Air Conditioning System - Keeping Your Cool

This ain't no installation tech feature (though we may get to that in the near future), but it's more an overview of air conditioning systems, how they work, why they sometimes don't work, and how to get the best from a system once installed.

Air conditioning systems have been available in passenger cars since the 1940s; along with most things over the past 60 years, they've become ever more complex with such improvements as automatic temperature control, as well as the changeover in recent years from R-12 Freon refrigerant to R-134a-an efficient absorber and carrier of heat without the ozone layer-damaging effects of R-12. We are also spoilt for choice today with a number of aftermarket companies offering complete A/C systems so we can enjoy the comfort levels of our modern daily drivers in our rods and customs.

How does an A/C system work?
Let's get right down to the basics. You may think an A/C system's main function is to produce cold air to blow into your car's interior, but that is actually a byproduct of its main purpose, which is to remove heat and humidity from the interior. How does it do this? The explanation requires looking at the system as a whole. OEM A/C systems can be broken down into three different types, but the concept of all three is essentially similar.

The components comprise a compressor, condenser, evaporator, thermal expansion valve or orifice tube, and a receiver-dryer or accumulator. The compressor is mounted on the engine and is a beltdriven pump, drawing refrigerant gas from the evaporator, compressing it and pumping it into the condenser, where heat dissipation takes place. Just like your car's radiator, the condenser radiates heat, and again like the radiator, airflow is important, which is why the condenser is often mounted in front of the radiator, utilizing the engine or electric fan to aid airflow (as an aside, you'd do well to employ a shroud, as not only do the condenser and radiator need all the cooling help they can get, but a shrouded fan moves approximately double the air of an unshrouded one). As the refrigerant comes into the top of the condenser as a hot gas, it cools, condenses and exits the bottom of the condenser as a high pressure liquid.

From here the liquid goes to the evaporator inside the vehicle, usually mounted behind the dashboard. This is where heat absorption and dehumidification takes place. The refrigerant enters the bottom of the evaporator as a low pressure liquid and boils as the warm air passes over the cooling fins, aided by the A/C blower fan. This very low boiling point means the refrigerant is capable of absorbing heat, which causes it to change state and become a gas again, before passing back to the compressor. The moisture in the air condenses on the surface of the fins, which is then drained off, and can be seen as water forming a puddle under the car. An added benefit is that dust and pollen in the air collect on the wet surface of the evaporator fins and are drained away with the water. So as the warm air condenses it has the effect of cold air being blown back into the interior through the A/C vents.

That's the basic theory but, as is usually the case, there's more to it. The ideal operating temperature for the evaporator is 32 degrees F or 0 degrees C, and to maintain the temperature the pressure and rate of flow of the refrigerant into the evaporator has to be regulated. A number of pressure regulator devices have been used over the years, and different manufacturers still use different methods, which is why in there are a couple of either/or mentions in the list of components here.

A/C systems will employ either an orifice tube or a thermal expansion valve to regulate the pressure. Most commonly used on OEM systems is the former, located on the inlet side of the evaporator, either in the line or at the evaporator's inlet tube. The thermal expansion valve is most often seen on import and aftermarket systems. It senses temperature and pressure, and the sudden drop in pressure it provides changes the refrigerant from a liquid to a very cold vaporizing liquid spray before it enters the evaporator.

A/C systems have a high-pressure side (between the compressor and the expansion valve or orifice tube) and a low-pressure side (from the inlet of the evaporator to the inlet of the compressor). On the high side of systems that use a thermal expansion valve, between the condenser and the valve, there's a component called a receiver-dryer. Its main purpose is to separate liquid and gas, should there be any of the latter after the refrigerant has passed through the condenser. It also removes moisture and dirt. There are various types of receiver-dryers, and they employ a number of different desiccants to remove moisture, not all of which are compatible with R-134a. The desiccant type can usually be found on a sticker on the receiver-dryer body, with desiccant type XH-7 compatible with both R-134a and R-12 refrigerants, and XH-9 compatible with R-134a only.

Systems that use an orifice tube, however, employ an accumulator, which is located immediately after the evaporator outlet. It stores excess liquid refrigerant so no liquid can reach the compressor, which is designed to only compress gas. The accumulator also removes moisture and dirt. Moisture is the single biggest cause of problems in an air conditioning system, and combined with refrigerant can form a corrosive acid.

Getting the best from your A/C systemOK, so now we know how the A/C works, how can we make sure it stays working? According to Rick Love from Vintage Air, "Air conditioning is no different than a brake system. You can have all the fancy hardware in the world, but if the master cylinder is the wrong bore size it won't work properly. You have to understand how A/C works and look at it as a system. Using the brake analogy again, if you don't bleed all the air from the system it won't work, and it's the same for air conditioning. If it's not evacuated properly to start with, it won't work properly." Evacuating involves drawing a vacuum on the system to remove moisture and other contaminants, and should be done with a new system or after servicing or replacing components. Vintage Air recommends evacuating for between 30 and 60 minutes so the system reaches at least 29 inches of vacuum for five minutes at 85 degrees, while Classic Auto Air recommends a minimum of 30 minutes.

Charging can begin once the system has been evacuated. This can be done by the home-builder, but we'd recommend calling a local specialist to charge the system, as the compressor can be damaged if liquid enters it. Gas charging is the safest method.

But it's not just the actual A/C system itself that ensures it's working at its most efficient. "A popular misconception is how an A/C system works," Rick said. "It doesn't make cold air; it removes hot air from the interior, which is why insulation is so important. The most heat will enter the interior through the roof and firewall, yet these areas are often not insulated. It's like turning on the A/C in your house and leaving all the windows open!" So ensure you have comprehensively insulated the whole interior, and that includes the floor, which can draw heat from the exhaust system as well as the motor and trans, and even the trunk area. Plug all the holes in the firewall, as excess hot air under the dash will be drawn straight over the evaporator cooling fins and make the system work harder than it needs to. While we're talking about that underdash area, ensure it's not totally sealed in, as the air from the interior needs to be able to circulate to the blower fan in order to reach the evaporator. In fact, the location of the evaporator should be considered very early in the build stages of a project. Install it first and position other components (wiper motor, heater, brake vacuum booster, etc.) around it. Always size the evaporator to the car, and use the largest possible in the space