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Homework Help: Saturated water tables:use temp or pressure

  1. Jun 19, 2013 #1
    1. The problem statement, all variables and given/known data
    In my class we use two Saturated Water (Liquid-Vapor) tables. One is tabulated based on temperature and the other based on pressure.

    For example if a state is defined by p=10 kPa and T=40 C and I need to find hf I can get either 167.57 or 191.82 depending on which table I use.

    I am unsure which table to use.

    3. The attempt at a solution
    Looking in terms of the temperature table at 40 C the given pressure is 7.384 kPa which is less than the state's pressure.

    Looking in terms of the pressure table at 10 kPa the given temperature is 45.81 C which is more than the state's temperature.

    I have looked at a few problems and whenever this happens one table will yield results were the two properties will be less then the two properties from the other table. In these questions the values needed are taken from the table where the two property values are smaller.

    Thus hf=167.57.

    Is my logic correct in using the table were the properties have the lowest values. Could I run into a state where one value is higher and the other one is lower ? Then what ?
  2. jcsd
  3. Jun 20, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    As long as you're in the mixed-phase region (saturated water to saturated vapor, or 0% < quality < 100%) there is only one possible temperature for a given pressure, or only one possible pressure for a given temperature. The combination you cite (10 kPa and 40C) is not in that region, so it is not saturated water.
  4. Jun 20, 2013 #3
    It seems that if you look at the saturated water tables @T you get Psat. If Psat < P than the substance is actually sub cooled.

    Since the internal energy is mostly a function of temperature using the values found using the table @T gives you the same answers (with the table precision).

    You can find the actual values using the table @ T using:

    hsubcooled(T) = hsat + vsat(Psubcooled-Psat)

    Where hsat, vsat and Psat are found from the table @Tsubcooled

    Hopefully that makes sense.
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