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

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.