Joule Thomson (or Joule Kelvin) effect When a gas at high pressure is allowed to pass through a porous plug, to a low pressure region, a decrease in the temperature of the gas occurs. The gas in this case expands against a constant external pressure and therefore has to do external work. An internal work by the gas molecules also takes place because they are taken apart against the force of attraction. The process is done adiabatically. Since D Q = 0, i.e. no heat enters or leaves the system, the enthalpy*, H = U + pV, remains constant in the throttling process. The quantity (¶ T/¶ p)H is given by the equation,

(¶ T/¶ p) = - [partial diff. of U w.r.t. p at constt. T +

partial diff. of (pV) w.r.t. p at constt. T] C_p^-1

The first term in right is due to the deviation of Joule's law and is always negative for real gases. The second term is due to the deviation form Boyle's law. Beyond an inversion temperature the second term in the equation is positive and large enough to completely cancel the first term. Therefore, beyond the inversion temperature the Joule Kelvin expansion will produce heating.

Joule Kelvin effect is used for the liquefaction of gases. For air, oxygen the inversion temperature is higher than the room temperature. But for gases like hydrogen and helium the inversion temperature is low. Therefore these gases are pre-cooled before they are allowed to undergo Joule Kelvin expansion. In table JI the maximum inversion temperatures for some gases are shown.

Table JI

Maximum Inversion Temperature of some gases