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Thermodynamics question help? is this right?

  1. Jun 21, 2012 #1
    Thermodynamics question help?? is this right??

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

    A boiler is supplied with water at a rate of 0.3 kg/s through a pipe of diameter 50mm

    a) calculate the energy per second that must be supplied to the boiler,raising the water temperature to 100C, when the water is supplied at 15C

    b) If the input power to the boiler is 3.1GJ/h, Calculate the efficiency of the boiler and heat transfer process


    The density of water - 1000kg/m^3
    specific heat capacity for water - 4.7kj/kgK
    latent heat evaporation for water - 2.26MJj /kg
    steam pipe outside diameter - 55mm
    steam pipe wall thickness - 2.5mm
    modulus of elasticity for steel - 210GPA
    coefficient of linear expansion of steel - 18 x 10^-6/C

    2. Relevant equations

    Q = MCAO + ML (mass flow rate x heat capacity x change in temp) + (mass flow rate x latent heat for evaporation of water)

    Thermal efficiency = Q out / Q in

    3. The attempt at a solution

    M = 0.3 kg/s
    c = 4.7 KJ/kgK in J = 4700J/kgK
    AO = 100C - 15C = 85C
    L -= 2.26MJ/KgK in J = 2260000J/kgK

    so Q = MCAO + ML
    Q = (0.3)x(4700)x(85) + (0.3) x (2260000)
    Q = 797 850 J/s or 0.79785MJ/s


    Thermal Efficency = Qout / Qin
    Thermal Efficency = 797.850J / 3100000000J
    =???? (please help me out on this one)
     
  2. jcsd
  3. Jun 21, 2012 #2
    Re: Thermodynamics question help?? is this right??

    Hint: Watch your units.
     
  4. Jun 21, 2012 #3
    Re: Thermodynamics question help?? is this right??

    for the efficiency??
     
  5. Jun 21, 2012 #4
    Re: Thermodynamics question help?? is this right??

    Hint: Look at the numeric information given in the problem statement more closely.
     
  6. Jun 21, 2012 #5

    rude man

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    Re: Thermodynamics question help?? is this right??

    Hint: how does the 50mm figure in?
     
  7. Jun 21, 2012 #6
    Re: Thermodynamics question help?? is this right??

    all the values in the list there are at the bottom of the page i have, they are for a number of questions, i think there are 6 but only some will apply for this question. So i don't have to use them all!
     
  8. Jun 21, 2012 #7

    rude man

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    Re: Thermodynamics question help?? is this right??

    Good, 'cause you can toss that one out.
     
  9. Jun 21, 2012 #8
    Re: Thermodynamics question help?? is this right??

    c = 4.7 KJ/kgK in J = 4700J/kgK
    L -= 2.26MJ/KgK in J = 2260000J/kgK

    did i transpose on of these wrong
     
  10. Jun 21, 2012 #9

    rude man

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    Re: Thermodynamics question help?? is this right??

    Well, there's a question here. Is the water just raised to 100oC or is it then changed to steam at 100oC? The question doesn't say. If the former, do you need L? (I suppose 'boiler' does imply steam but who knows ...)

    Anyway, like Lawrence told you - watch your units. What should the units of L be, assuming you need L in the first place?
     
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