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Homework Help: Second Law of Thermodynamics problem

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data

    As shown in the figure, a system undergoing a power cycle develops a net power output of 1MW while receiving energy by heat transfer from steam condensing from saturated vapor to saturated liquid at a pressure of 100 kPa. Energy is discharged from the cycle by heat transfer to a nearby lake at 17 deg. C. These are the only significant heat transfers. Kinetic and potential energy effects can be ignored. For operation at steady state, determine the minimum theoretical mass flow rate, in kg/s, required by any such cycle.


    2. Relevant equations

    (the book has letters with dots above them, these are represented by 'd')

    dW = dQH-dQC
    l_3c57a33c37ca42b6acadd05371bce2b4.jpg (equals zero)

    3. The attempt at a solution
    So I just did this problem again, and here is what I get:

    I calculate efficiency, which is 1-(Tc/Th), T being in Kelvin. I get 41/186, or ~0.2204

    Then I use the formila Q=W/n, n is efficiency, so it is 1000000/(41/186) = ~4536585.4

    Then plugging this Q into the mass flow formula, where gz and V^2/2 are zero:

    4530585.4-1000000+m(2675-417), 2675 and 417 being enthalpy values at the states given

    I get a negative number, but mass flow rate can't really be negative per say, so I end up with m=1566.2468

    Am I right?

    Last edited: Oct 16, 2008
  2. jcsd
  3. Oct 16, 2008 #2
    I updated the problem with my current solution (or what I think is the solution)

    Please let me know if I am on track

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