Air Conditioning: How it Works

Understand the Basic Technology Behind Keeping Your Cockpit Cool

Dan Kahn Mar 31, 2005 0 Comment(s)

Walk through the pits of a vintage drag race or cruise the main drag of a car show, and you'll likely hear a handful of "old timers" belly-aching about the good ol' days before billet aluminum and fiberglass snuck their way into the heart of hot rodding. One thing you won't hear any complaining about, however, is the plethora of highly adaptable and efficient air conditioning units now available on the aftermarket for rods of just about every year, make, and model. While A/C was once considered a luxury item reserved for only the most well-equipped luxury cars, climate control is now standard on virtually every new vehicle on the showroom floor. Furthermore, many of the same folks who tell tales of trekking through the snow barefoot on the way to school would never even consider making a midday rod run without their A/C blasting. Air conditioning has become a way of life for many of us, and new technology makes these systems lighter, more efficient, and more environmentally friendly than ever before. In this article we'll start with the basics--how air conditioning works--then move on to some tips for setting up a new system, as well as making your existing system work better. With a little planning, forethought, and a few days' worth of work, your interior should be as cold as Martha Stewart's heart.

PHYSICS 101
A common misnomer about air conditioning is that the system somehow magically creates cold air, which it then blows out the vents into your car. The truth of the matter is that air conditioning is all about heat. If that sounds confusing, read on. A properly functioning A/C system is basically a giant circuit designed for transferring heat. Essentially, heat is pulled from the air inside your car, carried into the engine compartment via hoses running through the firewall, and transferred into the fresh air flowing through your grille. All of this happens thanks to a few basic rules of physics, namely heat transfer, evaporation, condensation, and pressure.

The basic rule of heat transfer is that "cold" doesn't actually exist; cold is the absence of heat. The only time something seems cool is when its heat has been removed and transferred somewhere else. Furthermore, heat is constantly trying to travel from warmer to colder areas in an attempt to create balance, and the greater the temperature difference, the faster it moves. An example of this occurs when you look across a highway in the middle of a summer afternoon. The ripples you see coming off the ground are actually heat waves; the asphalt is transferring its heat into the cooler air surrounding it. As the asphalt cools down, the air is heating up. Another example occurs when you put a hot cup of joe on the counter, only to come back an hour later to a cold cup of coffee. The coffee doesn't magically change temperature; the heat of the liquid was transferred/absorbed into the cooler atmosphere of the room. This type of temperature change will always occur when the ambient air temperature is different from the object in question; heat is constantly trying to stay consistently dispersed.

The second principal necessary for air conditioning is called Latent Heat of Evaporation. Without getting too complicated, this rule basically states that it takes a lot of energy to turn a liquid into a gas. In most cases that energy comes in the form of heat, and if a liquid absorbs enough heat, it will evaporate into a gas, taking the heat with it as it goes. For example, anyone who has spent time in the hot sun knows that the best way to stay cool is to dump a bottle of water over your head. Even if the water is room temperature, rather than ice cold, you'll still cool off. Here's why: In order for the water to evaporate off your body, it absorbs tremendous amounts of heat/energy as it transforms into a gas. This process essentially transfers the heat from your skin to the surrounding atmosphere, cooling you off in the process. Your skin didn't get cooler, you just removed some of the heat.

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 WORKS
Now 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 SETUP
Now 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.

"The most important thing about putting air conditioning in a car is preparation," Chisenhall explains. "You've got to have room to install all the components, the car should run cool, and a good fan is vital. In fact, you should try to accommodate the biggest fan you can fit between the engine and the radiator, because that can affect the entire system. In the initial planning stages of your project, leave yourself plenty of room for cooling options." Moving motor mounts and changing the engine setback is a lot easier in a custom-built street rod than it is in a factory application, but most Detroit iron has plenty of room up front for a large-diameter fan anyway. Chisenhall is running an 18-inch clutch fan on his small-block-Chevy-powered '39 Ford and a 19-inch clutch fan on his F-100 pickup. The climate control guru also warns against using a factory radiator from a car that didn't originally come with A/C, as it probably won't be up to the task of keeping things cool once you throw slap a big condenser in front. There are several aftermarket radiator companies offering a wide range of products more aptly suited to an A/C application, and whether you choose copper/brass or aluminum, either will do an adequate job with sufficient airflow.

"The way to remove heat from refrigerant or coolant is either to increase the face area of the heat exchanger (radiator or condenser) or to move more air through it," Chisenhall says. "Since the face area is fixed by the grille shell size in most hot rods, your only other alternative is to move more air. That's why planning for a big fan beforehand is vital."

