= Joule-Thomson effect
{c}
{wiki=Joule–Thomson_effect}
The <Joule-Thomson effect> is the <temperature> response of a <fluid> flowing from higher to lower <pressure> through a valve or a plug of <porous medium>, when the throttle is insulated and the other <steady-flow energy balance> assumptions hold. Its equilibrium endpoints have equal <specific enthalpy>. The <Joule-Thomson coefficient> determines whether a small drop cools or warms; cooling is not universal.
\Image[/joule-thomson-throttle-enthalpy-balance.png]
{title=<Enthalpy> accounting for an insulated <Joule-Thomson effect> throttle}
{id=joule-thomson-throttle-enthalpy-balance}
{description=Original schematic of the <steady-flow energy balance>, with $u$ denoting <internal energy> per unit <mass>. The arrows include inlet and outlet <pressure> <work>. The reduced balance requires negligible bulk <kinetic energy> and <potential energy> changes. No particular apparatus or measured <temperature> drop is depicted.}
A useful contrast is between three insulated expansions. In <free expansion> of a gas-only <closed system> into a <vacuum>, no external <work> occurs and $U_2=U_1$; an <ideal gas> returns to its initial equilibrium <temperature>. In a throttle, continuous flow carries <pressure> <work> across the boundary and preserves <specific enthalpy> under the stated assumptions; an <ideal gas> again has no endpoint <temperature> change. In a reversible <adiabatic expansion> of a loaded <closed system>, the <gas> supplies <work>, loses <internal energy> and cools, even if it is an <ideal gas>. That process preserves <entropy>; throttling generally produces <entropy>. For a small constant-<enthalpy> step, $dH=T\,dS+V\,dp=0$ gives $dS=-V\,dp/T>0$ when $dp<0$ and $T,V>0$. This equilibrium comparison computes endpoint <entropy> differences without supplying a reversible path through the plug.
The effect is useful in <refrigerators> because an appropriately prepared <real gas> can cool while passing through a simple valve. The inlet must lie in a suitable positive-<Joule-Thomson coefficient> region. The <Joule-Thomson inversion curve> makes that requirement precise; a pressure drop by itself is not a guarantee of refrigeration.
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