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Air conditioners

by Chris Woodford. Last updated: June 10, 2013.

What's the best way to cool down your kitchen on a hot summer's day? If your immediate answer is "Open the refrigerator door," you're way off target! Every bit of heat a refrigerator sucks in through its cool box is pumped straight out of the metal fins at the back. If anything, because of the sheer inefficiency of the machine, you'll make the room even hotter. But using a refrigerator to cool a home isn't such a mad idea as it might seem: with a few slight modifications, it's almost exactly how an air conditioner works. Let's take a closer look!

Photo: A typical air conditioner unit outside a restaurant. This is the fan that blows away the hot air. There's another fan you can't see, circulating cool air inside the building. Most air conditioners are permanently fixed in one place, but you can get small portable air-conditioning units too.

How not to cool your kitchen

Refrigerator with the door open

A basic law of physics called the conservation of energy says you can't make or destroy energy: if you have some energy you don't want (such as heat in your kitchen), you can't get rid of it completely. All you can do is change it into another form or move it to another place. If you open your refrigerator door in the hope that you'll cool the kitchen, all the heat that gets drawn in has to go somewhere else. The only place it can go is out of the back of the machine. You may have noticed that the grid of fins on the back of a refrigerator gets pretty hot—and that's why: they're giving off all the heat that would normally be inside. You can find out more in our article on how refrigerators work.

Photo: Physics tells us you can't cool your kitchen by leaving the refrigerator door open!

How to build an air conditioner

But all's not lost! Instead of letting the power of science defeat us, we just have to use it the right way.

Suppose you take a refrigerator and build your house around it, so half the machine (the chiller cabinet) is inside your home and the other half (the grid of hot fins at the back) is outside. Now if you leave the door open, what you have in effect is a fully fledged air conditioner. It draws in heat from inside your home and belches it out again outside, gradually cooling your home in the process.

The simplest air conditioner units work in almost exactly this way, except they have fans on both sides to circulate air more rapidly. They also have a heating element in them so they can warm the air in a room on cold days as well as cool it down on warm days. Machines like this are sometimes called HVACs (heating and ventilation air conditioning units). More elaborate air conditioners use long ducts to pipe the warmed or cooled air throughout an entire building, but they still work in essentially the same way.

How an HVAC air conditioner works

artwork showing the basic components of an air conditioning unit and the order in which they work

  1. Warm air from the room is sucked in through a grille at the base of the machine
  2. The air flows over some chiller pipes through which a coolant fluid is circulating. This part of the machine works just like the chiller cabinet in a refrigerator. It cools down the incoming air and a dehumidifier removes any excess moisture.
  3. The air then flows over a heating element (similar to the one in a fan heater). On a hot day, this part of the unit may be turned right up so the HVAC works as a heater.
  4. A fan at the top blasts the air back through another grille into the room. If the heating element is turned down, the air re-entering the room is much cooler, so the room gradually cools down.
  5. Meanwhile, coolant (a volatile liquid that evaporates easily) flows through the chiller pipes. As it does so, it picks up heat from the air blowing past the pipes and evaporates, turning from a cool liquid into a hotter gas. It carries this heat from inside the room to the outside of the building, where it gives up its heat to the outside air. How? Just like in a refrigerator, the coolant flows through a compressor unit and some condensing pipes, which turn it back into a cool liquid ready to cycle round the loop again.
  6. What happens to the heat? In the unit outside the building, there are lots of metal plates that dissipate the heat to the atmosphere. An electric fan blows air past them to accelerate the process.
  7. Over time, the heat inside the building gradually pumps away into the outside air.

Daikin air conditioner: front view Daikin air conditioner: side view and closeup of heat dissipating metal plates
Photos: Where does the heat go? Look around the side of an air conditioner like this and you'll see it's jam-packed with metal, heat-dissipating plates. You may even feel some heat being given off as the fan sucks or blows air past them. The image on the right is a close-up of the black area outlined in the middle photo.

