Contrary to popular perception, Air Conditioning (AC) is not about adding cool air to the room, but more about drawing heat away from it. The end result is a space with significantly less heat, which makes it feel cooler to occupants. Air conditioning takes advantage of the effects of evaporation, much like a swab of alcohol makes a person's skin feel cooler as the liquid evaporates. The alcohol doesn't lower the person's skin temperature, but rather draws away heat from the air as it turns to a gas.
Air conditioning units contain a special chemical called a refrigerant, which has the unique ability to change from a gas to a liquid in a short amount of time. A refrigerant called freon is commonly used in AC units, although there are other commercial refrigerants available. The refrigerant is pumped into the unit at the factory, along with a small amount of lubricating oil for the compressor.
The parts of a typical AC unit usually form a closed system consisting of a compressor, a condenser, an expansion valve and a thermostat. Motorized fans help to circulate the conditioned air, while thin metal fins allow heat to dissipate quickly. The heaviest part of a typical air conditioning is often the compressor, since it must be strong enough to withstand a significant amount of pressure.
The process of cooling an area begins with the refrigerant entering the compressor, usually located at the bottom of the unit. At this point, the refrigerant is a cool gas. As the gas enters the compressor's inner chamber, the compressor squeezes the refrigerant and the gas becomes a very hot gas under high pressure. This hot gas goes through a series of condensing coils placed outside of the room being cooled. The heat dissipates into the outside air, much like a car's radiator dissipates heat from the engine coolant. Once the refrigerant reaches the end of these coils, it is significantly cooler and in liquid form.
This liquid is still under high pressure, like the contents of an aerosol can. In the case of air conditioning, the liquid refrigerant is forced through a very tiny opening called an expansion valve. The liquid refrigerant comes out of the other end of the expansion a very small amount at a time. Because the refrigerant evaporates at a much lower temperature than water, it begins to evaporate while traveling through another set of coils. It is this evaporation action that draws heat out of the surrounding air, including the air contained in the room. The unit's fan blows across metal fins placed over these coils, causing the sensation of cooling in the room.
At this point, the liquid refrigerant has become a cold gas again and re-enters the compressor, where the entire process begins again until a thermostat registers a specific temperature and shuts off the compressor. When the room warms up, the thermostat senses the added heat and the compressor kicks back on to create more of the hot pressurized gas. At some point, the temperature of the room may equal the cooling power of the air conditioner and the compressor will shut off again. The air conditioning systems of most houses do benefit from energy-saving steps such as using window shades and keeping doors closed, since they don't have to work as hard to keep the room at an acceptable level of cool.