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Scanning-electron microscope image of typical air-laid filter medium, showing wide range of fibre diameters, random structure and binder bridges. Test methods and standards for evaluating the filter media for air filtration applications are now being formulated by the industry. Generally accepted test methods exist for fabricated air filter cells in applications ranging from residential air conditioning to nuclear reactor safety systems. Nevertheless, media test methods are needed for product quality control, and to assure design engineers and users that media will deliver promised performance under the environmental stresses of specific applications.
A survey of current standards relating to media testing shows that test methods for many media attributes have already been addressed by national and international standards-writing organisations.
Test standards for filter media, like test standards for filtration devices, fall into two general categories - those intended for disposable filter cells, and those intended for devices having means for cleaning the filter media.
Filter media in this group include those formed from fibrous materials, including natural or synthetic organic fibres and glass fibres.
The table of contents of this draft standard shows the many details which such a standard must address.
An important feature of the standard is that the Most Penetrating Particle Size (MPPS) must be determined. Media used here include forms similar in geometry to disposable media, but with characteristics which make them suitable for harsher environmental conditions and the stresses of cleaning.
In the 1980s, the group at the University of Karlsruhe developed test procedures for flat sheets of cleanable media, from which came VDI 3926. The test duct and instrumentation, including the holder for the test filter sample and the cleaning mechanism, are specified in great detail.
A specific nozzle design and compressed-air pulse form are required for the reverse air flow cleaning pulse.
The compressed-air cleaning pulse is activated whenever the pressure drop across the media sample reaches 1000 Pa, or for one test set, 1800 Pa. The automation of pressure drop measurement and the inclusion of a computer in the system means that the pattern of pressure drop over many cycles of each test phase can be recorded and included in a report. These systems face a heavy concentration of an aerosol which easily plugs filters, and is also highly flammable and odourous.
Fibrous droplet capture filters (demisters) are a necessary part of some chemical processes. Some standards also exist for evaluating dust control equipment using cleanable fabrics and nonwovens in various forms. Although some test results, required by government agencies for protecting public health, are openly available, other results can be kept under confidentiality agreements, to protect manufacturer trade secrets, while using rigorous, fully-defined test methods. The bulk of disposable filters are used in building HVAC systems, clean rooms and other applications having relatively benign environmental conditions.
In their gross structures, these media have the form of nonwoven fabrics, wet-laid papers, or air-laid glass fibre mats spun from continuous fibres or blown from molten glass.
Both national and international standards bodies, government agencies, technical societies and individual manufacturers have laboured to produce test standards for different attributes of this category of filters, and the media used in them. Separate counters may be used, or a single counter may be switched from downstream to upstream. More statistically valid procedures which could reveal media non-uniformity are available in several standards for nonwovens, especially TAPPI 1007, which deals with glass fibre papers. The holder illustration is obviously intended for tests on thin media with sufficient resistance to flatten the flow velocity profile over the exposed area. The suggestion that DOP is an acceptable aerosol should be removed, since it is a known potentially carcinogen. These include organic, metallic and ceramic fibre media with random structures, often fabricated into cylindrical canisters or pleated structures.
This standard, which had been formalised by 1992, ignores the effects of gross dust collector geometry, dusts, and cleaning techniques (shaking, reversing-flow and compressed-air pulses) in favour of a strictly defined flat sheet procedure. This draft standard was submitted to ISO member organisations for public enquiry in late 2009. This is a practical solution, but hazardous, since one cannot always rely on a single manufacturer, and the quality controls are only vaguely defined. The form and magnitude of the pulse must be demonstrated at least once a year, using a high-speed pressure transducer.
A series of five test phases is required; the details of these are too extensive to present here.

A loss-of-coolant accident will expose reactor safety systems to steam and free water; particulate filters in these systems are required to demonstrate their ability to survive such exposure. Control of the aerosol usually requires a combination of a louvre-type aerodynamic collector and downstream fibrous (expanded metal) filters. They cannot be removed from media, however, and generation and detection of test aerosols is quite different in the two cases. Similar filters are often incorporated in compressed air supplies, capturing condensed water vapour.
Test methods for the filtration media used in the filter elements of these systems have been slower to arrive. The exact mix of fibres, thickness, and other media physical properties are often crucial to the performance of a filtration product. Cleanable media, however, are used in industrial processes, stack-gas cleaning and other applications which may require operation at high temperatures, wet or corrosive gas conditions, and much higher dust concentrations than the other group. Filter media for these applications may be identical with those used in the other two groups, but they require different test methods, and demonstration that the mechanical properties of the media can withstand the special environmental conditions of their applications.
It is, therefore, a comparative procedure for media, and cannot, and does not pretend to replace tests on complete dust control devices with the actual aerosols and gas conditions they face in industrial situations. Deviations from the specified system are allowed, but the user of non-conforming systems must show tests made on three filter media which compare with tests made on the specified system within specified error bounds. The dust feeder is not defined, nor are particle diameter size distributions specified for the test dust as a bulk powder or dispersed in the airstream.
Concentration and dust feeder conditions are set by a preliminary test measuring the weight gain of a filter sample in the media holder. The assumption of uniform dust concentration seems unwarranted, but it is immaterial; the standard simply compares media performance under the defined conditions.
This is perhaps the most precisely defined element of the standard, indicative of the importance of cleaning patterns on performance results.
Residual conditions - the mass of dust remaining on the filter sample and the pressure drop across it after a set of cleaning cycles are measured during all five phases, and the downstream average dust concentration is measured at the ends of three phases. No special attention to statistics is required, because no particle counts are involved in this test method. Test procedures have been developed by manufacturers and organisations such as the American Petroleum Institute [search API 1581, Pall GAS-4300b for discussions]. However, complete test methods for filtration media - analogous to ISO TS 21220 test for fabricated filter cells, for example - are now passing through the international consensus-standards process for both disposable and cleanable filter media. The performance of a pleated filter is not easily predictable from the performance of flat sheet media.
The solids content of these media is low, with the fibre volume typically less than 10 % of the volume of the media.
Longer straight sections would probably make the face velocity more uniform for low-resistance media, and would allow a reasonable space for static taps for resistance measurement.
Because both the gross geometry of filter elements and media cleaning are important factors in performance of this type of dust separation equipment, earlier test methods were applicable to complete cylinders, pleated cartridges, or bags. There is also an implication that concentration can be determined by the weight change of the entire dust feeder during the test combined with the duct flow measurement, but this procedure is not detailed. It does not use actual paint spray aerosols, which would wreak havoc with users’ test ducts, but uses what is essentially EN 1822-5 with a (dry) KCl aerosol and then with a (liquid) oleic acid aerosol.
Because of the importance of the diameter and surface properties of liquid droplets and the fibres used to capture them, actual process streams are often tested.
The performance of cleanable media is highly dependent on the geometric configuration of the separator device and its cleaning means. In addition, thicker media could be accommodated without distortion, using edge seals if necessary. Whether the scope of the standard would include a dust holding capacity test, and what media physical attributes to include, has not been decided.
Loose specifications for sampling limit the usefulness of this ISO system to fine particle diameters. It should be possible to incorporate isokinetic sampling and minimal sampling-line particle losses into the standard.

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