How does air conditioning work?

An air conditioner is a device which uses a special type of substance which readily changes from its normal gas state to a liquid one (Typically freon or another refrigerant such as R-20 [chloroform] or R-134 [tetrafluoroethane]). The gas is contained in a closed circuit of pipes connected to a pump. The pump compresses the gas so hard that the pressure is great enough for it to turn into a liquid. In doing this the gas/liquid has got hot (If you try to compress a gas, it will almost always get hot - think of a bike pump when you pump up a tyre (tire), the greater the pressure in the tyre, the hotter the pump gets).

Now the hot liquid travels round a set of pipes which allow the heat to escape. Next the liquid under pressure passes through a valve into a pipe where the pressure is much lower, and the liquid evaporates back into its gas state. In doing this it needs to take in heat from its surroundings, thus making the pipes colder. The gas now gets back to the pump and the whole cycle starts again.

The pipes are usually arranged so that there are fans to blow air over both the hot part of the pipes and the cold part. The hot part is frequently put outside the house, and the cold part inside. This is called either a 'split pack', or a 'twin pack' depending on which country you are in There are also portable versions which have both parts in the same box, with the cold air blowing out of the front, and the hot air blowing out of a flexible hose which you put put outside the room to allow the hot air to dissipate.