Refrigerant that cools itself

Not ice cold, but 7 °C

When the pressure drops to about 10 atm, the refrigerant’s boiling point drops to about 7 °C. For a liquid at 40 °C, that is more than hot enough, so about 21% of the refrigerant boils into gas right there. The heat needed to boil is taken from the rest of the refrigerant, so all of it cools down to about 7 °C.

Why?It is the same reason rice comes out undercooked high on a mountain. Up there the air pressure is low, so water boils below 100 °C (about 90 °C at around 3,000 m). Lower pressure means a lower boiling point.

Learn moreA spray can gets cold when you spray it for a long time for a similar reason. The refrigerant does not get ice cold. If the indoor pipes dropped below 0 °C, water from the air would freeze on them and they would take in heat poorly, so the system is set to boil at around 7 °C, as in this example.

Temperature before and after the valve

40 → 7 °C

Before the valve

40 °C · 28 atm · liquid

After the valve

7 °C · 10 atm · 21% gas

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