Thermodynamic Breakdown of a Fan

Discussion in 'General Science & Technology' started by Layman, Jun 10, 2014.

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  1. exchemist Valued Senior Member

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    On the contrary, one of the first things you learn in school physics is how every form of energy turns to heat in the end.

    The power consumption of the fan ALL ends up as heat, just like the light from a bulb, the power expended by a car or aeroplane in motion, the energy of a falling object hitting a surface…..you name it, the energy all finally ends up as heat.

    An "efficient" motor simply mean one that converts a high proportion of the power consumed into mechanical work, but the mechanical work is dissipated through friction in the air, which converts it to heat.

    (The "power supply" doesn't have a wattage by the way, it is the load on the power supply, in this case the fan motor, that has a wattage.)
     
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  3. origin Trump is the best argument against a democracy. Valued Senior Member

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    First of all that is a pretty impressive fan that only pulls 14.5 watts. Secondly I am pretty sure I screwed up a my calculations somewhere because the temperature rise of 5 deg per hour for a 150 watt output (even in a perfectly insulated room) seems too high. Probably a screwed up a conversion factor.

    I did run a experiment using uncalibrated tools and a generally lousy set up last night (just to get a peek in the box) and with a ceiling fan on high was able to get a 1 def F increase in a closed room relative to a control thermometer right outside the room after about 1.5 hours.
     
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  5. origin Trump is the best argument against a democracy. Valued Senior Member

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    The feeling that you are getting is not very helpful unless you test that feeling. But the test have been done and exchemist is right.

    Here is the deal, think of it this way; what happens to the air movement when the fan is turned off? After the fan is turned of in just a few second the air stops moving, right? Where did that kenetic energy go, it sure as hell didn't just dissapear, did it?
     
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  7. Russ_Watters Not a Trump supporter... Valued Senior Member

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    With your numbers I get half a degree per hour.
     
  8. origin Trump is the best argument against a democracy. Valued Senior Member

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    Much more reasonable! I'll have to see where I screwed up when I get a chance, thanks for the correction.

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  9. Layman Totally Internally Reflected Valued Senior Member

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    Then in 8 hrs it would be 4 degrees hotter. If you let it run for 24 hours it would be 12 degrees hotter. That's a lot of extra heat for leaving a fan on all day so that it would be cooler...
     
  10. origin Trump is the best argument against a democracy. Valued Senior Member

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    That small amount of heat would transfer to the surroundings through the walls of the room. So in reality the room would maintain the same temperature as the rest of the house and the breeze from the fan would cool you.

    So it is all good.
     
  11. exchemist Valued Senior Member

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    But if you're sensible you don't leave it on all day because it doesn't cool the room, as an air conditioner does. As has been explained, a ceiling fan cools people, because people sweat.. The air flow evaporates water from the skin, which absorbs Latent Heat of Evaporation from the body.

    Leaving a ceiling fan on in an empty room just (very slowly) heats the room - totally counterproductive, as you say!
     
  12. Layman Totally Internally Reflected Valued Senior Member

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    I thought since you said you tested this out that you would need to switch the fan into the control room. It could be that room you had the fan in would just be naturally hotter. Then the experiment wouldn't be valid unless the control room also just got a degree hotter than the other room. Then I seriously doubt that either room would get 10 degrees hotter after a day of having a fan in it.
     
  13. Layman Totally Internally Reflected Valued Senior Member

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    Wow, you people will believe anything if it is in agreement with mainstream science.
     
  14. Aqueous Id flat Earth skeptic Valued Senior Member

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    You're conflating mainstream with correct which is philosophically OK but it's a misnomer.
     
  15. exchemist Valued Senior Member

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    Well yes, if by "mainstream" you mean "well-established", we do. Obviously. We are scientists.

    If you are surprised by that, then you are a bigger idiot than I took you for.
     
  16. Russ_Watters Not a Trump supporter... Valued Senior Member

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    Yes, I'll believe anything that is true. Shame on me!
     
  17. origin Trump is the best argument against a democracy. Valued Senior Member

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    This is not "rocket science", this is more like common sense.:shrug:
     
  18. James R Just this guy, you know? Staff Member

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    It seems to me that there have been several very clear explanations in this thread of how a fan in a closed room will heat the room.

    The rebuttal of those posts seems to be along the lines of "I find that hard to believe." and "Wow, you guys will believe anything science says."

    Are we done here? Time to close the thread?

    Does anybody have anything useful to add?
     
  19. exchemist Valued Senior Member

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    No James, we've had enough, I think.
     
  20. origin Trump is the best argument against a democracy. Valued Senior Member

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    We should put this whole sad episode behind us....
     
  21. Russ_Watters Not a Trump supporter... Valued Senior Member

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    One last thing I'm not sure I said before:

    While this all sounds very vague and theoretical for a house fan due to the small heat output vs room size and wall heat transmission, for a commercial air conditioning fan it can be a very big deal. If a cooling coil has a 15F drop across it, then the fan has a 3F rise, that's 20% error in net cooling capacity if you fail to account for it.
     
  22. Trippy ALEA IACTA EST Staff Member

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    You took the words right out of my mouth.
     
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