Freezers don't actually work by making things cold. They work by taking away the heat. They use a process of evaporation to draw out heat and leave the food in your freezer frozen. To complete this process, some very special components are used.
Componants of a Freezer
There are a couple of different refrigerant types commonly used today. The most common is freon. However, freon which is quickly being replaced with pure ammonia.
The other most important components are the compressor and expansion valve. These components--along with the thermostat and coils--make up a pretty simple process that enables you to keep food on hand for a good length of time.
How it Works
A freezer works by drawing the heat out of the freezer compartment. To start the process, the refrigerant--in gas form--goes into the compressor where the compression process causes it to get very hot. The hot gas moves through more coils and starts to cool off. This causes the gas to turn into a liquid. Gas is forced into an expansion valve while in the liquid form. This valve has a very small opening which--when the refrigerant is forced through it--turns into a very cold mist.
As the mist goes through the coils under the freezer compartment, it starts to evaporate and turn back into a gas. The temperature of the mist is around -27 degrees at this point, and it takes the heat from the freezer compartment with it.
The refrigerant starts to warm up again as it draws out the heat, and it is sent back to the compressor to start the process over again. Each time the refrigerant goes through it cycle, it takes more heat from the freezer compartment until there is none left. This is what leaves you with frozen food.
Thermostats and Refrigerants
The thermostat detects the temperature inside the freezer compartment and tells the compressor when to come on. This is why you will sometimes hear the freezer cycle on and off. This is also one of the most common repairs needed on a freezer. If the thermostat doesn't work, the freezer will either keep running or not come on at all.
The freezer system is sealed, and you should never see refrigerant. Freon and pure ammonia are highly toxic and should not be handled by anyone who is not certified to do so. If you see a leak, call for a repairman right away.