S

Fig. 11-7 Temperature-Entropy (T-s) Diagram of Steam Turbine Reheat Cycle.

Another method used to increase the thermal efficiency of a steam power plant is a regeneration cycle. With regeneration, steam is extracted from the turbine after it has partly expanded and used in a boiler feedwater heater. This produces a higher average temperature of heat addition (i.e., higher average boiler temperature). Since ideal efficiency is proportional to 1 — Tout/Tin, the increase in average temperature of heat addition produces an overall increase in power plant efficiency.

Another way to look at this is, since the fluid temperature entering the boiler is higher, less energy is required to vaporize and superheat the steam and, therefore, less fuel is needed. However, the turbine also develops less work since some of the steam is bled from the second stage. Operating conditions, therefore, must be chosen so that the decrease in net work is more than compensated for by boiler fuel savings. A single regenerative heater can improve overall cycle efficiency by 2% or more. Figure 118 shows the flow loop of a regenerative cycle and Figure 11-9 shows the cycle in a T-s diagram.

In the figures, steam enters the turbine at state E. After expansion, to an intermediate state F, some of the steam is extracted and enters the feedwater heater. The steam that is not extracted is expanded to state G and condensed in the condenser. This condensate is pumped into the feedwater heater, where it mixes with the steam

Fig. 11-6 Schematic Representation of Steam Turbine Reheat Cycle.

Boiler

Power

Turbine

Boiler

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