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Air conditioning

Note: in the broadest sense, "air conditioning" can refer to any form of "heating, ventilation, and air-conditioning." This article is specifically about the use of refrigeration for this purpose.

An air conditioner (often abbreviated to AC in the United States and air-con in Australia) is an appliance or mechanism designed to extract heat from a humanly occupied space air temperature using a refrigeration cycle. To many people, air conditioners are symbolic of the Western world.

An earlier form of air conditioning was invented in Persia (Iran) thousands of years ago in the form of wind shafts on the roof, which caught the wind and passed it through water and blew the cooled air into the building [1] (http://www.metafilter.com/comments.mefi/26911). The electrical version of air conditioning was invented by Willis Haviland Carrier (18761950) around 1902 to control temperature and humidity for improved manufacturing process control. Later, expensive air conditioning systems were applied to increase productivity in the workplace. Later still, air conditioning use was expanded to improve comfort in homes and automobiles.

Types of air conditioning

Refrigeration cycle

In the refrigeration cycle, a heat pump pumps heat from a lower temperature heat source into a higher temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it pumps the heat out of the interior into the room in which it stands.

The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine's crankshaft. Since evaporation absorbs heat, and condensation releases it, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the hot compartment (the evaporator coil), where the low pressure causes it to evaporate into a vapour, taking heat with it. In the other compartment (the heat exchanger), the refrigerant vapour is compressed, forcing it to condense into a liquid, releasing the heat.


Refigeration air conditioning equipment usually reduces the humidity of the air processed by the system. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.


"Freon" is a trade name for a family of chlorofluorocarbon (CFC) refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine bearing refrigerants reach the upper atmosphere when they escape. The chemistry is poorly understood but general consensus seems to be that CFCs break up in the stratosphere due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle forming a stable molecule.

Evaporation coolers

The aforementioned Persian cooling systems were evaporation coolers. In very dry climates, such affectionately called "swamp coolers" are popular for improving comfort during hot weather. The evaporative cooler is a device that draws outside air through a wet pad. The sensible heat of the incoming air, as measured by a dry bulb thermometer, is reduced. The total heat (sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite comfortable. These coolers cost less and are mechanically simple to understand and maintain. An early type of cooler, using ice for a further effect, was patented by John Gorrie of Apalachicola, FL in 1842, who used the device to cool the patients of his malaria hospital.


Air conditioner equipment power in the U.S. is often described in terms of "tons". A "ton" is defined as the cooling power of one ton US (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This is equal to 12,000 BTU per hour, or 3516.85284 watts (http://physics.nist.gov/Pubs/SP811/appenB9.html). Residential central refrigeration cycle air conditioners can be 1 to 5 tons (3 to 20 kW).

The use of electric/compressive air conditioning puts a major demand on the nation's electrical power grid in warm weather, when most units are operating under heavy load. In the aftermath of the 2003 North America blackout locals were asked to keep their air conditioning off. During peak demand, additional power plants must often be brought online, usually natural gas fired plants because of their rapid startup. A 1995 study of various utility studies of residential air conditioning concluded that the average air conditioner wasted 40% of the input energy. This energy is lost, ironically, in the form of heat, which must be pumped out.

There is a huge opportunity to reduce the need for new power plants and to conserve energy. In an automobile the A/C system will use around 5 hp (4 kW) of the engine's power.


Insulation reduces the required power of the air conditioning system. Thick walls, reflective roofing materical, curtains and trees next to building also cut down on system and energy requirements.

See also


  • Carrier, H. W., U.S. Patent 808897 --- "Apparatus for treating air" -- Janurary 2, 1906

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