Question

1. Steam undergoes a state change from 450°C and 3.5 MPa to 150°C and 0.3 MPa.

Determine ΔH and ΔS using the following:

a. Steam table data

b. Ideal gas assumptions (be sure to use the ideal gas heat capacity for water).

(ANS: 0.0717 kJ/kg-K; -576.8 kJ/kg; 0.143 kJ/kg-K; -555.71 kJ/kg)

Answer 1

Steam undergoes a state change from T1 = 450 C and P1 = 3.5MPa to T2 = 150 C and P2 = 0.3 MPa.

a) Inlet condition : T1 = 450 C and P1 = 3.5 MPa

By steam table,

Specific enthalpy h1 = 3337.86 kJ/kg

Specific entropy s1 = 7.007 kj/kg.k

Final condition : T2 = 150 C and P2 = 0.3 MPa

Specific enthalpy h2 = 2761.18 kj/kg

Specific entropy s2 = 7.079 kj/kg.k

Change in enthalpy, ΔH = h2 - h1 = 2761.18 - 3337.86 = -572.86 kJ/kg

Change in entropy Δs = s2 - s1 = 7.079 - 7.007 = 0.072 kj/kg.k

B) ideal gas assumptions (ideal gas heat capacity)

For steam

Cp = 1.8723 kj/kg.k

Change in enthalpy,

ΔH = Cp(T2 - T1) = 1.8523Kj/kg.k(150 - 450) = -555.71kj/kg

Change in entropy :

T1 = 450 C = 450 + 273 = 723 K , P1 = 3.5 MPa

T2 = 150 C = 150 + 273 = 423 K , P2 = 0.3 MPa

R = 8.314 J/mol.k =( 8.314 J/mol.k )/(18gm/mol) = 0.461 J/gm.k =0. 461 kJ/kg.k

Δs = Cp*ln(T2/T1) - R*ln(P2/P1) = 1.8523kj/kg.k * ln(423/723) - 0.461kj/kg.k * ln(0.3/3.5)

Δs = 0.139 kj/kg.k