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Homework Help: Heat transfer in a double pipe HE

  1. Nov 23, 2012 #1
    1. The problem statement, all variables and given/known data
    I performed an experiment using a double pipe heat exchanger. Readings were taken at steady state.

    Cold water entered the inner tube at 9°C, and its exit temp was 81°C. (flow rate was measured).

    The outer tube contained steam at 133°C and this temperature remained constant. Condensate flow rate was measured.

    2. Relevant equations

    i) q=U A ΔT(lmtd)
    ii) q=m h[itex]_{fg}[/itex] (where hfg is heat of vaporization)
    iii) q=m Cp ΔT(bulk)

    3. The attempt at a solution
    Since the steam did not change temperature but condensed, there was latent heat transfer. q was calculated using ii). (result q=28kW)

    I then calculated q using equation iii) applied to the cold water stream. (result q=59kW)

    I don't understand why these two results are so far apart. In a perfect system, wouldn't they be equal?
  2. jcsd
  3. Nov 24, 2012 #2
    Yes. And, even if there were heat losses, q ii would be higher than q iii, rather than lower. Show us the data.
  4. Nov 24, 2012 #3
    Yes my mistake, I mixed them up. The higher q came from equation ii). I guess that makes sense since not all the released energy from the condensation ends up warming the cold stream.
    So this large difference can be attributed to heat losses?

    I'm also asked to do an energy balance to verify the accuracy of the heat transfer rates. Isn't that what I just did when I compared the two q values? How would that verify anything, as they would always be different?
  5. Nov 25, 2012 #4
    Let's see the data. Flow rates, heat of vaporization at 133, Cp used?
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