Joule–Thomson effect

In a throttling process, a flowing fluid passes through a restriction such as a valve, capillary or porous plug from higher to lower pressure. In the usual steady-flow model, the device is insulated, performs no shaft work, and changes little in kinetic and potential energy. Under these conditions, the fluid's specific enthalpy at the outlet is approximately equal to its value at the inlet. The temperature can nevertheless change because enthalpy depends on both temperature and pressure for a real fluid. This pressure-driven temperature change at constant enthalpy is called the Joule–Thomson effect; it is not the same as a reversible, adiabatic expansion that keeps entropy constant.

For a single-phase fluid, the Joule–Thomson coefficient is defined as μJT = (∂T/∂p)H, the change in temperature with pressure at constant enthalpy. With this convention, a positive coefficient means that lowering pressure tends to lower temperature, while a negative coefficient means that a pressure drop tends to raise temperature. The value depends on the fluid and its thermodynamic state, and it can change sign. The boundary in pressure–temperature space where the coefficient is zero is called the inversion curve; on opposite sides of that curve, throttling has opposite temperature effects.

The effect arises from the non-ideal behavior of real fluids, whose molecules interact and whose enthalpy is not a function of temperature alone. In the ideal-gas model, enthalpy depends only on temperature, so constant-enthalpy throttling produces no temperature change. For a real gas, the result depends on the starting pressure and temperature: expansion can cool in one region and warm in another. It is therefore inaccurate to claim that every gas cools when it expands. A fluid's inversion conditions and phase behavior must be considered when predicting the outcome, especially near transitions between liquid and vapor phases.

Joule–Thomson cooling is used in gas refrigeration and liquefaction. In Linde–Hampson-type systems, high-pressure gas is precooled and then throttled; the colder low-pressure stream can pass through a heat exchanger to cool incoming gas, enabling successive cycles to reach lower temperatures and, under suitable conditions, liquefy part of the stream. The degree of cooling depends on fluid composition and operating state. Throttling also occurs in ordinary pressure regulators and pipelines, where the temperature change may need to be included in engineering calculations rather than treated as a useful cooling effect.