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The cooling of air.

  1. Jul 4, 2008 #1

    I've really been wondering how air can possibly get cooled with a fan. I read somewhere on the internet that a fan creates a "draught" or a "draft." It seemed to imply that this somehow cooled the air, but I really don't know.

    I've heard, and it seems reasonable, that a fan just blows air around so that it is just more circulated, and that really, when you feel cooler by a fan, it's just that the thermal boundary layer around your body has been blown away or something. But: then why does a fan cool off CPUs inside computers? Is it because the air outside the CPU is cool so that the incoming cold air cools it off? (I guess that sounds reasonable, but is it true?)

    I heard somewhere that having a fan blow air out of your window on a hot day takes away the heat. Is this true? It doesn't seem like it would change the average kinetic energy of each particle. Is it possible that the air that stays behind is cooler, or am I completely out of my mind?! (Maybe I'm nuts, but I tried this, and it seems that my room was cooler for some reason, maybe a co-incidence. Suffice it to say that my room is much cooler inside than it is outside. This is driving me nuts.)

    It does, conversely, seem reasonable that, if it is cold outside on hot inside, that having a fan go the opposite way would make it cooler inside. It almost seems obvious that incoming cool air would cool something hot, but what about blowing out air out of the window on a hot day? How could this possibly make it better?

    I mean, really what I am asking is: how can air be cooled by a fan? O.K. I mean really, can air be cooled by mere movement alone?

    What about this. Suppose I have a huge fan on the wall of my dorm room that is opposite of my window (the window is open, the fan is running). Could it be the case that the air that goes over every point of my room is carrying away heat and leaving my window?

    Hey, this stuff's pretty neat. Thanks in advanced!
  2. jcsd
  3. Jul 4, 2008 #2


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    Staff: Mentor

    Fans do not cool the air, they just move it. The moving air cools you because the air is cooler than you.
    Only if it is hotter inside than outside.
  4. Jul 4, 2008 #3


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    The fan does, at a minimum, two things:
    1] it moves the air around you, which, because you're warm-blooded, tends to be warmer than ambient air, and
    2] it aids in evaporating moisture off your skin which, being an endothermic reaction, extracts heat from you.
  5. Jul 5, 2008 #4
    Please remember that without a fan, your processor could cook breakfast. It produces a LOT of heat. So as far as your processor is concerned, the air in your house is freezing cold by comparison. Moving it away as fast as possible (before the processor can heat it up) is a good idea.
  6. Jul 6, 2008 #5


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    Staff: Mentor

    The movement of air results in 'forced convection', in which the air moves downstream as it is heated and replaced by cooler air - assuming that the ambient air is cooler than the object being cooled.

    It works the other way too with heating. Turn on the heater in the car after it has operated about 5 minutes and feel the hot air.
  7. Jul 6, 2008 #6
    Hey, thanks!
  8. Jul 7, 2008 #7
    I concur that it is only about circulation.

    The fan itself does not decrease air's temperature, or actually it heats it a little because of its own thermal dissipation.

    An object which is dissipating heat (your body, the CPU) heats up the air around it. As the air temperature increase, the transfer slows down. The fan merely helps replacing the air which has already absorbed some heat with "new" air.
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