Heating and Air Conditioning: Description and Operation

The air conditioning system removes heat and moisture from the passenger compartment. It accomplishes this through the application of four principles: materials absorb heat as they change from a liquid to a gas (evaporate); materials give off heat as they change from a gas to a liquid (condense); the boiling point of a liquid varies with its pressure; and heat always flows from hot to cold. For example, the first principle is demonstrated by wetting your hand and then blowing on it. As the water evaporates, it takes some heat with it and your hand feels cooler.

In the air conditioning system, the heat in the passenger compartment is absorbed by the refrigerant (R-12) in the evaporator and causes the R-12 to evaporate (boil). The R-12 is then circulated through the condenser where it is cooled, condensing back into a liquid and releasing the absorbed heat into the atmosphere. It is this cycle that removes the heat from the passenger compartment.

Moisture in the passenger compartment is removed at the evaporator in the same way that water drops form on a cold glass. The moisture drips onto the water tray beneath the evaporator (located behind the dash) and is routed outside. This is why a water puddle can often be seen under the car when the A/C is operating.

The compressor forces the R-12 through the system and at the same time pressurizes the R-12, raising its boiling point to make it more easily condensed. The compressor is engaged by an electromagnetic clutch which is actuated when the A/C is turned on. A thermostatic switch automatically disengages the clutch when the temperature in the passenger compartment reaches the level set on the operating controls. The condenser, which looks like a small radiator, is located in front of the engine radiator. The receiver/drier removes small amounts of moisture and dirt from the system. The expansion valve maintains a steady temperature by metering the flow of R-12 to the evaporator to prevent system freeze-up.

Safety Features
The fresh air blower and the radiator cooling fan come on at low speed whenever the A/C is turned on. To prevent system freeze-up a low-pressure switch prevents engagement of the compressor clutch if the R-12 charge is too low. A high-pressure switch runs the radiator cooling fan at high speed to cool the R-12 when the pressure in the system reaches a predetermined level. An additional high pressure switch disengages the compressor if the pressure in the system is excessive. A blow-out plug releases if system pressure exceeds approximately 590 psi. The three pressure switches and the blow-out plug are all located on the receiver/drier.