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Urgent chemical engineering. specific enthalpy. Help me please

  1. Sep 26, 2013 #1
    1. The problem statement, all variables and given/known data

    Prior to entering a combustion furnace, air is heated up from ambient temperature (25 °C) to 250 °C. This change in temperature causes the specific enthalpy to change to 3745 J/mol. Assume that the pressure of the air leaving the heater is 150 kPa and the flow rate is 1.3 m3 /min. Assuming ideal behaviour, calculate the heat required in units of kW.

    2. Relevant equations

    PV=nRT
    H=U+PV
    dH=Q
    3. The attempt at a solution
    I can find P1 and V1 from the equation T1/T2=P1/P2 and V1/V2=P2/P1
    P1=85.487 kpa
    V1=136.863m^3
     
    Last edited by a moderator: Sep 27, 2013
  2. jcsd
  3. Sep 27, 2013 #2
    Since you know the exit temperature and exit pressure, you should be able to determine the exit molar density using the ideal gas law. What is the exit molar density (moles/m^3)?
     
  4. Sep 27, 2013 #3
    It will be 34.487 mol/m^3
     
  5. Sep 27, 2013 #4
    If the exit molar density is 34.387 moles/m^3, and the exit flow rate is 1.3 m^3/min, what is the flow rate in moles/min?
     
  6. Sep 27, 2013 #5
    It should be 1.3 x 34.387 = 44.7031 mol/min. I guess
     
  7. Sep 27, 2013 #6
    In the molar flow rate is 44.7 mol/min and the change in enthalpy per mole is 3745 Joules, how much heat is being added per min? How much heat is being added per second?

    Chet
     
  8. Sep 27, 2013 #7
    Omg thank you very much now I get it . but do you think I have to find H for the entering flow ?
     
  9. Sep 27, 2013 #8
    What do you think?
     
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