Finally, we get to pressure. Stay with me, this is all going to come together in a minute. Any mechanic worth his salt knows that the coolant in a car's radiator boils a lot sooner with the cap off than with the cap on. This is because increasing atmospheric pressure raises the boiling point of a liquid. The higher the pressure, the higher the boiling (and evaporation) point. This principal applies to the refrigerant in an A/C system just as it applies to the coolant in your radiator. Unlike a radiator, however, an air conditioning system can regulate the pressure of its refrigerant, thereby controlling the condensation and evaporation point and the heat transfer properties of the system.
How It WorksNow that we've covered some of the basic principals involved in the process, it's time to explain exactly how an A/C system transfers heat. Basically, an automotive air conditioning system is split into two sections, a high-pressure side and a low-pressure side (see illustration), which are separated by the compressor (the big thing bolted to the front of your engine with hoses coming out of it) and the expansion valve or expansion tube. Starting at the beginning of the cycle, the compressor pushes refrigerant in superheated gas form through high-pressure lines into the condenser (the thing that looks like a small radiator behind your grille), where cool, fresh air absorbs and carries away most of the heat trapped in the hot gas. As the refrigerant cools off, it condenses, turning into a sub-cooled, high-pressure liquid. The liquid then flows into what is called the drier, a small canister somewhere in the engine compartment that filters out impurities in the refrigerant and separates pure liquid from the gasses.
Next, the sub-cooled, high-pressure liquid leaves the drier and travels to the expansion valve, where it is metered into small droplets that travel through a small orifice into an evaporator or heat exchanger inside the cabin. This effect is similar to placing your thumb firmly over the end of a garden hose and cranking the valve open. Rather than a stream of water pouring out, you get a big cloud of mist. As we discussed earlier, the evaporation process requires quite a bit of heat/energy, so the refrigerant cools down as it moves through the evaporator, which resembles a small radiator mounted in a box somewhere under the dash. Fans circulate warm cabin air through the box and over the fins of the evaporator, where the heat is sucked out of the air and absorbed by the refrigerant. The air temperature drops significantly, and since cool air has less capacity to retain moisture than warm air (that's why Florida in summer is muggy, and Maine in winter is dry), moisture collects on the fins of the evaporator and eventually runs down a drain tube and out the bottom of the car. Finally, cool, dry air is blown out the vents in the dash into your face, while the heated, low-pressure gas travels from the evaporator back to the compressor, where it will be turned back into a high-pressure vapor in order to start the whole cycle over again.
Proper SetupNow that we've covered the A/C basics, it's time to discuss some of the details involved in properly setting up a climate control system in your hot rod that will not only work, but work efficiently. The more efficient a system is, the cooler the air will be and the longer your setup will last. The beauty of buying a pre-engineered air conditioning setup specifically designed for your make and model of car is that someone has already done most of the setup work for you; all that's left is bolting everything together. With that in mind, we contacted Jack Chisenhall, president and founder of Vintage Air.
According to Jack, just because a system is designed for your type of car doesn't necessarily mean you can simply slap it in and go. Many factors contribute to the proper functioning of an A/C system.