Cool-hand Jack goes on to explain that when Vintage Air was faced with changing environmental legislation, forcing a switch from the standard R12 refrigerant to the more environmentally friendly R134a, he saw an opportunity to improve the efficiency of the entire system. "When R134a first came out, people didn't think it worked very well, but that's only because they were putting it in systems designed for R12, which works about as well as running alcohol in a motor set up for gasoline. R134 carries more heat than R12, so you have to get rid of more heat too. The combination of a bigger condenser and R134 works great, but you can't get a standard tube and fin condenser efficient enough to handle an R134 system to fit into the grille of a pre-WWII hot rod. That's why we made the switch to the newer serpentine-style condenser, which flows the refrigerant through oval tubes rather than round tubes, providing more surface area for the refrigerant to touch and thereby increasing heat transfer. Unfortunately, the oval tubes had these small passages, which individually were restrictive to the refrigerant flow when you tried to circulate it through one tube at a time, so the pressure of the system would skyrocket. The solution was to put tanks on both ends of the condenser like a radiator, rather than having the refrigerant snake back and forth through a series of S-curves. Thus the Superflow condenser was born. An added benefit to all this is that when condenser capacity increases, more heat is dissipated, pressure drops, and the compressor becomes easier to turn. This, in turn, means that your engine doesn't have to work so hard, and you get a boost in usable horsepower."

CONCLUSION
Unlike the big, heavy A/C systems of the '50s and '60s, today's climate control setups have benefited from advances in condenser and compressor technology that make installation a breeze. A little forethought and planning will allow you to cruise in comfort with a minimal sacrifice in weight, engine operating temperature, or parasitic power loss. Whether you order an A/C system from an aftermarket vendor or piece together your own, a thorough understanding of the technology at hand as well as an effort to properly prepare your car beforehand will pay off in spades. Now, what are you standing there sweating for? Get to work!

We would like to thank Jack Chisenhall of Vintage Air for his invaluable assistance with the making of this story. For a more detailed look at hot rod air conditioning function and installation, pick up a copy of "How To Air Condition Your Car" by Jack Chisenhall and Timothy Remus.

14

This is the latest in aftermarket A/C technology, as offered by Vintage Air. The benefit of buying an aftermarket kit is that everything is pre-engineered and ready to work, right out of the box.

An automotive air conditioning system can be split into two parts, a high-pressure side and a low-pressure side. The engine-driven compressor compresses the gas, sends it through the condenser where it gives off heat, condensing and changing state to a liquid, gets filtered through the drier, then passes through the expansion valve which atomizes the refrigerant and turns it into a stream of cool, low-pressure droplets. This vapor travels through a heat exchanger in the passenger compartment of the car and sucks the heat out of the air, then is sent back to the compressor again.

This nicely detailed engine compartment is complete with a polished axial compressor and neatly routed refrigerant lines. These Texas-developed compressors (which were licensed and produced by the Japanese for many years) are lighter, smaller, and smoother than earlier types.

Shown here is one of the earliest A/C systems, commonly adapted for use in street rod applications. Found in early Mustangs and other mid-'60s Fords, these under-dash-type units were almost completely self-contained and could be easily bolted to just about any car.

New axial compressors can be polished or chrome-plated, and with new bracket setups they can even be tucked in the valley of the motor, providing clearance in even the tightest pre-war applications.

The newer parallel flow-style condensers utilize fins like these sandwiched between rows of oval tubes to transfer heat out of the refrigerant and into the fresh air flowing though the grille.

Early condensers used round tubes that would snake back and forth, which flowed fairly well but weren't quite efficient. Newer models with oval tubes use tanks on either end like a radiator, so all the refrigerant flows through at once, bringing down head pressure and increasing efficiency.

This is Vintage Air's newest serpentine condenser, called the SuperFlow.

The key to a properly dialed-in climate control system is to plan in advance. This Chevy features a matching polished aluminum condenser and radiator, both of which are large to ensure efficient operation.

PROPER A/C PREP CHECKLIST
Insulation: Do a good job and the cool air stays in, the hot air stays out.
Engine Fan Size/Location: Use the biggest fan you can; it will keep the motor running cooler and the condenser working as efficiently as possible.
Cooling System: An oversized aftermarket radiator, high-flow water pump, and fresh coolant make a big difference in how well your A/C system works.
Muffler Heat Shields: Heat always travels upward. Mufflers are underneath your floor. 'Nuff said.
Tinted Windows: They don't just look cool, they keep the sun out and keep you cool too.
Double Panel Floors: If possible, a double panel floor will keep most of the drivetrain heat out of the car and allow your A/C system to work more efficiently.
Window Seals: Since the A/C circulates cabin air through the system, you want to keep out as much hot outside air as possible. Plus, noisy window seals are annoying.

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