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Boiling water produces how much air?

  1. Aug 5, 2008 #1
    Is there a way to calculate this?

    We are interested in how boiling water creates air bubbles. We want to generate a volume of 500 cubic feet per minute of air at a depth of 100 feet (which would be approx 2000 cfm at surface) in a large water tank (100x18x8). We want to generate this volume of air by placing a heating element either under the tank or at the bottom of the tank in order to boil the water and produce the air bubbles. We have a cap kilowatt usage goal for the heating element (or elements) of 200 kw constant. We have no idea how to calculate how much air can be produced though. Any thoughts?

    kit
     
  2. jcsd
  3. Aug 5, 2008 #2

    Vanadium 50

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    Before going further, you do realize that the gas that's generated is not air, don't you? Is this a problem?
     
  4. Aug 5, 2008 #3

    turbo

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    The only gas that you can get from water is the very small amount of gas that has been dissolved and/or entrained in the water. As Vanadium 50 has pointed out, the bubbles coming off the heating element are not air, but steam - water in its vapor phase.
     
  5. Aug 5, 2008 #4
    Thank you for replying. I understand what you are both explaining. Let me ask this - If you were to hover a large bowl above the rising steam bubbles, would you get any air at all in that bowl that would allow the bowl to rise to the surface (1 cubic foot of air lifts 63 lbs)?

    Kit
     
  6. Aug 5, 2008 #5

    turbo

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    If you want to generate lift by capturing and containing water vapor (steam) that's a whole different question. The lift generated by the minuscule amount of dissolved gases that might be liberated by heating water is insignificant.
     
  7. Aug 5, 2008 #6

    russ_watters

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    The lifting capacity of water vapor at 212F would be more than that of air - water vapor is less dense than air.
     
  8. Aug 5, 2008 #7

    berkeman

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    Interesting question, though. Which is more energy efficient -- boiling water to generate a steam bubble, or dissociating water into separate hydrogen and oxygen bubbles.... ?
     
  9. Aug 5, 2008 #8

    DaveC426913

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    OK, so that means this experiment is valid, right? Turning water to steam would* raise the lifting body.

    * I say "would" because they have something else to contend with: namely that they will have a tremendously difficult time boiling the water.

    1] Water conducts heat so well that they wil have trouble keeping the heat localized enough to heat the water to boiling, it will want to heat the entire tank before boiling.
    2] The depth of the tank will raise the vapour pressure, compounding the difficulty of vapourizing any water.
     
  10. Aug 5, 2008 #9
    Actually we were originally thinking of what the lifting capability would be of a rotary air compressor with an intake cfm of 2000 which would be 500 cfm at 100 ft depth of water. Then we wondered if you could get more lifting ability if you were to use the same energy which is 200 kw constant and place some sort of heating element at the bottom of the tank that would boil the water and possibly give a better result.



    Kit
     
    Last edited: Aug 5, 2008
  11. Aug 5, 2008 #10

    Vanadium 50

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    200 kW will vaporize 600g of water at 100C per second. I know your water isn't at 100C, so this is an upper limit. 600g of water occupies 750 liters at STP, or about 26 cubic feet. That's an equivalent intake of 1500 cfm. So even theoretically, you will do worse than pumping air. Putting in practicalities makes this worse still.

    The weight difference between air and steam is negligible - what matters is the volume of water displaced.
     
  12. Aug 5, 2008 #11

    russ_watters

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    Off the top of my head, I'd say the dissociation energy would have to include the vaporization energy. Not certain, though...
     
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