Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Isentropic Efficiency

  1. Feb 7, 2016 #1

    y2j

    User Avatar

    Hi
    can any one help me about the following ?
    if I have the experimental data of inlet and outlet of expander (temperature and pressure) working in ORC.
    Now if I want to find the Isentropic efficiencie :
    the Isentropic expansion can be found by (entropy in = entropy out)
    but how to find the actual enthalpy at expander exit of actual expansion
    (i.e. I can consider it in the mixture or superheated phase?)
    thank you
     
  2. jcsd
  3. Feb 8, 2016 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    entropy in = entropy out gives you an enthalpy. Enthalpy difference * efficiency gives you a new enthalpy. Pressure and enthalpy give you a temperature (or a composition at saturation).

    See e.g. here ( I Googled "isentropic efficiency turbine" and looked at the first image)
     
  4. Feb 8, 2016 #3

    y2j

    User Avatar

    thanks for reply
    but i am not have the Isentropic Efficiency

    I am only have pressure and temperature of experimental system and I would like to have the Isentropic Efficiency if the exit conditions locate in tow phase zone
     
  5. Feb 8, 2016 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Then you work backwards from the enthalpy observed....
     
  6. Feb 8, 2016 #5

    y2j

    User Avatar

    I don't understated you
    what do you mean?
    thanks again
     
  7. Feb 8, 2016 #6

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Perhaps you should post your work so far.

    You have enthalpy in ##H_{in}##

    entropy in = entropy out gives you an enthalpy under isentropic conditions ##H_{isent}##

    observed enthalpy ##H_{out}##

    $$\eta_{isent} = {H_{in} - H_{out}\over H_{in} - H_{isent}}$$
     
  8. Feb 8, 2016 #7

    y2j

    User Avatar


    I am not have the Isentropic Efficiency
    I am only have pressures and temperatures at inlet and outlet of the
    turbine
    how can obtain Isentropic Efficiency in this case
     
  9. Feb 8, 2016 #8

    y2j

    User Avatar

    thanks for attention
     
  10. Feb 8, 2016 #9

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Is it clear now ?
     
  11. Feb 8, 2016 #10

    y2j

    User Avatar

    not yet
     
  12. Feb 8, 2016 #11

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Writing the same text in red does not help me to understand where you have a problem. Can you post your work so far ?
     
  13. Feb 8, 2016 #12

    y2j

    User Avatar

    OK
    I have an experimental data of inlet and outlet conditions of turbine (temperature and pressure ) working in a rankine cycle
    when the exit condition from turbine in saturated phase
    how I can find the actual enthalpy at exit (##H_{out}##)and that used to find the Isentropic Efficiency
     
  14. Feb 8, 2016 #13

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No need to get desperate...
    Enthalpies can be looked up in a steam table. Do you have that available in some form or other ?
     
  15. Feb 8, 2016 #14

    y2j

    User Avatar

    Enthalpies in the mixture zone given for vapor and liquid
    in this case to find Enthalpy we must have another parameter is (quality x) or dryness fraction
    and the later I have not
     
  16. Feb 8, 2016 #15

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I agree. That makes it difficult to get the correct value of ##\eta##. All I can imagine is that you assume no water comes out of the tubine, only steam...
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook