Air Filters used in General ventilation and HVAC systems are mainly tested for their capability to effectively capture and remove airborne contaminants such particulate matter, dust and molecular gases. This is technically called the removal efficiency of an air filter. The Air filters are also subject to testing to know their properties such as dust holding capacity and pressure drop during clean and dust accumulated conditions. In addition to this Air filters are also rated for their power consumption and Energy efficiency. Air filters plays an important role in maintaining good Indoor Air Quality index. They are also subject to combustion and smoke test to classify them deemed safe for the public safety under UL 900 standards.To measure many of these parameters, standards are needed in the air filtration industry to ensure that filter tests are conducted in a consistent and reliable manner.
Filter classifications and laboratory tests
In Europe, particulate air filters for general ventilation applications are classified according to the ISO 16890: 2018 test standard. In the U.S., ASHRAE 52.2: 2017 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.
Test based on ISO 16890: 2018
The new standard specifies two test phases for evaluation of air filters.
In Phase 1 of the testing, the pressure drop across the filter is determined as a function of volumetric air throughput. Then the filter’s fractional Efficiency / arrestance is determined over a particle size spectrum of 0.3 μm – 10 μm (12 particle sizes). Two different test aerosols are used in the testing: DEHS (di-ethylhexyl sebacate) aerosol for particulate sizes up to 1 μm and, for larger particulate sizes, KCl (potassium chloride) aerosol.
Phase 2 of the testing concentrates on the filter’s performance by discharging electrostatic charge. Therefore, in Phase 2 the filter is electrostatically discharged in a treatment chamber before its fractional Efficiency and pressure drop are again determined under more realistic conditions.
The results of both test phases are then averaged, providing the average efficiency for each of the fractions PM1, PM2.5 and PM10. Since the IPA conditioning is expected to overpredict the loss in efficiency in real use for charged filters, the average of the two values in each size range is then calculated as representative of the filter’s efficiency in use. Before the filter is classified in an ISO Filter Group, its efficiency results are weighted using theoretical particle size distributions for urban and rural air (In ISO 16890 the amount of particulate matter a filter will be exposed to in situ is represented by two different particle size distributions (psd) called rural and urban. The rural psd is used for calculating the mass removal efficiency for the ePM10 rated filters while the urban psd is used for the ePM2.5 and ePM1 rated filters). The ISO Filter Group classification is done in accordance with these weighted efficiency results.
In the case of ISO Coarse filters, testing of dust holding capacity is mandatory as stipulated by ISO 16890. For all other Filter Groups this is an optional determination. The testing particulate specified for use in the dust retention test is Test Dust ISO Fine.
For eg: an air filter qualifies for Filter Group ISO ePM1 or ISO ePM2,5 if it achieves a minimum efficiency of 50% or greater for particulate sizes ≤1 μm or ≤2.5 μm . Along with the Filter Group achieved, the complete ISO 16890 classification also includes the filter’s average efficiency. Minimum efficiency is defined as the efficiency achieved following electrostatic discharge of the filter before testing. Average efficiency is calculated by averaging the filter’s efficiencies in the untreated state, i.e., before discharging, and in the discharged state.
For example: if a filter achieves a minimum efficiency of 45% in the PM1 size spectrum and 56% in the PM2.5 size spectrum, it does not qualify for the ISO ePM1 Filter Group – having missed out by 5 % – but does qualify for the ISO ePM2,5 Filter Group. Assuming that the average efficiency achieved by this filter for ISO ePM2,5 particulates was 68%, this percentage is rounded down to the nearest 5% increment (i.e., rounded down to 65 %) and the filter’s ISO 16890 classification is therefore ISO ePM2,5 65%.
To be classified in Filter Group ISO ePM10 a filter must achieve an average efficiency of ePM10 ≥ 50 %. Filters with an average efficiency < 50% for this particle size range are classified in the Filter Group ISO Coarse and according their initial gravimetric arrestance.
Although the efficiency tests in ISO 16890 are performed very similarly to those of ANSI/ASHRAE 52.2, the reported values (ePM vs MERV) do not report the same thing. ISO is a mass-based result and MERV is directly related to individual particle removal. Health effects of particulate matter (PM) vary based on the particle size. Across various organizations, including U.S. Environmental Protection Agency (EPA), the World Health Organization (WHO) and the European Union PM10, PM2.5 and PM1 are considered important size fractions. Use of the various levels of ePM gives the user a measure of how much of the PM in air will be removed by a filter.
