Phase-change cooling refers to a chemical process of liquid changing to gas that cools computer parts and prevents them from over-heating. In computers, the central processing unit (CPU) within the computer system can heat up, thus causing the PC to be too hot. Much like an air conditioner, phase-change cooling describes the process where coils move very hot gases that have been heavily compressed through the cooling system in a computer.
Techopedia explains Phase-Change Cooling
Phase-change cooling is considered a high-end computer cooling process. These hot, compressed gases are forced through the apparatus as fans help them travel through the coils, thus cooling them through a refrigerator-type process and turning them into liquid. As this very cool liquid this travels further, the pressure drops substantially with the liquid now turning back into a gas form.