I was thinking the other day about how air conditioners and regrigerators work. I get that the compressor pump compresses the freon, which gets cold, absorbs heat, expands, and radiates the heat on the other side via the radiator.
What I don’t get is why something would get cold when you compress it. It seems to me that compressing something would be adding energy to it, just like sticking it above a fire or zapping it with electricity. So why does it get cold when you’re adding energy?
Actually, you’ve got it backwards - it’s the expansion that cools the fluid; compression heats it. Think of how hot an air compressor gets when it’s running. Same thing here.
As usual, HowStuffWorks has a great answer.
Doh! I feel dumb now. 
(Can I post this before someone links to How Stuff Works? Probably not…)
It doesn’t get cold when you’re adding energy, the refrigerant gets cold when it is allowed to expand.
Air conditioners work roughly along the lines of the ideal gas law:
PV=nRT, where:[ul]P=pressure
V=volume
n=number of gas molecules present
R=a constant
T=temperature (degrees Kelvin for the Ideal Gas Law, but I doubt things are this linear here.)[/ul]
The air conditioner has a compressor that pushes the refigerant through a closed loop with a restriction in it. Upstream of the restriction, the refrigerant is compressed, which heats it. As it goes through this area, it passes through a radiator (the condensor), where it dissipates heat and cools into a liquid. It then goes through the restrictor. Because of the reduced pressure on the downstream side, the refrigerant cools. It then passes through another radiator (the evaporator) where it absorbs heat and evaporates into a gas. From here, it goes on to the compressor, then back to the condensor.
by the time I got to this it was already answered.