EN 779:2012 Test Standard
This European standard preceded the ISO 16890 test standard. In this standard, the free-flowing pressure drop of the filter is determined. Then the filter is loaded with ASHARE dust to measure its dust-holding capacity and is challenged with a test aerosol to determine 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 2 below). Measurements of efficiency is 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 are determined by their average efficiency on particles 0.4 micrometres (μm) in size, which is interesting from a filtration point of view because 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 depend 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 therefore 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 2).
Tests according to ASHRAE 52.2
The American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) 52.2-2017 published this standard for evaluating air filters that is applied in HVAC systems. The standard describes a method for laboratory testing for measuring the performance of general ventilation air filters. It provides filters initial efficiency as a function of particle size as well as a numeric value called as Minimum Efficiency Reporting Value (MERV) for filter selection. MERV is a single number expressed on a 16-point scale which helps in easy filter selection.
The filters performance over a range of 12 particle sizes is determined using an aerosol such as Potassium chloride. The twelve size ranges are placed in three larger groups E1, E2 and E3 (Composite average Particle size efficiency). The table below shows the particle size in Micro meter and the sizes assigned to each group.
The average efficiency of these three groups is referenced against the MERV parameters and the appropriate MERV value is selected for the filters. For assigning a MERV rating all three specific range of parameters for E1, E2 & E3 needs to be satisfied. If anyone parameter is not met, the filter will go down in rating.
To measure the dust-holding capacity, the filter is loaded with standard ASHRAE test dust.
With the introduction of ASHARE 52.2, the standards committee had recognized the need to provide a standard that allows the user to select a filter based on specific contaminant based upon that contaminant’s particle size. ASHRAE has also given importance to respirable contaminant and tried to address the same using MERV ratings. Another important criterion in developing this Standard was the goal of providing a ‘lowest point filter efficiency’ or, in most cases, initial filter efficiency. In this manner, the user would know how efficient the filter is at its lowest point, which in most cases is as soon as it is installed in the system.
Eurovent 4/21- Energy certification
A filter consumes energy by creating a resistance to the air flow through it. This pressure drop means that the ventilation fan has to work harder in order to move the air. The effort required is directly related to the energy consumed by the fan motor. There are other contributory factors, but put simply, if the pressure drop of the filter is lowered, the fan works less hard and consumes less energy.
Eurovent 4/21- 2018 provides a uniform and validated method of testing, classifying and presenting the energy efficiency of an air filter, making comparison of different products simple. Fine dust filters according to ISO EN 16890 with a face dimension of 592 x 592 mm from participating (Eurovent certified) manufacturers are eligible to be rated.
Depending upon their expected energy consumption, products are then awarded one of six ratings from A+ (the best) to E (the worst). For the certification of air filters at Eurovent. Filter manufacturers submit test reports from independent laboratories for each filter. Eurovent checks the data and makes spot checks at production sites.
Eurovent 4/21 – 2018 is structured to place a percentage of products into each category, so only the very best filters will achieve the higher classifications.
- Grade A+: 1%
- Grade A: 5%
- Grade B: 15%
- Grade C: 30%
- Grade D: 50%
- Grade E: 50%
The definition is more precise than the old system according to EN779 and increases the transparency about the energy consumption of the filters for operators of air conditioning and ventilation systems and simplifies filter selection. Table below shows Eurovent Energy classification based on ISO 16890.
Test standards for molecular filters
Molecular filters have been used to remove gas phase pollutants from the air stream. A uniform international test standard for them was established only recently. Furthermore, there is no classification system for molecular filters, such as ISO 16890. The raw materials are tested separately as such or the filter is assembled with media and is subjected to testing.
The major standards that we follow for filter testing is based on ASHARE standard 145.1 and ASHRAE standard 145.2.
ASHRAE 145.1 compares and tests different kinds of absorbent loose granular media to arrive at an optimum solution. Here the loose granular media is installed in a test equipment. An air stream challenges this test apparatus with the gases under a fixed state of temperature and relative humidity. The testing is conducted at higher concentrations of gas phase contaminants than actually detected at site. Kindly note that this is a test only for media and not for the filter element.
ASHARE 145.2 helps in comparing the performance of filters which utilize the media selected by using 145.1 test method. Here the filter is arranged in in-line duct mounted fashion. Here the gas phase media is arranged in air-cleaning device and is installed in a test apparatus and challenged with an air stream of test gases under fixed state condition of temperature and relative humidity. The testing is conducted at elevated levels of gas level concentrations when compared to actual site conditions and hence the real-life performance of the filter is tested.
