MET 401 Chapter 2 improvement_to_rankine_cycle

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  • 2. HOW CAN WE INCREASE THE EFFICIENCY OF THE RANKINECYCLE?The basic idea behind all the modifications to increase the thermal efficiencyof a power cycle is the same: Increase the average temperature at which heat istransferred to the working fluid in the boiler, or decrease the average temperature atwhich heat is rejected from the working fluid in the condenser.Lowering the Condenser Pressure (Lowers Tlow,avg) To take advantage of the increased efficiencies at low pressures, the condensers of steam power plants usually operate well below the atmospheric pressure. There is a lower limit to this pressure depending on the temperature of the cooling medium Side effect: Lowering the condenser pressure increases the moisture content of the steam at the final stages of the turbine. The effect of lowering the condenser pressure on the ideal Rankine cycle. 2
  • 3. Superheating the Steam to High Temperatures (IncreasesThigh,avg) Both the net work and heat input increase as a result of superheating the steam to a higher temperature. The overall effect is an increase in thermal efficiency since the average temperature at which heat is added increases. Superheating to higher temperatures decreases the moisture content of the steam at the turbine exit, which is desirable. The temperature is limited by metallurgical considerations. Presently theThe effect of superheating the highest steam temperature allowed at thesteam to higher temperatures on turbine inlet is about 620°C.the ideal Rankine cycle. 3
  • 4. Increasing the Boiler Pressure (Increases Thigh,avg)For a fixed turbine inlet temperature, Today many modern steam powerthe cycle shifts to the left and the plants operate at supercriticalmoisture content of steam at the pressures (P > 22.06 MPa) and haveturbine exit increases. This side effect thermal efficiencies of about 40% forcan be corrected by reheating the fossil-fuel plants and 34% for nuclearsteam. plants.The effect of increasing the boiler A supercritical Rankine cycle.pressure on the ideal Rankine cycle. 4
  • 5. THE IDEAL REHEAT RANKINE CYCLEHow can we take advantage of the increased efficiencies at higher boiler pressures withoutfacing the problem of excessive moisture at the final stages of the turbine?1. Superheat the steam to very high temperatures. It is limited metallurgically.2. Expand the steam in the turbine in two stages, and reheat it in between (reheat) The ideal reheat Rankine cycle. 5
  • 6. The single reheat in a modern power plantimproves the cycle efficiency by 4 to 5% byincreasing the average temperature at whichheat is transferred to the steam.The average temperature during the reheatprocess can be increased by increasing thenumber of expansion and reheat stages. Asthe number of stages is increased, theexpansion and reheat processes approach anisothermal process at the maximumtemperature. The use of more than tworeheat stages is not practical. The theoreticalimprovement in efficiency from the secondreheat is about half of that which resultsfrom a single reheat.The reheat temperatures are very close or The average temperature at whichequal to the turbine inlet temperature. heat is transferred during reheating increases as the number of reheatThe optimum reheat pressure is about one- stages is increased.fourth of the maximum cycle pressure. 6
  • 7. THE IDEAL REGENERATIVE RANKINE CYCLE Heat is transferred to the working fluid during process 2-2’ at a relatively low temperature. This lowers the average heat-addition temperature and thus the cycle efficiency. In steam power plants, steam is extracted from the turbine at various points. This steam, which could have produced more work by expanding further in the turbine, is used to heat the feedwater instead. The device where the feedwater is heated by regeneration is called a regenerator, or a feedwater heater (FWH). A feedwater heater is basically a heatThe first part of the heat-addition exchanger where heat is transferred from theprocess in the boiler takes place at steam to the feedwater either by mixing therelatively low temperatures. two fluid streams (open feedwater heaters) or without mixing them (closed feedwater heaters). 7
  • 8. Open Feedwater HeatersAn open (or direct-contact) feedwaterheater is basically a mixing chamber, wherethe steam extracted from the turbine mixeswith the feedwater exiting the pump.Ideally, the mixture leaves the heater as asaturated liquid at the heater pressure. The ideal regenerative Rankine cycle with an open feedwater heater. 8
  • 9. Closed Feedwater HeatersAnother type of feedwater heater frequently used in steam power plants isthe closed feedwater heater, in which heat is transferred from the extractedsteam to the feedwater without any mixing taking place. The two streams now canbe at different pressures, since they do not mix.The ideal regenerative Rankine cycle with a closed feedwater heater. 9
  • 10. The closed feedwater heaters are more complex because of the internal tubing network, andthus they are more expensive. Heat transfer in closed feedwater heaters is less effectivesince the two streams are not allowed to be in direct contact. However, closed feedwaterheaters do not require a separate pump for each heater since the extracted steam and thefeedwater can be at different pressures. Open feedwater A steam power plant with one open heaters are simple and three closed feedwater heaters. and inexpensive and have good heat transfer characteristics. For each heater, however, a pump is required to handle the feedwater. Most steam power plants use a combination of open and closed feedwater heaters. 10
  • 11. Example (regenerative cycle)Consider a steam power plant operating on theideal regenerative Rankine cycle with one openfeedwater heater. Steam enters the turbine at 15MPa and 600°C and is condensed in the condenserat a pressure of 10 kPa. Some steam leaves theturbine at a pressure of 1.2 MPa and enters theopen feedwater heater. Determine the fraction ofsteam extracted from the turbine and the thermalefficiency of the cycle. 11