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Previous: EN 779 - particulate filter testing

EN 1822 - high efficiency filter testing

The European standardisation body introduced EN 1822 for the classification and testing of HEPA (High Efficiency Particulate Air) and ULPA (Ultra Low Penetration Air) filters, based on filter efficiency at the most penetrating particle size (MPPS). In many applications the integrity and suitability of HEPA/ULPA filters is evaluated in their installed condition. In-situ testing is performed in, for example, the microelectronics and food industries to ensure the final product quality, whilst in the pharmaceutical industry it is mandatory to protect humans against health hazards.

Testing according to EN 1822 is normally performed with the filter in its new condition, using an aerosol probe that can be moved over the entire surface of the filter. This moving or scanning of the aerosol probe results in the measurement of many local collection efficiencies. These local efficiencies can be used to calculate the overall efficiency of the filter or the 'leak rate' of a specific area of the filter. The overall efficiency calculation is also known as the integral value, while the leak rate is termed the local value.

The most important measurements using EN 1822 are:

. Pressure drop at a nominal volumetric flow;

. Overall (integral) particle collection efficiency for the particle size with the greatest penetration (MPPS) at the nominal volumetric flow;

. Local collection efficiencies for the MPPS at the nominal volumetric flow;

. Freedom from leaks in Class H13 and upwards.

The results are used for allocation of a filter to a class designated between H10 and U17. Determination of the collection efficiency and the MPPS can be difficult, so for Classes H13 and H14 the standard permits evaluation for freedom from leaks by use of the oil thread test, in which case the filter is not scanned.

ASHRAE 52.2

This standard assesses the ability of a filter to remove particles from an air stream and its resistance to the airflow. The loading dust is a mixture of SAE Standard J726 fine dust, powdered carbon and milled cotton linters. The test procedure uses laboratory generated potassium chloride particles in the size range 0.3 to 10 µm dispersed in air as the test aerosol, and a particle counter to count the particles in twelve size ranges. Particle counts are taken both upstream and downstream of the filter to determine its efficiency.

The separation challenge imposed on an air filter is dependent on the nature of the dust in the incoming air or gas stream, and in the case of HVAC applications the challenge changes with the environment inside and outside of the building.

The test results enable a MERV (Minimum Efficiency Reporting Value) to be ascribed to the filter. The range of MERV parameters is shown in Table 2, from which it is seen that the MERV defines a specific range of parameters that the filter is able to meet. By way of example, a filter that has an E3 efficiency of between 50 and 70% would have a MERV of 7; a filter that has an E1 efficiency of between 85 and 95% would have a MERV of 15. A filter must also be operated with a minimum final pressure drop that is consistent with the reporting value in Table 2.

Dust holding capacity is not a parameter considered in ASHRAE 52.2, and it is not possible to estimate service life from the test results. However, a user can select a filter based on the size of the contaminant particles in the air stream, including respirable size dusts.

ASHRAE 52.1 and 52.2 are under review, with a proposal to creating a single ASHRAE test standard for determining the efficiency of HVAC filters. This would reduce the number of tests needed on a filter and allow a single test rig to perform all the required tests. As part of this updating, it is proposed that the dust spot efficiency test requirements is removed since the particle size efficiency used in 52.2 provides more direct and useful data.

Air filter safety

The characteristics, components and materials used in HEPA filters in relation to risk of fire or electric shock or injury to people are covered by UL 586, which defines minimum constructional requirements along with a minimum criterion flame test. The combustibility and amount of smoke generated by air filter units of both washable and throwaway types in their clean condition are covered in UL 900. The UL 900 test simulates a fire that impinges on the filter within a duct. The test filter, mounted inside a standard size duct with a specific airflow through it. The test is started when a methane flame is ignited upwind of the filter, resulting in flame impingement on the filter. For classification, limits are placed on the amount of smoke and flames that may pass through the filter:

. Class 1: no flames or sparks may pass through the filter and only a small amount of smoke is generated;

. Class 2: limited flaming and sparking are acceptable and a larger amount of smoke may be generated.

Conclusions

Filter efficiency, dust holding capacity and differential pressure generally provide the basic measures used to assess a filter's efficiency, and each can be measured in a variety of ways, although to conform to a standard each must be measured in accordance with the recommendations in the standard.

The performance of an air filter changes over time, and the efficiency tends to increase as the filter becomes loaded with dust. The separation challenge imposed on an air filter is dependent on the nature of the dust in the incoming air or gas stream, and in the case of HVAC applications the challenge changes with the environment inside and outside of the building. The application of an air filter relates to the environment in which the filter is to be used. Hence standards for, for example, indoor air quality (e.g. EN 13779), air quality in controlled environments such as clean rooms (e.g. ISO 14644-1), or cabin air (e.g. DIN 71460) may need to be consulted.

Most air filters are installed in a system for many months or sometimes years before they are replaced; yet testing of the filters occurs within a matter of hours. In service, an air filter will experience many environmental changes such as temperature, humidity, airflow velocity, and dust particle load; but its original testing was done in a controlled environment. The imperfections of testing methodologies and the varied motivations of the people developing test methods have to be recognised, from which it should be concluded that it is important to fully understand how to interpret the results of any air filter test prior to using the results to make important decisions.

Contact:
Richard Wakeman
www.richardwakeman.co.uk

For bibliography, see the print edition.

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