The freezer in your kitchen is at about -22 degrees Celsius (-7.6 degrees Fahrenheit) despite the fact the outside temperature is more like 20 degrees Celsius (68 degrees Fahrenheit). It achieves this using a gas, such as tetrafl uoroethane, that has a boiling point not too far below 0 degrees Celsius (32 degrees Fahrenheit). The freezer compresses the coolant, which heats it, and then passes the hot vapour around the coils at the back. Heat radiates from the coils to the kitchen, and the coolant temperature drops. As the coolant is under pressure, it condenses, even though it is above its normal boiling point. Then it is pumped to the interior of the freezer and a pressure valve lets about half the coolant boil off. This drops the temperature of the rest of the coolant, which allows heat to flow from the freezer compartment to the coolant. Once boiled off, it returns to the compressor in order to restart the cycle.
Vapour compression cycling using different gases can achieve much lower temperatures – as low as liquid helium (around -269 degrees Celsius/-452 degrees Fahrenheit). Below that you can use a laser to reach temperatures a fraction of a degree above absolute zero. This uses the momentum from the photons themselves to slow down individual atoms.
Answered by Luis Villazon, How It Works contributor.