chapter 8. Production of Power from heat-student

chapter 8. Production of Power from heat-student - Chapter...

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Chapter 8 Production of Power from Heat The efficiency of conventional fossil-fuel steam-power plants rarely exceeds 35%. However, efficiencies greater than 50% can be realized in combined-cycle plants : from advanced-technology turbines. from steam-power cycles operating on heat recovered from hot turbine exhaust gases. efficiency of a fuel cell can be great as 85%.
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Figure 8.1 Simple steam power plant. Figure 8.2: Carnot cycle on a TS diagram. Problems of Carnot engine: vapor Liquid/vapor c Liquid Boiler Turbine Boiler Condenser Carnot Cycle
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The Rankine cycle An alternative model cycle taken as the standard, at least for fossil-fuel- burning power plant. 1 → 2: A constant pressure heating process in a boiler: heating of subcooled liquid water to its saturation temperature, vaporization at constant temperature and pressure, and superheating of the vapor to a temperature well above its saturation temperature. . 2 → 3: Reversible, adiabatic (isentropic) expansion of vapor in a turbine to the pressure of the condenser. 3 → 4: A constant-pressure, constant- temperature process in a condenser to produce saturated liquid at point 4. 4 → 1: Reversible, adiabatic (isentropic) pumping of saturated liquid to the pressure of the boiler, producing compressed (subcooled) liquid. Liquid vapor c Liquid/vapor
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Figure 8.4: Simple practical power cycle. Real power plants operate slightly different from Rankine cycles because of the irreversibility (steps of 2-3 and 4-1 are not isentropic). Two major differences:
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Example 8.1: Steam generated in a power plant at a pressure of 8600 kPa and a temperature of 500°C is fed to a turbine. Exhaust from the turbine enters a condenser at 10 kPa, where it is condensed to saturated liquid, which is then pumped to the boiler. (1) What is the thermal efficiency of a Rankine cycle operating at these conditions? (2) What is the thermal efficiency of a practical cycle operating at these conditions if the turbine efficiency and pump efficiency are both 0.75? (3) If the rating of the power cycle of part (2) is 80000kW, what is the steam rate and what are the heat-transfer rates in the boiler and condenser? 8600kPa 500 o C 10 kPa 10 kPa saturated liquid 8600kPa saturated liquid
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(1) The thermal efficiency of a Rankine cycle is the efficiency of that both turbine and pump operate in a irreversible and adiabatic process
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(2) With a turbine efficiency of 0.75: ( 29 kg kJ H H turbine W S s 6 . 955 75 . 0 2 . 1274 ) ( 3 2 3 2 , - = × - = = = - - η kg kJ H H pump W S s 6 . 11 75 . 0 7 . 8 ) ( 1 4 , 1 4 = = = = - - kg kJ H H H 4 . 203 6 . 11 8 . 191 1 4 4 1 = + = + = - kg kJ H H Q 2 . 3188 4 . 203 6 . 3391 1 2 = - = - = 2961 . 0 2 . 3188 0 . 944 | | | ) ( | = = boiler s Q net W For the condenser, the enthalpy of saturated liquid at 10 kPa: kg kJ H 8 . 191 4 = kg kJ net W s 0 . 944 6 . 11 6 . 955 ) ( - = + - = | | | | | ) ( | , , , , real boiler pump S Turbine S real boiler s Q W W Q Rankine W - = Q boiler,real is different from that Q boiler in reversible condition So
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power rating of 80000kW ) ( ) ( net W m net W s s = (3) s kg m 75 . 84 0 . 944 80000 = - - = s kJ boiler Q m boiler Q 270200 2 . 3188 75 . 84 ) ( ) ( = × = = ) ( 75 . 84 ) ( ) ( 3 4 H H condenser Q m condenser Q - × = = For boiler, n
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chapter 8. Production of Power from heat-student - Chapter...

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