Page 172 - Advanced Thermodynamics for Engineers, Second Edition

7.7 PROBLEMS 159

Table 7.1 Excerpt from Steam Tables, Showing Values on the Saturated Liquid and Vapour
Lines (Rogers and Mayhew (1994)).
Specific Change of Entropy of Enthalpy of
Temperature Pressure Volume Specific Liquid, s f Liquid, h f
3

( C) (bar) (m /kg) Volume, v fg (kJ/kg K) (kJ/kg K)
115 1.691
120 1.985 0.001060 0.8845 1.530 505
125 2.321

dp 2:321 1:691 5 3

At 120 C, the slope of the saturation line is z 10 ¼ 6:30 10 .
dT 10
From the Clausius–Clapeyron equation, Eqn (7.82)
s 2 s 1 dp s g s f
¼ ¼ ;
v fg dT v fg
giving

dp 3 3
s g ¼ s f þ v fg ¼ 1:530 þ 0:8845 6:30 10 =10
dT
¼ 1:530 þ 5:57235 ¼ 7:102 kJ=kg K
The specific enthalpy on the saturated vapour line is

h g ¼ h f þ Ts fg
¼ 505 þð120 þ 273Þ 5:57235
¼ 2694:9kJ=kg K
These values are in good agreement with the tables of properties; the value of s g ¼ 7.127 kJ/kg K,
while h g ¼ 2707 kJ/kg at a temperature of 120.2 C.

7.6 CONCLUDING REMARKS
Thermodynamic relationships between properties have been developed which are independent of a
particular fluid. These can be used to evaluate derived properties from primitive ones and to extend
empirical data.

7.7 PROBLEMS

P7.1 For a van der Waals gas that obeys the state equation

RT a
p ¼
v b v 2