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Vapor and Combined
Power CyclesThe steam cycle and more
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Carnot Cycle
The standard all others are measuredagainst
Not realistic model for vapor cycles
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Rankine Cycle, Ideal
1-2 isentropiccompression (pump)
2-3 constant pressure
heat addition (boiler) 3-4 isentropic
expansion (turbine)
4-1 constant pressureheat rejection(condenser)
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Rankine Cycle, Ideal
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Rankine Cycle Energy Analysis
Energy balance, each process
For pump
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Rankine Cycle Energy Analysis
For boiler
For turbine
For condenser
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Rankine Cycle Energy Analysis
Thermal efficiency
Heat rate: amount of heat (Btu) to
generate 1 kWh of electricity
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Real vs. Ideal Cycle
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Increase Efficiency?
Lower condenser pressure
Increase superheattemperature
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Increase Efficiency?
Increase boiler pressure
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Reheat
Materials limit temperature of steam, but can we takeadvantage of higher steam pressures and not havequality of steam issues?
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Reheat
Equations become:
Purposes of reheat: keep turbine inlet
temps within limits, increase quality ofsteam in last stages of turbine
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Ideal Regenerative Rankine Cycle
Regeneration:effective use ofenergy
Open (direct contact)feedwater heaters(mixing chambers)
Closed feedwaterheaters (heatexchangers)
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Ideal Regenerative Rankine Cycle
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Ideal Regenerative Rankine Cycle
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2nd Law Analysis
Ideal Rankine cycle is internally reversible
Analysis indicates where irreversibilitiesare
Again for steady-flow system:
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2nd Law Analysis
For a cycle:
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Cogeneration
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Combined Gas-Vapor Power Cycle
Use of two cycles to maximize efficiency
Gas power cycle toppinga vapor powercycle
Combined cycles have higher efficiencythan either independently
Works because:
Gas turbine needs high combustion temp tobe efficient, vapor cycle can effectively userejected energy
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