A simple introduction to phase-change cooling

This section describes the basic principles behind the phase-change process. It is meant as a primer for the uninitiated only, and therefore it is not 100% complete nor 100% correct.

The word 'phase' in phase-change refers to the different states in which a substance can exist. The most common phases are solid (ice), liquid (water), and gaseous (steam). There are a few more like liquid crystal, but those are not important for the phase-change cooling principle.

Now, when a substance changes it's phase, it absorps or releases energy. For example, when you try to freeze water (a liquid->solid phase change), you have to pull heat out of it. When you want to boil water (liquid->gaseous phase change), you have to put heat into it. Whenever steam condenses (gaseous->liquid phase change), it releases heat, and the surface will become hot. The same principle is used in a phase-change cooling system, such as used in your refrigerator. Let's see how a normal fridge works:


Schematic diagram of a refrigerator.
(source of original picture unknown)

A normal fridge is filled with a refrigerant like R134a, which already boils at a low temperature (-26 °C for R134a under atmospheric pressure). Refrigerant vapour is compressed into the condenser by a compressor. When the vapour is being compressed to a liquid, it releases heat in the condenser (pump up the tyre of your bicycle, and feel how hot the pump gets). From the condenser, the liquid is transported to the evaporator with a capillary tube. The function of the capillary tube is to provide resistance to the refrigerant flow, so the compressor can indeed build up enough pressure to compress the refrigerant into a liquid.
As soon as the refrigerant enters the evaporator, it starts boiling. This extracts energy from the environment around the evaporator. Thus, the environment cools down. The resulting vapour from the boiling refrigerant is then fed back to the compressor which can compress it again to close the loop.
The heat exchanger between the capillary tube and the suction line increases the efficiency of the system by pre-cooling the just-compressed hot liquid using the cold vapour coming from the evaporator.
The dryer removes water from the refrigerant to prevent the forming of ice inside the capillary tube.
The last uncovered part is the accumulator. Since it is impossible to compress a liquid, any liquid that is has not evaporated somehow is supposed to be captured in the accumulator so it does not damage the compressor.