How A/C works in your vintage car

It’s time to dive into some nice, cold air. Last week, I explained why a guy from Massachusetts became so obsessed with having working A/C in his vintage cars and also described why the A/C needs of a vintage car are different from those of a car built after the switch from R12 to R134a refrigerant. This week, I’ll give you the theoretical underpinnings for how A/C works in the automotive environment.

Think of it as eating your string beans before you have your pie.

The theory behind A/C is actually very simple. When a liquid boils and evaporates into a gas, it expands and cools. You know this from shaking a can of spray paint or deodorant, feeling the liquid sloshing inside, then spraying it and feeling the can get cold. You’ve also experienced it when you’ve mounted a propane tank on a barbecue grille, felt the liquid propane slosh in the tank, and felt how the tank becomes cold to the touch once the grille is turned on and the propane has a path to leave the tank, vaporize, and expand. Nearly all conventional refrigeration and air conditioning works on this principle. It’s just that you don’t think of it as “boiling,” even though that’s exactly what the liquid refrigerant is doing, because you’re accustomed to boiling water. Anything that’s pressurized in a can has a much lower boiling point. For R12, it is -21 °F; for R134a, it’s about -15 °F.

It would be woefully inefficient to have a refrigeration system that endlessly releases pressurized refrigerant so it can evaporate, expand, and cool. So, instead, refrigeration and A/C systems work in a closed-loop fashion. They take some kind of refrigerant that is naturally gaseous at room temperature and pressure, pressurize it until it changes from a gas into a liquid, then allow it to evaporate and expand into a gas inside a container, and cool. The system then takes the cold created by the expansion (sort of), and blows it at you to cool you off. The gas is then pressurized back into a liquid, completing the cycle.

There’s one thing we left out. In thermodynamics, there’s a saying that there’s no free lunch. If you’re pressurizing a gas into a liquid so it can expand and cool, you must be generating heat somewhere else, and this heat must be dumped to the outside, or it will heat up the very space you’re trying to cool.

You can, if you want, get very geeky and technical about all this (look up reversed Carnot cycle and adiabatic expansion), but, in terms of theory, that’s really all you need to know. An A/C system is essentially like a can of spray deodorant or paint or propane where the sprayed substance is released so it can expand and cool, but is continuously captured, compressed, and put back in the can to spray and cool again. There, those green beans aren’t so nasty, are they?

With the theory down, we can identify the major components of an A/C system and what they do:

  • The compressor is a pump that takes the low-pressure gaseous refrigerant and compresses it into a high-pressure gas. You probably already know that liquid is not compressible; that’s why an engine gets ruined if you drive into water deep enough that the water gets drawn into the intake. Thus, the refrigerant drawn into the compressor can’t be in a liquid state; it must be a low-pressure gas.
  • A modern, compact, rotary-style compressor.
    A modern, compact, rotary-style compressor. Rob Siegel
  • The condenser is like a radiator. It takes the high-pressure gaseous refrigerant supplied by the compressor, and allows it to cool into a high-pressure liquid, dumping the heat to the ambient outside air when it does so. An auxiliary cooling fan on the front of the condenser helps the heat to shed.
  • Condenser with auxiliary cooling fan.
    Condenser with auxiliary cooling fan. Rob Siegel
  • The expansion valve provides a variable restriction to the high-pressure liquid, like a spray nozzle at the end of a garden hose. The refrigerant drops in pressure as it passes through the expansion valve, allowing it to, as the name implies, expand, and then cool. The amount which the expansion valve opens is determined by the refrigerant pressure in the evaporator. On a vintage car, you can sometimes hear the evaporator “sigh” as the expansion valve opens and lets refrigerant spray through it.
  • The evaporator core is the place where the cooling action is. Not unlike the condenser, the evaporator core is like a radiator with metal tubes surrounded by fins, but it works in reverse from a radiator in that it doesn’t dump heat; it absorbs heat. The liquid refrigerant sprays from the expansion valve into the evaporator core, where it drops in pressure, boils, evaporates, and expands, thereby cooling. Ideally, the evaporator core runs at about 32°F. Since heat always flows from a warmer body to a cooler one, the air surrounding the evaporator core surrenders its heat and becomes cold. An evaporator fan blows the chilled air into the passenger compartment.

Evaporator core and expansion valve.
Evaporator core and expansion valve. Rob Siegel

As far as the major components of the A/C, that’s pretty much it. Note that in a home window-mounted A/C unit, all of these components are contained inside the square box, with the evaporator core located on the front of the box so the chilled air can be blown inside the house, and the condenser located on the back of the box so it can dump the heat to the outside.

