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Need help with thermo problems

  1. Sep 23, 2004 #1
    Need urgent help with thermo problems...

    Here are my problems:
    1) A rigid tank contains 5 lb of a two-phase liquid vapor mixture of water, initially at 260 F with a quality of .6 Heat transfer to the contents of the tank occurs until the temperature is 320 F. Determine the initial mass of vapor, in lb, initially present in the tank and the final pressure, in psi.

    For T=260, vf (specific volume of saturated liquid) = .01708 and vg (specific volume of saturated gas) = 11.77
    I know quality is .6, so I can use the equation x=(v-vf)/(vg-vf) to find v.

    For T=320, vf=.01765 and vg=4.919.
    *Can I assume quality is the same and use the same equation as above to solve for v? How do I find initial and final pressures? How do I know the initial mass of the vapor?*
    Hints please!

    2) Water vapor is heated in a closed, rigid tank from saturated vapor at 160 C to a final temperature of 400 C. Determine the initial and final pressures, in bar, and the initial and final specific volumes, in m^3/kg.

    *Here i'm having trouble discerning the relationships between p, v, and t. Can someone clarify for me?*

    Thanks in advance,
  2. jcsd
  3. Sep 23, 2004 #2


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    Yup. What's "unknown" in your second use of the equation?

    You've access to excerpts from the steam tables in your text (appendices), course materials (handouts), or library (CRC Hndbk, Int. Crit. Tables, JANAF tables, NBS500, yada-yada-yada). Phase rule: 1 component, 2 phases, you're given T, P is fixed --- it's the "vapor pressure." Given volumes and densities of the phases, do you need any other information to calculate mass?

    This looks like a freebie --- you probably want to check the problem statement again. As it stands, water vapor (saturation density at 160 C, and vapor pressure at 160 C) is heated at constant volume to a supercritical temperature, again single phase. Specific volumes don't change --- all you've got to do is pick the pressure from the tables, Molier diagram, whatever your instructor's pet representation of the steam tables happens to be.
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