Regenerative Rankine Cycle
With a high average temperature of the combustion gas in the boiler possible by exhaust regeneration, we now turn to the question of how to raise the average temperature during heat addition in a steam power plant. The basic Rankine cycle was already discussed in Sec. 10.1, where we introduced reheating between turbine stages as one means to this end, Fig. 12.3 repeats the T-s-diagram and the schematic, with air preheater added to the sketch. A higher pressure in the steam generator implies higher average temperature.
However, direct expansion from high pressure into the condenser (3-4’) results in unacceptably low values for steam quality at the turbine exit, and turbine blade damage through droplet formation. Reheat at intermediate pressure is used so that the quality at turbine exit is larger, which implies fewer droplet, and low blade damage.
Heat is added between states 2 and 3, with rather low temperatures in the liquid region right after the pump, the boiler feedwater. In a regenerative Rankine cycle internal heat exchange is used to preheat the feedwater between pump and boiler. For this, some steam is bled-off after the high pressure turbine stage (state 4), and then used to heat the feedwater.
One distinguishes between open feedwater heaters, in which vapor and water are mixed isobarically, and closed feedwater heaters, where vapor and feedwater are running through a heat exchanger at different pressures. In both cases modifications of the cycle are necessary, in order to feed the bled- off flow back into the cycle, and to adjust pressures. Real life power plants employ an array of closed and open feedwater heaters to optimize efficiency and thus increase fuel utilization as much as possible.