text: in a refrigerator, what is the compressor, and the expansion valves used for?   the compressor compresses the ammonia gas. the compressed gas heats up as it is pressurised (orange).  the coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. the ammonia gas condenses into ammonia liquid (dark blue) at high pressure.  the high-pressure ammonia liquid flows through the expansion valve.  you can think of the expansion valve as a small hole. on one side of the hole is high-pressure ammonia liquid. on the other side of the hole is a low-pressure area (because the compressor is sucking gas out of that side).    the liquid ammonia immediately boils and vapourises (light blue), its temperature dropping to -27 f. this makes the inside of the refrigerator cold.  the cold ammonia gas is sucked up by the compressor, and the cycle repeats.