When a high-pressure gas is blown adiabatically through a narrow opening (a porous plug), its temperature changes even though it has received no heat from outside. And it does not necessarily go down. In ① we look at what happens before and after the plug and confirm that the sign of μ = (∂T/∂p)H is determined by the gas and the temperature. In ② we build this “throttling” operation into an air conditioner as its expansion valve and turn it into a device that cools or warms a room.
① is calculated with the van der Waals equation of state (the values of a and b are tabulated values fitted to the critical point). For an ideal gas μ = 0, so this phenomenon appears only with a real gas. The refrigerants in ② are calculated with correlations: the Lee–Kesler vapor-pressure equation and the Riedel–Watson latent-heat equation.