[Lawrence Coppari]

Reason why charging with compressed air will not cool much is because there is no phase change. When the refrigerant goes from one phase to another it needs a lot of heat or to expel a lot of heat. Air at reasonable pressures will not do that. You do get some cooling by expanding air. Look at the exhaust on a jack hammer. You can see it because it is quite cool. Gases cool when expanded but the change in energy is nothing like what happens when there is a phase change. All the refrigerant does is move the heat from the cabin in your car to the air passing through the condenser that is in the front of your car. It is merely a transfer medium so you want it carry as much energy (heat) as it possibly can. It can store a lot of energy due to its phase change. For instance to change saturated liquid freon 12 from 10 F to 20 F with no change to vapor, about 2.2 Btu are needed per pound of the liquid. To change the liquid freon R12 from a liquid to a vapor at 15 F(without any temperature change), you need to supply about 67 Btu per pound. That is why is moves so much heat.