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by Anonymous on July 17th, 2006

Anonymous

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Why does an electric fan make it feel cooler? Shouldn't it just move the hot air around a bit?

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Answers. 7 helpful answers below.

  • by davoomac on July 17th, 2006

    davoomac

    Wow what a great question!

    I'd like to answer it myself but the experts at howstuffworks.com already did all the work. ;)

    (electric) fans actually add heat to a room. One way to think about it is like this: If you have a perfectly insulated room and you put an electric fan in it, then the room will get warmer. All the electricity that is driving the fan turns directly into heat.

    So a fan does not cool the room at all. What a fan does is create a wind chill effect.

    When weatherpeople talk about wind chill ( http://science.howstuffworks.com/question70.htm ) on a cold winter day, what they are referring to is how the wind increases convective heat loss (see How Thermoses Works: http://science.howstuffworks.com/thermos.htm for details on convection). By blowing air around, the fan makes it easier for the air to evaporate sweat ( http://science.howstuffworks.com/sweat.htm ) from your skin, which is how you eliminate body heat. The more evaporation, the cooler you feel.

    Full article:
    http://science.howstuffworks.com/question22.htm

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  • by JackNicklaus on October 19th, 2006

    JackNicklaus

    As perspiration evaporates from your skin, it absorbs energy or heat. This cools your skin. A fan speeds up the evaporation process thus cooling the skin more rapidly. The fans motor produces heat. This heat is released into the air.

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  • by Steeley on September 16th, 2006

    Steeley

    Your body naturally tries to reduce your temperature when you are hot by "sweating". Water can absorb more heat than just dry air, and as your sweat evaporates into the air it carries heat with it. The evaporation process cools you. The moving air from the fan evaporates the moisture (sweat) faster.

    But yes, if you put a thermometer in front of a fan and another away from the fan, you'll discover that the still air and moving air temperatures are essentially the same.

    Now, if you blow the fan through a wet cloth or straw matting or similar medium, the evaporation of the water from the wet medium pulls heat from the air passing through and the air on the other side of the medium *is* actually cooler.

    This is the principle that works in evaporative coolers, only the fan is usually inside a "box" of wet material (kept wet by a water pump) and the fan draws air inside through the wet medium by forcing air out the bottom or one side of the cooler box. The air blown out of the box draws air in through the wet medium, which is cooled by evaporation, and then blown by the fan into the area being cooled, drawing in more air.

    However, these work best in dry climates - if its humid outside evaporative coolers don't work very well because the water doesn't "evaporate" as easily - the humid outside air is already saturated with hot moisture so there's no place for the water in the medium to evaporate to.

    And that is why you can actually feel hotter when it's, say 85 degrees and 75% humidity, than when it's 110 degrees and 20% humidity. In the latter case the evaporation process is more efficient and so you feel cooler, even though the actual temperature is much hotter.

    Moisture is the key. And why you want to drink a lot of water when it's hot.

    Heat Stroke occurs when your body is out of water - you can't sweat any more, and you quickly overheat and can die.

    A fan won't help someone suffering heat stroke, any more than the fan won't cool the thermometer placed in front of it.

    By the way, it's evaporation that keeps water from getting hotter than 212 degrees (at sea level) - at that temperature the water turns to gas and carries away the heat as rapidly as the heat can be input. If you go up in altitude, say to the top of a tall mountain, the air pressure is less, evaporation occurs more readily, and water boils at a lower temperature (say 185 degrees).

    A fun experiment is to bet your friends that you can boil water in a styrofoam cup directly in a camp fire. As long as there is water in the cup it will keep the temperature of the styrofoam below the melting point, and contain the water until it boils. As soon as the water evaporates away the styrofoam cup immediately melts.

    As you all will see from that experiment - evaporating water is very efficient at "cooling" (drawing heat away from something hot), and all a fan does is speed up the evaporation process.

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  • by Anonymous on May 28th, 2007

    Anonymous

    Any moving air mass, whether caused by an electric fan or other means, will flow over your skin at a greater rate. Provided that the air is cooler than you are, it will absorb heat from your body. Because the air is moving, more air can absorb more heat than if it were not moving. So, if you block the air from touching you by wearing clothes, you will effectively prevent the air from circulating and removing the heat from you - wetsuits work on this principle.

    The really neat part is that even if you're stuck in a box, like a room with no ventilation, you will continue to be cooled by this process known as convection. This is because heat is lost within the system.

    This waste of heat is unavoidable and it means that no system can ever be perfectly efficient. As more and more waste occurs, more and more heat is just plain lost. It's called entropy and it's a general principle that says that nothing is completely efficient and eventually everything will run out of steam. But, for staying cool in a room, it works in our favor!

    For a slightly more technical explanation of this stuff, including ways to predict how much heat you will lose, how much heat the air will lose, and how to measure the efficiency of this cooling process you should read about:
    Entropy
    Convection
    Carnot Heat Engine
    Thermodynamics
    Heat Death of the Universe

    Stay curious!

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  • by JackNicklaus on September 6th, 2006

    JackNicklaus

    Yes the motor on a fan will actually cause some heat to be added to the location of the fan. The benefit you'll receive is the air will be moved. This sounds simple enough, but the advantages of moving the air must be considered. If you are confined in a small room with allot of heat being generated within it the net effect will be to cool the room's temperature down by exchanging the hotter air inside the room with some cooler air from outside the room. The other benefit you could receive is if the wind causes the sweat on your skin to evaporate faster, you will be cooled down. The process of evaporation, turning a liquid into a gas, consumes energy or heat. You will feel cooler.

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  • by tjatherton on September 6th, 2006

    tjatherton

    I think Jack has the best answer.
    Heat rises and if you put a fan at floor level it will pick up the coolest air... but circulation can cause a chill factor.. and the evaporation of sweat can cool you off.. all good thoughts.

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  • by nukedout on January 18th, 2010

    nukedout

    OK, Actual answer

    by increasing the mass flow rate of the air across the skin you actually increase the rate of heat transfer out of the body. eveyone always tries to say that cool and hot are dependant on temperature alone, but they are NOT when you are talking about heat transfer.

    the rate of heat transfer out of the body is dependent on: temperature inside the body, temperature outside the body, and the total film coefficient of the skin such that (rate of heat xfer out of the body) = (total film coefficient) x (difference between the temperature inside and outside of body)

    total film coefficient is dependent upon the surface area of the skin, mass flow rate of air moving across skin, and the thermal coefficient of the skin

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