Soaking up heat by boiling

Boiling refrigerant takes heat

Cold refrigerant at about 7 °C flows into the winding pipes inside the indoor unit (the evaporator). When 27 °C room air brushes past these pipes, heat passes from the warm air into the cold refrigerant. The refrigerant takes in that heat and boils, turning from liquid into gas. Watch the small dots (liquid) turn into large dots (gas).

Why?Turning a liquid into a gas takes a lot of heat. So boiling refrigerant pulls a lot of heat from its surroundings. It is the same reason you feel cool when sweat dries after exercise: the sweat turns into gas and carries your body heat away.

Learn moreThe pressure inside these pipes is about 10 atm, so the refrigerant boils at about 7 °C instead of −52 °C. While it boils, its temperature stays the same even as it keeps taking in heat, because the heat goes into turning liquid into gas. Boiling away 1 kg of refrigerant at 7 °C takes about 304 kJ of heat. A liquid turning into a gas is called evaporation, and these pipes are called the evaporator.

Refrigerant entering the indoor unit

7 °C

Coming in

7 °C · 79% liquid + 21% gas

Going out

12 °C · 100% gas

calculated from R-32 property tables (CoolProp) · example: room 27 °C, outdoors 35 °C