Knowledge Building: Air Conditioning Systems (July 01, 2004)

On one level, the air conditioning system of a passenger vehicle is not a complicated arrangement, but the interplay of temperatures and pressures, with the added fun of a variety of control systems and safeguards operated by increasingly sophisticated vehicle computers, means that suspected problems with a non-functioning system can sometimes be difficult to diagnose.

When a car is spewing coolant all over the engine bay, even a non-technician understands that there is a serious loss of integrity in the system, and can usually determine the location of the fault. But failures on an air conditioning system can be invisible and sometimes can drive even the best technician batty.

A good example of this is a vehicle that would not engage its air conditioning system. The refrigerant level seemed up to snuff, but upon further investigation, the compressor clutch refused to engage. The technician checked the air gap, which was within spec, and did a hard-wired functional test–applied a voltage–just to be sure.

The problem appeared to be that the system, even when activated, refused to energize the clutch coil, yet it all appeared to be responding properly when prompted.

The technician spent quite a bit of time on the vehicle looking for problems, but when he decided to do an overall system check, he found that the battery was in poor condition–delivering less than 10 volts–and had activated a sort of failsafe system on the vehicle which locked out systems such as the air conditioning to prevent the vehicle from stranding its passengers. Interesting, yet, unexpected.

Thankfully, most problems with an air conditioning system are more firmly routed in the functions of its own components.

The basic components of a typical five-part A/C system are: the compressor; the evaporator; the condenser; the expansion valve or orifice tube; and the accumulator or receiver drier. In addition, switches and controllers as well as refrigerant and lubricant are indispensable.

There are four basic types of A/C systems: the basic cycling clutch system; the control valve system; the clutch cycling orifice tube system (CCOT, also known as the accumulator orifice tube system); and the variable displacement orifice tube system (VDOT).

All systems cool by the same principle: converting a liquid to a gas (the refrigerant) pulls heat from air passing through the evaporator.

Although they are closed-loop systems, A/C systems are divided in two sections: High Side and Low Side, referring to the pressure levels at each part. The system area “downstream” of the compressor, including the condenser and receiver/drier, are on the High Side. The orifice tube or expansion valve marks the start of the Low Side, where the pressure in the system is dropped from as much as 250 psi (or more) to, for example, 30 psi.

It is the easy evaporation of refrigerant as a result of this drop in pressure which provides cooling. Low-pressure refrigerant is routed through the evaporator.

The Compressor literally drives the system. In any system, it provides the pressure necessary (e.g. 250 psi) to condense the refrigerant at normal operating temperatures. The refrigerant is, however, still in a gaseous state, not becoming a liquid until it has passed through the Condenser. Compressor types include: two piston, multiple piston, radial and variable displacement. All serve the same function: to increase the pressure on the refrigerant, causing it to condense more easily once it passes through the condenser, improving its ability to absorb heat as it passes through the evaporator.

The Compressor Clutch is only found on systems that cycle the compressor on and off, such as the basic cycling and CCOT systems. (Clutches are not found on Control Valve and VDOT type A/C systems.) The Compressor Clutch is driven by a belt from the crank. When the system is turned on, it engages the compressor. The cycling action comes under the control of a Thermostatic Cycling Switch (or an Evaporator Pressure Switch) in order to prevent the evaporator from freezing. It uses a capillary tube filled with refrigerant and is located at the evaporator. In “clutchless systems” a Suction Throttling Valve (STV or POA or Control Valve) regulates evaporator pressure, preventing freezing by limiting the pressure.

The Condenser, essentially the A/C system’s radiator, allows the heat collected by the refrigerant during the interior air cooling process to be passed to the outside air. The condenser is usually located ahead of the car’s radiator, but is much smaller. The refrigerant enters the condenser as a high-pressure gas and leaves as a liquid.

The Receiver/Drier or Filter/Drier is located after the Condenser, on the high pressure side of the system. Its job is to remove moisture (using a desiccant) from the refrigerant, as well as to remove any particles of contamination. It also serves as a liquid storage device. These are used in basic cycling systems and control valve (STV) systems, which use Expansion Valves.

The Accumulator is the equivalent of the Receiver/Drier but are located on the low-pressure side of a system and used in CCOT and VDOT systems, which both use Orifice Tubes. Although it too contains a desiccant for drying, its most important job is to keep liquid refrigerant from reaching the compressor by separating any liquid from the vapour. If the compressor were to get a “mouthful” of liquid refrigerant, which cannot be compressed, it would fail.

Both the Expansion Valve and the Orifice Tube devices serve the same function: to regulate the liquid refrigerant flow to create a low-pressure area downstream between it and the compressor (which also results in a high-pressure zone on the upstream side). Expansion Valves vary the amount of refrigerant flow in response to temperature/pressure changes in the evaporator. Orifice Tubes cannot vary their size but are balanced to the system. Because they are fixed, there is the possibility that some liquid refrigerant could enter the evaporator, hence the need for the low-side accumulator on orifice tube systems.

Variable Displacement Orifice Tubes (VDOTs) have entered the aftermarket as a modification over the standard orifice tube in some systems. These are generally used to compensate for some of the situations occurring in R-134a retrofits of R-12 systems, and act in a method similar to Expansion Valves, to regulate pressures.

The Evaporator, located in the heater core area, receives the low-pressure refrigerant (as small droplets) and when hot air from the passenger cabin is blown through it, the refrigerant droplets absorb heat from the air and boil as they do so. The cooled air is routed back to the cabin through ducts and is regulated by blend doors. Moisture from the air also condenses on the cold evaporator and is collected and routed to the ground.

There are, of course, many Safety Switches and System Controllers: Low Pressure Cutout Switches; Ambient Temperature Switches; High Side and Low Side Safety Switches; and Fan Controllers. These are generally designed to protect the system from damage due to freeze-up, low refrigerant, or refrigerant overcharge.