How Air Conditioners Work

A brief description of how a Freon based compression/evaporation cycle refrigeration unit works.

Air conditioners and refrigeration units are a modern convenience that many people take for granted. There are many different cycles used to provide refrigeration in large buildings, and industrial settings: ammonia cycle refrigeration, lithium bromide refrigeration, and even cooling towers all provide means of selectively cooling specific areas. The most common refrigeration cycle used, however, is Freon cycle refrigeration. There are some similarities between Freon refrigeration units and the others mentioned: ammonia refrigeration, for example, works in almost the same way, simply using ammonia as the cooling agent, instead of Freon. Cooling towers don't work with an internal evaporation/compression cycle; they cool by evaporation to the outside air, much like perspiration cools humans. Lithium bromide plants are a bit different in that they do not use an evaporation/compression cycle instead relying on the heat of solution to provide the cooling necessary.

So, what is this evaporation/compression cycle that I'm talking about?

It's time to review some high school physics and chemistry. Don't worry - we're not going to get too complicated. Normally, when heat is added into a compound it is directly measurable as some kind of increase in temperature - this is called "˜sensible heat.' You put heat in, and the compound being heated rises in temperature; nice and simple. Obvious, even. When a fluid is taken to its boiling point it will begin to start going into the gaseous phase. But, it can't all convert at once, because in addition to needing to be heated to the temperature at which the fluid will boil, it needs a little extra energy to go from being a liquid at that temperature to being a gas. Now, because there is a measurable difference in the energy the compound will contain as it goes from liquid to gas, while remaining at the same temperature, chemists and physicists have a special name for that.

The problem begins when you look at water, for example, as it boils. To take a liter of water from 100 degrees Celsius to the same mass of steam at 100 degrees Celsius you have to keep heating the water for some time - without seeing any change in the water's temperature. The water is still absorbing that heat, you can tell because more and more of it is steam, and less is left in the pot, but it is not expressing that absorption through a rise in temperature. It is using that energy to break the attraction between the water molecules so that they are liberated to become gaseous. This is called "˜latent heat.' Every phase shift has an associated latent heat: freezing/melting and boiling/condensing are the two latent heats that most people are familiar with.

Now - one more thing to point out - the energy that a compound absorbs as it boils, or goes to the gaseous phase, is the same amount of energy that it will release as it condenses, or goes back into the liquid phase. This equivalency is at the heart of the Freon cycle refrigeration unit.

The other factor to consider is that based on the pressure at which a gas or liquid is at, the boiling point will vary. Everyone is familiar with the high altitude baking directions on cake mix boxes - because at a higher altitude the atmospheric pressure is less the cook has to adjust cooking times to make sure that things bake properly, this is an example of how the variations in boiling points are commonly dealt with.

In a Freon cycle refrigeration unit, there are two sections of the unit - a low pressure section, and a high pressure section. In both sections there is a phase change going on - in the low pressure section the Freon is absorbing heat energy and going into the gaseous phase. By absorbing heat, it takes the heat from the area it is cooling - in thermodynamic terms it acts as a heat sink. Then the gaseous Freon passes through a compressor, pushing it into the high pressure section of the refrigeration unit. Here the gaseous Freon releases heat energy to the outside, using the outside environment as a heat sink, and condenses back to a liquid. Then the liquid is stored in a reservoir until there is a demand for more cooling in the area being cooled. At that point an electrically controlled throttle valve, called the thermo-expansion valve opens to allow some of the high pressure fluid from the reservoir into the low pressure side of the refrigeration unit, where it again evaporates, and continues the cycle.

This is the cycle that all evaporation/condensation cycle cooling units use, no matter which coolant is running through the system: ammonia cooling units operate in the same manner, just with a different coolant.

© Demand Media 2011