This article is meant to help those owners who don't understand the Air Conditioning System very well, if at all. It is meant to show how the system works in our GM E-bodies and K-Body cars, but it won't go into detail and it is not meant to be a troubleshooting tool.
To many owners the Air Conditioning System can be confusing, and even a source of consternation so I hope this will help you.
There aren't many parts to the Air Conditioning System and the big thing is to identify them and understand their purpose. For completeness I will discuss some parts that were used on older systems also.
Since 1977 General Motors has used an Accumulator instead of a Receiver-Dryer and an Orifice Tube instead of an Expansion Valve. Each newer item serves the same purpose as it's older counterpart.
Typical Layout of Components (picture 1; Author Unknown)
Since this is a closed system that works only when all of the parts work correctly, we need to start somewhere, and that would have to be at the Compressor. When the System first starts it is the Compressor which generates the High Pressure gas that starts the whole process.
The Function of the CONDENSOR
The Condensor, is the area in which heat dissipation occurs. The condenser is located in front of the radiator. On our front wheel drive vehicles it's not necessary to have a supplemental fan unless the car is operated in extremely high temps. The reason why it's not necessary is because our engines are mounted longitudinaly with the fan behind the radiator. The radiator cooling fan is usually sufficient enough to cool both the radiator and the Condensor.
As hot compressed gasses are introduced into the top of the condenser, they are cooled off by the passing cooler air across the fins and tubes. As the gas cools, it condenses and leaves at the bottom of the condenser as a high pressure liquid.
The Function of the EVAPORATOR
Located inside the vehicle and hidden from view, the Evaporator is probably the least understood of all the components. It serves several functions, its primary duty is to remove heat from the inside of the cabin, but how it's done seems to mystify owners.
A secondary benefit is dehumidification. As warmer air travels through the aluminum fins of the cooler evaporator coil, the moisture contained in the air condenses on its surface. Dust and pollen passing through stick to its wet surfaces and drain off to the outside. On humid days you may have seen this as water dripping from the bottom of your vehicle.
Now you may wonder since the blower fan would seem to blow warm air across the elements and warm the liquid, how can the fan cool the air and yet remove the warmth and high humidity at the same time?
To understand this you need to remember that in physics, warm air or anything else that's warm has more energy than cold and warmth will move into a cooler area.
What happens doesn't just occur on the surface of the tubes and fins but on the inside of the tubes. The warm air flowing across and through is absorbed by the refrigerent causing it to boil. The cold from the liquid enters the cabin by the fan air and displaces the warm air. At the same time the heat causes the refrigerant to boil inside the tubes and become a gas again. It's at this time that the air flowing across the elements sends the cold into the cabin and the refrigerant picks up the heat and boils the liquid and produces a gas.
The warm air passing through the evaporator fins causes the refrigerant to boil (refrigerants have very low boiling points). As the refrigerant begins to boil, it can absorb large amounts of heat. This heat is then carried off with the refrigerant to the outside of the vehicle.
The ideal temperature of the evaporator is around 32 Fahrenheit or 0 Celsius. Refrigerant enters the bottom of the evaporator as a low pressure liquid.
Several other components work in conjunction with the evaporator. Temperature and pressure regulating devices must be used to control its temperature. While there are many variations of devices used, their main functions are the same; keeping pressure in the evaporator low and keeping the evaporator from freezing; A frozen evaporator coil will not absorb as much heat.
PRESSURE & TEMPERATURE REGULATING DEVICES
Controlling the evaporator temperature can be accomplished by controlling refrigerant pressure and flow into the evaporator. Our E-Bodies use an Orifice Tube and a Compressor Clutch Cycling Pressure Switch to control both, along with temperaure sensors in the cabin.
The orifice tube is located in the inlet tube of the evaporator, or in the liquid line, somewhere between the outlet of the condenser and the inlet of the evaporator. This point can be found in a properly functioning system by locating the area between the outlet of the condenser and the inlet of the evaporator that suddenly makes the change from hot to cold. It is not uncommon for these tubes to become clogged with small debris. While inexpensive, the labor to replace one involves recovering the refrigerant, opening the system up, replacing the orifice tube, evacuating and then recharging. With this in mind, it might make sense to install a larger pre filter in front of the orifice tube to minimize the risk of of this problem reoccurring.
CLUTCH CYCLING PRESSURE SWITCHis located on the Accumulator and Cycles the Compressor Clutch on if the pressure is low and off when the pressure is sufficently high enough.
Relative Pressures through each component:
Accumulators are used on systems that accommodate an orifice tube to meter refrigerants into the evaporator. It is connected directly to the evaporator outlet and stores excess liquid refrigerant. Introduction of liquid refrigerant into a compressor can do serious damage. Compressors are designed to compress gas not liquid. The chief role of the accumulator is to isolate the compressor from any damaging liquid refrigerant. Accumulators, like receiver-driers, also remove debris and moisture from a system. It is a good idea to replace the accumulator each time the system is opened up for major repair and anytime moisture and/or debris is of concern. Moisture is enemy number one for your A/C system. Moisture in a system mixes with refrigerant and forms a corrosive acid. When in doubt, it may be to your advantage to change the Accumulator or receiver in your system. While this may be a temporary discomfort for your wallet, it is of long term benefit to your air conditioning system.
The accumulator is placed at the evaporator outlet and is used to separate liquid refrigerant from vaporized refrigerant, as well as remove any moisture contained in the refrigerant. Refrigerant enters the accumulator as it leaves the evaporator. Any liquid refrigerant is trapped in the accumulator, while vaporized refrigerant and refrigerant oil is allowed to pass on to the compressor. A desiccant bag is placed in the accumulator to remove any moisture that may have contaminated the air conditioning system.
For completeness in this Basic A/C page I'm including two parts that aren't part of our systems but were used on earlier American A/C systems and on some import systems.
THERMAL EXPANSION VALVE
Also known as a TXV. Commonly used on import and aftermarket systems. This type of valve can sense both temperature and pressure, and is very efficient at regulating refrigerant flow to the evaporator.
The receiver-drier is used on the high side of systems that use a thermal expansion valve. This type of metering valve requires liquid refrigerant. To ensure that the valve gets liquid refrigerant, a receiver is used. The primary function of the receiver-drier is to separate gas and liquid. The secondary purpose is to remove moisture and filter out dirt. The receiver-drier usually has a sight glass in the top. This sight glass is often used to charge the system. Under normal operating conditions, vapor bubbles should not be visible in the sight glass. The use of the sight glass to charge the system is not recommended in R-134a systems as cloudiness and oil that has separated from the refrigerant can be mistaken for bubbles. This type of mistake can lead to a dangerous overcharged condition. There are variations of receiver-driers and several different desiccant materials are in use. Some of the moisture removing desiccants found within are not compatible with R-134a. The desiccant type is usually identified on a sticker that is affixed to the receiver-drier. Newer receiver-driers use desiccant type XH-7 and are compatible with both R-12 and R-134a refrigerants.