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The rate of evaporation at high temperatures

  1. Feb 3, 2009 #1
    To whom it may concern
    I'm a University of Portsmouth student in the UK and have a conundrum for one of my projects. I'm trying to evaporate a reservoir of water at the bottom of a venting system to cool down the airflow in a gap between two membranes for a period of time. The reservoir is at the bottom of the void and the temperature of the air above the water is 360 degrees C. How long would it be for 2 litres of water to evaporate?
    I have looked for a simple solution for this and have come across Langmuir evaporation rate but I'm struggling to make sense of it. If this the correct way forward?
    According to my findings I need to use the equation
    (mass loss rate)/(unit area)= (vapour pressure- ambient partial pressure)* sqrt((molecular weight)/(2*pi*Gas constant*temperature in Kelvin))
    The Surface area (unit Area) of the water will be 0.1m²
    the vapour pressure of water at 360 degrees C is 18666
    the pressure at ambient (20degrees) is 2.3392
    Molecular weight of water is 0.014kg/mole
    Gas Constant (R) = 461.5 for water vapour
    Temperature in kelvin = 360+273=633

    Therefore this would mean through calculus that the rate of evaporation is 0.185 kg/m²/sec(3 d.p.). Meaning 2 litres of water with a surface area of 0.1m² and an air temperature above of 360C will evaporate dry in roughly 108 seconds.
    This surely can not be right.
    Can anyone advise on the errors of my ways?
    Thank you in advance if you can.
  2. jcsd
  3. Feb 3, 2009 #2


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    "Surely" because of an observation, or "surely" because it just doesn't sound right? I would be surprised if water could hang around for too long so high above its boiling point. Unfortunately, my oven doesn't get that hot, so I can't test it. But perhaps I will see how long it takes for 2 L of water at 561 K (288 C) to boil away.
  4. Feb 4, 2009 #3
    Surely in the sense that this time seems far too short a time for 2 litres of water to last. Air entering the vent system is 20 degrees and the air 250mm up is 360. That figure seems just too quick to me.
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