Steam Entropy

Basic steam thermodynamics - entropy diagram.

The entropy diagram for steam shows the relationships between

Steam - water vapor - entropy vs. temperature diagram

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

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.

The change of entropy can be expressed as:

dS = log_e(T₁/T) (1)

where

T = absolute temperature (K)

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

dS = log_e(T₁/273) (2)

Change of Entropy during evaporation

dS = dL/T (3)

where

L = latent heat (J)

The entropy of wet steam can be expresses as:

dS = log_e(T₁/273) +ζ(L₁/T₁) (4)

where

Change of entropy during super-heating can be expressed as

dS = c_p log_e( T/T₁) (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( T₁/273) + L₁/T₁ + c_p log_e( T_s/T₁) (6)

where

T_s = absolute temperature of superheated steam

T₁ = absolute temperature of evaporation