Vintage Air University
Installing a climate control system in your vehicle may seem like a daunting challenge. But like many projects we car guys get into, when taken one step at a time, the component installation process is easy to manage by an experienced auto enthusiast. This introductory chapter will provide an overview of the basics of performance air conditioning and factors affecting the selection of the main components needed to air condition any vehicle. You may choose to contract with a professional shop for your installation, but it is always good to have an understanding of the functions and components of these systems. If you really want more in-depth knowledge about the theories and technology involved in a modern climate control system, we recommend ordering the book “How To Air Condition Your Hot Rod”, written by our own Jack Chisenhall and Norm Davis, available directly from Vintage Air.
Air Conditioning Systems
Simplified: The Basic Functions
We’ll begin under the hood with the engine-mounted compressor. It pumps refrigerant, in gas form, into the high-pressure gas discharge line. This gas is loaded with heat it has absorbed from air blown by the system’s fan over the evaporator coil inside the vehicle’s cabin. (It is important to remember that an air conditioning system doesn’t “make cool air”, it just removes ambient heat from the space you want to be cooled). Next, as the heated high-pressure gas flows on through the condenser, the heat is carried off by air flowing through the condenser, normally mounted in front of the vehicle’s radiator. The refrigerant then condenses into a liquid which becomes heavier, requiring less space. The more efficiently the refrigerant is condensed, the less room it uses in the system. This allows the refrigerant to lower the pressure in the high-pressure side of the system. The refrigerant, now gas and liquid, then flows into the receiver/drier, where the liquid falls to the bottom of the drier tank. The pick-up tube in the drier almost reaches the bottom of the tank. The open end of the tube is always below the liquid level in the receiver/drier if the system is fully charged. This provides pure liquid refrigerant to the liquid line (between the drier pick-up tube and the expansion valve). The expansion valve is an orifice that contracts or expands in size according to the temperature of the evaporator coil. By changing size, the expansion valve meters the refrigerant according to the demands of the evaporator. The orifice is largest when warm, and gets smaller
as the evaporator becomes colder. This orifice provides a pressure drop with the resulting drop in refrigerant temperature. The high pressure liquid pours into the larger opening of the evaporator, and the low pressure liquid droplets begin to pick up heat, which expands the refrigerant to a low pressure (super heated) gas, which then returns back to the compressor to start the cycle all over again (See drawing above).
Basic Components: Selecting A Condenser
The most difficult challenge in building an efficient system is getting a condenser large enough, in a place where the air is cool enough, and can flow across it well enough, to remove the heat and condense the refrigerant adequately. The condenser has to provide a low resistance path for the condensed liquid to flow so pressure build-up on the system’s refrigerant can be avoided. (Excessive pressure here tends to reduce heat loss which tries to reduce condensation.)
The old vertical vs. horizontal tubes confusion in the condenser comes into play here. On a tube and fin condenser, the tubes must run horizontally. On a parallel flow condenser, the tanks run vertically, but tubes must run horizontal as well. Why? Because lubrication oil flows with the refrigerant in the system and will settle in the lower loops of the condenser, thereby obstructing the flow of liquified refrigerant. We have seen this single factor increase the internal pressure of the high-pressure part of the air conditioner by 50%, reducing its ability to work properly.
With the conversion from R-12 to HFC-134a, refrigerants in automotive air conditioning systems, we would have had to increase the surface contact area of conventional tube and fin condensers by about 20% to maintain proper efficiency, but unfortunately many classic car and truck applications just won’t allow that size increase. A new type condenser was the correct solution and Vintage Air pioneered that technology for the performance aftermarket.
Our new design SuperFlowTM condensers, introduced back in 1991, provided that increase in capacity without additional external size! By using flat tubes manifolded together so that the refrigerant flows through multiple tubes on each pass, we get virtually 100% contact of the refrigerant with the condenser tube walls. This design also offers very low restriction in the pathway through the condenser. The SuperFlow condensers actually deliver up to 40% more efficiency than a comparable sized conventional copper tube-and-fin type condenser. This means we can get more capacity with less space, a good thing for smaller vintage cars and trucks!
The condenser has to have good air flow and must be mounted properly. It must be matched to a compressor of approximately nine (9) cubic inches or slightly less, using a standard aftermarket evaporator. Using this as a basic minimum standard, we have tailored each of our systems with a combination of components optimized to deliver the best performance. We always recommend using the largest parallel flow condenser possible. The condenser should cover as much of the radiator core as possible.
Air flow to the condenser is as important as size: the more the better. Ambient air temperature (outside air temp) is more important to an air conditioner condenser than it is to the engine cooling radiator because when the refrigerant is exposed to temperatures above approximately 100°F, the chemical expands at a very rapid and disproportionate rate, affecting system performance. Water/antifreeze expands more proportionately at much greater temperatures than refrigerant because of the higher boiling point. So, we position the condenser in front of the radiator, or in the coolest air stream possible, to keep the ambient air flowing over the condenser below or as near the century mark as possible. More time and consideration should be spent on selecting the condenser than on any other component of your system.