How air conditioners take hot and humid air, and make it drier and cold.
In: 36
Warm air passes over cold coils and fins. These are made cold by refrigerant.
The humidity condenses on these cold coils as contact with the air is made.
The cool air is then blown via a fan into a room. The condensation from the process either leaves the unit, or evaporates within it.
The AC passes the air over a very cold radiator. That greatly reduces the temperature. At lower temperatures, air can’t hold as much water, so the water condenses on the outside of the radiator. The water drains down to a pipe that lets it drip outside. The very cold air is this mixed with the air in the room to cool the room.
The radiator is connected to a heat pump outside, which warms the outside to make the radiator inside the HVAC system cold.
Kinda hard to eli5
But basically 3 parts and a refrigerant.
A refrigerant is basically a substance that can change it’s temperature based on pressure so when you drop pressure it makes cold.
An air conditioner is a evaporator condenser and compressor.
The refrigerant gets compressed so it’s at a high pressure. It then goes then goes to the evaporator, which usually has a small limiting valve at it’s entrance. This takes the high pressure refrigerant and only lets some through.
The refrigerant is now lower pressure. It then literally sucks the heat out of the room as it moves through the coils. A fan usually blows over the coils to help spread the cold around.
The refrigerant heats up and eventually dumps it’s heat at the condenser. This is the hot side of your air conditioner. Usually it faces or is completely outside of the area.
The refrigerant is compressed again. And it keeps going around.
This process is extremely efficient and can even be reversed. That’s what a heat pump is.
If you’ve ever used an air duster can or any type of aerosol can, the can turns cold when you use it. In short simple terms, when you expand a gas (all else equal), you decrease the temperature, when you compress a gas (all else equal), you increase the temperature. Air conditioners play with this concept to move energy around.
They compress a gas outside in pipes to increase its temperature above the temperature of what it is outside. Fans cool the gas to the outside temperature. Then, they bring that gas expand it which brings it to below freezing. Fans then blow hot air from your house onto these freezing pipes which condenses any water in the air and cools the air. The cool air is now blown into your house.
What is required to keep this process going is energy. This is why air conditioners use so much electricity
It’s basically magic. Google the “vapor-compression refrigeration cycle”.
The ELI5 version is that your refrigerator (or water cooler or AC unit) takes heat from a cold source and rejects it to a hot source. Normally, heat wants to flow from a hot source to a cold source. That’s the magic part. Your refrigerator does this by putting the refrigerant into a state where it really wants to boil (evaporate) at the right temperature – usually around 40-45 degrees F. When it evaporates, it sucks heat out of its environment….which can be the air in your refrigerator, or the air in your room, or the drinking water in a water cooler. In doing so, it cools the air or water.
The more complicated / complete version is that there’s four stages in the vapor-compression refrigeration cycle….which acts on a refrigerant. (Refrigerants are just chemicals that are typically gas at normal conditions, but which can be put into liquid states where they really “want” to boil or evaporate.)
So the four stages of the cycle are compression, condensation, expansion, and evaporation…..which occurs on the refrigerant.
The compression is done by….the compressor. The refrigerant is a gas at this point, and is compressed to a higher temperature and pressure. That’s the part of your refrigerator (or water cooler or window AC unit) that makes that kind of low rattling / humming noise. If you put your hand on it, it will be hot.
The condensation is done by….the condenser. At this point, the refrigerant starts as a hot gas and condenses into a hot liquid, still at high pressure. This corresponds to the heat being rejected from your refrigerator. The condenser coil is the part of your refrigerator that blows hot air into your kitchen.
The expansion is done by…the expansion valve. The refrigerant passes through the valve, and expands to a lower pressure. You don’t see this.
