MODERN THERMAL POWER PLANTS

There are three major reasons for the reduction of efficiency when operating a
turbine in the wet area:
• thermodynamic losses
• braking losses
• frictional losses
4.6.3.1 Thermodynamic losses
When steam expands rapidly into the wet area across the saturation line, the
nucleation time is limited and the steam will become subcooled while remaining
dry. The steam temperature will therefore undergo the local saturation temperature
and will not be at thermodynamic equilibrium, hence, T < T(p). At a certain point
called the Wilson point, maximum subcooling is reached and droplets start to form.
The Wilson point is not a fixed value, and depends on the rate of expansion. Once
condensation has been initiated and the droplet surface is created the process
proceeds very rapidly. When droplets are formed, the latent heat produced heats the
flow, which speeds up the process of reaching the equilibrium temperature. Sudden
condensation leads to a sheet of discontinuity when the volume of the flow is
reduced. The event has many similarities with an adiabatic shock wave in ideal gas
[23].
Figure 12: The condensation shock in a h-s diagram
The phenomenon can be illustrated in an enthalpy versus entropy diagram, as
shown in Figure 12, but it should be noted that the expansion before the
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