A refrigerator and a vehicle air conditioning system have a lot in common. In fact, the automotive air conitioning system is basically a refrigerator. We don't call it that, because most folks don't like the idea of riding around in a mobile refrigerator, but technically, that's what a car with the A/C running amounts to.

The cooling cycle (or refrigeration cycle) takes advantage of some inherent properties of matter. The first is a basic heat transfer principle; Heat always flows from areas of higher temperature to areas of lower temperature. When you put your hand around a glass of ice, it feels like the cold is flowing into your skin, but really, heat is flowing from your skin to the glass faster than your body can replace it.

A/C systems also exploit changes of state. In order for a liquid to change into a gas (boil), it must absorb heat. Think for a minute about boiling a pot of water. As you turn the flame up under the pot, a droplet of water at the bottom of the pot absorbs enough heat to change to steam (a gas). It flows up through the liquid to escape into the air above.

At normal atmospheric pressure boiling occurs at 212 degrees F. As long as there is water in the pot, we can turn the flame as high as we like, and the water (and the pot's inside surface) will never rise above 212 degrees F. Only when the water is gone does the pot burn, because we've lost the cooling effect of the evaporating water.

In an air conditioning system, we take advantage of the same phenomenon by blowing air across a heat exchanger (the evaporator) that has a pressurized liquid in it. As the air passing over the evaporator coils gives up heat to the cooler liquid inside, the liquid evaporates (boils). Each drop of liquid that converts to a gas absorbs a large amount of heat from the air flowing across the outside of the heat exchanger. This cooled air is conveyed into the passenger compartment of the vehicle.

The evaproated refrigerant, now a gas, flows into an accumulator, which acts as a storage tank. The accumulator also separates from the gaseous refrigerant any liquid fraction that may still remain, and allows only the gas to go on to the compressor inlet. The gas is drawn into the compressor, which raises the pressure (and thus the temperature) of the gas and pumps it through the system.

After the compressor, the next stop for the hot, gaseous refrigerant is the condenser, which is simply another heat exchanger. In the condenser, the hot gas gives up its heat to the cooler outside air flowing across the condenser tubes. As the refrigerant cools at the high pressure, it condenses again into a liquid.

Next, it flows through a restriction of some kind (usually an orifice tube), which lowers the liquid's pressure before it returns to the evaporator to provide more passenger compartment cooling.

And that's the cycle. In the evaporator, refrigerant absorbs heat from the air as the refrigerant changes state from a liquid to a gas; the cooled air flows into the passenger compartment. At the other end of the cycle, the gaseous refrigerant gives up its latent heat to the outside air as it changes state back into a liquid. The work necessary to make this happen is provided by the vehicle's engine, which drives the compressor (nothing is free) by way of a drive belt and pulley assembly.

The internal moving parts of the compressor are lubricated by a special oil that dissolves into the refrigerant and travels through the system with it. Different refrigerants require different oils. Some newer system components can withstand exposure to most of these oils, but many use materials only compatible with one type or another.

During normal operation, the evaporator tubes become so cold that moisture in the air condenses on the tubes and drains off as water. This accounts for the puddle we often see under recently parked cars in the summer, especially in humid weather. But if refrigerant pressure inside the evaporator should fall too low, the evaporator fin temperature can drop below 32 degrees F, and the condensation on the external surface of the evaporator's fins will actually freeze. This, in turn, reduces heat transfer efficiency.

To eliminate this problem, the A/C system must be controlled to keep evaporator temperature above a certain level. In many systems, the control scheme takes advantage of the fact that refrigerant temperature and pressure are linked. As pressure rises, so does temperature.

Overall operation in most mobile A/C systems is controlled by cycling a clutch on the compressor drive pulley on and off. When evaprator temerature falls too low, the compressor is cycled on, raising the pressure (and thus, the temperature). When temperature rises to a satisfactory level, the compressor is cycled off again. This process can repeat itself many times each minute, but it happens automatically, so we're rarely aware of it.

Some systems, instead of using a fixed orifice and cycling the compressor on and off, use an expansion valve that modultes the pressure drop across the valve to regulate evaporator pressure. The principle is the same, though the components used in the system are different.

An A/C system's operation is also affected by the operation of the vehicle's cooling fan, which affects the volume and rate of air flow over the condenser, and by the blower fan, which controls the flow of air over the evaporator and into the vehicle's interior.

Operating the blower fan at too low a speed, especially on humid days, can lead to evaporator icing, and a loss of cabin cooling.

The basic functional requirements for an A/C system refrigerant are relatively straight-forward.

1. It must condense (become liquid) at temperatures significantly higher than the outside air's when reasonable pressure is applied (so that heat can be transfered out of the system, to the outside air).

2. It must evaporate readily at 32 degrees F to 40 degrees F when the pressure is reduced (so that air destined for the cabin can transfer heat into the system).

3. It must not corrode or otherwise harm aluminum, steel, plastic, rubber, or the other materials from which system components are normally made.

Beyond these, there are other practical requirements, including that it not cause ozone depletion, that it not be toxic to humans or animals in case it should leak into the air flowing into the passenger compartment, and that it be available at an economically acceptable price.

While these latter characteristics don't actually affect it's ability to provide cooling, they are the factors that have driven refrigerant selection in the last half-decade. With the exception of depleting the ozone, Freon, or R-12 offered high performance in all categories. Of course, causing huge holes in the ozone is no small problem, so we now face the transition to R-134a, and perhaps to other alternative refrigerants.

Again, we aren't going to discuss the pros and cons of the various refrigerants here, but the choice of refrigerant does have some practical impact on the A/C system hardware. The most notable and obvious is that all fittings must be exclusive to each refrigerant. This is an EPA requirement, and if a system is retrofitted from R-12 to another refrigerant, every fitting in the system must also by changed.

The problem of cross-contamination, that is, getting one refrigerant into the recycling and/or reclamation equipment that's supposed to be dedicated to another refrigerant, can cost the service provider much money and aggravation. The unique fittings provide a physical reminder that each refrigerant must be kept and handled separately from the others.

Even with the precautions of unique fittings and mandatory on-vehicle lables, A/C service providers still risk costly contamination if a mixture of refrigerants has been introduced into a vehicle's A/C system before it gets to them. For that reason, many are routinely using SAE-approved refrigerant identifiers--devices that "sniff" and identify specific refrigerants. If it cannot determine the exact type, it will indicate that the system contains an unidentified refrigerant or mixture of refrigerants.