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A particulateair filter is a device composed offibrous, orporous materials which removesparticulates such assmoke,dust,pollen,mold,viruses andbacteria from theair. Filters containing anadsorbent orcatalyst such ascharcoal (carbon) may also removeodors and gaseous pollutants such asvolatile organic compounds orozone.[1] Air filters are used in applications whereair quality is important, notably in buildingventilation systems and in engines.
Some buildings, as well asaircraft and other human-made environments (e.g.,satellites, andSpace Shuttles) use foam, pleated paper, or spunfiberglass filter elements. Another method,air ionizers, use fibers or elements with astatic electric charge, which attract dust particles. The air intakes ofinternal combustion engines andair compressors tend to use eitherpaper,foam, orcotton filters. Oil bath filters have fallen out of favour aside from niche uses. The technology ofair intake filters ofgas turbines has improved significantly in recent years, due to improvements in theaerodynamics andfluid dynamics of the air-compressor part of the gas turbines.
Do-it-yourself air cleaner are low-cost alternative to commercial portable air cleaners.[2]
High efficiency particulate arrester (HEPA),[3][4] originally called high-efficiency particulate absorber but also sometimes called high-efficiency particulate arresting or high-efficiency particulate arrestance, is a type of air filter. Filters meeting the HEPA standard have many applications, including use in clean rooms for IC fabrication, medical facilities, automobiles, aircraft and homes. The filter must satisfy certain standards of efficiency such as those set by theUnited States Department of Energy (DOE).
Varying standards define what qualifies as a HEPA filter. The two most common standards require that an air filter must remove (from the air that passes through) 99.95% (European Standard)[5] or 99.97% (ASME standard)[6] of particles that have a size greater than or equal to 0.3 μm.
Filters containing anadsorbent orcatalyst such ascharcoal (carbon) may also removeodors and gaseous pollutants such asvolatile organic compounds orozone.[7]
The cabin air filter, also known in the United Kingdom as a pollen filter, is typically a pleated-paper filter that is placed in the outside-air intake for the vehicle's passenger compartment. Some of these filters are rectangular and similar in shape to the engine air filter. Others are uniquely shaped to fit the available space of particular vehicles' outside-air intakes.
The first automaker to include a disposable filter to keep the ventilation system clean was theNash Motors "Weather Eye", introduced in 1940.[8]
A reusable heater core filter was available as an optional accessory onStudebaker models beginning in 1959, includingStudebaker Lark automobiles (1959-1966),Studebaker Gran Turismo Hawk automobiles (1962-1964) andStudebaker Champ trucks (1960-1964). The filter was an aluminum frame containing an aluminum mesh and was located directly above the heater core. The filter was removed and installed from the engine compartment through a slot in the firewall. A long, thin rubber seal plugged the slot when the filter was installed. The filter could be vacuumed and washed prior to installation.
Clogged or dirty cabin air filters can significantly reduce airflow from the cabin vents, as well as introduceallergens into the cabin air stream. Since the cabin air temperature depends upon the flow rate of the air passing through theheater core, theevaporator, or both, clogged filters can greatly reduce the effectiveness and performance of the vehicle'sair conditioning and heating systems.[9]
Some cabin air filters perform poorly, and some cabin air filter manufacturers do not print aminimum efficiency reporting value (MERV) filter rating on their cabin air filters.[citation needed]
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The combustion air filter prevents abrasive particulate matter from entering the engine's cylinders, where it would cause mechanical wear and oil contamination.
Mostfuel injected vehicles use a pleated paper filter element in the form of a flat panel. This filter is usually placed inside a plastic box connected to thethrottle body with duct work. Older vehicles that usecarburetors or throttle body fuel injection typically use a cylindrical air filter, usually between 100 millimetres (4 in) and 400 millimetres (16 in) in diameter. This is positioned above or beside the carburetor or throttle body, usually in a metal or plastic container which may incorporate ducting to provide cool and/or warm inlet air, and secured with a metal or plastic lid. The overall unit (filter and housing together) is called theair cleaner.
Pleated paper filter elements are the nearly exclusive choice forautomobile engine air cleaners, because they are efficient, easy to service, and cost-effective. The "paper" term is somewhat misleading, as the filter media are considerably different frompapers used for writing or packaging, etc. There is a persistent belief amongtuners, fomented by advertising for aftermarket non-paper replacement filters, that paper filters flow poorly and thus restrict engine performance. In fact, as long as a pleated-paper filter is sized appropriately for the airflow volumes encountered in a particular application, such filters present only trivial restriction to flow until the filter has become significantly clogged with dirt. Construction equipment engines also use this. The reason is that the paper is bent in zig-zag shape, and the total area of the paper is very large, in the range of 50 times of the air opening.[citation needed]
Oil-wettedpolyurethane foam elements are used in some aftermarket replacement automobile air filters. Foam was in the past widely used in air cleaners on small engines onlawnmowers and other power equipment, but automotive-type paper filter elements have largely supplanted oil-wetted foam in these applications. Foam filters are still commonly used onair compressors for air tools up to 5 horsepower (3.7 kW). Depending on the grade and thickness of foam employed, an oil-wetted foam filter element can offer minimal airflow restriction or very high dirt capacity, the latter property making foam filters a popular choice in off-road rallying and other motorsport applications where high levels ofdust will be encountered. Due to the way dust is captured on foam filters, large amounts may be trapped without measurable change in airflow restriction.[citation needed]
Oiledcottongauze is employed in a growing number of aftermarket automotive air filters marketed as high-performance items. In the past, cotton gauze saw limited use in original-equipment automotive air filters. However, since the introduction of theAbarth SS versions, the Fiat subsidiary supplies cotton gauze air filters as OE filters.
