Joule Thomson effect
The cooling occurs because work must be done to overcome the long-range attraction between the gas molecules as they move farther apart.
The frequency of atomic collisions decrease as air expands, therefore the air gets cooler. Temperature is just the average heat of a substance. As the energy needed to increase it's temperature must be supplied from somewhere, and the gas does not takes the energy from the surrounding system giving the effect of cooling.
Most of the real gases need more work downstream at ambient temperature, due to the effects of compressibility.
P 1 ×V 1 <P 2 ×V 2
The indicates that the internal energy decreases when the gas passes through the restriction.
It can be generalised that for many real gases, the temperature decreases during a reduction in pressure, but not true for every gas and condition. Depressuring is an isenthalpic process which reveals that enthalpy doesn’t change. The temperature can either decrease or increase for any gas based on how the internal energy changes to maintain the enthalpy constant.
The Joule–Thomson coefficient:
The Joule–Thomson coefficient ηJT is defined as the variation of temperature with pressure at constant enthalpy.
Joule-Thomson coefficient is defined as the rate of change of temperature with pressure during an isenthalpic process or throttling process. Joule-Thomson coefficient gives slope of constant enthalpy lines on temperature-pressure diagram. Thus, it is a parameter for characterizing the throttling process.
The Joule–Thomson coefficient of an ideal gas is zero.
JTC = (∂T/∂p)
Where Enthalpy is constant.
The JTC of an ideal gas is equal to zero since its enthalpy depends on only temperature.
Inversion temperature:
The temperature of a gas at which the reduction in pressure does not lead to any change in temperature is known as inversion temperature.
The gas gets heated up on expansion above inversion temperature and cools down below inversion temperature.
Above inversion temperature, slope of curve is negative. So will have warming up of gas when it is expanded.
Below inversion temperature, slope of curve is positive. So will have cooling of gas when it is expanded.
For derivation of JTC:
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