What is the Joule-Thomson effect and when is it important?

Joule-Thomson effect is temperature change during isenthalpic (constant enthalpy) throttling. Joule-Thomson coefficient: mu_JT = (dT/dP)_H. For most gases at normal conditions, mu_JT > 0 (cooling on expansion); for hydrogen and helium at room temperature, mu_JT < 0 (heating on expansion). Inversion temperature: where mu_JT = 0. Important in: refrigeration cycles (expansion cooling), natural gas processing (cooling can cause hydrate formation), cryogenic processes, and pressure letdown systems (temperature prediction for material selection).