The Magic Inside an Air Conditioner

Last time we talked about the history of getting cool. We looked at evaporative cooling, cold water cooling, and the eventual rise of Willis Carrier’s air conditioner. What we didn’t do was look at how a modern air conditioner works. Inside that little thing in your window or the twenty ton monster on your roof, machinery is exploiting physics to move energy from one place to another.


The 3 Part Process

We can simplify the whole AC Process down to three things: A Condensor Coil, an Evaporator Coil, and a Compressor. The Condenser Coil is outside, during operation it becomes incredibly hot, radiating heat into the air. The Evaporator Coil is inside your facility with a fan blowing over it. It becomes incredibly cold during operation. The Compressor is usually outside as well, but there are exceptions to the rule. It squeezes the refrigerant in your system, forcing it to give off heat and cool down.

I bet you can see where this is going. So, we’ll start at the condenser. Your refrigerant is compressed into a liquid. As a liquid, it’s forced to give off it’s energy, it’s heat. The heat is dissipated into the  Condenser Coils. It moves out into the fins of the coil and is given out into the air passing over them. The liquid refrigerant is then sent inside to the Evaporator Coil. In these coils, it rapidly picks up heat. The coils pull heat out of air passing over their fins, and put it into the refrigerant. Once turned into a gas, the refrigerant is pushed outside to repeat the process.


Exploiting the Laws of Physics

Now, the question no one ever asks: WHY does this work? If you compressor gasoline, it goes boom. If you compress gun powder, it goes boom. If you compress your boss’s computer in the trash compactor, he goes boom. Most of the time when you squeeze something, it tends to push back with the same or greater force.

Well, first off, don’t use gunpowder as a refrigerant. Second, we’re using what is known as Charle’s Law. Charle’s Law states: When pressure is constant, the temperature and volume of a gas are directly related. In our little system, we are compressing the gas to reduce its volume. Low volume means low temperature. Low volume means the gas is “saturated” with all the energy it can handle. You could perhaps consider this analogous to pouring so much sugar in your coffee that a sugar-sludge collects at the bottom. The coffee can’t hold any more sugar. The compressed gas cannot hold any more air.

At the Evaporator Coils, we suddenly have an area of greater volume. The refrigerant can expand. As the refrigerant expands, it’s temperature must go up. Well, we have another law about that. The Conservation of Energy. Energy has to come from somewhere. You can’t magic up a bundle of energy out of thin air (if we could, I’d be magic’ing up gas for my truck). As a result, the gas is instead forced to absorb energy. It has a higher capacity for energy and readily takes it out of the coil around it. This allows the coil to become cold. Once it’s cold, air passing over becomes cold.  In the case of our coffee analog, if you heat the coffee up, it can dissolve more sugar. If you make the coffee hot enough, maybe pressurize it a bit like a mad scientist, it won’t have the sugar sludge anymore… But it will probably melt your Styrofoam cup, so don’t do that.

At this point, we’re ready to repeat the cycle. We compress the gas, forcing the energy out of it. The gas becomes a liquid. The liquid is ready to take on energy. Here we go again.

Refrigerant Safety

There is one last thing to cover: The Humble Refrigerant. This is important to touch on because while the process remains the same, there are good, bad, and ugly refrigerants in the wild. There’s some safety material we want to dump on you just in case you walk by a coil making a very strange hissing sound. If you hear such a noise, run. Don’t walk. Run. Call your HVAC/Maintenance Crew or maybe a Fire Department unless you know what that refrigerant is. If you’ve got kids or animals, get them far from there, they may be more sensitive or receive a higher dose.


As we covered earlier, not everything takes kindly to compression. Compressing gun powder, your boss’s computer, and water will lead to tears in a hurry. As a result, only a few compounds are ever actually used as refrigerants. These include Sulfur Dioxide, Ammonia, Propane, and various forms of chlorofluorocarbons (CFCs). Most of these compounds are either prone to catching fire and making giant fireballs or prone to killing you after brief contact. I once got a little exposure to what I think was Freon. I broke my mini-fridge, it leaked, and I got lucky. It just gave me an awful headache and I was tired for a while. We drained it safely and now it’s waiting for me to fix it with a new coil. Work is ongoing to find better and safer refrigerants, but at the end of the day, it’s better safe than sorry. If you hear hissing from an HVAC System, let someone with an air tank, a gas detector, and a fireproof suit deal with it.

That wraps up today’s post! What do you want us to cover next? Seriously, someone send us a suggestion at We’ve got a thousand topics and we don’t even know where to start anymore! We want to do something about soldering pipes, something about pools and pool safety for the summer, maybe some stories about frost build up in AC Systems, and there’s even a glimmer of thought about an article on a DIY Smart Thermostat.


Most of all however, we want to produce content you’ll read. What are you most interested in?

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