General Question

andrew's avatar

How does adding energy into a system cool it down? (How does air conditioning work)?

Asked by andrew (16346points) April 30th, 2009

Inspired by my previous question, I’ve always been a little curious as to why air conditioning requires so much energy to run.

If I understand A/C correctly, you’re passing air over a super-cool element (say, a tube filled with freon). The energy passes from the air into the freon, heaiting it slightly.

The freon is then pumped into a condenser, which then releases (?) the heat back into the atmosphere, away from your nice cool home or car.

So, is it the case that you’re just transporting the heat to another place where it doesn’t matter, and that’s what requires the energy? Is my understanding of the condenser correct?

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7 Answers

FrankHebusSmith's avatar

I’m not 100% on how an A/C works, but I do know a basic concept of how adding energy will actually cool something.

Every molecule will become a liquid at some temperature (EVERYTHING). For some molecules that temperature is VERY low, for example they use liquid methane to “burn” off warts as it has a very low liquid temp. So theoretically if you could condense a molecule into it’s liquid form, it would be very cold. So in essence I would think that if you could condense a molecule (in this case freon) into a liquid, and then run it thru a series of tubes in front of a fan, the air from the fan would become cold.

However the energy needed to condense those molecules, would be rather significant. Hence you would have to put energy in, in order to get cold air.

You should also keep in mind that it’s not only cold air coming out of your A/C. The side that faces outside actually pumps out quite a bit of hot air.

Harp's avatar

Air conditioners take advantage of the principle that when substances go from a liquid to a gaseous state (evaporate) they absorb heat energy from the environment, and when they go from a gaseous state to a liquid state (condense), they release heat energy. The AC puts a refrigerant (freon) through both of these changes, by alternately compressing and decompressing it.

When compressed, the freon goes into a liquid state, which causes it to release heat. The hot pressurized liquid passes through a coil over which a fan blows. This heated air is directed outside your house. The cooled, but still liquid, freon is then directed through another coil, where it is allowed to depressurize, becoming a gas again. In the process, it absorbs heat from the air surrounding this coil. Air from the interior of the house is passed over this coil and blown back out into the room.

So the energy used by the AC is mostly consumed by the action of compressing the freon, which requires a hefty electrical motor.

RocketGuy's avatar

Conceptually, you are using electrical energy to pump heat energy from a warm place (the room) to a hot place (outside). Since this is counter to the natural flow of heat energy (hot to cold), energy is needed. @Harp accurately described this process. Note that the heat energy output into the environment is equal to the heat energy pumped out of the room + the electrical energy to do it. Not a very efficient process, and results in more heat all together.

andrew's avatar

@Harp So obviously the energy is used to push the freon around the system… is the “compression” just a product of a smaller tube?

Lupin's avatar

There’s another type of heat pump. We use the Peltier effect in our thermoelectric coolers to keep our high speed/high power circuits from overheating. The device is solid state and moves heat from one side to the other when you apply current. They are expensive, and have limited efficiency but cannot be beat when you have to work in a small space. Seebeck and Peltier were smart boys
Here’s a good explanation .

Harp's avatar

The compressor functions as a pump, using a piston to compress the freon (here’s a schematic of the compressor). The decompression further down the line happens when the freon is released through a small aperture in the form of a spray into the evaporator (cooling) coil. So that aperture acts as the bottleneck in the system, in effect resisting the push from the compressor.

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