I have difficulties to understand the Joule Thomson coefficient given on the wikipedia page. It says that $(\partial_p T) = \frac{V}{C_p}( T \alpha -1)$. Now my problem is that I don't know about which temperature, pressure and volume we are talking in this equation. I mean there are two temperatures, pressures and volumes in the Joule Thomson process( before and behind the trottle). So to which quantities does this equation refer to?
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The throttling process occurs at constant enthalpy, the free (control) parameter is the pressure that moves the gas from one side of the plug (valve) to the other. The equation tells you how the temperature of the gas changes as function of pressure (the control parameter) at constant enthalpy, so it holds for either side of the throttle.
answered Jan 18, 2015 at 16:23