Joule–Thomson effect

In <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, the Joule–Thomson effect (also known as the Joule–Kelvin effect or Kelvin–Joule effect) describes the temperature change of a <a href="/facts/Real_gas/Lne2E6dL">real gas</a> or <a href="/facts/Liquid/t9GkLWzt">liquid</a> (as differentiated from an <a href="/facts/Ideal_gas/80EfYGFj">ideal gas</a>) when it is expanding; typically caused by the pressure loss from flow through a <a href="/facts/Expansion_valve_(steam_engine)/5dEo3SQq">valve</a> or <a href="/facts/Enthalpy/bBou8lCE">porous plug</a> while keeping it insulated so that <a href="/facts/Adiabatic_process/rJj42U5h">no heat is exchanged</a> with the environment. This procedure is called a throttling process or Joule–Thomson process. The effect is purely due to deviation from ideality, as any ideal gas has no JT effect.
At room temperature, all gases except <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, <a href="/facts/Helium/yRul93TG">helium</a>, and <a href="/facts/Neon/5bF1FwGL">neon</a> cool upon expansion by the Joule–Thomson process when being <a href="/facts/Throttle/kgdj3cuj">throttled</a> through an orifice; these three gases rise in temperature when forced through a porous plug at room temperature, but lowers in temperature when already at lower temperatures. Most liquids such as <a href="/facts/Hydraulics/KSIqLovc">hydraulic</a> oils will be warmed by the Joule–Thomson throttling process. The temperature at which the JT effect switches sign is the <a href="/facts/Inversion_temperature/BshyIToz">inversion temperature</a>.
The gas-cooling throttling process is commonly exploited in <a href="/facts/Refrigeration/Bs8HHbYd">refrigeration processes</a> such as <a href="/facts/Liquefied_gas/bHUThq0A">liquefiers</a> in <a href="/facts/Air_separation/WWTx03pP">air separation</a> industrial process. In hydraulics, the warming effect from Joule–Thomson throttling can be used to find internally leaking valves as these will produce heat which can be detected by <a href="/facts/Thermocouple/FW9cYU5D">thermocouple</a> or <a href="/facts/Thermal_Imaging_camera/dplJK39u">thermal-imaging camera</a>. Throttling is a fundamentally <a href="/facts/Irreversible_process/k7PH6Muk">irreversible process</a>. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits their performance.
Since it is a constant-enthalpy process, it can be used to experimentally measure the lines of constant enthalpy (isenthalps) on the 
 
 
 
 (
 p
 ,
 T
 )
 
 
 {\displaystyle (p,T)}
 
 diagram of a gas. Combined with the <a href="/facts/Specific_heat_capacity/9P0U6YgI">specific heat capacity at constant pressure</a> 
 
 
 
 
 c
 
 P
 
 
 =
 (
 ∂
 h
 
 /
 
 ∂
 T
 
 )
 
 P
 
 
 
 
 {\displaystyle c_{P}=(\partial h/\partial T)_{P}}
 
 it allows the complete measurement of the <a href="/facts/Thermodynamic_potential/rBnf2hB6">thermodynamic potential</a> for the gas.

Joule–Thomson effect open-in-new

Joule–Thomson effect