However, in a car, the components are distributed. The evaporator core, expansion valve, and fan are usually together in a box referred to as the evaporator assembly that lives inside the car, up under the dashboard, surrounded by the center console. The compressor is mounted to the engine and driven by a belt from the crankshaft pulley. There is a magnetic clutch on the compressor that allows the pulley to free-wheel when the compressor isn’t engaged. The condenser and auxiliary cooling fan are mounted in the nose of the car, in front of the radiator, to allow ambient air blown over it by the car’s motion to carry the heat away.

On most vintage cars with A/C, you’ll find an evaporator assembly housing the evaporator core, expansion valve, and blower fan inside the center console.
On most vintage cars with A/C, you’ll find an evaporator assembly housing the evaporator core, expansion valve, and blower fan inside the center console. Rob Siegel

I’ve left out two pieces. There are hoses and fittings to interconnect the above components, and there is a receiver-drier, a canister that provides extra volume for the liquid refrigerant, as well as a filter to clean it.

When everything is taken into account, an automotive A/C system with an expansion valve looks like this:

The components and their interconnection in a system with an expansion valve.
The components and their interconnection in a system with an expansion valve. Rob Siegel

Note that not all systems have expansion valves. There is another type of system called a Clutch Cycling Orifice Tube (CCOT) system. Whereas an expansion valve, generally located inside the evaporator assembly, provides a variably-sized restriction through which the liquid refrigerant passes, a CCOT system has an orifice tube with a fixed-diameter opening, slid inside the metal pipe leading into the evaporator. Because, unlike the expansion valve, the size of the opening in an orifice tube is fixed, there needs to be another mechanism to vary the amount of refrigerant passing through it. For this reason, instead of a receiver-drier, a CCOT system has an accumulator with a pressure switch on it, mounted at the outlet of the evaporator instead of at the inlet like on an expansion valve system. The pressure switch opens when the pressure in the accumulator reaches a certain level. This shuts off the compressor. When the pressure falls, the switch closes, turning the compressor back on. The components in a CCOT system look like this:

The layout of components in a system with an orifice tube instead of an expansion valve.
The layout of components in a system with an orifice tube instead of an expansion valve. Rob Siegel

With all of those components visualized, we now can talk about the “low side” and the “high side” of the system. The low-pressure side, usually colored in blue on diagrams, is where the liquid is evaporating and cooling. It consists of the section starting at the outlet of the expansion valve or orifice tube, through the evaporator, through the accumulator on a CCOT system, and to the inlet or suction side of the compressor. The high-pressure side of the system, usually colored in red on diagrams, is where the refrigerant is condensing and heating up. It runs from the outlet or discharge side of the compressor, through the condenser, through the receiver-drier on an expansion valve system, to the inlet of the expansion valve.

In the diagram below, we combine the physical components with the heat transfer mechanisms occurring in each. To walk it though, low-pressure gaseous refrigerant is drawn into the suction side of the compressor. It’s discharged by the compressor out the “high side” as high-pressure gaseous refrigerant. It passes through the condenser, where it condenses into a liquid, surrendering heat to the outside ambient air. The high-pressure liquid passes through the receiver-drier and into the “low side” through the expansion valve or orifice tube. There, the restriction in the expansion valve causes it to drop in pressure and spray, like a nozzle on a garden hose, into the evaporator core. In doing so, the refrigerant boils, evaporates, expands, and cools. This causes the evaporator core to be much colder than the air in the cabin. The cold core thus absorbs heat from the surrounding cabin air, cooling it. The evaporator fan blows the cold air into the cabin (everybody say “ahhhhhhh”). The low-pressure refrigerant, warmed by the heat it absorbed from the cabin, is then drawn into the suction side of the compressor, and the cycle begins again.

Heat transfer and cooling in an A/C system.
Heat transfer and cooling in an A/C system. Rob Siegel

The diagram shows the heat transfer process for an expansion valve system, but it’s very similar for a CCOT system. The main difference is that, instead of a receiver-drier on the inlet side of the evaporator, there’s an accumulator on the outlet side with a pressure switch.

Really, that’s most of it. For this week, the main take-away messages from our class in automotive A/C theory is this: When an A/C system isn’t working, and you’ve ruled out simple stuff like blown fuses, it could be because the gaseous refrigerant isn’t being compressed (bad compressor) or isn’t expanding (clogged expansion valve or orifice tube), but most of the time, especially in a neglected vintage car, it’s because the refrigerant has leaked out and there’s nothing left to compress and expand.

Next week: Refrigerants.


Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel magazine for 30 years. His new book, Just Needs a Recharge: The Hack MechanicGuide to Vintage Air Conditioning, is now available on Amazon. You can order a personally inscribed copy here.

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