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ChemE 260 Entropy Balances On Open and Closed Systems

ChemE 260 Entropy Balances On Open and Closed Systems. Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 8: A & B CB 6: 10 + Supplement. May 10, 2005. Entropy Balance: Closed System. 2 nd Law, Internally Reversible Processes:. 1 st Law:.

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ChemE 260 Entropy Balances On Open and Closed Systems

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  1. ChemE 260 Entropy BalancesOn Open and Closed Systems Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 8: A & BCB 6: 10 + Supplement May 10, 2005

  2. Entropy Balance: Closed System • 2nd Law, Internally Reversible Processes: • 1st Law: • Boundary Work, Internally Reversible Processes: (Usually assume Wtot = Wb.) • Gibbs 1st Equation: • Entropy Balance Equation • Integral Form: • Differential Form: • Rate Form: Baratuci ChemE 260 May 10, 2005

  3. Entropy Balance: Open System • Steady-state, SISO: • General: • Entropy generation within the system: Baratuci ChemE 260 May 10, 2005

  4. The 1st Law and Entropy • 1st Law, Steady-state, Internally Reversible, SISO: • From the definition of entropy: • Gibbs 2nd Equation: • Integrating for an open system: Baratuci ChemE 260 May 10, 2005

  5. Mechanical Energy Balance Equation • Combine all the equations from the previous slide: • The MEBE: Bernoulli Equation • If Wnot b = 0 : Bernoulli Equation (incompressible fluid) Usually,Wnot b = Wsh • If Ekin = Epot = 0 : Baratuci ChemE 260 May 10, 2005

  6. Shaft Work & PV Diagrams • Polytropic Processes or: • 1 : •  = 1 : Baratuci ChemE 260 May 10, 2005

  7. PV Diagram: Polytropic Baratuci ChemE 260 May 10, 2005

  8. Summary: Wsh Polytropic Processes • Wsh is actially all work other than flow work. Baratuci ChemE 260 May 10, 2005

  9. Next Class … • Problem Session • After that… • Isentropic Efficiency • Define an efficiency for a process by comparing actual performance to the performance of an isentropic process • Nozzles, compressors and turbines • New diagram: HS Diagram • Multi-Stage Compressors • Intercooler HEX’s reduce work input requirement Baratuci ChemE 260 May 10, 2005

  10. Example #1 • Air is compressed from 1 bar and 310 K to 8 bar. Calculate the specific work and heat transfer if the air follows a polytropic process path with d = 1.32. Assume air is an ideal gas in this process. Baratuci ChemE 260 May 10, 2005

  11. Example #2 • A turbine lets down steam from 5 MPa and 500oC to saturated vapor at 100 kPa while producing 720 kJ/kg of shaft work. The outer surface of the turbine is at an average temperature of 200oC. Determine the heat losses from the turbine and the entropy generation in the turbine in kJ/kg-K. Baratuci ChemE 260 May 10, 2005

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