Stainless steel mesh is another example of medium which allow more air to pass through. Stainless steel mesh comes with different mesh counts, offering different filtration standards. In an extreme modified engine lacking in space for a cone based air filter, some will opt to install a simple stainless steel mesh over the turbo to ensure no particles enter the engine via the turbo.
An oil bath air cleaner consists of asump containing a pool of oil, and an insert which is filled with fiber, mesh, foam, or another coarse filter media. The cleaner removes particles by adhering them to the oil-soaked filter media rather than traditional filtration, the openings in the filter media are much larger than the particles that are to be filtered. When the cleaner is assembled, the media-containing body of the insert sits a short distance above the surface of the oil pool. The rim of the insert overlaps the rim of the sump. This arrangement forms alabyrinthine path through which the air must travel in a series of U-turns: up through the gap between the rims of the insert and the sump, down through the gap between the outer wall of the insert and the inner wall of the sump, and up through the filter media in the body of the insert. This U-turn takes the air at high velocity across the surface of the oil pool. Larger and heavier dust and dirt particles in the air cannot make the turn due to theirinertia, so they fall into the oil and settle to the bottom of the base bowl. Lighter and smaller particles stick to the filtration media in the insert, which is wetted by oil droplets aspirated there into by normal airflow. The constant aspiration of oil onto the filter media slowly carries most of the finer trapped particles downward and the oil drips back into the reservoir where the particles accumulate.
Oil bath air cleaners were very widely used in automotive andsmall engine applications until the widespread industry adoption of the paper filter in the early 1960s. Such cleaners are still used in off-road equipment where very high levels of dust are encountered, for oil bath air cleaners can sequester a great deal of dirt relative to their overall size without loss of filtration efficiency or airflow. However, the liquid oil makes cleaning and servicing such air cleaners messy and inconvenient, they must be relatively large to avoid excessive restriction at high airflow rates, and they tend to increaseexhaust emissions of unburnedhydrocarbons due to oil aspiration when used on spark-ignition engines.[citation needed]
In the early 20th century (about 1900 to 1930), water bath air cleaners were used in some applications (cars, trucks, tractors, and portable and stationary engines). They worked on roughly the same principles asoil bath air cleaners. For example, the originalFordson tractor had a water bath air cleaner. By the 1940s, oil bath designs had displaced water bath designs because of better filtering performance.[10]
Bulk solids handling involves the transport of solids (mechanical transport, pneumatic transport) which may be in a powder form. Many industries are handling bulk solids (mining industries, chemical industries, food industries) which requires the treatment of air streams escaping the process so that fine particles are not emitted, for regulatory reasons or economical reasons (loss of materials). As a consequence, air filters are positioned at many places in the process, especially at the reception of pneumatic conveying lines[11] where the quantity of air is important and the load in fine particle quite important. Filters can also be placed at any point of air exchange in the process to avoid that pollutants enter the process, which is particularly true in pharmaceuticals and food industries. The physical phenomena involved in catching particles with a filter are mainly inertial and diffusional[12]
Under European normalization standards EN 779, the following filter classes were recognized:
| Usage | Class | Performance | Performance test | Particulate size approaching 100% retention | Test Standard |
|---|---|---|---|---|---|
| Coarse filters (used as Primary) | G1 | 65% | Average value | >5 μm | BS EN779 |
| G2 | 65–80% | Average value | >5 μm | BS EN779 | |
| G3 | 80–90% | Average value | >5 μm | BS EN779 | |
| G4 | 90%– | Average value | >5 μm | BS EN779 | |
| Fine filters (used as Secondary) | M5 | 40–60% | Average value | >5 μm | BS EN779 |
| M6 | 60–80% | Average value | >2 μm | BS EN779 | |
| F7 | 80–90% | Average value | >2 μm | BS EN779 | |
| F8 | 90–95% | Average value | >1 μm | BS EN779 | |
| F9 | 95%– | Average value | >1 μm | BS EN779 | |
| Semi HEPA | E10 | 85% | Minimum value | >1 μm | BS EN1822 |
| E11 | 95% | Minimum value | >0.5 μm | BS EN1822 | |
| E12 | 99.5% | Minimum value | >0.5 μm | BS EN1822 | |
| HEPA | H13 | 99.95% | Minimum value | >0.3 μm | BS EN1822 |
| H14 | 99.995% | Minimum value | >0.3 μm | BS EN1822 | |
| ULPA | U15 | 99.9995% | Minimum value | >0.3 μm | BS EN1822 |
| U16 | 99.99995% | Minimum value | >0.3 μm | BS EN1822 | |
| U17 | 99.999995% | Minimum value | >0.3 μm | BS EN1822 |
European standard EN 779, on which the above table is based, remained in effect from 2012 to mid-2018, when it was replaced by ISO 16890.[13]
