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Street Race Truck More... The Toolbox

A/C 101 on a '65 Mustang

5/9/2017

As Speedway Motors employee Alanna continues the process of installing a Vintage Air SureFit Air Conditioning Kit in her 1965 Mustang, she thought it might be a good idea to have an overview of how the A/C system of your vehicle actually works. If you missed the teardown of the Mustang, you can check it out first by clicking here.

Compressor. Condenser. Dryer. Evaporator. Compressor. Condenser. Dryer. Evaporator. Yea, it was all Greek to me too. When my dad the car guru and my husband the mechanical engineer talked about installing a new A/C kit in our ’65 Mustang, I was lost. Like really lost. At one point my husband tried to explain the cooling process to me and said, ‘it’s the first law of thermodynamics.’ Right, like I listened in my 9th grade physics class. So of course I turned to my trusty friend Google that told me more about the ‘law’ of thermodynamics - energy can be transformed from one form to another, but can be neither created nor destroyed. In the case of an a/c system, written in normal people terms, energy (refrigerant) can move from a liquid to a gas and back to a liquid, and nothing will be lost in the transition back and forth. If you’re still confused, check out physicsforidiots.com. Yes, that’s a real site.

Anyway, for anyone as lost as I was, let’s talk about the basics of air conditioning. Because really, you should know how/why an air conditioner works before you try to install one.

For future reference:

  • A/C Compressor = Yellow
  • A/C Condenser = Green
  • A/C Dryer = Purple
  • Expansion Valve = Pink
  • Evaporator = Orange

But first, DID YOU KNOW cold really isn’t a thing? Coldness is just a lack of heat. So to make something cooler, you just remove the heat… I know, mind blown.

So let’s start with the term refrigerant. The refrigerant is a liquid or a gas with a low boiling point. Water boils at 212 degrees Fahrenheit, whereas a refrigerant boils at 15 degrees below zero. Refrigerant is the stuff that runs through an a/c system, turning from liquid to gas back to liquid, changing pressures and temperatures, etc. (See ‘Law of Thermodynamics’ definition above.) When the refrigerant in a system boils or evaporates into a gas state, it’s absorbing heat from its surroundings, therefore leaving the air cooler.

Moving on to the A/C system, we’ll start with the A/C compressor. The air conditioning compressor is the work horse of the system. Imagine that it’s the heart in a human body – it’s the component that keeps things running throughout the system, just like a heart keeps blood pumping and the entire body working. The A/C compressor is powered by the engine – and is connected to the crankshaft of the engine by a belt. The engine runs, which makes the belt move, which makes the A/C compressor work. Refrigerant pumps through the system, and when it reaches the A/C compressor it’s already in a cool temp, low pressure gas state. So you have this gas flowing through, and then you add the work/energy of the compressor – again, the heart of the system – and the gas is magically turned into a high temperature, high pressure gas. Just kidding, it’s not really magic. The gas (refrigerant) is pressurized, which makes it turn the high temperature, high pressure state. Once it’s reached this stage, it’s ready to move on to the next phase of the system, so the gas releases from the high side port of the A/C compressor and is ready to move on to its next destination, the A/C condenser.

The high pressure, high temp gas flows into the A/C condenser, which is located right in front of the engine’s radiator. It’s placed here so it can get air drawn through it by the engine’s cooling fan. Once the high temperature, high pressure, gas reaches the A/C condenser, the fan on the engine and the air moving through the A/C condenser work to cool the gas. Once cooled, the gas is turned into a high pressure liquid (again, thermodynamics) and is ready to travel out of the A/C condenser and onto the dryer.

The high pressure liquid flows through the dryer, which catches all excess moisture that may contaminate the refrigerant. You may ask, "isn’t the refrigerant a liquid at this point? Isn’t liquid considered moisture?" No. In this case, ‘moisture’ is considered water. Water contamination is baaaaad, because it can cause terrible things to happen in the system under the hood.

Next, the high pressure liquid passes through the expansion valve, which removes pressure from the liquid refrigerant, and at that point turns into a low pressure liquid. The low pressure liquid flows out of the expansion valve and passes through the evaporator, where it starts to boil. As the liquid boils, it turns back into a low pressure gas and absorbs heat. Remember the DID YOU KNOW at the top where I told you that things only become cold because there’s a lack of heat? Here’s where that comes into play. The gas absorbs the heat, which chills the walls of the evaporator. (Again, the walls are chilled because the gas has absorbed a lot of the heat.) At that point, the blower motor pushes the cool air from the chilled walls of the evaporator and blows it into the vents in the dash, giving you the air conditioning you long for on a hot day.

Again, the Law of Thermodynamics states that nothing will be lost in this process, so even though there’s an output of cold air, there’s still a low pressure gas working the behind the scenes because it can’t be ‘lost’ – unless you have a leak of course, but that’s beside the point. After the low pressure gas has flowed through the evaporator, it then travels to the A/C compressor and the cycle starts over again.

It’s a lot to process, I know. But really, if you’re thinking about installing an air conditioner in your car it’s super helpful to know what everything does. And if my ‘Air Conditioner 101’ doesn’t help you understand it at all, give physicsforidiots.com a shot.

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