In calculating downstream temperatures in a 50 psig compressed air system, I cane across an apparent puzzle. I know the Joule Thomson coefficient for air at my conditions is roughly 0.25 °C/atm so an expansion from 3 atm -> 1 atm should cool a little less than 1°C (correct?). I did an experiment pumping a Coke bottle to 50 psig, let the bottle cool back to ambient, and release the pressure. I haven't found a way to measure the temperature, but I feel sure the air cools more than predicted by the JT effect. I would guess it is cooling 5-10°C and there is often visible condensation. The gas velocity inside is nearly zero throughout the process, so the kinetic energy change in the bottle should be negligible. Any thoughts?
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