Knowledge Building: Air Conditioning Basics

The Farmer’s Almanac is promising a scorcher of a summer (so I’ve been told) which seems hard to believe from the late start it seems to be getting in certain parts of the country. Nevertheless, even in moderate weather conditions, a vehicle’s air conditioning system can fail and cause the car’s owner to overheat.

A modern vehicle’s air conditioning system is inseparably connected to the overall climate control and defogging system and, as a result, the air conditioning parts market continues to expand and demand flatten somewhat throughout the year, though summertime peaks are still the norm. The market is also rife with provincial regulations that vary from region to region, but the principles that govern air conditioning systems remain the same which is good because it is a market which can demand much of counterpeople.

Whenever you are thinking about air conditioning systems it is useful to remember some high school physics or chemistry: with gasses, temperature and pressure vary in kind. Raise the pressure of the gas, its temperature will rise. Lower the pressure, its temperature will drop. This property is why A/C works at all and is at the root of most troubleshooting.

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 an 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 a closed-loop, A/C systems are divided in two sections: High Side and Low Side, referring to the pressure levels at each. 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 pounds per square inch (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 type system, it provides the system pressure necessary to condense the refrigerant at normal operating temperatures (e.g. 250 psi). The refrigerant is, however, still in a gaseous state, not becoming a liquid until it has passed through the Condenser. There are a number of configurations of compressor: 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. These are the basic cycling and the 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 (to cool the air, it gains the heat) to be passed to the outside air. The condenser is located usually 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 the system. Its job is to remove moisture (using a desiccant) from the refrigerant, as well as 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.

Accumulators are 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 vapor. 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 a system’s standard orifice tube in some cases. These are generally used to compensate for some of the situations occurring in R-134a retrofits of R-12 systems, acting 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 to 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.