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chng 2804

Background. Practically all chemical plants require the movement of liquids and gasesThis movement requires a pressure driving forceCompressing gases is energy intensive and expensiveThe flow of compressible fluids is more complex than incompressible fluids. . Power Plant. Expansion of Gas. Compression of Liquid.

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chng 2804

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    1. CHNG 2804 Physical Thermodynamics Lecture 5 John Kavanagh

    3. Power Plant

    4. This Lecture Gas Compression Real Ideal Adiabatic Ideal Isothermal Ideal Multistage Liquid Compression Steam Expansion Compressible Fluids Throttles Nozzles Sonic Velocity

    5. Compression of Real gas

    6. Compression of Real gas

    7. Compression of Ideal Gas How much Power is required to compress 1mol/s of an ideal diatomic gas from 25oC 1 atm to 10 atm? What is the final temperature? Assume reversible adiabatic compression

    8. Adiabatic Compression

    9. Isothermal Compression

    10. Adiabatic vs Isothermal Compression

    11. Adiabatic vs Isothermal Compression Adiabatic compression requires significantly more energy than isothermal. Can we build an isothermal compressor? Would require an extremely large heat transfer area Generally use multistage adiabatic compressors with inter-cooling Can approximate isothermal as number of stages increases

    12. Staged Compression

    13. Compression of Liquids The work required to compress liquids is far less than for gases Specific volumes are much smaller How much work is necessary to pressure liquid water at room temperature from 1 atm to 10 atm? (assume incompressible)

    14. Turbines - Expansion of ideal gas

    15. Turbines - Expansion of ideal gas

    16. Compression and Expansion of Steam Winnick and other thermo text books have a large range of data for steam. Use enthalpy and entropy balances and steam tables

    17. Example What is the minimum power to adiabatically compress 200lb/min of saturated steam from 25 psia to 100 psia? What is the power required if the compressor is 70% efficient? What is the state of the outlet stream in b (T, h and s)

    18. Solution to Example A Use steam tables to determine hI (1160.9 BTU/lb) and sI (1.7142 BTU/lboR) Use entropy balance to determine sO (1.7142 BTU/lboR) and hO (1279.3 BTU/lb) Use energy balance to determine W (23,686.7 BTU/min or 558 hp)

    19. Solution to Example B Use efficiency to determine W reqd (33,838 BTU/min or 797 hp) C Determine ho (1330 BTU/lb) from energy balance Use steam tables to determine To (601F)and so(1.7821 BTU/lboR)

    20. Energy Generation from Steam Cycle

    21. Expansion of Real Gas For most gas expansion questions the downstream temperature will be unknown. For the expansion of real gasses you need to use the method of corresponding states If the downstream pressure is in the high density region a trial and error approach may be req’d

    22. Compressible Flow In 2801 you used Bernoulli’s equation to solve problems with in-compressible flow Bernoulli’s equation is simply an energy balance (PE, KE, Work and Friction) For compressible flow it is necessary to determine whether the system is isothermal or adiabatic Ideal or non-ideal We will look at a few simple examples here, for more information consult C&R vol 1 Chapter 4.

    23. Throttles Adiabatic Valve Often referred to as a Joule Thompson device For an ideal gas there is no temperature change Temperature increases and decreases are possible with real gases Temperature decreases are the basis of vapour compression refrigeration/air conditioning systems

    24. Joule Thompson Expansion

    25. Heat Pumps

    26. Nozzles A nozzle is a carefully designed adiabatic section of pipe Gently converging then diverging Converts enthalpy to kinetic energy Example Calculate the exit temperature and velocity if air at 40oC enters a horizontal isentropic nozzle at 1.5m/s and 200kPa, and exits at 100kPa

    27. Nozzle Answer System: Nozzle Accounting Period: time for 1g-mol to flow through TO = 258.7K Dh = - 1637.2 J Dv = 336 m/s vo = 337.5 m/s This is close to the speed of sound

    28. Compressible Flow

    29. Speed of Sound Sound travels as compression waves Winnick has a derivation from p 156-158

    30. Super Sonic Flow Under certain conditions it is possible to generate flows in excess of the speed of sound See Winnick 6.8.2 or C&R vol 1 Chapter 4 for more details

    31. Summary Gas Compression Real Ideal Adiabatic Ideal Isothermal Ideal Multistage Liquid Compression Steam Expansion Compressible Fluids Throttles Nozzles Sonic Velocity

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