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Until the mid 1980's Britain used a rather poor unrepresentative test referred to as BS 2831, which was made use of synthetic dust to measure filter ‘efficiency’. The results were artificially high when compared with the ASHRAE standard 52 which uses atmospheric air to measure efficiency by a series of test sample discs briefly explained later.
On the Continent the Eurovent 4/5 standard was adopted which was equivalent to ASHRAE 52-68 (now ASHRAE 52.1) and in Britain this was accepted as BS 6540 Part 1. EC regulations have given us a new reference, EN 779, and thus we now have BS EN779.
A brief description of the particulate air filter tests is given below:
For
primary and secondary filters we us BS EN779, and for higher efficient filters
such as semi HEPA, HEPA and ULPA, we have BS EN 1822. All tests are based
on measuring the effect of challenging a test filter with a known air volume
containing a particulate level of either synthetic or atmospheric dust or,
in the case of high efficiency filters, a known particulate challenge size.
BS EN1822 filters are tested using MPPS technology.
• BS EN779 "Arrestance"
The measure of a filters' ability to capture and retain a known weight of synthetic dust, fed into the filter at a specific rate to a final differential pressure drop of the filter. Results are expressed as a "by weight" arrestance.
• BS EN779 Efficiency
The efficiency test for air filters - referred to as the "atmospheric dust spot efficiency test" is carried out using atmospheric air only, and comparing the discolouration of a high efficiency filter disc, one upstream and one downstream of the test filter. The test filter will remove some of the staining portion of the atmospheric dust, and the downstream target (after the filter) will discolour more slowly than the upstream target. A number of tests are conducted to establish an efficiency curve (increase in efficiency) using synthetic dust to load the filter and accelerate an otherwise lengthy test procedure. The principle that filters tested to BS EN779, increase in efficiency with dust loading is the basis for the test procedure. Filters with average efficiencies below 20% or above 98% are not considered suitable for this test method.
• BS EN 1822 High Efficiency Filter Test
We have included this range of filters in the document because the lower efficiency or semi-HEPA range have found application in some building ventilation (HVAC) systems.
Usually expressed as the maximum penetration of a new filter, the BS EN1822 test is applied to confirm that high efficiency particulate air (HEPA) filters meet the minimum performance standards required. This test is also applied to filters which have an efficiency slightly lower than standard HEPA and these filters are called semi-HEPA grades. Some of these filters are installed in computer room environmental control (air conditioning) systems.
Mechanical media type air filtration is
based on moving a known volume of air through a filter media which is designed
to give the required efficiency, at an agreed pressure drop. As the efficiency
requirement increases, then media area must also be increased to compensate
for the resulting increase in filter pressure drop for the same air volume.
A combination of filter types may be required to provide the design level
of air filtration. An example would be primary plus secondary to achieve F9
final filtration.
The following air filter grade list is for BS EN779 and BS EN1822 tests. The
tests apply to filters used for HVAC, controlled zones and other process control
requirements.
| BS EN 779 arrestance | Test type/application | |
| G1 G2 G3 G4 |
<65 65<80 80<90 >90 |
Average value for collection of large particles using synthetic dust. Filters installed to prevent mechanical system fouling and as pre-filters to secondary and semi-HEPA range. |
| BS EN 779 efficiency % | ||
| F5 F6 F7 F8 F9 |
40<60 60<80 80<90 90<95 >95 |
Average percentage value (for atmospheric dust spot efficiency) using atmospheric air. Filters installed to keep buildings and process spaces clean and free from airborne pollution. |
| BS EN 1822 minimum MPPS* % |
||
| H10 H11 H12 H13 H14 |
85 95 99.5 99.95 99.995 |
EN 1822 – Oil mist aerosol MPPS.
Filters for specific (high efficiency) air quality control. |
Higher efficiency grades available.
*MPPS - Most Penetrating Particle Size
The testing of gas phase filter media to determine residual filter life is not straightforward. Physical adsorption (physisorption) and chemical adsorption (chemisorption) are two dissimilar processes, and many physisorption products contain a degree of chemisorption technology in their design. To avoid dependence on the "nose test" which will indicate that breakthrough has occurred, samples of media can be sent to a recognised laboratory for analysis. However, tests are not conclusive and the information required from the user is often not easily obtained. Chemisorption, using a colour change to determine a life cycle is available, but again the colour change does not fully indicate the state of the media, which is expensive to replace.
Tests to confirm the efficiency of electronic air cleaners are also not straightforward. ASHRAE standard 52 test method does not offer a procedure for testing such filters, and manufacturers often claim very high efficiency levels at particulate sizes down to 0.01 micron. In reality there are a number of factors which influence the efficiency of such filters and these include the particle size of the dust, dust collection loading on the plates, air speed through the filter, voltage collection plate design and ionizer spacing. If the electrical power fails the filter efficiency is zero, and a high percentage of the collected dust would be released downstream of the filter. The production of ozone is also a consideration.
Fractional efficiency tests carried out in laboratory
conditions on electronic air cleaners indicate an efficiency at one micron
of about 60%, with air speeds equivalent to mechanical filters in an HVAC
system. The efficiency reduces with filter operation, which is the opposite
for mechanical filters.
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