Effect of Pressure on Boiling or Condensation

The saturation temperature (the temperature where boiling or condensation occurs) of a liquid or vapor increases or decreases, according to the pressure exerted on it.

In the air conditioning system, high-pressure liquid Refrigerant-12 from the condenser enters the fixed orifice tube in the liquid line. When the liquid Refrigerant-12 is released into the evaporator by the orifice tube, the resulting decrease in pressure and partial boiling lowers its temperature to its new boiling point. As the Refrigerant-12 flows through the evaporator core, passenger compartment air passes over the outside surface of the core. As Refrigerant-12 boils, it absorbs heat from the air and thus cools the passenger compartment. The heat from the passenger compartment is absorbed by the boiling refrigerant and hidden in the vapor. The refrigeration cycle is now under way. To complete cycle, the following remains to be done:

1. Dispose of the heat in the vapor.

2. Convert the vapor back to liquid for reuse.

3. Return the liquid to the starting point in the refrigeration cycle.

   The compressor and condenser perform these functions. The compressor pulls the refrigerant vapor (containing the hidden heat) out of the evaporator and suction accumulator/drier and forces it under high-pressure into the condenser which is located in the outside air stream at the front of the vehicle. The increased pressure in the condenser raises the Refrigerant-12 condensation or saturation temperature to a point higher than that of the outside air. As the heat transfers from the hot vapor to the cooler air, the Refrigerant-12 condenses back to a high-pressure liquid. The liquid under high-pressure now returns through the liquid line to the fixed orifice tube for reuse.

   It may seem difficult to understand how heat can be transferred from a comparatively cooler vehicle passenger compartment to the hot outside air. The answer lies in the difference between the refrigerant pressure that exists in the evaporator and the pressure that exists in the condenser. In the evaporator, the compressor suction reduces the pressure and the boiling point below the temperature of the passenger compartment. Thus, heat transfers from the passenger compartment to the boiling refrigerant. In the condenser, the compressor raises the condensation point above the temperature of the outside air. Thus, the heat transfers from the condensing refrigerant to the outside air. The fixed orifice tube and the compressor simply create pressure conditions that permit the laws of nature to function.