Thursday, December 18, 2008

How air conditioning works


The essential ingredients in an air conditioning system are a fan to blow air around, a cold surface to cool and dehumidify the air, a warm surface and a source of water vapour. In a large system there will also be a tangle of tubes to distribute the air and collect it again. Notice that the cold surface has two independent jobs to do: it is used to cool the air and it is also used to dehumidify, by condensing water from the air. This means that the air is cooled more than is necessary for temperature control, so that it must be heated again afterwards.
Air conditioning wastes energy. It also wastes space, because air has to be pumped around in quite large tubes, so that it doesn't make a rushing noise. It does, nevertheless, generate a disturbing background noise if the room is otherwise quiet. There is worse news to come: air conditioning is spreading to more and more buildings in climates that we once thought were so mild that they would be immune.

Simple air conditioning

A fan draws air from the room first through a cooling device, consisting of metal fins extending from a pipe through which cooling fluid circulates, at a rate determined by the thermostat or by the humidistat. The air next passes over a heater, usually electrical, which is energised on instructions from the room thermostat. Air conditioning has in the past been used where the climate is too hot for comfort. Cooling will increase the relative humidity of the air, so humidification is not usually built into these systems. If it is necessary, the usual method is to inject steam from electrically boiled water.

Large air conditioning systems

The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil.

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