Chapter10 - Chapter 10 Vapor and Combined Power Cycles...

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Chapter 10 Vapor and Combined Power Cycles Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach , 5th edition by Yunus A. Çengel and Michael A. Boles
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2 We consider power cycles where the working fluid undergoes a phase change. The best example of this cycle is the steam power cycle where water (steam) is the working fluid. Carnot Vapor Cycle
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3 The heat engine may be composed of the following components. The working fluid, steam (water), undergoes a thermodynamic cycle from 1-2-3-4-1. The cycle is shown on the following T-s diagram.
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4 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 0 100 200 300 400 500 600 700 s [kJ/kg-K] T [C] 6000 kPa 100 kPa Carnot Vapor Cycle Using Steam 1 2 3 4 The thermal efficiency of this cycle is given as η th Carnot net in out in L H W Q Q Q T T , = = - = - 1 1 Note the effect of T H and T L on η th, Carnot . •The larger the T H the larger the η th, Carnot •The smaller the T L the larger the η th, Carnot
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5 To increase the thermal efficiency in any power cycle, we try to increase the maximum temperature at which heat is added. Reasons why the Carnot cycle is not used: Pumping process 1-2 requires the pumping of a mixture of saturated liquid and saturated vapor at state 1 and the delivery of a saturated liquid at state 2. To superheat the steam to take advantage of a higher temperature, elaborate controls are required to keep T H constant while the steam expands and does work. To resolve the difficulties associated with the Carnot cycle, the Rankine cycle was devised. Rankine Cycle The simple Rankine cycle has the same component layout as the Carnot cycle shown above. The simple Rankine cycle continues the condensation process 4-1 until the saturated liquid line is reached. Ideal Rankine Cycle Processes Process Description 1-2 Isentropic compression in pump 2-3 Constant pressure heat addition in boiler 3-4 Isentropic expansion in turbine 4-1 Constant pressure heat rejection in condenser
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6 The T-s diagram for the Rankine cycle is given below. Locate the processes for heat transfer and work on the diagram. 0 2 4 6 8 10 12 0 100 200 300 400 500 s [kJ/kg-K] T [C] 6000 kPa 10 kPa Rankine Vapor Power Cycle 1 2 3 4 Example 10-1 Compute the thermal efficiency of an ideal Rankine cycle for which steam leaves the boiler as superheated vapor at 6 MPa, 350 o C, and is condensed at 10 kPa. We use the power system and T-s diagram shown above. P 2 = P 3 = 6 MPa = 6000 kPa T 3 = 350 o C P 1 = P 4 = 10 kPa
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Pump The pump work is obtained from the conservation of mass and energy for steady-flow but neglecting potential and kinetic energy changes and assuming the pump is adiabatic and reversible. ( ) m m m m h W m h W m h h pump pump 1 2 1 1 2 2 2 1 = = + = = - Since the pumping process involves an incompressible liquid, state 2 is in the compressed liquid region, we use a second method to find the pump work or the h across the pump. Recall the property relation:
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This note was uploaded on 04/20/2010 for the course M E 320 taught by Professor Deinert during the Spring '08 term at University of Texas.

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Chapter10 - Chapter 10 Vapor and Combined Power Cycles...

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