How Does Auto Air Conditioning Work?
Car air conditioners work very much like any other device that takes heat from one area and moves it to another. Freezers and refrigerators work on the same principle, as do home air conditioners. Heat pumps do it in both directions; when it is warm they pump hot air out of a home, and when it is cold they pump it back in.
Car air conditioners are applications of the combined gas law: pressure times volume equals temperature times a constant (PV=kT, with k being the constant). If pressure goes up and the space available does not change, the temperature must also go up. Air conditioners work by changing the pressure of a gas in its coils, moving the gas to another area, removing heat and then changing the pressure back.
The bean analogy
To make things clearer, assume your car contains not too much heat but too many beans. Five people help remove the beans using a bowl called the refrigerant. The helpers are called the evaporator, the compressor, the condenser, the receiver and the expansion valve.
The evaporator sits in the passenger compartment, puts some excess beans in the bowl and passes it to the compressor. The bowl is not full enough for the beans to slosh over the edge, so the compressor adds some more beans. Then the bowl passes to the condenser, who sloshes out as many beans as possible.
The condenser then passes the bowl to the receiver, who hands it to the expansion valve. This valve removes the extra beans that the compressor put in. Now the bowl has fewer beans than when it was handed to the compressor. The expansion valve then passes the bowl back to the evaporator and the cycle begins again.
In real life
Refrigerant is a gas that fills your air conditioner system. Cold refrigerant enters your evaporator, which is a set of coils usually mounted on the front wall of the passenger compartment under the glove compartment. A fan blows hot air over the evaporator's coils, heating the refrigerant and cooling the air. The cooled air blows back out the vents into the passenger compartment, and the heated refrigerant goes to the compressor.
Your engine's crankshaft runs the compressor, a pump that raises the pressure of the refrigerant coming out of the evaporator and sends it to the condenser. More pressure, remember, means a higher temperature. Now the difference in temperature between the hot refrigerant and the outside air is enough that air blowing through the condenser's coils cools it significantly.
The refrigerant passes to a receiver, and then to the expansion valve which lowers the pressure again. Less pressure means lower temperature, and since some of the heat energy was lost in the condenser, the air that the expansion valve sends back to the evaporator is cooler than it was when the evaporator sent it to the compressor, and can be used to further cool the passenger compartment.
Air conditioners are complicated pieces of equipment. The pressures involved can make them difficult to work with, and many jurisdictions regulate the sale of refrigerant. While air conditioner maintenance should be done by professionals, vehicle operators are free to study and understand the system. Everything you learn about a car makes you a better driver, and air conditioner function is no exception.