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Homework Help: Fundamentals of thermodynamics, specific volume stuff

  1. Jan 31, 2010 #1
    1. ammonia at 20 c with a quality of 50% and a total mass of 2 kg is in a rigid tank with an outlet valve at the bottom. how much liquid mass can be removed through the valve assuming that the temperature stays constant?

    and from so table in the text

    saturated press.= 857.5kps (@20 c)
    saturated specific volume Vf = 0.001638 [m^3/kg]
    saturated specific volume Vg = 0.1422 [m^3/kg]

    2. Relevant equations

    3. The attempt at a solution
    mass liquid =1 kg
    mass gas= 1kg

    Vf * 1kg = 0.001638 m^3
    Vg* 1kg= 0.1422 m^3

    v of tank = 0.150858 m^3

    (0.001638 m^3)/(Vg = 0.1422 [m^3/kg])= 0.0109770808kg

    1kg (of liquid) -0.0109770808kg ( mass of gas needed to fill the liquids space) = .9890229192 kg of liquid can be removed from the tank

    can anyone tell me if thats right?
  2. jcsd
  3. Feb 1, 2010 #2


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    Science Advisor
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    Gold Member

    Hi eibon,
    The problem gives you the quality (50%) of the saturated, 2 phase mixture. It also gives you the temperature and total mass. I'm going to assume you've correctly found the values of Vf, Vg and the saturation pressure.

    Knowing this information, you can determine the nominal density of the mixture by applying the equation for the definition of quality. I don't see that equation listed in your post so I can't be sure how you arrived at the tank volume, but I don't think your answer is correct (ie: .150858 m^3 doesn't seem right to me).

    Once you determine the tank volume, then knowing that temperature doesn't change as you remove liquid from the tank until the pressure hits the saturated gas point, you may assume the remaining ammonia is 100% saturated gas. Since you already have the specific volume for this, you should be able to determine the mass left in the tank. Then it's just a simple calculation to determine how much mass was removed.
  4. Feb 2, 2010 #3
    I've been working on the same question for an assignment. I think the volume of the tank is correct and I also got the same final answer (0.98902 kg).
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