Test methods and standards for air filters: Air filters utilise the fine fibres of their media and complex filtration mechanisms that are invisible to the naked eye, just like the particles and gases they capture.
We know that the most important property of an air filter is its efficiency, which is a measure of its ability to remove air- borne pollutants, such as dust, particles and gases. Other essential properties are a filter’s pressure drop development and its dust-holding capacity.
To measure these properties, standards are needed in the air filtration industry to ensure that filter tests are conducted in a consistent and reliable manner. Standards based on documented knowledge enable the classification and comparison of filters from different manufacturers.
In Europe, particulate air filters for general ventilation applications are classified according to the EN 779:2012 test standard. In the U.S., ASHRAE 52.2 is used. The choice of standard determines the type of test rig, the test procedure, the classification of the filter and how the test results are reported.
In this standard, a filter is loaded with a standard test dust to measure its dust-holding capacity and is challenged with a test aerosol to deter- mine its filtration efficiency. The dust-holding capacity is a measurement of the quantity of test dust (in grams) captured and held by the filter until the final pressure drop of the test is reached (see Table 1 below). Measurements of efficiency are made after regular doses of test dust until the stipulated final pressure drop is reached.
The results of the test are used to classify the filter. The filter class for medium filters and fine filters is determined by their average efficiency on particles 0.4 micrometres (μm) in size, which is interesting from a filtration point of view be- cause the majority of anthropogenic particles – those produced by human activities – are around
0.4 μm in size. To qualify for Filter Class F7, the filter’s average efficiency has to be in the range of 80-90% (≥80 <90 %) during the entire test cycle. However, in real service the filter’s efficiency will not increase.
Some filters use electrostatically charged polymer (synthetic) fibres. Their efficiency will de- pend on the strength of the charge, which weakens over time, often within a couple of months, as the filter is used. Measuring and reporting the Minimum Efficiency (ME) of these filters, after a potential diminished or lost charge, is there- fore included in the test and measurement procedures of the EN 779:2012 standard. To remove the electrostatic charge, the filter is treated with isopropanol. The ME value for an F7 filter is 35% (see Table 1).
ASHRAE 52.2:2007 is the American standard for testing and classifying filters. Salt particles (potassium chloride salt) are used to measure a filter’s efficiency on particle sizes in three different size ranges, 0.3-1 μm, 1-3 μm and 3-10 μm, and during incremental dust-loading steps.
To measure the dust-holding capacity, the filter is loaded with standard ASHRAE test dust. MERV – the Minimum Efficiency Reporting Value – is the basis for classifying air filters ac- cording to ASHRAE 52.2. MERV is the lowest measured initial efficiency or lowest efficiency rating during dust feeding.
In ASHRAE 52:2, Appendix J, the electrostatic charge is removed by an excess of potassium chloride test aerosol.
ISO 16890 is a new global filter standard that is being developed to replace EN 779 and ASHRAE
52.2. Filtration efficiency is the basis for the standard and it is measured in different ways, depending on the type of filter. For coarse filters, the gravimetric efficiency on a test dust is used. For medium filters, efficiency is tested on PM2.5 and PM10 particles. Efficiency on PM1 particles is used for fine filters.
The Swedish P-mark is the highest quality assurance rating for a filter today. No other quality system has higher requirements or is based on real-life conditions. Filters are tested in the lab according to EN 779:2012. To guarantee performance in actual operating conditions, the filters are also tested in the field for six months.
To certify that filters perform according to the manufacturer’s technical data, an independent authority, the Eurovent Certification Company, can be engaged to test and certify bag filters, compact filters and panel filters in the fine filter classes M5 to F9. Here, Eurovent uses an EN 779 accredited lab.
Approved manufacturers receive a Eurovent certificate and permission to use the Eurovent logotype in their marketing. Tests have to be repeated annually to retain the certificate.
Molecular filters have been used for more than 50 years, but a uniform international test standard for them was established only recently. Furthermore, there is no classification system for molecular filters, such as EN 779. Today’s molecular standards can be divided into test methods for raw material and for complete filter assemblies.
Gas Phase Air Cleaning Media (GPACM) is the acronym for filter media used to remove gases in air. Loose-fill media, e.g. pellets or granules, are tested according to the U.S. standard ANSI/ ASHRAE 145.1 by challenging them with a polluted air stream in a two-inch-wide test tube in which the airflow, temperature and humidity are kept constant. The time is measured until a 50% breakthrough of the initial challenge concentration is attained.
Besides loose-fill media, the international standard ISO 10121-1 also tests flat-sheet media and structures like honeycomb. Both standards are used to benchmark media; they are not used for testing final products.
Gas Phase Air Cleaning Devices (GPACD) is the acronym for products that remove gases from air. Initial efficiency, efficiency during the loading test, capacity, retentivity and pressure drop can be tested today according to the U.S. standard ANSI/ASHRAE 145.2 or the international standard ISO 10121-2. Both utilize different approaches to compare similar products.
ANSI/ASHRAE 145.2 tests only GPACDs that employ adsorbent media and users can choose among 30 different test gases at fixed concentrations. Capacity is reported as the amount of gas removed at 99% penetration, or after a maximum of four hours. ISO 10121-2 contains an open part in which the application-specific gas, concentration, flow, temperature and humidity can be used. A simplified part is based on pre- defined parameters. The ISO standard demands a minimum efficiency reduction to arrive at quality data and accepts all types of filters and air cleaning techniques.
EN 13779 is the European standard for general ventilation in all types of buildings, except residential. The standard specifies the level of function required for technical solutions, but it is not mandatory. EN 13779 helps to create a healthy and comfortable indoor climate with acceptable installation and operating costs.
The filter class is determined by the standard specifying the filtration stages needed to reach the targeted indoor air quality (IAQ) while considering outdoor pollution levels. Gas filters for molecular filtration, to remove gases and vapours, are required to meet the strictest requirements (see Table 2).
In Europe, the EN 1822 standard is used, which divides testing of high-efficiency EPA, HEPA and ULPA air filters into five parts.
High efficiency air filters are classified using the MPPS (Most Penetrating Particle Size) particle size – the particle that can penetrate the filter the easiest. Mandatory tests for leakage are required for filters in class H13 and higher.
“ISO 29463 – High-efficiency filters and filter media for removing particles in air” – is an ISO standard based on EN 1822. It is also divided into five parts that have the same headings as in EN 1822, although it also introduces “even” classes in between the EN 1822 classes, e.g. 99.90%, which is ISO 30E.
IEST, an international technical society of engineers based in the U.S., has instituted a number of test methods. IEST-RP-CC001, 007, 021 and 034 pertain to high-efficiency air filters. The different parts of this standard cover performance requirements, classification, design, design requirements and filter media tests.
ISO 14644 and ISO 14698 are currently the most common standards used for cleanroom facilities. ISO 14644 has replaced national standards like U.S. Federal Standard 209 and British Standard 5295.
ISO 14644 has ten parts and several more will be added eventually. ISO 14644 does not consider micro- organisms. ISO 14698 and GMP (Good Manufacturing Practices) are used by the food processing and pharmaceutical industries. These standards include microbiological contaminants.
*GF = Gas Filter (activated carbon filter and/or chemical filter)