ChemE 260 The Brayton Power Cycle and Variations

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ChemE 260 The Brayton Power Cycleand Variations

• Dr. William Baratuci
• Senior Lecturer
• Chemical Engineering Department
• University of Washington
• TCD 9 E FCB 8 1 - 3, 8 - 10

May 25, 2005
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The Brayton Cycle

• The ideal gas cycle for gas-turbine engines

Baratuci ChemE 260 May 25, 2005
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The Air-Standard Brayton Cycle

• Air-Standard Assumptions
• Air is the working fluid and it behaves as an
  ideal gas.
• The Brayton Cycle is modeled as as a closed
  cycle.
• The combustor is replaced by HEX 1. (External
  Combustion)
• All processes are internally reversible.

• Step 1-2 Isobaric heating
• Step 2-3 Isentropic expansion
• Step 3-4 Isobaric cooling
• Step 4-1 Isentropic compression

Baratuci ChemE 260 May 25, 2005
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PV TS Diagrams
Baratuci ChemE 260 May 25, 2005
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The Cold Air-Standard Assumption

• The heat capacities of air are constant and
  always have the values determined at 25oC.

• Compression Ratio

• Thermal efficiency of an internally reversible,
  cold air-standard Brayton Cycle

Baratuci ChemE 260 May 25, 2005
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Air-Standard Brayton Cycle Efficiency
Baratuci ChemE 260 May 25, 2005
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Improvements to the Brayton Cycle

• Regeneration
• Use the hot turbine effluent to preheat the feed
  to the combustor.
• Reheat
• Use a 2-stage turbine and reheat the effluent
  from the HP turbine before putting into the LP
  turbine.
• Intercooling
• Use a 2-stage compressor with an intercooler.
• Regeneration with Reheat and Intercooling
• Use all of the techniques listed above to achieve
  high efficiency.

Baratuci ChemE 260 May 25, 2005
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Regenerative Brayton Cycle
Baratuci ChemE 260 May 25, 2005
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Regenerative Brayton Cycle Efficiency
Brayton Cycle
Regenerative Brayton Cycle T1 / T4 0.30
Regenerative Brayton Cycle T1 / T4 0.20
Baratuci ChemE 260 May 25, 2005
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Reheat Brayton Cycle
Baratuci ChemE 260 May 25, 2005
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Brayton Cycle with Intercooling
Baratuci ChemE 260 May 25, 2005
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Regeneration, Reheat Intercooling
Baratuci ChemE 260 May 25, 2005
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Next Class

• Vapor-Compression Refrigeration Cycles
• Cycle corresponds to the vapor power cycle.
• TS Diagrams, Deviations from internal
  reversibility
• Selecting a refrigerant
• Enhanced Vapor-Compression Refrigeration Cycles
• Cascade V-C Refrigeration Cycles
• Two separate refrigeration cycles, Analogous to
  Binary Vapor Power Cycles
• One provides cooling to the other
• Two different refrigerants
• Can reach very low temperatures
• Multi-Stage V-C Refrigeration Cycles
• Similar to Cascade V-C Refrigeration
• Two cycles use the same refrigerant
• Instead of exchanging heat between two cycles,
  the refrigerant streams are mixed. This is more
  efficient than heat exchange.

Baratuci ChemE 260 May 25, 2005