Air Conditioning 101
How Air Conditioning Works
 Air conditioners use a gas called refrigerant. The refrigerant is special because like water it can change state from a liquid to a vapor, and then back to a liquid.
What creates the cold energy is the fact that when the refrigerant evaporates, it becomes extremely cold. Let us back up one step start the cycle outside first. When the refrigerant leaves the outdoor unit (often called the condenser or the air conditioner. You may even hear the word compressor, which is somewhat true; us trades people know what you’re talking about!), it is compressed to a much higher pressure (about 200 to almost 400 PSI, depending on which type of refrigerant and how hot it is in the house and outdoors). At this high pressure, it leaves the condenser as a liquid, and is sent (it has no choice) to the indoor AC coil (usually on top of the furnace, or the air handler). For any technical people out there, you noticed I skipped over the condensing cycle, when the super hot gas is cooled down, but this explanation is meant for the beginner!
When it reaches the indoor coil, it passes through a metering device that closely regulates how much liquid passes through it. Its like a nozzle on your garden hose, sort of. Once past the metering device (there are several types: TXV is the most popular, and Fixed Orifice is also very common), the refrigerant flashes into a vapor (not quite without liquid at this point, but well stick to the basic premise here). Because the refrigerant is not too far above freezing (32F) at this point, the warm and muggy air from the house drops off the sensible and latent energy. These are technical words for the heat we feel (sensible) and the humidity (latent) in the air.
Once the refrigerant leaves the indoor coil, it now is a vapor. This pipe is called the suction line and will be the larger of the two lines if you even wondered which pipe was which. It will also be insulated, because it is very cold compared to the surrounding air temperatures and could quite easily sweat, and make a mess or cause damage to finished basement ceilings. The suction line heads back to the air conditioner so that the process can happen all over again. The smaller line in the liquid line, and its usually warm to the touch when the AC system is running.
Many factors have to come together for the AC system to operate as efficiently as it was designed to. Clean indoor and outdoor coils, a properly evacuated (no non-condensables in the refrigerant) piping system, proper air flow with a clean blower wheel, and a properly controlled zoning system (if applicable) are all important factors for your AC to work properly.
As a closing thought on this subject, Air Conditioning has the magical illusion that its producing cold air, but in reality what it is actually doing is removing the heat from the living space and rejecting it outside. The cold air you sense is a result of the hot and muggy air being taken away!
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