Thermodynamics

expansion process takes place in two stages. In the first stage (the highpressureturbine), steam is expanded isentropically to an intermediate pressureand sent back to the boiler where it is reheated at constant pressure,usually to the inlet temperature of the first turbine stage. Steam then expandsisentropically in the second stage (low-pressure turbine) to the condenserpressure. Thus the total heat input and the total turbine work output for areheat cycle becomeandq in q primary q reheat 1h 3 h 2 2 1h 5 h 4 2w turb,out w turb,I w turb,II 1h 3 h 4 2 1h 5 h 6 2(10–12)(10–13)The incorporation of the single reheat in a modern power plant improvesthe cycle efficiency by 4 to 5 percent by increasing the average temperatureat which heat is transferred to the steam.The average temperature during the reheat process can be increased byincreasing the number of expansion and reheat stages. As the number ofstages is increased, the expansion and reheat processes approach an isothermalprocess at the maximum temperature, as shown in Fig. 10–12. The useof more than two reheat stages, however, is not practical. The theoreticalimprovement in efficiency from the second reheat is about half of thatwhich results from a single reheat. If the turbine inlet pressure is not highenough, double reheat would result in superheated exhaust. This is undesirableas it would cause the average temperature for heat rejection to increaseand thus the cycle efficiency to decrease. Therefore, double reheat is usedonly on supercritical-pressure (P 22.06 MPa) power plants. A third reheatstage would increase the cycle efficiency by about half of the improvementattained by the second reheat. This gain is too small to justify the added costand complexity.Chapter 10 | 565T3High-pressureturbine3Reheating5BoilerReheater 4High-PturbineLow-Pturbine4Low-pressureturbine2P 4 = P 5 = P reheat5CondenserPump62161sFIGURE 10–11The ideal reheat Rankine cycle.