Thermodynamics

566 | ThermodynamicsTThe reheat cycle was introduced in the mid-1920s, but it was abandonedin the 1930s because of the operational difficulties. The steady increase inboiler pressures over the years made it necessary to reintroduce singlereheat in the late 1940s and double reheat in the early 1950s.The reheat temperatures are very close or equal to the turbine inlet temperature.The optimum reheat pressure is about one-fourth of the maximumcycle pressure. For example, the optimum reheat pressure for a cycle with aboiler pressure of 12 MPa is about 3 MPa.Remember that the sole purpose of the reheat cycle is to reduce the moisturecontent of the steam at the final stages of the expansion process. If wehad materials that could withstand sufficiently high temperatures, therewould be no need for the reheat cycle.T avg,reheatsFIGURE 10–12The average temperature at which heatis transferred during reheatingincreases as the number of reheatstages is increased.EXAMPLE 10–4The Ideal Reheat Rankine CycleConsider a steam power plant operating on the ideal reheat Rankine cycle.Steam enters the high-pressure turbine at 15 MPa and 600°C and is condensedin the condenser at a pressure of 10 kPa. If the moisture content ofthe steam at the exit of the low-pressure turbine is not to exceed 10.4 percent,determine (a) the pressure at which the steam should be reheated and(b) the thermal efficiency of the cycle. Assume the steam is reheated to theinlet temperature of the high-pressure turbine.Solution A steam power plant operating on the ideal reheat Rankine cycleis considered. For a specified moisture content at the turbine exit, the reheatpressure and the thermal efficiency are to be determined.Assumptions 1 Steady operating conditions exist. 2 Kinetic and potentialenergy changes are negligible.Analysis The schematic of the power plant and the T-s diagram of the cycleare shown in Fig. 10–13. We note that the power plant operates on the idealreheat Rankine cycle. Therefore, the pump and the turbines are isentropic,there are no pressure drops in the boiler and condenser, and steam leavesthe condenser and enters the pump as saturated liquid at the condenserpressure.(a) The reheat pressure is determined from the requirement that theentropies at states 5 and 6 be the same:State 6:Also,Thus,State 5:P 6 10 kPax 6 0.8961sat. mixture2s 6 s f x 6 s fg 0.6492 0.896 17.49962 7.3688 kJ>kg # Kh 6 h f x 6 h fg 191.81 0.896 12392.12 2335.1 kJ>kgT 5 600°Cf P 5 4.0 MPas 5 s 6 h 5 3674.9 kJ>kgTherefore, steam should be reheated at a pressure of 4 MPa or lower to preventa moisture content above 10.4 percent.