The refrigerant in the air-conditioning system must have a low boiling point and must also be able to change its state in a continuous cycle of vaporization and condensation.
One commonly used refrigerant known as R-134-A has a boiling point of minus 26.6°C.
This is a liquid which boils well below normal room temperatures, and in vaporizing, will absorb tremendous amounts of heat without increasing its own temperature.
This vaporization must take place inside the vehicle in a unit which can become cold itself and can be positioned so that the warm air inside the vehicle can be drawn into contact with it, and give up its heat. This is the function of the evaporator.
Once cooled, the air can be redistributed inside the passenger compartment.
Since the refrigerant must carry away from the evaporator the heat it has absorbed, a continuous flow of refrigerant must be provided. This is the function of the compressor.
The compressor circulates refrigerant through the system and also raises its pressure.
Pressure can only be created when there is a restriction to the flow.
This is provided for at the entry to the evaporator where the quantity of refrigerant entering the evaporator is controlled.
The restriction will also cause a drop in the pressure of the refrigerant as it enters the larger volume of the evaporator.
The drop in pressure ensures the liquid refrigerant will be able to vaporize inside the evaporator coils. Heat is absorbed from the air passing over the external evaporator fins into the vaporizing refrigerant. The low pressure low temperature refrigerant is drawn away from the evaporator by the intake side of the compressor.
The heat contained in the refrigerant must then be given up to the outside air and it must change its state back to a liquid to continue its cycle.This is the function of the condenser.
The restriction to flow at the entry of the evaporator allows the outlet side of the compressor to build up pressure on the high pressure side of the system.
The low pressure low temperature vapor drawn into the compressor is discharged as a high pressure high temperature vapor.
The temperature of this high pressure vapor is now considerably above ambient air temperature.
The high pressure vapor passes into the condenser coils and gives up its heat to the airstream flowing across the external condenser fins.
The removal of this heat causes the refrigerant to fall in temperature and in doing so, also causes it to condense back into a liquid.
This happens within the condenser coils and the high pressure, still high temperature liquid, exits from the condenser to flow on through the system.