The condenser is one of two heat exchangers that is included on every A/C system. Typically mounted at the front of the vehicle, directly in front of the engine cooling radiator, the condenser has only one function in the auto A/C system. That is to cool the refrigerant.

Air conditioning is all about heat transfer and the automotive A/C system uses R-134a refrigerant as a coolant to absorb and transfer heat. At the evaporator, which is the A/C systems second heat exchanger, the refrigerant absorbs heat from the air passing over it, before entering into the vehicle. When the refrigerant absorbs that heat, it actually boils and changes state to vapor refrigerant.

That vapor refrigerant then leaves the evaporator and is circulated by the compressor to the condenser. The sole function of the condenser is to remove the heat from the refrigerant that was absorbed in the evaporator. Doing that, the vapor refrigerant once again changes state from vapor back to liquid.

Although there is a change of state of the refrigerant, technically there is not a change in temperature. In the evaporator, all the heat that is absorbed is latent heat. Latent heat is hidden heat as it can not be measured using a thermometer. However, that latent heat is consumed to cause some change in the substance, other than temperature. In the case of the refrigerant in the evaporator, the change that is caused is a change of state. The liquid refrigerant changes state to become a vapor refrigerant, but it is at the same temperature as the liquid refrigerant.

At the condenser, that latent heat is removed. Therefore, the vapor refrigerant entering the condenser should be the same temperature as the liquid refrigerant leaving the condenser. However, in actual fact, because of the efficiency of the condenser, there is some amount of sub cooling occurring. That is, the liquid refrigerant is cooled to a temperature below its saturation point or the temperature that it changed state from vapor to liquid. That sub cooling assures that the liquid line from the condenser to the expansion valve or orifice tube is full of liquid and no vapor is present. The maximum amount of heat transfer occurs when the liquid refrigerant changes state to vapor. Returning vapor refrigerant to the evaporator would reduce the efficiency of the A/C system. Therefore, the condenser actually performs some amount of sub cooling when it cools the vapor refrigerant.