Air filter
From Wikipedia, the free encyclopedia
An air filter is a device which removes contaminants, often solid particulates such as dust, pollen, mold, and bacteria from air. Air filters are used in application where air quality is important, notably in building ventilation systems and in engines, such as internal combustion engines, gas compressors, diving air compressors, gas turbines and others.
Some buildings, as well as 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. The technology of air intake filters of gas turbines has improved significantly in recent years, due to improvements in the aerodynamics and fluid-dynamics of the air-compressor part of the Gas Turbines.
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[edit] Types of air filters
There are four main types of mechanical air filters: paper, foam, synthetics and cotton. Though all have varying degrees of practicality, it is generally recognized that cotton air filters are best because of their durability and technological advantages.
An example of pleated type air filters designed for in-duct use with home heating, ventilation, and air conditioning (HVAC) systems. The quality of the air filters in use affects the Indoor Air Quality greatly. Regarding Indoor Air Quality, the Department of Energy's Energy Efficiency and Renewable Energy section wrote "[Air] Filtration should have a Minimum Efficiency Reporting Value (MERV) of 13 as determined by ASHRAE 5.2.2-1999." [1] There are a variety of different types of air conditioner filters available. Many are inexpensive and not very efficient. Some options are panel, pleated, electrostatic, HEPA, electronic and media. ASHRAE recommends (MERV 6 or higher) air filters to lower the amounts of pollen, mold and dust that reaches the wet cooling coils in air conditioning systems. Wet coils with lots of pollen and dust can allow mold colonies to grow.
[edit] Paper
Paper filters are generally used in automobiles because they are inexpensive and disposable. Due to the characteristics of paper, good air-flow is sacrificed for good filtration.
[edit] Synthetics
Polyester fiber is commonly used to make web formations used for air filtration. Polyester has a high temperature rating (about 250°F)and is widely used in commercial, industrial or residential applications. Polyester 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 chemical resistance. Tiny synthetic fibers knows as micro-fibers are used in many types of HEPA (High Efficiency Particulate Air Filter) filters.
Many in-duct filters for home forced air heating and air conditioning systems are made from spun fiberglass fibers. These filters are inexpensive, disposable, 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 expensive and offer more resistance to the air. They also become more quickly "loaded" with contaminants and dust. They are considerably less expensive than pleated-paper filters for the same size.
[edit] Foam
Foam is used in many performance air filters, due the unique properties it offers. It is as free flowing as cotton gauze, but has a much higher dirt retention level. This makes foam filters a popular choice in rallying and other motorsport applications. As with cotton gauze, a dirt retention additive can be used to give higher levels of filtration along with increased air flow.
[edit] Cotton
Cotton is generally regarded as the best material for air filters because of its excellent filtration and high airflow. This is because the "holes" in cotton are much looser than paper, giving high airflow, but because of characteristics such as interception, impaction, and diffusion, cotton fiber filters can block particles much smaller than the actual "holes." High performance air filters use oiled layers of cotton gauze. Dust particles held suspended in the cotton gauze actually "becomes" part of the filter medium to all the more increase filtration without obstructing airflow to the engine, thereby outlasting paper filters in service life and performance. Although cotton filters are pricey, they can be cleaned and re-oiled at a long service interval of 10,000 miles, by which time a paper filter may have already been replaced on the vehicle.
One limitation of cotton gauze is the inability to produce different materials for different environments. The same material would be used for dusty desert environments, through to clean race tracks and snowy cold countries. Due to this, foam is fast overtaking cotton gauze as the preferred material for performance filters.
[edit] Oil Bath
Typically, rows of brushes are arranged in a cylinder shape very similar to the cylindric automotive air filters of today. The ends of the bristles are immersed in a resevoir of motor oil. Then, by capillary action, the bristles become coated with a thin layer of oil that attracts particles. When the bristles become coated with too many particles and can no longer effectively attract more, the bristles are cleaned and the oil is changed if needed.
[edit] Automotive air filters
Two main types of air filters are used in automobiles: the combustion air filter, and the cabin air filter. The combustion air filter prevents particulate matter from entering the engine's combustion chambers. This filter is commonly changed at oil-change time, but may be changed at longer or shorter intervals, depending on operating conditions of the vehicle.
Most modern, fuel-injected vehicles use a flat panel filter. This filter is usually placed inside a plastic box connected to the throttle body with a large hose. Occasionally these are replaced with a conical filter and cold air intake which, in most cases, includes a heat shield to protect the intake air from underhood temperatures, along with tubing to improve airflow into the throttle body. In many cases an improved air-intake system can produce an increase in power and efficiency.
Older vehicles that use carburetors or throttle body fuel injection typically use a cylindrical air filter, usually a few inches high and approximately a foot in diameter (the most common version is 14 inches in diameter and 3 inches tall). This is positioned above the carburetor or throttle body and secured with a metal lid.
The cabin air 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 combustion air filter. Others, such as in the Ford Taurus, are roughly triangular in shape, so as to fit in the narrow curving space of the outside-air intake. Cabin air filter replacement has recently become an opportunity for increased billings and profits at professional oil-change locations. Improper removal and reinstallation of this filter can lead to water leaks (by misalignment of the water diverter or seals) and in rare instances, a cracked windshield. This filter is often overlooked and clogged or dirty cabin air filters can significantly reduce airflow from the cabin vents, as well as introduce allergens into the cabin air stream. Periodic, proper replacement will increase cooling and heating efficiency. A filter should be replaced annually to ensure optimal efficiency. Filters can be changed by yourself or at an automotive service center.
[edit] References
[edit] See also
- Air purifier
- Clean Air Delivery Rate
- Cyclonic separation
- Pneumatic Air Filter
- Respirator
[edit] External links
- Engineered Systems Magazine Filter Selection: A Standard Procedure (June 2000) MERV designations (Minimum Efficiency Reporting Value)
- ASHRAE At least a MERV 6 filter upstream of any wet-surface devices, such as air conditioning coils
- U.S.E.P.A. Indoor Air Quality, Federal Interagency Committee
- Furnace Filter
