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Evaporation rate

  1. Dec 29, 2013 #1
    i have question.. i need some to help me to solve it..

    the Fact said that " when we reduce the pressure exerted on free water surface, the evaporation rate decrease"

    so i am searching for formula that related between the evaporation rate and pressure and volume
    Last edited by a moderator: Mar 21, 2016
  2. jcsd
  3. Dec 29, 2013 #2
    That statement is wrong. When air pressure decreases, evaporation generally increases. If the air pressure is greatly reduced, water can begin to boil.
  4. Dec 29, 2013 #3


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    In addition to what voko said, I would add that "the fact said ... " is not an adequate reference for this forum. Where did you hear this nonsense?
    Last edited by a moderator: Mar 21, 2016
  5. Dec 29, 2013 #4
    yes you are right, there was something wrong in my statement but now my question is what is mathematical equation that relate between evaporation rate and the atmospheric pressure
  6. Dec 29, 2013 #5


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    Oh my god.....Please use the default Font and Font size.
  7. Dec 29, 2013 #6
    hhhhhhhhhhhhhhh ok lol
  8. Dec 29, 2013 #7
    what is mathematical formula that relates between evaporation rate and atmospheric pressure
  9. Dec 31, 2013 #8
    Do you mean vapor pressure? Evaporation is more closely related to the vapor pressure. There is a relationship between the boiling temperature and the atmospheric pressure, as Voko mentioned.

    In any case, try googling on the Clausius-Clapeyron Equation.
  10. Dec 31, 2013 #9
    the Clausius-Clapeyron Equation relates between pressure and temperature.
    i will give you example for what i want:

    if i have vessel contains 100 m^3 of water and the surface area is 500 m^2 and the relative humidity is 50% and the speed of air is 0.5 m/s what is pressure drop that i need to evaporate this volume of water during 24 hours?
  11. Dec 31, 2013 #10


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  12. Dec 31, 2013 #11
  13. Dec 31, 2013 #12
    Firstly, just what do you mean by “pressure drop”? Vapor pressure? Atmospheric pressure? And how do you plan to drop it? Is this in a controlled laboratory setting, or out in the open air?

    Secondly, you have provided insufficient information with which to solve the problem.

    We need to know:

    1) Surface temperature of the water and whether this temperature will be maintained as the enthalpy of vaporization removes thermal energy from the water during vaporization. Surface temperature affects the gross rate of vaporization and the enthalpy of vaporization also varies with the temperature.

    2) Temperature of the air. If it is the same as that of the water, fine. If it is different, then we need to know whether this temperature differential will be maintained throughout the process. Air temperature affects the gross rate of condensation and the relative humidity.

    3) Pressure of the air. Along with the RH and the wind speed, this affects the gross condensation rate. Net vaporization is gross vaporization minus gross condensation.

    4) Surface energy of the water and whether this energy will be maintained throughout the process (minerals in solution?). Surface energy affects the gross rate of vaporization and varies with the temperature and the strength of the various surface bonding forces.

    5) Will the RH be maintained at a steady 50% as net evaporation increases?

    6) If natural cooling of the water is permitted, will the water be able to draw heat from its environment? If so, how much and at what rate?

    7) The evaporation of one-hundred cubic meters of water at a temperature of 25°C and 1000 hectopascals of pressure will produce roughly 130,000 cubic meters of water vapor. Where is all that vapor going to go?

    As you can see, the problem is not a simple one. I suspect, however, that engineers have come up with approximate solutions. Have you tried the Engineering Toolbox?
  14. Mar 21, 2016 #13
    I like this problem. It is deceptively complicated.
    Try to summarize it without formulas or words like 'Enthalpy'.

    The evaporation rate will be somewhere between the two extremes of zero (no evaporation) and boiling (maximum rate of evaporation).

    Assume the temperature of the air and water are always equal.

    We know and understand:
    Evaporation rate is increased by an increase in Surface area, temperature, and airspeed.
    Evaporation rate is decreased by an increases in pressure and relative humidity.

    If the air was still, Would it be possible to lower the pressure to zero (vacuum) and still have 50% relative humidity ?
    Will the 50% relative humidity determine the evaporation rate, regardless of pressure, temperature or air speed ?

    What is evaporation ?

    You have molecules leaving the surface and flying into the air, and molecules flying out of the air and joining the surface.
    If there was 100% relative humidity there would be equilibrium, and no net evaporation...

    But you have set this rate to 50% by setting the relative humidity at 50%.

    Regardless of temperature, air movement, or pressure..

    If there is 50% the amount of saturation, then you are going to have 50% rate of evaporation approximately ?

    You can lower the pressure all you want.
    Your theoretical relative humidity of 50% is going to set the evaporation rate is it not ?
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