The ASTM standard D6646 tests the Activated carbon media for removing Hydrogen sulphide gas from a humidified gas stream. Here the breakthrough performance of the media is tested. Here the actual site conditions are not replicated and hence this test is mainly intended for comparing the performance of various granular media in removing H2S.
GPACM- Gas Phase Air Cleaning Media
ISO 10121-1:2014 aims to provide an objective laboratory test method, a suggested apparatus, normative test sections and normative tests for evaluation of three different solid gas-phase air cleaning media (GPACM) or GPACM configurations for use in gas-phase air cleaning devices intended for general filtration applications. ISO 10121-1:2014 is specifically intended for challenge testing and not for general material evaluation or pore system characterization. The three different types of GPACM identified in ISO 10121-1:2014 are GPACM-LF (particles of different shape and size intended for e.g., Loose Fill applications), GPACM-FL (Flat sheet fabric intended for e.g., flat one layer, pleated or bag type devices) and GPACM-TS (three dimensional structures that are many times thicker than flat sheet and e.g. used as finished elements in a device). The tests are conducted in an air stream and the GPACM configurations are challenged with test gases under steady-state conditions. Since elevated gas challenge concentrations (relative to general ventilation applications) are used, test data should be used to compare GPACM within the same configuration and not for the purpose of predicting performance in a real situation.
GPACD- Gas Phase Air Cleaning Devices
There is an increasing use and need for gas-phase filtration in general filtration applications. This demand can be expected to increase rapidly due to the increasing pollution problems in the world together with an increasing awareness that solutions to the problems are available in the form of filtration devices or, phrased more technically, gas-phase air cleaning devices (GPACD). The performance of devices relies to a large extent on the performance of a gas-phase air cleaning media (GPACM) incorporated in the device. Still applications and device performance are often poorly understood by the users and suppliers of such media and devices. Media tests may also be adequate to offer data for real applications if actual low concentrations (< 100 ppb) and longer exposure times (>weeks) can be used in the test, provided that the geometrical configuration, packing density and flow conditions of the small-scale test specimen are equal to those used in the real applications. Such tests are however not included in the scope of ISO 10121.
Indoor Air Quality and EUROVENT 4/23
The EUROVENT 4/23 provides a guidance for the selection procedure of the filter class, considering all relevant factors, which matches the recommended minimum filtration efficiency with both the outdoor air and supply air category. To maintain consistency on an international level, the method refers to limit values recommended by WHO.
It refers to ISO 16890 for filter selection based on particle efficiency.
The new EN 16798-3:2017 standard, which supersedes the globally known EN 13779, is perceived as the main guidance for HVAC consultants on how to design filtration in ventilation systems.
High efficiency air filters
EN 1822-2019 is the most accepted international test standard for the filter classification of HEPA and ULPA filters developed by the European Standardization institute. In this standard the efficiency is determined on the basis of a particle counting method using a liquid (or alternatively a solid) test aerosol and allows a standardized classification of these filters in terms of their efficiency, both local and integral efficiency.
During the test procedure as a first step the Most Penetrating Particle Size (MPPS) is determined for a flat sheet filter media for a given media velocity. The filter is manufactured with specified media grade and amount of media to comply with the defined media velocity.
To test the filter element, it is subjected to a constant airflow from an aerosol with an average
particle size which corresponds to the hardest particle size to capture (MPPS). Using movable
probes, local particle concentrations are measured on the downstream side, which give together with the particle concentrations measured on the upstream side the results for the local penetration and the local collection efficiencies, respectively. If the local penetration values stay within the limits set out in the standard for each individual filter class, the filter can be classified as leak-free. Similarly, the overall efficiency of the filter is also tested to meet the specified values set forth as per the standard.
“ISO 29463 – High-efficiency filters and filter media for removing particles in air” – is an ISO standard based largely on EN 1822.
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.
Standards for cleanrooms
ISO 14644 is the most common standard used for cleanroom facilities. ISO 14644-1 and ISO 14698 are non-governmental standards developed by the International Organization for Standardization (ISO). The former applies to clean rooms in general (see table below); the latter to cleanrooms where biocontamination may be an issue. ISO 14644 does not take into account microorganisms.
ISO 14698 and GMP (Good Manufacturing Practices) are used by the food processing and pharmaceutical industries. These standards include microbiological contaminants.
(All above information & Data are derived from Book records of Filtration Standards & Other related standards, research papers and its related webpages)