The final stage, where the actual cooling occurs, is evaporation. The refrigerant is in a state where it’s just dying to boil, and changes phase from a liquid into a gas. Whenever anything boils (evaporates), it sucks heat out of its environment. This occurs in the evaporator – which can be an air coil, or a water heat exchanger. In your refrigerator, this happens in the little blower coil that’s inside the refrigerator, near the top of your refrigerator. In a water cooler, it’s a copper heat exchanger that has refrigerant in one side and drinking water in the other side.
The important point of all this is that these three stages (compression, condensation, expansion) are all there to just get the refrigerant into a state where it really “wants” to evaporate….and suck heat out of its environment….thus causing a cooling effect.
I’m actually going to try a slightly more ELI5 version here. Wish me luck.
All heat really is is molecules zooming around. Like anything that zooms around, molecules have momentum. When they bump into stuff, some of that momentum transfers into that stuff. Like pool balls on a table or a Newton’s cradle.
That’s one of the main ways heat moves. The reason why when you put a hot thing next to a cold thing they both end up kind of warm is that because the molecules in the hot thing are moving so much faster than the molecules in the cold thing, there are a ton more hot-smacking-cold collisions than cold-smacking-hot collisions. Eventually, when the hot molecules have given up enough momentum that the cold molecules are going just as fast, the hot and cold things stay the same temperature because their molecules hit each other at the same rate. Neither side is faster.
Heat moves because of how many hot-smacks-cold collisions versus cold-smacks-hot collisions there are in a given space.
So suppose you have some air in a syringe. Its molecules are going a specific speed, room temperature, and smacking into the walls of the syringe say 100 times a second. The walls of the syringe are at room temperature so its molecules are hitting back at 100 times a second. For every molecule that smacks the wall, the wall smacks back. It’s balanced.
Now say you plug up the syringe’s end with your finger and squeeze the plunger to squish the air molecules closer together. Now, the air molecules are going the same speed as before, but because they have less room to move around and smaller walls to hit, they hit the walls much more often. That means they start hitting the walls of the syringe (that they’re still exposed to) much more frequently than the walls are hitting back, because the walls didn’t change. That means they’re going to be giving the walls more momentum than they get back, and that means the walls will heat up, even though you didn’t add any heat.
Eventually the wall’s molecules will get enough momentum from the air that it’ll be the same, and eventually the wall will even lose that heat to the room air because the room air is cooler than the wall now. It’ll all be balanced again.
Now suppose you un-push the plunger. The air in the syringe gave its molecules’ momentum up, but now they have all the space back and bigger walls to hit—which means they hit the walls less frequently. So now the wall’s molecules hit the air molecules more frequently than the air molecules hit back. More momentum goes into the air than comes back out. *The air gets cold.*
Now suppose you unplugged your finger from the end of the syringe and, real quick, hooked the syringe to a metal pipe that poked into a sealed, insulated box. The air’s cold now, so when it goes into the pipe, the heat from the pipe moves into the air, making the pipe cold, and then the heat from the air from the box moves into the pipe, making the air cold.
Now suppose you wanted to do this over and over so you could make the box cold enough to hold your ice cream without melting.
You could take an air pump (or maybe there’s something that works even better than air…) and hook it to a metal tube. When you pump the air into the tube, if the tube has a valve to keep the air from getting out, it’ll compress like the air in the syringe, make the pipe hot, then the heat will dissipate out into the air in the room. Then if that valve were to let the air out (slower than it was coming in so it would still be squished on the pump side) just as the pipe poked through to the box, the air would expand and get cold just as it went in. You could keep the pipe looping through the box for a while, long enough for the heat from the air in the box to go into the pipe and into the previously-squished air, then once the pipe air was warmed up, the pipe could punch back out of the box, and go right back into the pump.
Voila. Refrigeration.
If you want to cool a very hot piece of metal, you can spray it with water, and it will make the water boil and produce steam. Since boiling water takes a lot of energy, it cool hot things very well. You can also control at which temperature you want to cool thing, since if you just spray until it stops boiling, then the piece of metal is at 100C (212F), which is the boiling temperature of water.
