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Homework Help: Calculating h & u for values not on tables (Thermodynamics)

  1. Nov 21, 2013 #1
    Hi, most of my thermodynamics class we have been looking up enthalpy on tables when the pressure and temperature is provided. For some homework questions, the values are not on the tables, but fall in between them. What is the correct approach to finding these when the quality is not provided? For example:

    Which has the capability to produce the most work in
    a closed system—1 kg of steam at 800 kPa and 180°C or 1 kg
    of R-134a at 800 kPa and 180°C? Take T 0 = 25°C and PQ =
    100 kPa. Answers-. 623 kJ, 5.0 kj

    u = 2594 . 7 kJ/kg
    P = 800 kPa
    v = 0 . 24720 m /kg
    T = 180 ° C
    s = 6 . 7155 kJ/kg ⋅ K

    u 0 ≅ u f @ 25 ° C = 104 . 83 kJ/kg

    v 0 ≅ v f @ 25 ° C = 0 . 001003 m /kg
    P 0 = 100 kPa
    s 0 ≅ s f @ 25 ° C = 0 . 3672 kJ/kg ⋅ K
    T 0 = 25 ° C

    (Edited in correct values)

    What is the correct approach to get here?
    The textbook I am using is Thermodynamics 7e (cengal)

  2. jcsd
  3. Nov 21, 2013 #2


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    Can't interpret the example without being told what values are in the tables and what values you are trying to look up. (Try not to assume anyone wanting to respond to you has the same book.)
    As a general matter, there's interpolation and extrapolation. If the values you have lie between those in the tables then it would be usual to assume (in the absence of glaring evidence to the contrary) that the function is about linear in between adjacent the values in the tables. So you can just do a weighted average. Do you know how to do that?
  4. Nov 21, 2013 #3
    Look up interpolation. That's how you find values that are in between.
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