How  ‘Bout Them Air Conditioners?

A primer on A/C system basics...

by Richard McCuistian

Air conditioning works because evaporating liquid absorbs heat.  If you ever get out of the shower or the pool and feel cold, well, your skin is being air conditioned by the evaporation of water.  Different liquids have different boiling points, too, so some liquids work a lot better than others  when it comes to heat absorption.

Air conditioning on vehicles depends on a compressor to move the refrigerant (R134a) through a radiator/type heat exchanger called a ‘condenser’ or, as it is known on newer vehicles, a ‘subcooler.’   The compressor, when it is energized and working, squeezes gaseous refrigerant into a high pressure gas and forces it through the condenser mounted in front of the radiator, where it gives off heat and condenses into a high pressure liquid.  The condenser invariably has a fan that pulls air across it when the vehicle isn’t moving down the highway.

 Common Swash Plate Compressor

On older systems, the hot high pressure liquid leaves the condenser to make its way through an expansion valve (a variable orifice) or a fixed orifice that atomizes it into a low pressure liquid so it can absorb heat from the air that is passing through another radiator style heat exchanger called an evaporator.  That air is the cold air that blows out of your register and cools the inside of the car.  Newer cars have a subcooler instead of a condenser, and the subcooler cools the high pressure liquid refrigerant after it condenses so it’ll work even better absorbing heat in the evaporator.

After the evaporated refrigerant leaves the evaporator, it passes through a larger line back to the compressor, which squeezes it back into a high pressure gas and sends it back to the condenser to dump the heat it picked up in the evaporator.

Vehicle A/C systems are outfitted with pressure switches designed to trigger the operation of the compressor and the electric cooling fan (if equipped).   Low pressure in the evaporator means low temperature in that heat exchanger, and if the evaporator is allowed to get too cold, it’ll freeze up, because it sweats like a glass of tea when the air conditioner is operating.  That’s bad, so the compressor will generally stop compressing (it’s energized by an electromagnetic clutch on most cars) if the pressure/temperature drop too low in the evaporator. 

High pressure means high temperature in the condenser, and if the temperature goes too high, the compressor may be shut down for that reason, and the electric cooling fan may be switched to a higher speed.

In the dash there is a plenum (air box) that contains the blower fan, the evaporator heat exchanger, several doors and flaps for directing air, and a heater core which is a small radiator-type heat exchanger that carries warm engine coolant so you won’t freeze during the winter. The heater hoses carry coolant from the engine to this small in-car radiator. The doors and flaps that control airflow are operated by cables, vacuum from the engine, or electric motors, depending on the vehicle.

Since the evaporating refrigerant dries the air (thus the water that drips under the car when the air conditioner is operating), the compressor is engaged any time defrost is selected on the car’s control panel so as to clear the windshield more rapidly.  If the driver selects warm air, the flow of air in the plenum is directed through the evaporator core first and then through the heater core.   The door that blends cold and warm air is called the ‘blend’ door.

If a malfunction occurs in the A/C airflow control system, most systems are designed to default airflow to the windshield defrost vents for safety purposes.