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Heat exchangers

  1. Nov 24, 2014 #1
    1. The problem statement, all variables and given/known data

    1. If the flue gases exiting the boiler are used to preheat the water fed to the boiler from a temperature of 28 C to 90 C and assuming:
    • a mean specific heat capacity for water over this temperature range to be 4.2 kJ kg–1 K–1

    • a mean molar heat capacity for the flue gases up to 300oC to be 31 kJ kmol–1 K–1

    • 10% of the heat required to heat the water is lost in the heat exchanger

    • all water entering the system is converted to steam

      determine the final outlet temperature of the flue gas
    2. Relevant equations

    pw = (mole fraction)water × total pressure


    3. The attempt at a solution
    I'm not sure where to start off on this once as it does not state any pressure the steam is at, I thought you needed a pressure to multiply by the mole fraction to then obtain a temperature using steam tables?
    Any help/suggestions on equations would be much appreciated -thanks
     
  2. jcsd
  3. Nov 24, 2014 #2

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    The boiler feed is in the liquid phase.
     
  4. Nov 24, 2014 #3
    Thanks for your reply, so is there a different equation that is used? Because you do not need to use specific heat capacity when using the method I described I believe
     
  5. Nov 24, 2014 #4

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    This is the second (nth?) part of the flame temperature question isn't it? Vapor pressure of water at 90 C is less than 1 atm, and the steam generation occurs in the boiler.

    There are some parameters missing here --- water feed rate, flue gas generation/flow rate?
     
  6. Nov 24, 2014 #5
    Yes it is the final part there is one after the flame temp question that I believe to have completed just the notes are not very clear,
    The flow rate of flue gas is 1400 kmol h^-1
    And it does not state any water feed rate, what is the equation that you are intending on using please
     
  7. Nov 24, 2014 #6

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    From the earlier part, "steam at 5atm?" Gauge or absolute? So, you've got 1400 kmol/h at whatever heat content from 300 C to flame T (again from earlier) generating however much steam it takes to soak up that much heat (liquid heat capacity from 90 C to 5atm steam T plus enthalpy of vaporization at 5 atm.), be it gauge or absolute, giving you the water feed rate.

    'Nuff of a start?
     
  8. Nov 24, 2014 #7
    Thanks for the explanation would you be willing to give me a formula to use I might be able to tackle it and give it a go that way
    Thanks
     
  9. Nov 24, 2014 #8

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    Total heat in (flue gas flow rate at flame T x heat content) = Total heat out (sum of pre-heat/0.9, plus heat content of steam (or, heat content of flue gas x flue gas flow rate between flame T and 300 C, which is the steam generated, minus losses if they mention it's not a perfect boiler) ) --- I think I got everything, but double-check it.
     
  10. Nov 24, 2014 #9
    Thanks mate I will give it ago using this!
     
  11. Nov 25, 2014 #10
    What would the usual difference be between inlet and outlet temperatures with regards to flue gas as I have worked it out although it barely differs from the inlet temp (around4 degrees) surely I have gone wrong?
     
  12. Nov 25, 2014 #11

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    This is for the feed water pre-heat? What did you get for feed rate?
     
  13. Nov 25, 2014 #12
    2113 kg/hr? I used the liquid heat cp 90c + 5bar + enthalpy of vaporisation 5 bar
    4.208+2.32+2107=2113
     
  14. Nov 25, 2014 #13

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    Lemme see --- 2113 x 4.2 x 62 = 555 kJ/h; and (31kJ/kmol x 1400 kmol/h)/.9 = 48.2 kJ/h; goes into 555 11.4 times for an 11.4 K delta T. And, you evaporated water in the pre-heat, I'll bet ---- pre-heat T gets to 90 C, at 5 atm., so it can't evaporate.

    "Are we there yet?"
     
  15. Nov 25, 2014 #14
    I'm not sure hahah! So what is the 11.4 difference in temperature?
     
  16. Nov 25, 2014 #15

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    That's how much the flue gas exhausted from the boiler at 300 C is further cooled by running it through the pre-heater.
     
  17. Nov 25, 2014 #16
    Ah so it is simply 300- 11.4 to give the outlet temperature of the flue gas?
     
  18. Nov 25, 2014 #17

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    That's the way I'd play it.
     
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