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- 10 October 2007 -

Bulk chemicals and petrochemicals: Air filter testing

When working with gases and oils, air filters play an important role in stopping unwanted charges into air and protecting expensive industrial systems from damage - and at some point the filters will need testing. Richard Wakeman explains all.

Air filters are generally simple in concept yet they are technically complicated devices. The filter helps to mitigate the health effects of such air pollutants by improving the quality of air or preventing unwanted discharges into air, or protects air handling equipment or costly systems like gas turbine compressors. The contaminants are numerous and an air filter is the primary defence measure against airborne solid particles such as grit (particle sizes >76 µm) and dust which includes particles such as minerals and pollen (sizes <76 µm), smoke (<4 µm), fume (<1 µm), and liquid aerosols that arise in mists (<10 µm) and fogs (>10 µm).

Like most other process equipment, filters have developed to improve their in-service performance, for example reduced pressure losses or improved particle retention through more advanced filter media utilising various treatments to filter fibres or the application of nanofibres. The performance of an air filter is generally evaluated on four parameters:

. Contaminant removal efficiency - tested by challenging the filter with contaminant on the upstream side and measuring the contaminant that passes through the filter to its downstream side.

. Contaminant holding capacity - determined by measuring the mass of contaminant retained by the filter when it reaches its maximum allowable differential pressure.

. Resistance to airflow - determined by measuring the pressure of the air upstream and downstream of the filter. The value of resistance to airflow should be accompanied by the value of air velocity in order to understand the performance of the filter.

. Safety, particularly with reference to the ability of the filter to support or spread a fire when there is no other fuel source, or release sparks when exposed to a flame, or to generate smoke.

Contaminants are numerous and an air filter is the primary defence measure against airborne solid particles.

There are many standards that relate to either the testing or application of air filters; this article presents only those that can be used to compare air cleaners on a standardised basis irrespective of their application.

Air filter testing

Many air filter testing methods have been developed for predicting and comparing the performance of filters of differing designs; organisations involved in setting filter standards include the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), the European Committee for Standardisation (CEN), the International Standards Organization (ISO), and Underwriters Laboratories (UL). Although the purpose and focus of these organisations differs there is overlap of their standards and testing methods.

A widely accepted test for primary and secondary filters, mainly panel and pocket media type filters, is based on the ASHRAE 52 standard which was adopted in the UK (BS 6540) and other European countries (EUROVENT 4/5); the European Community later adopted the standards as EN 779.

The terms 'efficiency' and 'penetration' are often referred to when reporting results from air filter tests, and are related by

A further term, 'arrestance' is often used to describe a filter's ability to capture and retain dust and is defined as the mass removal of loading dust; the average arrestance is the ratio of the total amount of loading dust retained by the filter to the total amount of dust fed up to the final pressure drop.

EN 779 - particulate filter testing

Guidance to testing primary and secondary filters is given in EN 779. More strictly, EN 779 is applicable to air filters having an initial efficiency of <98% with respect to 0.4 µm particles when tested at an air flow rate between 850 and 5400 m3 h-1.

The tests are based on measuring the effect of challenging a test filter with two synthetic aerosols. Efficiency of finer Class F filters, is measured using a fine synthetic dust, DEHS (DiEthylHexylSebacate), produced by a generator capable of producing sufficient particles in the 0.2 to 3.0 µm size range. A coarser dust which is composed mainly of Arizona road dust (ISO 12103-1) is used to obtain information about dust holding capacity and, in the case of coarser Class G filters, filtration efficiency with respect to coarse loading dust (arrestance).

The revised EN 779:2002 provided better knowledge about the performance of filters than previous standards and made it possible to evaluate filter performance properties in relation to indoor air quality requirements and process demands.

EN 779 grades filters according to their efficiency (arrestance) under a defined set of test conditions - an air flow rate of 3400 m3 h-1 and a maximum pressure drop of 250 kPa for coarse (Class G) filters, or 450 kPa for fine (Class F) filters.

The efficiency of air filters is measured through a sequence of tests that use dust to load the filter to accelerate what would otherwise be a very length test, and in this way aims to simulate the filter in service. The principle for the test procedure is that filter efficiency increases with dust loading; filters with average efficiencies of less than 20% or greater than 98% are unsuitable for this test.

The revised EN 779:2002 provided better knowledge about the performance of filters than previous standards and made it possible to evaluate filter performance properties in relation to Indoor Air Quality (IAQ) requirements and process demands, and offered improved agreement between laboratory test results and actual installations, as well as giving a faster, simpler method that is easier to understand.

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Next: EN 1822 - high efficiency filter testing