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Homework Help: Pressure change at the inlet of a steam turbine in a small electric power plant

  1. Apr 4, 2017 #1
    1. The problem statement, all variables and given/known data
    A steam turbine in a small electric power plant is designed to accept 4500 kg of steam at 60 bar and 500oC and exhaust the stem at 10 bar.

    Part C.

    In off-peak hours, the power output of the turbine in part a) (100% efficient) is decreased by adjusting a throttling valve that reduces the turbine inlet steam pressure to 30 bar while keeping the flow rate constant. Compute T1, the steam temperature to the turbine, Tr, the steam temperature at the turbine exit, and the power output of the turbine.

    2. Relevant equations
    Sautrated steam table, superheated water table, linear interpolation

    P=m (H2-H1)

    Entropy balance

    3. The attempt at a solution
    I know a valve is isenthalpic so I worked out for the new tempature of the steam and got 484C . I know turbines are isentropic so the entropy will remain the same from inlet to outlet. My only question is will my outlet pressure change from changing my inlet pressure from 60to 30 bar? or do I keep it the same as the problem statement in the first part which is 10 bar? Or is the pressure changing at the outlet as well? If it is how is it changing?

    Any and all help is appreciated

    Best regards,

  2. jcsd
  3. Apr 9, 2017 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
  4. Apr 14, 2017 #3


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    You could make an assumption that the output is designed to eject at 10 bar - do you have enough information to calculate that it would be different? If it is different, we would assume it would be less, since in the first situation it take energy out of 60 bar steam, then ejects 10 bar steam. Starting off at a lower pressure, if it take the same percentage of energy out, would it be close to half (5 bar) at the output. Probably not something as simple as that - atmospheric pressure is a little more than 1 bar. Since it is getting ejected, it will definitely have to be higher than that. (or is this gauge pressure?). I hope these ideas help you.
  5. Apr 16, 2017 #4
    They have helped me in my approach thank you. The problem statement did not clarify if there was any change in the outlet stream so I proceeded to solve it with the same outlet pressure of 10 bars.

    It is also how my professor solved it.

    Best regards,

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