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Steam vs Fuly saturated air: Separation

  1. Jul 18, 2011 #1
    For water separation purposes, what's the difference between steam and fully saturated air? I was looking at a certain product:
    http://www.nciweb.net/combinat.htm
    And I'm not sure if such a thing would be suitable for separating the water from a fully saturated air stream at about 60degC.
    Anyone can help? Not used a product like this before.
    Thanks
     
  2. jcsd
  3. Jul 18, 2011 #2

    jambaugh

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    Steam is pure water vapor.

    In considering air and water mixtures you look at the partial pressures of water vs air components. The total pressure will equal the sum of the partial pressures and you can look up how that relates to e.g. mass ratios.

    Pure steam has a partial pressure for air of zero. A steam air mix is a water vapor air mix.

    Then looking at the temperature and comparing it to the vapor pressure of water at that temperature you can see if the mixture is saturated (partial pressure = vapor pressure). In the case of steam it is always saturated since it is a pure gas.

    Now if you drop the temperature of a saturated gas mixture it will become super-saturated and the water component may condense. You get fog, or clouds, or mist, or rain.

    The device in your link appears (to me) to be designed to remove liquid droplets from a gas, be it steam or saturated air. It will have no effect on water vapor, only on water droplets which have condensed.

    [edit]
    One application may be when compressed air (with some water vapor in it) expands from a tank to feed a pneumatically powered device, it cools and may cool enough to condense some of the water. Have you ever noticed a little wisp of fog when disconnecting a compressor hose? The water droplets may be detrimental to the device and you would want to filter them out.
     
    Last edited: Jul 18, 2011
  4. Jul 19, 2011 #3
    Engineers define steam as water vapor above the boiling point of water.
     
  5. Jul 21, 2011 #4

    jambaugh

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    Which reminds me of another point or two. The partial pressure of water vapor in air at a given temperature is the pressure at which this temperature is the boiling/condensing point.

    Typically saturated air will, along with its 100% humidity, contain some liquid water since it is right at the condensation temperature.

    Take then the case where one is below saturation (dry gas), then the boiling point at the partial pressure is below the current temperature. This includes the case of 100% water vapor a.k.a. steam (dry steam).
     
  6. Jul 25, 2011 #5
    jambaugh, thanks for your help, I appreciate it.

    With reference to the exponential P vs T diagram here: http://en.wikipedia.org/wiki/Vapour_pressure_of_water

    Is it right to say that at points above this line, condensation will occur, whereas superheated steam will result at points below the line? Or is it more complicated than that - I just saw a phase diagram that confused me a little.
     
  7. Jul 25, 2011 #6
    And, if that is the case, it would also suggest to me that if you aim to increase the pressure of the saturated gas, you can achieve more condensation.
    But wouldn't increasing the pressure also cause an undesirable temperature rise - which means you would have to find a way of preventing a temperature increase..
     
  8. Jul 25, 2011 #7

    jambaugh

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    Right!

    Generally you cool the gas to encourage condensation. As you surmise increasing pressure will increase temperature unless accompanied by cooling. Note the reverse case does happen (I think), decreasing pressure can cool the gas enough to induce condensation... recall my mention of seeing fog when disconnecting a compressed air line?
     
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