Handbook 752 Air Conditioning and Climate Control Expansion and If a pressurized liquid is released into an area of lower pressure, it will evaporate into a gas and absorb heat from that area (assuming the temperature is above the boiling point of the liquid). This is the principle that causes the spray from an aerosol can to feel cold to your skin. Likewise, a liquid that changes from liquid to vapor at a low temperature (e.g. alcohol) will feel cool on the skin as it evaporates (phase change). This is the situation you feel when you get out of a swimming pool or shower. The evaporating water on your skin absorbs heat from your body even though the air temperature is quite high.
Evaporation The device that regulates pressure in an A/C system is the expansion valve. The area where heat transfer takes place is the evaporator (heat exchanger). Compression and Condensation After the gaseous refrigerant absorbs heat, the system pressurizes it to change it back into a liquid. As this happens, the gas gives off much of the heat that was absorbed. This is what the compressor and condenser do in the A/C system. In order for the gas to actually give up heat while in the condenser (heat exchanger), it must be hotter than the air around it. The compressor makes this possible by increasing the line pressure, and therefore, the temperature of the refrigerant gas.
14 Technical Training