How does the A/C work?

There are two basic types of automotive air conditioning systems. The type of system is determined by the device used to control the expansion of the high-pressure, high-temperature liquid to a low-pressure, low-temperature liquid. One system uses an expansion valve while the other uses an orifice tube.

The first component in the A/C cycle is the compressor. The compressor, usually referred to as the “heart” of the A/C system, pumps low-temperature refrigerant gas and compresses it into a high-pressure, high-temperature gas.

The refrigerant is then sent to the condenser which sits in front of the radiator. The condenser removes some of the heat from the refrigerant via the ambient air which passes through its fins, thereby causing the refrigerant to change phase from a hot gas to a warm liquid.

In the expansion valve A/C system, the warm liquid is then passed through a receiver-drier which removes moisture (via a desiccant bag in the drier) from the refrigerant to maximize the efficiency of the heat exchange capability of the refrigerant. No phase change occurs as the refrigerant passes through the receiver-drier.

From there, the refrigerant is then passed through the expansion valve. The expansion valve has a sensing line that is attached to the suction of the A/C compressor. The expansion valve senses the pressure at the suction of the A/C Compressor and modulates to maintain the pressure at the compressor suction.

In the orifice tube system, the warm liquid refrigerant passes directly from the condenser to the orifice tube. The orifice tube is a fixed expansion device. The pressure drop across the orifice tube is proportional to the pressure into the orifice tube. So, the orifice tube system maintains the pressure at the suction of the A/C compressor by cycling the compressor on and off via a clutch. This is the type of system typically used by GM and many Ford vehicles.

The expansion of the warm liquid through the expansion valve or orifice tube causes a pressure drop and hence a temperature drop in the refrigerant. So, out of the expansion device, we get a low-temperature liquid refrigerant.

The next component in the A/C system is the evaporator. Air from the passenger’s compartment passes across one side of the coils in the evaporator. The low-temperature liquid refrigerant passes through the other side of the coils. The refrigerant removes heat from the air in the passenger’s compartment and returns to the A/C compressor suction as a low-temperature gas (phase change).

In the orifice tube system, there is an accumulator located between the evaporator and the A/C compressor suction. The accumulator has moisture removal desiccant just like the receiver-drier used in the expansion valve system. However, it also collects any unevaporated refrigerant to prevent liquid lock of the compressor. This can occur due to the cycling action of the compressor.