Thermodynamic Breakdown of a Fan

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Layman said:
Why do I get the feeling that if you worked a problem that way on a physics test it would be wrong? I really don't think the wattage of the power supply could be directly related to the amount of heat a fan would give off due to the transfer of it's kinetic energy.
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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.)
 
Kittamaru said:
Er... what? How do you figure on that?

Take the AERATRON E503 model fan - it draws 2.4 watts on a 120 volt circuit, giving a total draw of 20mA on low speed, and draws 14.5 watts at on a 120 volt circuit for a maximum draw of 121mA on its highest setting. It is energy star certified to have an electrical efficiency of at least 80% even at maximum draw. If it converted 100% of that energy to heat, a simplistic heat output would be:

BTU/h = (V * I) * 3.412

BTU/h = (120V * .121A) * 3.412
BTU/h = 49.54

This means it can raise roughly one square foot per hour of air by 49.54 degrees Celsius... (assuming I have done my math and research right on this) or, in a 500 square foot room, it'll raise the temperature .099 degrees celsius per hour.
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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.
 
Layman said:
Why do I get the feeling that if you worked a problem that way on a physics test it would be wrong? I really don't think the wattage of the power supply could be directly related to the amount of heat a fan would give off due to the transfer of it's kinetic energy.
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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?
 
origin said:
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.
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With your numbers I get half a degree per hour.
 
Russ_Watters said:
With your numbers I get half a degree per hour.
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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...
 
Layman said:
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...
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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.
 
Layman said:
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...
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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!
 
origin said:
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.
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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.
 
exchemist said:
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!
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Wow, you people will believe anything if it is in agreement with mainstream science.
 
Layman said:
Wow, you people will believe anything if it is in agreement with mainstream science.
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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.
 
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?
 
James R said:
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?
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No James, we've had enough, I think.
 
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.
 
James R said:
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?
Click to expand...

You took the words right out of my mouth.
 
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