For any gas, the sign of the Joule-Thomson effect depends on tanperature and pressure. The positive effect for each gas is observed only in the limited interval of temperatures and pressures. For each gas there are values of temperature and pressure at which the Joule-Thomson effect is equal to zero (no temperature changes occur at gas expansion in vacuum). These points (T, p,) are called points of inversion. At these points, the influence of forces of attraction is completely compensated for by the influence of repulsion forces consequently the gas temperature does not change. The set of inversion points forms an inversion curve in a p-T diagram. [Pg.229]

From either of these last two expressions it is evident that p.j,x. = 0 for an ideal gas, because each partial derivative is zero for such a substance. It is interesting that p,j X. does not equal 0 for a real gas at zero pressure except at the inversion temperatures (see below). This result suggests that our assumption that a real gas approaches the properties of an ideal gas at the limit of zero pressure is not entirely correct. [Pg.101]

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