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Homework Help: Need Help Mech. Eng. -- An ideal Rankine cycle

  1. Apr 18, 2015 #1

    I need help verifying the correct answers for the following questions.

    2. An ideal Rankine cycle is operated with a turbine inlet pressure of 500 psia and an inlet temperature of 600 oF. The steam is isentropically expanded through the turbine to 2 psia as shown in the following T-s diagram. Under these operating conditions, find the following .

    (a) Using the Mollier diagram on page 9 of this exam, find the entropy at the turbine inlet and turbine outlet. Clearly identify the turbine inlet and outlet entropies on the given Mollier diagram.

    I was given 1765 btu/R from another source

    My Ans. 1.56 Btu/lb-R

    (b) Using the Mollier diagram, find the enthalpy at the turbine inlet and turbine outlet. Clearly identify the turbine inlet and outlet enthalpies on the given Mollier diagram.

    Another source 1560 BTu/lbm

    My ans. h4=1300 BTu/lbm h5= 900 BTu/lbm

    (c) Using the Mollier diagram , determine percent moisture of the steam at the turbine outlet.

    another source 8%

    My ans 22%

    (d) Using the steam tables, determine the enthalpy at the pump inlet and pump outlet (neglect pump work).

    Work done by the pump is equal to w/pump = h2 - h1

    since pump work has to be neglect h2 ≅ h1

    Enthapy at inlet and outlet of pump is 89.575 Btu/lbm

    (e) Using values obtained from part (a), part (b), and part (d), determine the cycle efficiency (neglect pump work)

    Efficiency = Net work done / Heat input cycle

    395/1205 = 32.81%

    (f) Determine the work output of the turbine.

    W = h3 - h4 = 1300 -905 =395 Btu/lbm

    (g) Determine the heat rejected by the condenser.

    Heat rejected by condenser = h4 - h1 = 905 -94 = 810 Btu/lbm

    (h) Determine the heat gain or heat added to the boiler.

    Heat added to boiler = h3 - h3 = 1300 - 95.5 = 1204 btu/lbm

    Attached Files:

    Last edited: Apr 18, 2015
  2. jcsd
  3. Apr 18, 2015 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Your "another source" is in wild disagreement with the Mollier chart you posted. I would discard this other source entirely, as it will only confuse and mislead you.

    I don't read an s for your turbine of 1.58 BTU/lb-R, but you are close.

    Answering "not sure" to the rest of these questions is not a sufficient attempt at providing a solution. If you care to take a stab at providing actual answers, then more help will be forthcoming.
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