air conditioning

[Elizabeth]

How does air conditioning work? What are the different types of unit available for the home (not commercial) and what are their advantages and disadvantages?

[KARL]

The most common types operate on precisely the same principle as a refrigerator. Simplistic description: you have two loops and pump a gas through the circuit. As the pump (compressor) compresses the refrigerant gas it turns to a liquid but in so doing it gives off a lot of heat and you let this heat dissipate into the surrounding air at one loop (outside the 'fridge or outside the room), then as the refrigerant passes along the second loop (on its way back to the compressor) it evaporates, but for this it needs to absorb some heat (as when you boil water). It gets this heat from the surroundings and thus this part of the circuit is positioned in the environment you want to cool (inside fridge or room). The gas then goes through the compressor and the whole process is repeated again. By passing air over the evaporator circuit you cool the air (in air conditioning, or within a fridge) and transfer the absorbed heat from inside to outside - it is called a heat pump principle. Some airconditioners of this type are contained in a single box, others are "split units" with the cooling coil (evaporator) inside the space and the heat discharge side (compressor circuit) outside the building. There are systems which operate on electricity alone (no refrigerant) relying on the behavior of different materials when current passes through them - most notably coolboxes. These units currently have less transfer capacity than the refrigerant type. Finally, there are so-called desert coolers which are a modern version of a very ancient principle. Water is cascaded through a duct where air also passes through (powered along by a fan) and the evaporation of the water chills the air, but also imparts humidity to it. These devices are not effective where the ambient air has high humidity levels to start with nor do they effect as big a comfort improvement generally as the first type, but they are very cheap to run and simple to maintain.

[sft42]

Just as an afterthough to the very good answer by Karl....the first building in the world to be air conditioned wasn't in the US...It was the Royal Victoria Hopital in Belfast, and it had nothing to do with bombs blowing holes in the building!