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Homework Help: Thermodynamics - Help me understand phase diagrams

  1. Feb 2, 2013 #1
    1. The problem statement, all variables and given/known data

    So my thermo professor has been putting a huge emphasis on phase diagrams, which I didn't really expect. He basically skips most of the analytical/calculus type problems and gives us the graphical phase diagram/ "real world" type of problems for homework. I don't understand phase diagrams because they never put any labels on the axes, and it is just like a little parabola and then he draws two horizontal lines through it, but nothing is labeled.

    So here's the problem :

    For H2O, determine the specific property at the indicated state. Locate the state on a sketch of the T-V diagram.

    a) p = 300 kPa, v (specific volume) = .5 m^3/kg Find T, in Celsius.

    2. Relevant equations

    PV^n = k, k is a constant

    3. The attempt at a solution

    I have a table where I can look up these things but it says that water at 300 kPa (3 bar) is supposed to be .6058 m^3/kg not .5 m^3/kh. So I guess the water in this problem is not saturated and I cant use my table? How can I tell?

    And once I find the temperature how do I draw the diagram?
  2. jcsd
  3. Feb 2, 2013 #2
    Google T-V diagram. One of the figures it will show you will be a t-v diagram for water. Click on it. Find your point on the diagram.
  4. Feb 3, 2013 #3
    Ok I have done this. But the graph is scaled logarithmically or something and my answer would not be exact. Am I just supposed to just guess what the temperature is between 100C and 200C?
  5. Feb 3, 2013 #4
    Yes. That's how you would use any graph. You have to interpolate as best you can by eyeball. Are you uncomfortable with logarithmic coordinates? If so, get used to it. Find a piece of logarithmic graph paper with a more detailed grid on it to see how the numbers space out, or get yourself a slide rule (which has logarithmic scales on it). Try to find the location of your data point as closely as possible on the graph and see if you can at least see what region it is in qualitatively. For example, is it in the superheated region or in the saturated region?

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