Joule-Thompson Coefficient

Joule-Thompson Coefficient

  • A throttling process produces no change in enthalpy; hence for an ideal gas the temperature remains constant. For real gases, however, the throttling process will cause the temperature to increase or decrease. The Joule-Thomson coefficient, μJ, relates this change and is defined as:
  • A positive value of μJ indicates that the temperature decreases as the pressure decreases; a cooling effect is thus observed. This is true for almost all gases at ordinary pressures and temperatures.
  • The exceptions are hydrogen, neon, and helium, which have a temperature increase with a pressure decrease, hence μJ < 0. Even for these gases there is a temperature above which the Joule-Thompson coefficient changes from negative to positive. At this inversion temperature, μJ = 0.