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I Does water vapour absorbtion depend on H2O conc or RH?

  1. Dec 21, 2017 #1
    If I blow air containing a fixed kg/m3 of water vapour over anhydrous CaCl2, will the absorbtion rate change if the air temp is different? To simplify, we neglect exotheric heat generated by hygroscopy.

    Thanks for your answers to this rather simple question.
  2. jcsd
  3. Dec 21, 2017 #2


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    Staff: Mentor

    Yes. You are describing the exact material and process sometimes used for for desiccant dehumidification. The absorption rate and capacity drops as temperature rises and above a certain temp, the absorption stops and it starts rejecting. You should be able to find charts giving exact performance for a theoretically perfect process. I found this, for example, but it doesn't have cacl2...
    Last edited: Dec 21, 2017
  4. Dec 25, 2017 #3
    May I first wish you and your family a Happy Xmas. Father Christmas and the grandchildren visited us yesterday so I'm free today to think .....

    I'll come back on absorbtion rates when I've finished a few observations here, but in the meantime could I again ask for your opinion on two schoolboy topics:

    1. A closed air-containg horizontal tube externally heated towards one end. Will air inside be at a lower density at the heated end?
    2. If the air is moist, will the g of water wapour per kg of dry air be different along the tube?
  5. Dec 25, 2017 #4
    1) The air will be at the same pressure, but lower density at the heated end.
    2) The absolute humidity (grams H20 per grams of dry air) will be the same, the relative humidity will be lower at the heated end.

    You might find it interesting to spend some time studying a psychrometric chart, which shows the relationships between temperature, water vapor concentration, relative humidity, and density.
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