Types of Air Filters: A Comprehensive Guide

Air filters are an essential part of many industries, from building ventilation systems to engines. They are used to remove solid particles such as dust, pollen, mold, bacteria, and other pollutants from the air. Different types of air filters are available, each with its own advantages and disadvantages. Also known as flat panel filters, fiberglass air filters consist of strands of glass spun together and reinforced by a metal grid.

Washable electrostatic air filters are based on an electrostatic filter medium, usually made of woven polypropylene or polyester. Particles suspended in the air encounter friction when they pass through the filter media, becoming charged and attracted to the inner layers of the filter media. The filter media retains particles by static electricity until they are washed to remove them. Its electrostatic property remains constant over time and is not affected by washing. Disposable fiberglass air filters are the most common type.

The layered fiberglass fibers are placed on top of each other to form the filter media and are typically reinforced with a metal grid that supports the glass fiber to prevent failure and collapse. These filters are similar to fiberglass filters, but generally have greater air flow resistance and a superior ability to stop dust. Particulate air filters are composed of fibrous or porous materials that remove solid particles such as dust, pollen, mold and bacteria from the air. Filters that contain an adsorbent or catalyst, such as charcoal (carbon), can also remove odors and gaseous pollutants, such as volatile organic compounds or ozone. Air filters are used in applications where air quality is important, especially in building ventilation systems and engines. Another method, air ionizers, use fibers or elements with a static electric charge, which attract dust particles.

The air intakes of internal combustion engines and air compressors tend to use paper, foam or cotton filters. Oil bath filters have fallen into disuse apart from specialized uses. The technology of gas turbine air intake filters has improved significantly in recent years, due to improvements in the aerodynamics and fluid dynamics of the air compressor part of gas turbines. The different standards define what qualifies as a HEPA filter. The two most common standards require an air filter to remove (from passing air) 99.95% (European standard) or 99.97% (ASME standard) of particles having a size greater than or equal to 0.3 μm.The cabin air filter is usually a pleated paper filter that is placed in the outside air intake of 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 the outside air intakes of certain vehicles. A reusable heating core filter was available as an optional accessory on Studebaker models beginning in 1959, including Studebaker Lark cars (1959-196), Studebaker Gran Turismo Hawk cars (1962-196) and Studebaker Champ trucks (1960-196). The filter was an aluminum frame that contained 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 groove when the filter was installed. The filter can be vacuumed and washed before installation. Some cabin air filters malfunction and some cabin air filter manufacturers do not print a minimum efficiency report value (MERV) filter rating on their cabin air filters. Combustion air filter prevents abrasive particles from entering engine cylinders, leading to mechanical wear and oil contamination. Oil-moistened polyurethane foam elements are used in some car replacement air filters. In the past, foam was widely used in air filters in small engines, lawn mowers and other electrical equipment, but automotive-type paper filter elements have largely replaced oil-moistened foam in these applications.

Foam filters are still commonly used in air compressors for pneumatic tools up to 5 Hp. Depending on the grade and thickness of the foam employed, an oil-moistened foam filter element can offer minimal airflow restriction or a very high dirt capacity, the latter property making foam filters a popular choice in off-road rallies and other motorsports applications where high levels of dust will be found. Because of the way dust is captured in foam filters, large amounts can be trapped without measurable changes in airflow restriction. Oiled cotton gauze is used in a growing number of aftermarket automotive air filters that are marketed as high-performance items. In the past, cotton gauze had limited use in original equipment automotive air filters. However, since the introduction of the Abarth SS versions, the Fiat subsidiary has been supplying cotton gauze air filters as OE filters. Stainless steel mesh is another example of a medium that allows more air to pass through.

The stainless steel mesh comes with different mesh counts, offering different filtration standards. In an extremely modified engine that lacks space for a cone-based air filter, some will choose to install a simple stainless steel mesh over the turbo to ensure that no particles enter the engine through the turbo. In the early 20th century (around 1900 to 1930), water bath air filters were used in some applications (cars, trucks, tractors, and portable and stationary engines). They worked on roughly the same principles as oil bath air purifiers. For example, the original Fordson tractor had an air filter in a water bath.

By the 1940s, oil bath designs had displaced water bath designs due to better filtering performance. Handling bulk solids involves transporting solids (mechanical conveying, pneumatic conveying) that may be in powder form. Many industries are handling bulk solids (mining industries, chemical industries, food industries) that require treatment of air streams escaping the process so that fine particles are not emitted for regulatory or economic reasons (loss of materials). As a result, air filters are placed in many places in the process, especially in the reception of pneumatic conveying lines where the amount of air is important and the loading of fine particles is very important. Filters can also be placed at any air exchange point in the process to prevent contaminants from entering the process which is particularly true in pharmaceutical and food industries. The physical phenomena involved in capturing particles must be taken into account when selecting an appropriate type of filter for any given application.