Joule–Thomson effect

The temperature change of a real gas during throttling at constant enthalpy. Its direction and size depend on the gas, its initial state, and the pressure drop.

Throttling process: An irreversible flow process in which a fluid loses pressure across a restriction with negligible heat transfer and external work. It produces the pressure drop while keeping upstream and downstream enthalpy approximately equal.
Linde process: A gas-liquefaction process that uses compression, cooling, and regenerative Joule–Thomson expansion. Repeated throttling and heat exchange let the process cool gas toward liquefaction.
First law of thermodynamics: The principle that energy is conserved and transferred as heat, work, and changes in stored energy. It explains how an insulated throttle can preserve enthalpy despite a pressure drop.
James Prescott Joule: An English physicist whose experiments helped establish the mechanical equivalent of heat and principles of thermodynamics. His free-expansion experiments with William Thomson established the work that preceded the effect's discovery.
Enthalpy: A thermodynamic state function equal to internal energy plus pressure times volume. Its near-constant value across a throttle constrains the gas's temperature change.
Gas liquefaction: The conversion of a gas into a liquid by lowering its temperature and, sometimes, increasing its pressure. Joule–Thomson cooling is one route to the low temperatures required.
Real gas: A gas whose behavior departs from the ideal-gas model because molecules interact and occupy finite volume. Molecular interactions make enthalpy vary with pressure as well as temperature.
William Thomson: A Scottish mathematical physicist later known as Lord Kelvin, noted for foundational work in thermodynamics. He collaborated with Joule on experiments showing temperature changes during gas expansion.
Joule–Thomson coefficient: The temperature change per unit pressure change during an isenthalpic process. Its sign and magnitude specify the local direction and strength of the effect.
Refrigeration cycle: A thermodynamic cycle that transfers heat from a colder region to a warmer one using work or heat input. Many vapor-compression systems use a throttling valve, though their cooling also involves evaporation.

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