Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do mist fans work?

  1. Aug 1, 2011 #1
    So, as far as I know evaporative cooling is when the top layer of a material (lets say water) evaporates- taking energy with it leaving the rest of the material cooler. Also leaving the air, which transported some heat to it, cooler.

    But in a misting fan, the mist evaporates, takes energy from the air, and then- it just remains there.
    So the overall energy level of the system should be the same, no? Same entlapy, same entropy.

    Then why when this mixture of air and evaporated water droplets blows- even that you suposedly get a system with the same energy level as before, it cools you down?

  2. jcsd
  3. Aug 1, 2011 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The energy liquid water may have the same as the energy of the water vapour but the temperature has decreased. The water mixing with the air results in an increase in potential energy of the water molecules. This potential energy comes at the expense of the kinetic energy of the surrounding air. So the temperature of the surrounding air decreases.

    The potential energy of water molecules in vapour comes from the fact that water molecules are polar - in liquid and solid form they stick together and it takes energy to separate them.

  4. Aug 2, 2011 #3
    Thank you for the succint answer.

    That raises another question then...
    So, youre saying that water vapor is cooler than the water it escapes from?
    But when water evaporates, doen't it increase its particle speed?
  5. Aug 3, 2011 #4
    The mist is liquid not vapor. That's why it is visible.

    Also when you said "same entropy", the statement is wrong.
  6. Aug 3, 2011 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    It is a bit complicated because the water molecules in liquid form are stuck together and have vibrational kinetic energy. When they separate into individual molecules (vaporization) they no longer vibrate against each other but have translational kinetic energy. The average translational kinetic energy of those water molecules can be less than the original liquid water but because they are not stuck to other water molecules they stay in vapour form. The energy required to separate the molecules is at the expense of the kinetic energy of the surrounding air molecules. So the temperature of the air decreases as more water is vaporized.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook