ME 201 Michigan State University（我201年密歇根州立大学）.pdfVIP

ME 201 Thermodynamics Steam Power Plants All steam power plants are based upon the ideal Rankine cycle shown below. Boiler Turbine Pump Condenser The four devices are Constant Pressure Boiler Isentropic Turbine Constant Pressure Condenser (fluid exits as saturated liquid) Isentropic Pump In actual steam power systems, two additional devices are often included to improve performance and efficiency of the system: reheater and feedwater heater. Both of these are discussed below. 1. Reheater: In analyzing ideal Rankine cycle performance it is seen that the large the pressure drop across the turbine the higher the thermal efficiency of the cycle. However, as the pressure drop rises the quality of the steam leaving the turbine decreases. This is a maj or problem since if we consider the wet steam to be composed of small droplets suspended in a gas (vapor), one could imagine these small droplets impacting on the turbine blades and causing considerable erosion and mechanical fatigue. In fact, most modern turbines can only withstand an exit quality greater than 95%. To work within in this constraint one needs to either lower the pressure drop (which will decrease the thermal efficiency), increase the inlet steam temperature, or apply reheating. For a variety of reasons the preferred 1 ME 20 1 Thermodynamics choice is often to employ a reheat leg. In a reheat leg the steam is not expanded all at once