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Homework Help: Problem with Pressure

  1. Sep 9, 2006 #1
    I'm having trouble beginning this problem. It doesn't seem that I have enough information to begin the problem, but I know that can't be the case. Can someone please provide a little direction?

    Consider two tanks, A and B, connected by a valve. Each has a volume of 200L, and tank A has R-12 at 25 degrees C, 10% liquid and 90% vapor by volume, while tank B is evacuated. The valve is now opened and saturated vapor flows from A to B until the pressure in B has reached that in A, at which point the valve is closed. This process occurs slowly such that all temperatures stay at 25 degrees C throughout the process. How much has the quality changed in tank A during the process?

    EDIT---> Ok, here's what I've got so far:

    Before the valve is opened:

    specific volume of R-12 liquid is .000763 m3/kg
    mass of the R-12 liquid= 20L/.763L/kg= 26.21 kg

    specific volume of R-12 vapor is .02685 m3/kg
    mass of the R-12 vapor= 180L/26.85L/kg= 6.70 kg

    After the valve is opened:

    mass of the R-12 vapor= 380L/26.85L/kg= 14.15 kg

    What I don't understand is how I determine how much vapor is still in tank A. To determine the quality, I need to know the specific volume of the vapor still in the tank. The formula I have for quality is:

    x(quality)= (υ – υf)/ υfg where υf is the specific volume of the liquid and υfg is the specific volume of the vapor-liquid mixture.
    Thanks for your help so far. If you could provide any more assistance I would be grateful.
    Last edited: Sep 10, 2006
  2. jcsd
  3. Sep 9, 2006 #2


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    Homework Helper

    The vapour pressure for a pure liquid is only temperature dependent. If this R-12 can be considered such then the amount of vapour will increase as it expands from 180 L to 380 L in order to keep its pressure the same. Since the expansion is adiabatically the vapour will stay in a state of saturation during all stages of the expansion. My physics intuition tells me that the density of the vapour (and the liquid) should stay the same under such conditions.
    Last edited: Sep 10, 2006
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