= Helium liquefaction
<Helium liquefaction> converts <helium> <gas> into a collectable <liquid>. It creates a bath for investigating material properties at very low <temperatures>, including <superconductivity>. The central question is how the incoming <specific enthalpy> compares with the <specific enthalpies> of the outgoing <liquid> and <vapor>. The <liquefaction enthalpy budget> makes that comparison explicit; a <temperature> decrease by itself does not specify a liquid yield.
In a <Joule-Thomson effect> cooling stage, a throttling <pressure> drop approximately preserves <enthalpy> under the assumptions of the <steady-flow energy balance>. For an <ideal gas>, <specific enthalpy> depends only on <temperature>: with positive <specific heat capacity at constant pressure>, equal <specific enthalpy> means equal <temperature>. Thus its <Joule-Thomson coefficient> is zero. An <ideal gas> also supplies no <liquid> branch. Cooling by throttling depends on <real gas> properties; expansion is not an automatic source of refrigeration.
For a local drop $dp<0$, $dT=\mu_{\rm JT}dp$ cools only where the <Joule-Thomson coefficient> is positive. Precooling must bring the <real gas> into a useful cooling region, with enough cooling to overcome unwanted <heat transfer>. A finite expansion can cross a <Joule-Thomson inversion curve>; its final state requires the full <enthalpy> balance, including <phase coexistence> if <liquid> forms. <Kamerlingh Onnes's 1908 helium liquefaction experiment> combined external precooling with regeneration to reach that regime. The <Joule-Thomson effect> article develops the throttling identities and their assumptions.
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