MODERN THERMAL POWER PLANTS

short blade the end-wall losses and tip clearance will become dominant and
manufacturing tolerance will also have larger impact as the component size is
reduced.
A high moisture content in the low-pressure turbine causes erosion of the blades
and reduced efficiency. To limit the moisture content and allow for a further
increase in the admittance pressure, reheating can be introduced to the cycle.
Reheating means that the steam is reheated in the HRSG before it enters the
intermediate turbine. The heat supplied to the reheating steam reduces the available
heat for the evaporation, as can be seen in Figure 8, and will therefore reduce the
mass flow of the cycle. Figure 8 also shows that the reduced mass flow causes the
economizer temperature profile to diverge even more than in the case without
reheating.
3.3 Multi-pressure CCPPs
Reheating and a high admittance pressure will lead to a large temperature difference
in the economizer. If this temperature difference, or at least part of it, is used to
drive a second Carnot cycle the efficiency can be further increased. This can be done
by adding an additional low-pressure Rankine cycle, i.e., by dividing the economizer
into two sections with a low-pressure evaporator between them, according to the
Figure 9: A dual-pressure combined cycle with reheating (Reheating is the last part on the hot
end)
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