An air conditioner (AC) is an appliance or mechanism designed to modify air temperature, generally cooling it down. It was invented by Willis Haviland Carrier (1876 - 1950) around 1902.

Air conditioning units may also reduce the humidity of the air processed by the system. Since drier air aids evaporative cooling of people, this improves the comfort provided.

From a mechanical engineering viewpoint, a modern air conditioner is a specialized form of a heat engine. It is designed for maximum efficiency when work is being applied to cool a living or storage volume by pumping heat from a low temperature heat source to a higher temperature heat sink. In electric units this is typically done by using an electric motor to drive a compressor. The compressor compresses a working fluid. During compression the working fluid is heated to a higher temperature than the higher temperature heat sink. The working fluid is circulated to outside coils where it cools to ambient air temperature. The working fluid is then expanded (which lowers the temperature of the fluid via adiabatic cooling) and circulated through an interior coil. The interior coil absorbs heat energy from the interior air and transfers it to the working fluid which then proceeds to the compressor to repeat the cycle.

In the past, "swamp cooling" was popular for improving comfort during hot weather. It is a simpler system where air is blown over a moist mesh to cool the air evaporatively, and it worked best in hot, dry climates.

Freon was commonly used as a working fluid due to its superior heat conducting and capacitance properties. Unfortunately, it has become very controversial as evidence has accumulated that it reachs the upper atmosphere when it escapes. The chemistry is poorly understood but consensus seems to have been growing that fluorocarbons act as a catalyst in the presence of ultraviolet radiation and may be doing severe damage to the ozone layer.

Air conditioner size is often measured in "tons". A "ton" is the ability to remove 12,000 BTUs per hour. Residential central air conditioners can be 1-5 tons.

The sale of the appliance depends on seasons and weather. The sale usually lifts a little or a lot in summer and particularly during heat waves.

The use of compressive AC is a major draw on electrical supplies in warm climates where there are many units installed.

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