Entropy change
What's the method to solve this problem please?
Air is throttled through a porous plug. The inlet conditions are 30 bar and 85°C and the outlet is 15
bar. The process is adiabatic and kinetic energies are negligible. Calculate the change in entropy
per mass.
Many thanks
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The throttling process is an isenthalpic process meaning the enthalpy of the throttled quantity of the substance remains constant. h1 = h2.
As we know the enthalpy of an ideal gas is function of temperature only. The temperature before throttling and after throttling remains same.
i.e, T1=T2=(85+273)=358 K.
Now, to calculate entropy change we use one of the 3 entropy change formulae for ideal gas involving pressure and temperature.
ds = Cp*ln(T2/T1) - R*ln(P2/P1)
first term cancel out. T1=T2
ds = - R*ln(P2/P1) ....... kJ/kg.K
The answer should be positive and a large change in entropy compared to other processes we know because the throttling process is highly irreversible.