System Controls:

                     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 modulates the pressure drop across the valve to regulate evaporator pressure. The principle is the same, though the components used in the system are different.

Related Components:

                     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.

Refrigerant Considerations:

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