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Superheated steam/isentropic expansion.

  1. Aug 3, 2012 #1
    I have a quiz tomorrow and I hope somebody sees this and answers in time.

    Alright, so I have superheated steam at 8600kPa and 500 degrees C. It expands isentropically to 2900 kPa.

    Since it's isentropic, S2 must be equal to S3, assuming it's ideal. S2 = S3' = 6.6858.

    Getting the saturation entropies at 2900 kPa, Sl = 2.6283 and Sv = 6.1969. x is 1.13.

    Now this is the problem; it's still superheated. How on earth do I get enthalpy (H) 3? I've been looking at the freaking steam table for three hours with no luck. apparently the answer is 3071 and I have no idea how did they even get that.

    Thanks.
     
  2. jcsd
  3. Aug 3, 2012 #2
    Re: Superheated steam/isentropic expansion. URGENT

    you will have to extrapolate the enthalpy from saturated vapour enthalpy point to the point you want.
     
  4. Aug 3, 2012 #3
    Re: Superheated steam/isentropic expansion. URGENT

    you can also use h-s chart.
     
  5. Aug 3, 2012 #4
    Re: Superheated steam/isentropic expansion. URGENT

    Can I get numerical values? All this thinking made my brain go broke.

    I just need to know the entire process of getting the value, then I'll probably rest a bit. Thanks.
     
  6. Aug 3, 2012 #5
    Re: Superheated steam/isentropic expansion. URGENT

    take a look at mollier diagram.a big chart,just follow the constant pressure line corresponding to the pressure and see what you get for that entropy.
     
  7. Aug 3, 2012 #6
    Re: Superheated steam/isentropic expansion. URGENT

    we're required to use steam tables though.
     
  8. Aug 3, 2012 #7
    Re: Superheated steam/isentropic expansion. URGENT

    OMG i finally got it. I take 2900kPa. Look for the appropriate interpolation wrt to 6.6858, then interpolate it to take the enthalpy which has the entropy of 6.6858. thanks!
     
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