Steam Entropy
Basic steam thermodynamics  entropy diagram.
The entropy diagram for steam shows the relationships between
 Pressure
 Temperature
 Dryness Fraction
 Entropy
With two factors given  the others can be found in the diagram. The ordinates in the diagram represents the Entropy and the Absolute temperature.
The diagram consist of the following lines
 Isothermal line
 Pressure lines
 Lines of dryness fraction
 Waterline between water and steam
 Dry steam lines
 Constant volume lines
The total heat is given by the area enclosed by absolute zero base water line and horizontal and vertical line from the respective points.
An adiabatic expansion is a vertical line. An adiabatic process is expansion at constant entropy with no transfer of heat.
 Critical temperature of steam is 375 to 3,380^{o}C
 Critical pressure is 217.8 atm
Total Entropy of Steam
Entropy of Water
The change of entropy can be expressed as:
dS = log_{e}(T_{1}/T) (1)
where
T = absolute temperature (K)
The entropy of water above freezing point can be expressed as:
dS = log_{e}(T_{1}/273) (2)
Entropy of Evaporation
Change of Entropy during evaporation
dS = dL/T (3)
where
L = latent heat (J)
Entropy of wet steam
The entropy of wet steam can be expresses as:
dS = log_{e}(T_{1}/273) +ζ(L_{1}/T_{1}) (4)
where
ζ = dryness fraction
Entropy of superheated steam
Change of entropy during superheating can be expressed as
dS = c_{p} log_{e}( T/T_{1} ) (5)
where
c_{p}= specific heat capacity at constant pressure for steam (kJ/kgK)
The entropy of superheated steam can be expressed as:
dS = log_{e}( T_{1}/273 ) + L_{1}/T_{1} + c_{p} log_{e}( T_{s}/T_{1} ) (6)
where
T_{s} = absolute temperature of superheated steam
T_{1} = absolute temperature of evaporation
Water and Steam Entropy  Imperial Units
Temperature (^{o}F)  Absolute Pressure (psia)  Entropy (Btu/lb ^{o}F)  

Water  Steam  
0  0.0185  0.3243  2.330 
10  0.0309  0.3141  2.283 
20  0.0505  0.3038  2.238 
30  0.0809  0.2936  2.195 
32 (0 ^{o}C)  0.0887  0.0000  2.187 
40  0.1217  0.0162  2.159 
50  0.1718  0.0361  2.126 
60  0.2564  0.0555  2.094 
70  0.3633  0.0746  2.064 
80  0.5074  0.0933  2.035 
90  0.6989  0.1116  2.008 
100  0.9503  0.1296  1.982 
110  1.277  0.1473  1.957 
120  1.695  0.1647  1.933 
130  2.226  0.1817  1.911 
140  2.893  0.1985  1.889 
150  3.723  0.2151  1.868 
160  4.747  0.2314  1.848 
170  5.999  0.2474  1.829 
180  7.519  0.2632  1.811 
190  9.350  0.2787  1.793 
200  11.54  0.2941  1.776 
210  14.12  0.3092  1.760 
212 (100 ^{o}C)  14.70  0.3122  1.756 
220  17.20  0.3241  1.744 
230  20.80  0.3389  1.729 
240  24.99  0.3534  1.714 
250  29.85  0.3678  1.700 
 1 psi (lb/in^{2}) = 6894.8 Pa (N/m^{2})
 1 Btu/(lb_{m} ^{o}F) = 4186.8 J/ (kg K)
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