1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Pressure Vessel

  1. Jun 23, 2004 #1
    I have a question for you all, say you have a pressurized cylinder with water in it, it is at 200*F and at a pressure of 300psig. Now you open a drain valve on the bottom to the atomosphere.

    Will it flash to stream or just come out as hot water or a combo of the two?

    What equations and info do tyou use to figure this out?

  2. jcsd
  3. Jun 23, 2004 #2
    Here is a calculation for flash steam:

    % flash steam = (100 * (hf1 - hf2)) / hfg2

    hf1 = enthalpy of liquid at pressure p1
    hf2 = enthalpy pf liquid at pressure p2
    hfg2 = latent heat of vaporization at pressure p2

    This calculation is from ASHRAE Systems 2004.

    Now if I'm interpretting this correctly, that equals

    % flash steam = (100 *(393.84 - 180.07)) / 970.3 =
    % flash steam = (100 *(213.7)) / 970.3 =
    % flash steam = 21377.00 / 970.3 = 22.03%

    So approximately 22% should flash to steam.

    Do not try this experiment at home!
  4. Jun 24, 2004 #3
    what units are you using? I used 80*F as the ambient temp.

    I got hf1=169 BTU/lb, hf2=49 BTU/lb and hfg2 970 BTU/lb.

    so I came up with 12% steam.
  5. Jun 24, 2004 #4
    Sat. Liquid Enthalpy hf of 393.84 at 300 psi Abs Pressure and 180.07 at atmospheric pressure of 14.696 psi Abs Pressure.

    I'm not positive that I am right, but the formula calls for the hf at the given pressure, not the given temperature. Maybe someone else can verify this.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook