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Confusion over Thermodynamic process (heat and termperature)

  1. Feb 10, 2009 #1

    I have been learning about the Carnot and Rankine Cycles, where process 4-1 is through a evaporator, process 1-2 a turbine, 2-3 a condensor and 3-4 a compressor. Sketching this on a T-s diagram gives straight horizontal lines for processes 4-1 and 2-3. This is where the heat is extracted and put in, so I dont understand how temperature does not increase and decrease. i.e, you add heat in the form of flame to water and its temperature increases. Probably simple but it has been driving me crazy!

    Any help would be most appreciated.

  2. jcsd
  3. Feb 10, 2009 #2


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    Staff: Mentor

    If the water is at 100 C and atmospheric pressure and you add a flame to it, what happens to the temperature?
  4. Feb 10, 2009 #3
    You are neglecting the phase change of the water. The heat being put into and removed from the fluid is in the form of latent heat. So the temperature remains constant but the fluid changes phase. Since you have heat transfer at a temperature, you have entropy creation or removal, hence the strait horizontal line.
  5. Feb 11, 2009 #4
    does so for carnot cycle, doesmt for rankine cycle. Rankine cycle also has a sensible heat gain part in the process 4-1(considering 4 is the pump outlet). If you have seen a modified rankine cycle, it has a sensible heat gain after the total evaporation(& its called superheat)
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