The Joule-Thomson coefficient is given by μJT=(∂T∂P)H=VCP(βT−1),
Answer
If you look up the coefficient of thermal expansion of air, you will find that it decreases with increasing absolute temperature. In fact, it decreases rapidly enough that even the product βT decreases with increasing temperature. For air at room temperature and 1 bar, for example, the product is about 1.01 while, at 200 C, the product is about 0.99.
In terms of the compressibility factor z, the product of β and T is given by: βT=1+(∂lnz∂lnT)P
Get yourself a plot of z vs reduced temperature and reduced pressure and note how the right hand side of this equation decreases with increasing reduced temperature. It is positive at low reduced temperatures, and reaches a value of zero at a reduced temperature of about 4.
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