In an AC you spray the inside of a small pipe (usually copper) with a special liquid that boils at 4C (38F) or so. You can then have a pipe that is kept at a cool temperature, and just blow air with a fan close to the pipe (which is usually in a serpentine or coil shape to save space, often with thin aluminum sheets perpendicular to the pipe, those are just to reduce the pipe length needed).
The humidity removal is the same phenomenon you see when you have a cold bottle of beer (4C from the fridge) on which the humidity in the air condense and eventually drips on the floor. This water (previously humidity) is removed from the air, making is dryer. Same thing with the cold pipe in you AC (it drips outside the window or in the drain).
The rest of the AC is just fancy gadgets that take the liquid you just boiled and bring it back to a liquid you can spray again.
Have you ever had one of those cans of compressed air for cleaning your computer? Do you notice how the outside of the can gets cold when you use it a lot? That’s because the changing pressure of a gas inside the can causes it to get cold. Air conditioners work on a similar principle.
Colder air can’t hold as much water. Like when you have a cold drink on a hot day, condensation forms on the outside of it because the air near the surface of the glass is much cooler. Or when you breathe on the cold day the water condenses on the way out of your mouth and you can see your breath in the form of water droplets.
Now as for how air conditioners work, when you put a fluid under pressure, it warms up, and when you take away that pressure, it cools down. Think about it like this, by compressing the fluid, it acts like a spring. You put energy in, and it stores that energy as heat. Now if you take that energy out by letting the heat radiate away, you can decompress the fluid and it’s much cooler than you started with because all of that heat energy radiated away. Air conditioners simply compress a fluid called a refrigerant, and let it cool off outside, then bring it inside and let it warm back up from the air inside, before sending it back outside and repeating the cycle. Matter also needs to absorb/release energy to change between phases, and refrigerant is actually a liquid on the hot side and turn to a gas on the cool side for extra energy storage per cycle, but that’s the basic idea.
Fun fact: refrigerators, freezers, dehumidifiers (that don’t just use a desiccant to pull moisture from the air), conditioners work in the exact same way
Fun fact: the air conditioner was invented by trying to make a dehumidifier, but then the air cooling benefits were an appreciated side effect.
Fun fact: a heat pump is just an air conditioner working backwards, taking heat from the outside and bringing it inside.
Air conditioners rely on some basic ideal gas laws that you probably learned in middle school. Let’s do a quick refresher.
* If you compress a gas, its temperature will go up.
* If you allow a gas to expand, its temperature will go down.
Here’s how your air conditioner uses these two laws to cool your house.
Outside your house, there is something called a compressor. This compressor pumps a gas into a copper tube that runs in a coil shape. At the other end of the long, coiled up tube is a tiny opening so that the gas cannot flow through quickly. The pumping action compresses the gas in the tube as it is blocked by the tiny opening, called the orifice.
As the gas is compressed, it gets hot. Like, really hot. Hot enough to burn your hand, and much hotter than the air is outside. Let’s say it gets up to 190°F, but the air outside is only 85°F. The compressor also has a fan that blows across the coils. The 85°F air blowing across the hot coils carries some of the heat away.
Inside your house is something called the air handler. It contains a fan that moves air around your house, and another, larger (diameter) coil of copper tubing that is connected to the compressor outside so that the two form a loop. Importantly, the orifice is also right at the beginning of the evaporator.
As the gas exits the orifice, it enters the larger diameter evaporator coil where it rapidly expands. Thanks to the ideal gas law we talked about earlier, this causes its temperature to drop rapidly. This causes the evaporator coil to get cold, because back when the gas was outside and very, very hot, we blew a fan across it to cool it down.
The fan in the air handler blows air from your home across the cold evaporator coil, cooling it. As the warm, humid air from inside your house hits the cold evaporator coil, condensation forms on the evaporator coil and drips into a condensation collection pan. The condensation pan has a pipe connected to it that carries the water away. Usually outside, but sometimes into a drain inside your house.
Latest Answers