the filter medium (MPPS).
In compliance with DIN EN 1822-2, a probe is used to scan the entire downstream side of the filter. During this process, a computer records the coordinates, the velocity and the measured particle flow of the probe so that possible leakage points can be clearly located. Particular attention is paid to the filter's boundary surfaces, as most leaks are expected to be detected at the edges between the filter and the frame, at seals or at corners.
In order to be able to exclude measurement inaccuracies, points with particularly high transmittance are subsequently checked again with a fixed probe. If no measured local transmittance exceeds the permissible maximum value, the filter is considered to be leak-free.
The measurements carried out at MANN+HUMMEL are fully automatic, so that the average filtration efficiency of the filter can be determined during the measurement. This value is used to determine the associated filter class at the volume flow rate used in accordance with DIN EN 1822-1.
Part 5: Separation efficiency test of the filter element.
During this test, the filtration efficiency of the filter is determined. For this purpose, the filter is clamped in the filter holder and the test air and test aerosol flow through it at nominal flow rate.
This test also always takes place at the particle size in the separation efficiency minimum of the filter medium (MPPS - see DIN EN 1822-3), in order to ensure an estimation to the safe side.As explained in the previous section, the downstream side of the filter is scanned with a probe without any gaps. During the so-called scanning process, particles are removed from the downstream side with the probe and fed to a particle counter, which in turn adds up the individual particles. The separation efficiency can then be determined by comparing the particles measured on the downstream side with the particles fed to the upstream side. This in turn is a measure of the efficiency of the filter.
The resulting minimum collection efficiency forms the basis for classification according to DIN EN 1822-1.
Alternative to the leak test of the filter (H13 /H14), as well as all non-planar filters.
A quick and inexpensive test for the leak tightness of the filter is the leakage test with test aerosol. Here, the filter is positioned in a dark test room in front of a black background. With the aid of a suitable test aerosol (at MANN+HUMMEL a triglycol mixture), a fine particulate aerosol is generated and applied to the filter on one side. On the other side, visual inspection for leaks is carried out with the aid of bright light sources positioned above the filter, which highlight rising aerosol plumes against the dark background. With this method, leakages at the gasket, frame and also filter medium are easily detectable. It is not possible to determine the filter efficiency.