Ensuring the quality of EPA, HEPA and ULPA filters
The European filter testing standard is the most important basis for testing and classifying absolute filters. The standard is based on state-of-the-art particle measurement technology and authorized procedures for determining the efficiencies. It has five parts. The filter is assigned to the relevant filter class using the results from sections 4 (local arrestance) and 5 (integral arrestance).
An individual test report and serial number are produced for filters in classes H13 and higher. Therefore each filter from H13 on can be assigned to its own individual test. Individual testing of EPA filters is not necessary according to the standard, and is possible with the testing procedure described. EPA filters are tested in the course of sample testing, whereby the arrestance is obtained as a mean value from individual, random measurements.
Part 1: Classification, Performance, Test and Identification
EN 1822-1:2009 sets three groups:
- Group E: EPA – Efficient particulate air filter
- Group H: HEPA – High efficiency-particular air filter
- Group U: ULPA – Ultra low penetration air filter
The absolute filters are classified according to the local and integral arrestance values determined during testing.
Part 2: Aerosol production, measuring equipment, particle counting statistics
This part describes the conditions for testing and the aerosol generators, the particle measuring technology and the statistical procedures to evaluate the counts.
Part 3: Testing flat sheet filter media (determining MPPS)
Part 3 describes the determination of the fractional efficiency and determination of the most penetrating particle size (MPPS) of the flat sheet filter media.
A test aerosol is applied to the filter media at the nominal flow velocity specified for later use of the filter. Partial flows of the test aerosol are taken upstream and downstream of the filter sample. The particle counting method determines the particulate concentrations and calculates the fractional efficiency curve. The particle size at which the fractional efficiency curve reaches its minimum is call the MPPS. Put in simple terms, this is the particle size at which the filter medium works worst for a defined flow velocity.
Part 4: Leak testing of filter elements (scan method)
This section addresses how to test filters for leaks. Leaks can occur due to faults in the filter media, improper sealing between the pleat pack and frame or irregularities when handling the components. On account of the high filtration efficiency expected of absolute filters, even the smallest leaks (that are hardly visible to the human eye) can produce increased local particle concentrations.
For the automated process (scan test), the filter element is set up in a test rig and a DEHS (Di-2-Ethylhexyl-Sebacat) test aerosol is then applied. The mean particle size of the aerosol must lie in the range of the MPPS. The flow side of the filter is approached using probes on computer-controlled linear axis. At each point on the clean air side, the local aerosol concentrations are measured to determine the local degree of penetration. If the aerosol concentration does not exceed the required limit at any of the points, the filter is deemed to be leak free.
The necessity to determine the local efficiencies also implies the necessity for individual testing of each filter element upwards of filter class H13.
Part 5: Determining the efficiency of filter elements
Part 5 describes the determination of the integral filter efficiency. This value is usually calculated as the mean of the local individual efficiencies measured in Part 4. Alternatively, an individual measurement with fixed sampling probes is also permissible.
Leak test alternative: Oil thread test (H13 and H14)
In this fast, low-cost leak-testing method, the filter is positioned in front of a black background in a brightly lit room, horizontally and leak-free on a diffuser. An oil-drop aerosol (liquid paraffin) is then applied to the filter. Then, the filter is inspected visually for leaks. The test procedure depends highly on the training and attitude of the test personnel. Therefore the results cannot be exactly reproduced in repeated tests. Furthermore, the oil thread test does not determine the filter efficiency.