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A/C System Basics

There are three primary components in a vehicle’s air conditioning system including the compressor, condenser, and evaporator. These parts are connected by tubes and hoses to form a continuous path with two distinct sections: a high-pressure side and a low-pressure side. In order to transfer heat from the vehicle’s interior to the outside air, a chemical refrigerant is circulated throughout the system. In theory, the A/C system accomplishes the same task as the engine’s cooling system, in that both absorb the heat from one area and release it to another (heat transfer). While coolant remains a liquid during the heat transfer process however, refrigerant repeatedly alternates between a liquid and a gas as it circulates throughout the air conditioning system.

A/C Components

Compressor
The compressor is a pump that pressurizes and circulates the refrigerant in the air conditioning system. It is mounted on the front of the engine and driven by the serpentine drive belt or its own individual belt. The compressor also serves as one of the two junctions separating the high and low sides of the system.

Condenser
Similar to the radiator, the condenser is a large heat exchanger located directly behind the grille. As part of the system’s high side, the condenser is used to liquefy the high-pressure vapor discharged from the compressor. A condenser consists of a series of tubes surrounded by thin fins, which provide a large surface area for heat dissipation. While there are various tubing arrangements used, refrigerant flow is always from top to bottom.

Evaporator
Like the condenser, the evaporator is also a heat exchanger consisting of tubes and fins. However, that’s where the similarity ends. Unlike a condenser, which is designed to release large quantities of heat, an evaporator is used to absorb large quantities of heat. An evaporator is also much smaller than a condenser and is part of the system’s low side. Evaporators are located in the air handling case along with the blower.

Orifice Tube
This is a fixed metering device located inside the liquid line between the condenser and evaporator. The orifice tube is enclosed within a plastic housing and protected by a fine mesh filter. The filter prevents debris from clogging the tube.

Expansion Valve
A variable metering device that varies refrigerant flow based on cooling demand. As demand increases, the valve opens wider to permit more refrigerant into the evaporator. Once the demand has been satisfied, the valve opening is reduced to decrease flow. Cooling demand is detected within the expansion valve or by a sensing bulb mounted on or near the evaporator.

Receiver Drier or Accumulator
Depending on the vehicle, the A/C system will either have a receiver drier or an accumulator. Both of these components contain a desiccant, which is a chemical that attracts moisture. The desiccant serves a vital function, since the combination of water and refrigerant forms corrosive acids. Acids not only reduce A/C performance, they can ultimately destroy the system. The receiver drier and accumulator also serve as temporary holding tanks for liquid refrigerant.

Although these two parts serve similar purposes, the receiver drier is connected to the condenser outlet (high-pressure side) and is used exclusively in expansion valve system. In contrast, the accumulator is attached to the evaporator outlet (low-pressure side), and is only found in orifice tube systems. The primary functional difference is that the accumulator is designed to prevent liquid refrigerant from being drawn into the compressor.