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Homework Help: Heat transfer and combustion

  1. Nov 1, 2017 #1
    1. The problem statement, all variables and given/known data

    A fuel gas consists of 75% butane (C4H10), 10% propane (C3H8) and 15% butene (C4H8) by volume.

    It is to be fed to the combustion chamber in 10% excess air at 25oC, where it is completely burnt to carbon dioxide and water. The flue gases produced are to be used to generate 5 bar steam from water at 90oC.

    1 If the flue gases exiting the boiler are used to preheat the water fed to the boiler from a temperature of 28oC to 90oC 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 and state if the dew point will be reached in both of the cases given in part (j)

    2. Relevant equations
    heat in=heat out (Enthalpy balance)

    3. The attempt at a solution

    On my previous attempt I got an outlet temperature of 288.58 degrees C. But my tutor marked that as completely wrong.

    I attempted the question again using

    Heat in = heat out.

    I have calculated the heat in by using the cv calculated in previous question plus the (mass x Cpmolar(31) × 62) - The heat lost due to additional water content in the air. Multiplied all this by 0.9 as 10% of heat is lost. I get roughly 2380 mj
    heat in = (2580+(mass*Cpmolar(31)*62)-0.3695 mj) *0.9 62 is change in temp
    = 2380j

    I then calculate the heat out by calculating the enthalopy of each product minus the heat of vapour caused by the 5 bar. I then used the grapgh to get a temp of between 1700 and 1800.
    heat out=sum of enthalpy of products-heat of water at 5 bar
    = 2266-1.18

    Is this the right process
    I feel I am completely wrong
    This is my last question and I have completed my mechanical engineering studies

    Thank you
  2. jcsd
  3. Nov 1, 2017 #2

    1. Can you please specify where do you apply the energy balance (preheater, boiler, chamber, system)?

    2. Why do you multiply mass*cpmolar*ΔT?

    3. Did you use the energy that is released from the combustion chamber in your calculations?

    I think the solution will include more than one equation. Also, you need to see your result (temperature). Is it rational?
  4. Nov 1, 2017 #3
    To be honest I am completely lost.

    as for your points
    1) I was trying to use the energy balance from the point of combustion to the point where the flue gas leaves. From the previous question the air intake has 0.8kg of water per mol entering the combustion chamber so that there will be more water as a product in the flue gas.

    2) I was trying to calculate the heat released during condensation of water vapour and the cooling of the condensate from 90 degrees to 28 degrees C. so that I would take this away from the input heat energy. I have calculated the flame temp roughy 1992 degrees C

    3) yes I used the energy that was released during combustion which is roughly 2580 mj which equates to the Tf (flame temp) above.

    The equation I have in my collage notes are as follows:

    Heat in = heat out

    Heat in =(sensible heat in fuel and air + latent heat released by combustion)

    Heat out=Σ(mass×mean specific heat capacity×Tf) for each of the products of combustion.

    A little confused as to how to take this
    Thank you
  5. Nov 1, 2017 #4
    1) If you have calculated the temperature of the flue gas (temperature in the exit of combustion chamber) to be 1992°C, then you can apply two energy balances. One for the boiler and one for the condenser.
    Heat in is the enthalpy of the incoming substances and Heat out is the enthalpy of the outcoming substances.

    2) I mean you must multiply mass*cpmass*ΔT, in order to use the right units.

    3) Maybe you do not need to use the combustion heat because you have already used it.
  6. Nov 1, 2017 #5
    There seems to be missing information. Can you please provide the complete problem statement, and also please provide a flow sheet showing the stream temperatures, flow rates, and compositions that you know.
  7. Dec 24, 2017 #6
    would you look at this document for me and see what i have done wrong and can i get some help on part (i) and k

    Attached Files:

  8. Dec 28, 2017 #7
  9. Jan 6, 2018 #8
    i was struggling to understand my learning material and so i came on here and tried ti have a look if someone els has had a go on the same question and i found they reply at the top so had a go and followed the steps with my own figures that were not that different.
    but if i did just 0.75 *34 that would be 25.5
    0.1*34 was then 3.4
    0.15*34= 5.1
    it all equals to 34 leaving no space for air or nitrogen
    do think that i should have done that instead
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