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Psychrometry question

  1. Aug 22, 2015 #1
    As we all know that in atmospheric pressure (1 Bar) saturation temperature of water is 100 deg C. And the air contains mixture of dry air and water vapour.
    My doubt is here that the water needs 100 deg Celsius to change from liquid to vapour state then how can a normal room temperature air contain water vapour. since it need 100 deg to start converting into vapour then how can water be in form of vapour at room temperature (say 20 deg).
    One thing i also have doubt that how a water spread on floor gets evaporated at normal temperature. I know i am doing wrong anywhere but please clear the concept.
  2. jcsd
  3. Aug 22, 2015 #2


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    Look/Google vapor pressure, partial pressure, phase rule, dew point, bubble point, phase transition, phase diagram.
  4. Aug 22, 2015 #3


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    The water doesn't start to evaporate at 100 °C, but earlier. Have you ever heard of Dalton's law? It says that the total air pressure is the sum of all the partial pressures of its components - the partial pressures corrispond to the volume and molucule fraction of the different gases. That means, that 1 bar of air pressure is roughly the sum of 0.79 bar "nitrogen-pressure" an 0.21 bar "oxygen-pressure". However there are more components than these two in the air, also steam. Depending on its temperature, water has a certain vapor pressure, which is the maximal possible (partial) pressure of water in gaseous phase. At 100 °C the vapor pressure is 1 bar, therefore all the water can evaporate. The vapor pressure of water at 20°C is 0,0234 bar. That means if the water has a temperature of 20°C and the pressure is 1 bar, water can evaporate till 2.34 % of the air consists out of steam.

    Find attached a diagram of the temperatur vs. pressure for water to find the boiling temperature depending on the pressure or the vapor pressure depending on the temperature

    PS.: The ratio of the actual partial pressure (volume/molecule fraction) steam in the air to the maximal possible (=the vapor pressure dependent on the temperature) is called the relative humidity.

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    Last edited: Aug 22, 2015
  5. Sep 10, 2015 #4
    There is about 2 ways of evaporating water.

    1) Heating
    2) Depressurization/ flash evaporation - on an isotherm
    Simple heating - increasing the water temperature at constant Pressure until vapor phase is reached and consequently it evaporates
    Flash evaporation by the way, is achieved when water is depressurized at constant temperature.

    Also air has capacity to absorb water to the point of (saturation) where it can no longer absorb further, thus condensation or precipitation occur.
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