Heating and Air Conditioning: Description and Operation
Ventilation is by a flow-through system. Fresh air enters the car at the air intake in the right side of the plenum chamber in the body cowl. Interior air exits through vents located in the side trim panels of the luggage compartment. A three-speed fresh air blower assists air flow.
A blend heating system is used for faster reaction and less fluctuation in passenger compartment temperature levels. In a blend system, coolant always flows through the heater core. Temperature is regulated by a blending door which controls the amount of air that passes through the heater core before it enters the passenger compartment. Two other doors control the flow of air to the windshield and the passenger compartment.
Cars with air conditioning also use a blend system. Fresh air is first cooled and dehumidified by the evaporator. A portion of the air can then be directed by the blending door through the heater core to maintain the desired temperature. In very hot conditions the interior air can be recirculated for maximum cooling.
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 from the passenger compartment boils (evaporates) the refrigerant (R-12) in the evaporator, causing the heat to be absorbed by the R-12. This heat is then released into the atmosphere when the R-12 is cooled and condensed into a liquid at the condenser. Moisture 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 the reason a water puddle may 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 electro-magnetic clutch which is actuated when the A/C is turned on. The thermostat 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 in the system by metering the flow of R-12 based on the temperature of the refrigerant as it leaves the evaporator.
To prevent system freeze-up, the fresh air blower and the radiator come on at low speed whenever the A/C is turned on. 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 if the pressure in the system is excessive. A blow-out plug in the receiver/drier releases, if system pressure exceeds approximately 500 psi. On diesel-engine cars, a thermoswitch turns the compressor off when the engine coolant temperature rises above 248°F (120°C).