The Magic of Cold, Part 1 - How Your Air Conditioner Works
If you've wondered how it is that your air conditioner actually keeps you cool in summer, you've found the right place. As Earth-shattering as this sounds, I've actually spent a lot of my half-century living in it! I know, I know. It's an amazing accomplishment, but really, I have to give all the credit to my parents, grandparents, Willis Carrier, and the internets.
An air conditioning system is an amazing thing. I spent the years 1988 to 1998 living in Louisiana and Florida driving a car without an air conditioner, so I appreciate cool air. If you've been a reader of this blog for a while, you probably know that I also appreciate HVAC systems done right.
If you've ever wondered how your air conditioner works and couldn't decipher the explanations you found in other places, I think I can make it intelligible for you. Today's post is going to cover the basics of air conditioning in plain English. No TXVs, subcooling, or evaporator coils! Please comment below and let me know how well I succeed. If you want to go a little deeper, tomorrow I'll cover the refrigeration cycle at an intermediate level.
The Fundamentals of Air Conditioning
An air conditioner is a device that moves heat from one place to another. It picks up heat from inside your home and moves it to the outside. In other words, it pumps heat from one place to another. Although we could call it a heat pump, we usually reserve that term for air conditioners that can pump heat in either direction - inside to outside or outside to inside. (I wrote about the paradox of getting heat out of cold winter air a while back.)
When you put your face in front of that AC vent, it may seem that an air conditioner creates cold, but in reality, it's removing thermal energy from inside your house and sending it outside. This transfer of heat from your home's air does indeed make the air cooler, and the air blowing out of the supply vents does feel cold. It's best to think of the process, though, as a heat flow from inside to outside. (For more about this, read What IS Heat Anyway?)
The Refrigeration Cycle, Simplified
What makes an air conditioner work is a thermodynamic cycle called the refrigeration cycle. It's a series of changes in temperature, pressure, and state (liquid/vapor) that the refrigerant undergoes as it removes heat from the house. The refrigerant is a special fluid that changes between liquid and vapor at convenient temperatures for pulling heat out of air that's at about 75° F and dumping it into air that's above 90° F. It's what travels through those copper pipes, one insulated and one uninsulated, that connect the indoor part of your air conditioner to the outdoor part.
I'm going to focus this discussion on the most common type of central air conditioning system - the air-source, split system. It's called air source because it dumps the heat from inside the house into the outside air, as opposed to a ground-source or water-source system that dump the heat into, well, the ground or some water. It's called a split system because there's a unit that sits outside making all that noise all summer long and another component that's inside the house somewhere, maybe in the attic or crawl space. Other types of air conditioners still follow the same refrigeration cycle, but the locations of some of the pieces differ.
The refrigeration cycle has four stages, so let's go through each of them. We'll start with the refrigerant collecting the heat from inside the house, labeled number 1 in the diagram below.
Step 1: Catch the heat from inside the house.
The inside part of your split system AC has a blower, that pulls air from the house and sends it over a very cold coil. This coil has cold, cold, cold refrigerant running through it, so the air passing over it gets cold. Well, OK, what's really happening is that heat from the air is flowing into the cold coil with the cold refrigerant.
When the air comes off the coil, the temperature has dropped about 20° F (if everything is working right). So if you're keeping your house at 75° F, the air coming off the coil is at 55° F.
The basic physics here is that heat likes to flow from something warmer to something cooler. Warmish house air moves over the cold coil, and heat flows out of the air and into the coil. The refrigerant picks up the heat, increasing in temperature, and it just so happens that the heat it picks up causes the refrigerant to boil. It changes from a liquid to a vapor inside this coil.
The warmer, vaporized refrigerant then flows on to step number 2.
Step 2: The refrigerant gets pumped up to a high temperature.
The still cool but vaporized refrigerant flows to the outdoor unit and enters the compressor. The compressor then pumps the refrigerant to a high pressure, which also causes the temperature to increase. Why do we want the refrigerant at a high temperature? Because heat flows from warmer to cooler. Right?
When the refrigerant comes out of step number 1, it's still cold. Find the copper tubes that carry the refrigerant between the indoor and outdoor parts of your AC. The refrigerant going from step 1 to step 2 is in the insulated copper line. Find a place where you can touch that pipe, and you'll see that it's still cold.
Heat doesn't flow from cold to hot, so if we want to get the heat out of that refrigerant and put it into 95° F air, we've got to increase the temperature. The compressor does that job and takes it up to a temperature well above ambient (outdoor) temperatures. Air conditioners work even in places like Phoenix, Arizona, where the outdoor temperature can get to 115° F and above, so the compressor has a lot of work to do.
Step 3: The refrigerant gives up its heat to the outdoor air.
After getting pumped up to a high temperature, the now hot, vaporized refrigerant passes through another coil. This is the coil that surrounds the compressor in the outdoor unit. A fan inside the unit pulls outdoor air through the coil and sends it out the top of the outdoor unit. The hot outdoor air passing over the even hotter coil causes heat to flow out of the refrigerant and into the outdoor air.
Heat flows from warmer to cooler!
As heat flows out of the refrigerant and into the outdoor air, it cools off below the condensation point, and the vapor condenses back into a liquid. The temperature of the refrigerant after coming out of step 3 is still pretty high, which you can verify by putting your hand on the uninsulated copper pipe coming out of the outdoor unit.
Step 4: The refrigerant gets cold.
As the refrigerant travels from outside to the indoor unit, it goes through a special device before it enters the coil I talked about in step 1. This special device lets the warm, liquid refrigerant expand into a bigger volume, which causes the temperature to drop - a lot!
Have you ever used one of those CO2 cartridges to pump up your bicycle tire? Have you ever used a can of compressed air to clean out the keyboard on your computer? If so, you've probably noticed that after you release the gas from the cartridge or can, the container gets very cold. That's exactly what happens in this part of an air conditioner.
Why do we want to lower the temperature?
Heat flows from warmer to cooler!
We need to get the refrigerant colder than the indoor air so that we can pull the heat out of it, so this step is critical. In fact, I like to say that this is where the magic happens in an air conditioner.
Another Way of Looking at the Refrigeration Cycle
Gravity, even though it's the weakest of the fundamental forces of nature, is something we all understand intuitively. We've lived within its constraints our wholes lives, unless we're among the few to travel on the vomet comet or into outer space, so let's look at a gravitational analog for the refrigeration cycle.
The diagram below is basically the whole cycle translated. A cup collects water at the bottom and dumps it at a higher point. Water, in this analog, takes the place of heat, height stands for temperature, and the cup is the refrigerant.
Just as heat flows from warmer to cooler (remember that?), water here wants to flow from higher to lower. To pick up water (heat) from the house, the cup (refrigerant) has to be lower (cooler) than the house. To dump that water (heat) to the outside reservoir, which is higher (warmer), the cup (refrigerant) has to be raised to a height (temperature) above the outside.
So there you have it. If what I wrote above makes sense to you, you now understand how an air conditioner works. It's basically the same for a refrigerator or freezer, with a different range of temperatures. Please let me know what you think in the comment section below.
To read a more advanced version of the refrigeration cycle, in which I name the components and give more detail on what's happening with the refrigerant, see:
The Magic of Cold, Part 2 - Intermediate Air Conditioning Principles