The Joule Thomson Effect

The Joule Thomson effect is the phenomenon in which a gas, when allowed to expand through a hole or a porous plug will experience a drop in temperature.  The Joule Thomson effect is an important property used in the liquefaction of gases and in refrigeration.  The process is adiabatic, that is, no heat is exchanged with the surroundings. 

Joule Thomson Coefficient
The ratio of the rate of change of temperature to the rate of change of pressure is known as the Joule Thomson Coefficient of the gas at a given pressure. 

The Joule Thomson effect also occurs in piping when the pipe diameter changes suddenly.   This has to be factored during the design.