Determine the change in entropy between the following states. a) water, P1 = 10

Question

Determine the change in entropy between the following states.
a) water, P1 = 10 MPa, T1 = 400oC, P2 = 10 MPa, T2 = 100oC
b) R-134a, h1 = 111.44 kJ/kg, T1 = 7oC, P1 = 2 bar, T2 = 327oC, P2 = 1 bar
c) air as an ideal gas, T1 = 7oC, P1 = 2 bar, T2 = 327oC, P2 = 1 bar
d) hydrogen (H2) as an ideal gas, T1 = 727oC, P1 = 1 bar, T2 = 25oC, P2 = 2 bar

Explanation / Answer

a) Use water table P1=10MPa , T1=400C -> s1=6.211kJ/kgK P2=10MPa, T2=100C ->s2=1.299 kJ/kgK s2-s1 = -4.912 kJ/kgK b) Use R-134a table P1=2bar , T1=7C -> s1=1.695kJ/kgK P2=1bar, T2=327C ->s2=XXXX kJ/kgK ----> Temp is extremely high here. Check this first s2-s1 = XXXX-1.695 kJ/kgK c) Air Cp = 1.02 kJ/kgK R=0.287 kJ/kgK s2-s1 = Cpln(T2/T1) - Rln(P2/P1) s2-s1 = 1.02 ln(600/280)-0.287ln(1/2) = 0.976 kJ/kgK d) H2 Cp = 14.5 kJ/kgK R=4.157 kJ/kgK s2-s1 = Cpln(T2/T1) - Rln(P2/P1) s2-s1 = 14.5 ln(298/1000)-4.5ln(2/1) = -20.674 kJ/kgK

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