Joule–Thomson effect – Joule Thomson coefficient
The changes of the temperature during throttling process are subject of the Joule-Thomson effect. At room temperature and normal pressures, all gases except hydrogen and helium cool during gas expansion. The cooling occurs because work must be done to overcome the long-range attraction between the gas molecules as they move farther apart. The effect depends on the value of the Joule-Thomson coefficient, which is defined as:
An application of the throttling process occurs in vapor-compression refrigerators, where a throttling valve is used to reduce the pressure and reduce the temperature of the refrigerant from the pressure at the exit of the condenser to the lower pressure existing in the evaporator.