What is the Joule Thompson Effect and why it is not applicable to Ejectors?

The Joule Thompson Effect is the change is the temperature of a real gas when it undergoes isenthalpic expansion i.e. through a valve. No heat is exchanged with the outside, no work is done and the process is irreversible. Although the temperature normally decreases (by about 1 degree F per bar), this not the case for gases like helium an hydrogen. It depends on molecular attraction / repulsion.

In an ejector, gas expansion is typically highly isentropic and reversible - the decreased pressure is mainly converted into increased gas velocity. The temperature decreases much more than for isenthalpic expansion and in ratio with the pressure decrease according to the equation below.

So why is this important! Because the term "Joule Thompson Effect" has become too general purpose for gas pressure reduction and we may have forgotten out thermodynamics.

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