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How to calculate the pressure in a sealed vessel with water.

  1. Nov 10, 2011 #1

    I cannot find any equation to help determine the resultant pressure from heating water in a sealed metal container, all I find is PV-nRt which is only gas - We are heating doing this in part to pretreat biomass. sealed tube: vol =170cc containing 80ml.s water: tube heated to 180C. I need to find appropriate relief valve.

    How do I determine pressure?

    Please show calculations

    I assume it does not matter how much water is in the tube, the "gas" part of the tube has no influence on the specific pressure generated by heat/water (?)

    Thanks so much!

  2. jcsd
  3. Nov 10, 2011 #2


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    Staff: Mentor

    Welcome to PF!

    There is no calculation to do here: just look it up on a steam table.
  4. Nov 11, 2011 #3
    I was thinking the pressure has to do with temp and the vol of the cylinder -

    If I heat up to 150C, a 200cc sealed container containing 100 MLs water, the pressure would be different if I heat up to 150C, 100 mls. of water in a 500 cc container... is this not correct? - if I continue to increase the size of the container, will not the pressure decrease?

    but actually in my case, not all the water has converted to steam -
  5. Nov 11, 2011 #4


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    Staff: Mentor

    No, it is not correct. Steam has a certain vapor pressure at a given temperature, so a volume of water will boil until that pressure is reached.

    As long as that pressure is greater than that provided by the air and not all the water boils, it will reach an equilibrium between water and saturated steam.

    What you describe is not fundamentaly different from the operation of a steam heating system.
    Last edited: Nov 11, 2011
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