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# 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 3,380oC
• Critical pressure is 217.8 atm

### Total Entropy of Steam

#### Entropy of Water

The change of entropy can be expressed as:

dS = loge(T1/T)                               (1)

where

T = absolute temperature (K)

The entropy of water above freezing point can be expressed as:

dS = loge(T1/273)                            (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 = loge(T1/273) +ζ(L1/T1)                              (4)

where

#### Entropy of superheated steam

Change of entropy during super-heating can be expressed as

dS = cp loge( T/T1 )                                (5)

where

cp= specific heat capacity at constant pressure for steam (kJ/kgK)

The entropy of superheated steam can be expressed as:

dS = loge( T1/273 ) + L1/T1 + cp loge( Ts/T1 )                                           (6)

where

Ts = absolute temperature of superheated steam

T1 = absolute temperature of evaporation

### Water and Steam Entropy - Imperial Units

Steam Entropy vs. Temperature
Temperature
(oF)
Absolute Pressure
(psia)
Entropy (Btu/lb oF)
WaterSteam
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 oC) 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 oC) 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/in2) = 6894.8 Pa (N/m2)
• 1 Btu/(lbm oF) = 4186.8 J/ (kg K)

## Related Topics

• ### Thermodynamics

Calculate heat, work, temperature and energy. The thermodynamics of steam and condensate systems. Water and Ice properties.

## Related Documents

• ### 2nd Law of Thermodynamics

Entropy and disorder.

Adiabatic flame temperatures for hydrogen, methane, propane and octane - in Kelvin.
• ### Compressible Gas Flow - Entropy

Calculating the entropy in a compressible gas flow.
• ### Properties of Saturated Steam - SI Units

Saturated Steam Table with steam properties as specific volume, density, specific enthalpy and specific entropy.
• ### Saturated Steam - Properties - Imperial Units

Steam table with sensible, latent and total heat, and specific volume at different gauge pressures and temperatures.
• ### Saturated Steam - Properties for Pressure in Bar

Saturated Steam Table with properties like boiling point, specific volume, density, specific enthalpy, specific heat and latent heat of vaporization.
• ### Standardized Enthalpies and Entropies

Standardized enthalpies and entropies for some common substances.
• ### Steam and Vapor Enthalpy

Vapor and steam enthalpy, specific enthalpy of saturated liquid, saturated vapor and superheated vapor.
• ### Superheated Steam - Entropy

The entropy of steam superheated to temperatures above saturation points.
• ### Third Law of Thermodynamics

The entropy of a substance is zero if the absolute temperature is zero.
• ### Water - Thermophysical Properties

Thermal properties of water at different temperatures like density, freezing temperature, boiling temperature, latent heat of melting, latent heat of evaporation, critical temperature and more.
• ### Water-Steam Mollier Diagram

Enthalpy-entropy diagram for water and steam.
• ### Wet Steam - Enthalpy

Wet steam, dryness fraction and enthalpy.
• ### Wet Steam - Specific Volume

Wet steam and specific volume.

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## Citation

• The Engineering ToolBox (2003). Steam Entropy. [online] Available at: https://www.engineeringtoolbox.com/steam-entropy-d_99.html [Accessed Day Month Year].

Modify the access date according your visit.

4.19.9

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