How air conditioners can harm the environment

Hole in the ozone layer 1998. Picture by NASA

You probably love the feel of freshly chilled air on a hot day, but don't forget that law called the conservation of energy! There's always a price to pay for getting something good in our universe. In this case, the price is the energy you have to use to run the air conditioning unit; using energy means there's an impact on your pocket and on the planet too in the shape of environmental problems like global warming. Environmentalists say we should use less air-conditioning, which sounds easier than it is in a really hot climate. It's important to remember that air-conditioning isn't just about luxury or comfort: an air-conditioned room can make you much more productive at work and it can have important health benefits too; some public-health doctors have suggested that the greater use of air conditioning in the United States is one reason why there are fewer heat-related deaths there than in Europe, where air conditioning is used less. It's sometimes argued that if people don't have air conditioning, they're more likely to use things like electric fans, which work very inefficiently (rearranging hot air instead of removing it and generating heat with their own electric motors). But the biggest electric desk fans (typically rated 25–50 watts) use a fraction as much electricity as the smallest air conditioners (typically rated at 750–1000 watts); you could use about 20–30 fans and consume the same or less power than a compact AC unit!

So what's the environmental damage? Let's consider the energy first. Every time you switch on the air conditioner in your car, you add an extra 10–20 percent to your fuel consumption (and an extra 10–20 percent to the price you pay at the gas station). At low speeds, opening a window instead is often a better option, though at higher speeds you create air resistance (drag) and waste more energy than you save. At home, using the air conditioner will add plenty to your electricity bill; when physicist Tom Murphy tested his air conditioning scientifically, he found he used "more electrical energy in two days than we normally expend in a month." You could try other strategies like opening your windows all night but shutting them tight first thing in the morning and throughout the daytime to keep hot air out of your home. In really hot climates, you might find you simply cannot do without the AC; even so, you can dramatically reduce how much it's costing you (and how much energy you're using) simply by turning the thermostat to a slightly higher setting.

Air conditioning units used to have another very harmful effect on the environment as well. Until the late 20th century, most used coolant chemicals known as chlorofluorocarbons (CFCs) (so called because they are made from the chemicals chlorine, fluorine, and carbon), which were also used widely in refrigerators. When old air conditioners and refrigerators were broken apart for scrap at the end of their lives, the coolant chemicals escaped into the atmosphere. Floating up into the stratosphere (the upper atmosphere), they rapidly damaged Earth's ozone layer: the natural sunscreen that helps to protect us from the Sun's harmful ultra-violet rays. Most modern air conditioners avoid CFCs (now banned in many countries under a global agreement called the Montreal Protocol) and use alternative coolant chemicals instead (typically halogenated chlorofluorocarbons or HCFCs). If you look closely at our top photo, you can see that the fan has a green "Ozone friendly" label on it, which means there are no CFC coolants inside.

Photo: The ozone hole over Antarctica that was caused by CFC pollution, mostly from air conditioners, refrigerators, and aerosols. Picture courtesy of Great Images in NASA.

Drawing of Willis Carrier's air conditioner patent from 1934, reissued 1941.

Who invented air conditioners?

If you couldn't live without your air-con, thank Willis Carrier (1876–1950). He was the man who pioneered this "cool stuff" in the early decades of the 20th century. Here's one of his early designs—and note how closely it resembles my quick sketch up above. How does it work? Warm air is pulled in from a room (1), mixed with fresh air (2), conditioned, and blown back into the room by a fan (3). Heat is removed by the refrigerator chiller pipes in the center of the duct (4), which are fed and controlled by a system of pumps, compressors, valves, and thermostats (5).

Photo: One of Willis Carrier's air conditioner designs. This diagram is part of Carrier's US patent #675,144, filed in 1933 and reissued in 1941, which you'll find among the references below. Picture courtesy of US Patent and Trademark Office.

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Air conditioners. Retrieved from http://www.explainthatstuff.com/airconditioner.html. [Accessed (Insert date here)]

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