Thermodynamic Breakdown of a Fan

Status
Not open for further replies.
Layman

Layman

Totally Internally Reflected
Valued Senior Member
It is known that a closed system will increase in heat if energy is added to it due to conservation laws. Say you added mechanical energy of several blades rotating around, and it would only naturally make sense that the mechanical energy would be converted to heat. Okay, now say that your bedroom is a closed system. Then the ceiling fan takes electrical power to create mechanical energy inside of your room that comes from the power company. Then your room gets cooler. It doesn't make any sense, because it was supposed to make the molecules move faster and increase their vibrations creating more heat. :bugeye: The friction with the air and the parts of the fan itself should get hotter according to thermodynamic principals...
 
Russ_Watters said:
To be more specific; the fan will heat the room.
Click to expand...
Then that is exactly what thermodynamics would say about the situation, but fans are an ancient technology that actually works at cooling things off...
 
Layman said:
Then that is exactly what thermodynamics would say about the situation, but fans are an ancient technology that actually works at cooling things off...
Click to expand...

This whole business is rather silly; the fan adds energy to the room thus heating it. The wind chill effect makes a human standing in it's wake *feel* cooler but that's ONLY an effect, nothing more.
 
Read-Only said:
This whole business is rather silly; the fan adds energy to the room thus heating it. The wind chill effect makes a human standing in it's wake *feel* cooler but that's ONLY an effect, nothing more.
Click to expand...
Try leaving your ceiling fan on high with the door closed and see how much wind chill you get when not directly under it. LoL, what have I done. I think I just convinced all of you that fans are a bogus device that doesn't actually work. They don't even have infomercials, for Gods sake, saying how well the work...
 
Instead of having your energy company provide you with the power to run the fan, how about you eat a good breakfast and go in the room and close the door. Now start spinning the fan. Faster! Faster!!! After about all of 30 seconds of working you will begin to break a sweat. Evidence that things are heating up!! So operating the fan really does heat things up!!
 
Fans don't cool the air. They just move air around.

You feel cooler when you stand under a ceiling fan because it causes air to move past your skin. If you're sweating, the movement of the air aids in carrying the evaporated water away from your skin. It is actually the evaporation that cools you, not the air movement. The air movement just decreases the humidity right next to your skin.

In a closed room, a fan will actually heat the room.
 
Well I give up. I must have just hallucinated my room getting cooler when I turn on my fan, even though I never have it blow directly on me.
 
Layman said:
Try leaving your ceiling fan on high with the door closed and see how much wind chill you get when not directly under it. LoL, what have I done. I think I just convinced all of you that fans are a bogus device that doesn't actually work.
Click to expand...
No, you haven't. Fans "work" fine.
Well I give up. I must have just hallucinated my room getting cooler when I turn on my fan, even though I never have it blow directly on me.
Click to expand...
I'm not sure "hallucinated" is exactly the right word, but yes, you are wrong about the fan actually cooling the room.
 
Layman said:
Well I give up. I must have just hallucinated my room getting cooler when I turn on my fan, even though I never have it blow directly on me.
Click to expand...

Read James's reply. You get cooler because you sweat. The way sweat cools is by the Latent Heat of Evaporation of water. Circulating the air accelerates the rate of evaporation, because it constantly carries away, from the surface of your skin, air saturated with water vapour and replaces it with unsaturated air. The closed system gets warmer overall but your body, within it, gets cooler, due the Latent Heat absorbed by the evaporating sweat.

If you measure a dry object in a room with a cooling fan, it will have (at equilibrium) a temperature equal to that of the air. But a wet rag will be at equilibrium be cooler than the air, due to the latent heat of evaporation.

Is it clear now?
 
Layman said:
Well I give up. I must have just hallucinated my room getting cooler when I turn on my fan, even though I never have it blow directly on me.
Click to expand...

It doesn't have to blow directly on you. It probably does some good even if it just circulates the air around a bit - unless the humidity is already 100% and it's actually raining in your room.
 
James R said:
It doesn't have to blow directly on you. It probably does some good even if it just circulates the air around a bit - unless the humidity is already 100% and it's actually raining in your room.
Click to expand...
Note, fans cool both by convection and evaporation, so it has to be both human body temperature in the room and 100% humidity for a fan not to help cool you.

Layman, it is actually not that difficult for you to test this: you could get a small cooler, a battery operated fan (the stronger the better) and an indoor-outdoor thermometer. Put the thermometer and fan in the cooler, turn the fan on, and watch the temperature rise.

Alternately, you could just measure the temperature of an airstream before and after a powerful fan in an air handling unit.
 
Layman said:
Then that is exactly what thermodynamics would say about the situation, but fans are an ancient technology that actually works at cooling things off...
Click to expand...
wait til you hear this:
the fan will heat the room and cool you off at the same time.
that's a fact jack.
 
Here is the secret layman, don't make your room a closed system - open the freaking door and windows. That way the room won't heat up and you will move the air around cooling you off a bit. Moving air does not ever decrease the termperature in and of itself.

There is a rather ingenious method of air conditioning that I have seen in greenhouses. On one side of the green house a bank of windows is open and a stream of water is directed down a very open and porous sponge type material on the other side of the greenhouse is a bank of fans blowing OUT of the greenhouse. This arrangement pulls air in through the wet spongy material causing the water to evaporate this cools the air by the latent heat of evaportation and the fans pull this cooler air into the greenhouse.
 
origin said:
Here is the secret layman, don't make your room a closed system - open the freaking door and windows. That way the room won't heat up and you will move the air around cooling you off a bit. Moving air does not ever decrease the termperature in and of itself.

There is a rather ingenious method of air conditioning that I have seen in greenhouses. On one side of the green house a bank of windows is open and a stream of water is directed down a very open and porous sponge type material on the other side of the greenhouse is a bank of fans blowing OUT of the greenhouse. This arrangement pulls air in through the wet spongy material causing the water to evaporate this cools the air by the latent heat of evaportation and the fans pull this cooler air into the greenhouse.
Click to expand...

I think he or she may be in India ("lakhs" were spoken of on another thread), where it is possible that the air outside is hotter than in the house during the heat of the day.
 
exchemist said:
I think he or she may be in India ("lakhs" were spoken of on another thread), where it is possible that the air outside is hotter than in the house during the heat of the day.
Click to expand...

Sounds like the job for an air conditioner. You know, the thing that makes more heat to cool a room?

"Cooler" is like ice compared to water, where the volume is greater when heat is removed, right? So "warmer" is like water compared to ice, where the volume is less.

So warmer means less volume and cooler means more volume, right?
 
Motor Daddy said:
Sounds like the job for an air conditioner. You know, the thing that makes more heat to cool a room?

"Cooler" is like ice compared to water, where the volume is greater when heat is removed, right? So "warmer" is like water compared to ice, where the volume is less.

So warmer means less volume and cooler means more volume, right?
Click to expand...
Well, an air conditioner does require a lot of power, and it makes up for most of your power bill. Then it would do a lot of work to the closed system of your house, therefore it would have to heat up your house as well. It would just seem like your air conditioner cools the house, because when you turn it on you start sweating and having hot flashes that just creates the illusion that it is actually colder in the house.

Hotter air is less dense and colder air is more dense. Think of a hot air balloon. Hot air rises, because it is less dense. Then it becomes buoyant to the other air around it.
 
Status
Not open for further replies.
Back
Top