Air cleaner, also called an air purifier, is a device that removes contaminants (impurities) from the air or other gases. Solid contaminants include dust, lint, pollen, and the tiny particles in smoke. Air cleaners also remove unwanted liquid and gaseous contaminants. By removing such contaminants, air cleaners can help prevent both indoor and outdoor air pollution.
Uses.
Special types of air cleaners help eliminate pollen and dust in homes, and bringing relief to individuals who suffer from hay fever and other allergies. Department stores may use air cleaners to keep merchandise clean, and to reduce fire hazard by collecting lint or other burnable material that may be deposited in the ventilation system. Hospitals use air cleaners to prevent the spread of infection.
Without air cleaners, many industries could not operate efficiently. For example, food processing and the manufacture of electronic equipment require relatively dust-free air. Air cleaners can reduce expense by recirculating conditioned air from heating and air-conditioning systems.
Air-cleaning systems also keep exhausted contaminants from reentering the factory. Sometimes industrial dust or odors must be removed to keep the air from harming people in the area nearby. Air cleaners also protect internal-combustion engines and other machinery from excessive wear.
Types of air cleaners.
Air cleaners are often designed to remove certain types and amounts of contaminants. Some air-cleaning systems cost more to buy, operate, and maintain than others. Air cleaners are classified according to their principle of operation. The chief types of air cleaners are (1) filtration, (2) electrostatic precipitator, and (3) inertial.
Filtration air cleaners
use a dry, uncoated filter made of such materials as wool felt, cotton batting, or cellulose fiber. The filters trap solid particles floating in the air. They come in both cleanable and throwaway types. Dry filters can hold large amounts of lint. But accumulations can clog the filter pores, slowing the air flow through the filter and reducing effectiveness. The most familiar dry filters are those used in the air cleaners of automobiles and in the heating and cooling systems of homes.
Special dry filters, called HEPA (high efficiency particulate air) filters, can remove more than 99 percent of solid contaminants from the air. They contain such material as pleated cellulose-fiber paper, deep sand beds, a combination of fiberglass and wool, or a compressed fiberglass. The properties of such materials enable HEPA filters to remove from the air pollen, smoke, and tiny dust particles that other filters cannot catch.
Electrostatic precipitators
are among the most efficient air cleaners in use today. They are especially useful for removing particles of dust and smoke, but they also remove pollen and even bacteria from the air.
Some types of electrostatic precipitators are designed only for cleaning ventilating air in such places as homes, offices, hospitals, and stores. Other types of electrostatic precipitators are widely used in industry to clean air and gas.
An electrostatic precipitator consists of an ionizer, through which air passes; a cell, or collector, which removes the contaminant; and a power pack, which provides direct-current electric energy. A fan blows the contaminated air past a number of small, electrically charged wires in the ionizer. The particles receive an electric charge. The charged particles are said to be ionized (see Ion ). The ionized particles then pass to the cell, which consists of a series of metal plates. Some of the plates carry a positive charge, but others carry a negative charge. The ionized particles are attracted to the oppositely charged metal plates, because opposite charges attract each other, and like charges repel each other. The particles stick to the metal plates, which can then be cleaned.
Inertial air cleaners
use the principle of centrifugal force (see Centrifugal force). They change the direction of the air flow so that particles are thrown out of the air stream. Inertial air cleaners are primarily used in industry for the continuous removal of dust, granular material, and other contaminants.
Other types of air cleaners.
Absorber cleaners use simple absorbing agents, such as water or alkali, to remove soluble gases in various industrial processes. The gases dissolve into the absorbing agent. Adsorption cleaners also remove gases, but the gas collects on the surface of the adsorbing material, instead of dissolving. Such materials include powdered charcoal and silica gel.
Combustion cleaners burn explosive industrial gases at a high temperature to ensure that they do not become dangerously concentrated in the air. Combustion cleaners also burn gases that have an unpleasant odor. Some scrubber cleaners use water droplets in steam to wash the air. Scrubber cleaners also absorb gases or collect solid particles with wet filters or containers full of ceramic material.
Other air cleaners create ultraviolet radiation to kill harmful organisms and viruses in the air, or to transform harmful gases into harmless substances. Such cleaners must be used in combination with other filters to actually remove the contaminants.
See also Air conditioning (Cleaning the air); Air pollution; Filter; Gasoline engine (The fuel system); Ventilation.
