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An FE Exam Problem

  1. May 4, 2005 #1
    Q. A heat engine cycle is executed with steam in the saturation dome. The pressure of steam is 1 MPa during heat addition and 0.5 MPa during heat rejection. THe highest possible efficiency of the heat engine is.

    How to calculate heat efficiency?

    W = QH / COP,hp

    and

    COP = 1 / ( TH /TL - 1 )

    Pl. note COP- denotes the coefficient of performance of the heat engine.

    Any ideas on how to proceed?

    Thanks, a lot!
     
  2. jcsd
  3. May 4, 2005 #2

    Clausius2

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    It seems to me that the engine is working with a Rankine Cycle. Anyway, during the change of state the pressure and temperature remains constant. If you have the pressure 1MPa then you have the temperature of saturation of water ([tex] T_h=180ºC=353K[/tex]) and the pressure 0.5 Mpa gives you [tex]T_c=150ºC=323K[/tex] aproximately (verify the data in a table of water).

    This means the engine, maybe an steam machine, is vaporizing water in the boiler at temperature [tex] T_h[/tex] which may be the hot focus (maybe it is not totally true in a Rankine cycle because the bolier superheats the vapor), and is condensing the vapor at temperature [tex]T_c[/tex] in the condenser. The second principle of the thermodynamics states than the maximum efficiency is reached by means of a Carnot engine working between the two extreme temperatures:

    [tex]\eta_{max}=\frac{W}{Q_h}=1-T_c/T_h[/tex]

    I don't really understand why you used the COP when we are talking about an engine.
     
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