Air filters are used in applications where air
quality is important, notably in building ventilation systems and in
engines, as internal combustion engines, gas compressors, diving air
compressors, gas turbines and others. An air filter is the
which removes solid particulates such as dust, pollen, mold, and
bacteria from air.
Some buildings, aircraft and other man-made environments (e.g.,
satellites and space shuttles) use foam, pleated paper, or spun fiberglass
filter elements. Another method uses fibers or elements with a static
electric charge, which attract dust particles. The air intakes of internal
combustion engines and compressors tend to use either paper, foam, or cotton
filters. Oil bath filters have fallen out of favor.
Climate control air filters
There are four main types of mechanical air filter media: paper, foam,
synthetics, and cotton.
Air filter is found in all forced-air heating, ventilation, and air
conditioning systems. The efficacy of the air filters in such systems
significantly affects the Indoor Air Quality. For example, the US Department
of Energy recommends air filters with a Minimum Efficiency Reporting Value (MERV)
of 13 as determined by the ASHRAE 5.2.2-1999 test protocol, and LEED advises
builders similarly. And ASHRAE recommends MERV 6 or higher air filters to
control the amounts of pollen, mold, and dust that reach the wet evaporator
coils in air conditioning systems. Wet coils contaminated with high levels
of pollen and dust can allow mold colonies to grow.
There are different types of HVAC filters available in
car parts. Many are inexpensive and not very efficient. Many in-duct filters for
home forced air heating and air conditioning systems are made from
loosely-spun fiberglass. These filters are disposable, inexpensive, and come
in various densities and sizes. Less-dense filters allow for higher airflow,
but do not remove as much dust. Higher density filters remove more
particles, but are more reconstructive of airflow. They also become more
quickly loaded with dirt.
In polyester and glass fibers are commonly used to make air filters. Both
materials have high temperature ratings of at least 120°C , and are
widely used in commercial, industrial and residential applications.
Polyester and glass fibers can be blended with cotton or other fibers to
produce a wide range of performance characteristics. In some cases
Polypropylene, which has a lower temperature tolerance, is used to enhance
Different engineering associations and test organizations issue standards
regarding the flammability of air filters for use in buildings, homes, car,
truck ,auto machine and other habitable spaces. The Underwriters
Laboratories (UL) Standard 900 for air filter flammability , for example,
classifies filters as Class 1 or Class 2. When attacked by flame, clean
class 1 air filters do not contribute fuel and emit only negligible amounts
of smoke. Clean class 2 air filters attacked by flame burn moderately or
emit moderate amounts of smoke, or both.
Automotive cabin air filters
The cabin air filters are typically a pleated-paper filters those are placed
in the outside-air intake for the vehicle's passenger compartment. Some of
these filters are rectangular and similar in shape to the combustion air
filter. Others are uniquely shaped to fit the available space of particular
vehicles' outside-air intakes. Being a relatively recent addition to
automobile equipment, these filter are often overlooked. Clogged or dirty
cabin air filters can significantly reduce airflow from the cabin vents, as
well as introduce allergens into the cabin air stream.
Paper filter elements are the nearly exclusive choice for automobile 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 from papers used for writing or packaging, etc.
There is a persistent belief amongst tuners, 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.
Internal combustion air filters
Internal combustion air filter prevents abrasive particulate matter from
entering the engine's cylinders, where it would cause mechanical wear and
In many older vehicles that use carburetors or throttle body fuel injection
typically use a cylindrical air filter, usually a few inches high and
between 6 and 16 inches in diameter. This is positioned above 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 oiled cotton gauze is employed in a small number of aftermarket
automotive air filters marketed as high-performance items.
Oil-wetted polyurethane foam elements are used in some aftermarket
replacement automobile air filters. Foam was in the past widely used in air
cleaners on small engines on lawnmowers and other power equipment, although
automotive-type paper filter car parts have largely supplanted oil-wetted
foam in these applications. 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 motor sport
applications where high levels of dust will be encountered.
Oil bath air cleaners are very widely used in automotive parts
and small-engine applications. 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 efficacy or airflow. However,
the liquid oil makes cleaning and servicing such air cleaners messy and
The oil bath air cleaner consists of a round base bowl containing a pool of
oil, and a round insert which is filled with fiber, mesh, foam, or another
coarse filter media. When the cleaner is assembled, the media-containing
body of the insert sits a short distance above the surface of the oil pool.
This arrangement forms a labyrinthine 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 base bowl, down through the gap between the outer wall of the insert
and the inner wall of the base bowl, 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 their inertia, so they fall into the oil and
settle to the bottom of the base bowl. Lighter and smaller particles are
trapped by the filtration media in the insert, which is wetted by oil
droplets aspirated there into by normal airflow.