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An air conditioner works by moving heat from inside to outside your house using a process of evaporating and condensing a liquid.
To understand this basic principle let us look at what happens when you boil water. You put the water in a kettle and heat it up. When it gets up to boiling point (100 degrees centigrade) the water doesn't get any hotter, it does, however, carry on absorbing heat but the heat energy doesn't increase the temperature any more instead it is used to evaporate the water changing it from a liquid into a gas, steam.
It takes a lot of heat to evaporate the water and this is known as the latent (hidden) heat of vaporisation.
Now if we cool the steam down it will lose the heat and condense, changing back into water.
But then there is another important thing about steam. If we compress it we can force it to turn back into water without cooling it, then if we release the pressure it will boil again (as long as we keep it hot).
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Let us see how this principle is applied in an air conditioner.
First we take a liquid that boils at a much lower temperature, it has to be well below the minimum temperature our air conditioner will run at, let us say −41 degrees centigrade. Obviously at room temperature it has boiled into a gas.
Now, outside our house, we put the gas into a pipe and feed it into a compressor which pumps the gas up to about 45 psi (pounds per square inch) the gas condenses and changes into a liquid. It will give up the heat energy and the pipe will get very hot.
Now we feed the pipe inside our house and we put it through a very narrow tube, the constriction of the tube keeps the liquid at high pressure on one side but allows it to slowly run through the tube and into a second wider pipe. As the liquid flows from the narrow tube into the wider pipe the pressure is released and the liquid evaporates sucking heat from the pipe which becomes very cold.
This second pipe is now fed back outside the house and back to the compressor ready to be compressed and start the cycle again.
Now let us do some modifications to make our air conditioner work better.
The first thing we can do is we put a fan to blow air onto the pipe outside the house just after the compressor to blow the heat away into the atmosphere.
We can also bend the pipe to form a coil and increase the surface area of the pipe so the fan can cool it down better.
If we add cooling fins to the pipe (like the fins on a motor cycle engine) this will further increase the fan's ability to cool the pipe.
We can repeat this to the pipe on the inside of the house just after the narrow tube adding a fan, coiling the pipe and adding cooling fins to increase the ability to absorb heat from the air in the room.
As the air from the room flows over the evaporation pipe it cools and any humidity in the air condenses into water drying the air and leaving drips of water on the cooling fins. We need to collect this water and install a drain to carry it away.
We will now add some further items to improve our air conditioner's operation:
1We can add an air filter to collect dust from the air inside the room.
By installing louvres on the inside fan unit we can control the direction the cool air blows.
A speed controller on the fan will allow us to set the flow of air to high or low.
Finally we can add a temperature sensor to switch the compressor off once the inside room is cool enough.
We now have an air conditioner.
Air conditioners come is several configurations though all work using this basic concept. There are variations most notably:
1 Electronic control circuits and an infra red sensor pick up signals from a remote control and provide the ability to set the temperature, fan speed, operating mode and the swing of the louvres and program timing on and off switches.
2 More sophisticated air conditioners use a pressure release valve operated by a temperature sensor instead of the narrow "capillary" tube.
3 Inverter air conditioners. In a standard air conditioner the compressor, which pumps the refrigerant around the system, is driven by an electric motor which runs on standard AC (alternating current) electrical supply which has a serious limitation- the speed cannot be varied, the motor is either on or off so the air conditioner is constantly switching on and off and this creates surges in the electrical supply every time the motor starts up.
4 The latest air conditioners have an inverter to convert the AC (alternating current) supply to DC (direct current) supply. The speed of DC motors can be varied and so the compressor can be slowed down or speeded up according to temperature. The air conditioner runs continuously and far more efficiently. |
