dstebbins

How do fans cool you down?

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If you really think about it, it doesn't make sense. If the air coming from the fan is moving faster than the other air in the room (which is the whole point of a fan), doesn't that mean the air from the fan has more kinetic energy than the still air?

And isn't kinetic energy basically just a fancy word for "heat?"

So by making the air move faster, shouldn't that make the air feel hotter than the still air?

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It is because our skin is damp and the movement of the air speeds evaporation from the surface. There is probably also an effect where we are surrounded by a layer of air warmed by our bodies, so a fan can replace that with cooler air (unless the ambient temperature is higher than body temp). 

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1 minute ago, Strange said:

It is because our skin is damp and the movement of the air speeds evaporation from the surface.

Like a whole-body blow dryer?

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BTW, one of the world's more bizarre superstitions is the belief in Korea that if you use a fan in a closed room YOU WILL DIE!!!!

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7 minutes ago, Strange said:

BTW, one of the world's more bizarre superstitions is the belief in Korea that if you use a fan in a closed room YOU WILL DIE!!!!

The K- Pop Idol - Samsung - Plastic Surgery - Korea or the False imprisonment - contempt for human rights and life - rogue state Korea?

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33 minutes ago, dstebbins said:

If the air coming from the fan is moving faster than the other air in the room (which is the whole point of a fan), doesn't that mean the air from the fan has more kinetic energy than the still air?

Do you know that air molecules have speed approximately 340 m/s.. ?

30 minutes ago, Strange said:

It is because our skin is damp and the movement of the air speeds evaporation from the surface. There is probably also an effect where we are surrounded by a layer of air warmed by our bodies, so a fan can replace that with cooler air (unless the ambient temperature is higher than body temp). 

Fans are widely used to cool down electronic equipment and devices e.g. CPU.. which has more than 70-80 C during operation.. There is no (significant) evaporation from surface of CPU (as it has no more water than is initially in air).. still fan is cooling it..

@dstebbins

Particle hitting at target can be scattered/reflected and having more kinetic energy than it used to have prior collision (taking part of energy of body which has been hit).

Particle hitting at target can give part of it's kinetic energy to body and excite it. But soon induced emission will happen and photon with higher energy will be emitted taking body energy with it.

 

 

Edited by Sensei

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8 minutes ago, Sensei said:

Fans are widely used to cool down electronic equipment and devices e.g. CPU.. which has more than 70-80 C during operation.. There is no (significant) evaporation from surface of CPU (as it has no more water than is initially in air).. still fan is cooling it..

I'm sure you know but the PC fan takes air from outside the computer case and draws it inside. Strange's description still applies. (aside for humidity)

Of course there are other methods to cool down your PC Sensei :D 

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As @Strange said it is because of evaporation. 

An experiment you can do at home is boiling 2 pots of water (equal amounts). Put a lid on one to prevent steam from escaping and leave the other one uncovered allowing the steam to escape. The covered pot will biol faster as less of its heat is being lost to evaporation. 

16 minutes ago, Sensei said:

Fans are widely used to cool down electronic equipment and devices e.g. CPU.. which has more than 70-80 C during operation.. There is no (significant) evaporation from surface of CPU (as it has no more water than is initially in air).. still fan is cooling it..

Air has moisture in it. The devices heat the air around them. Removing that air removes the heat stored in its moisture and providing new air to absorb more of the devices heat.

Edited by Ten oz

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I mistakenly thought it was because if the ambient temp is cooler, passing more of it over something with higher than ambient temp would allow for more air-object heat transfer interaction. Therefore more air movement means higher capacity for cooling.

How does this work then if the ambient temp is above body temp for example? Would a fan not work as intended in this situation?

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28 minutes ago, Scott of the Antares said:

I mistakenly thought it was because if the ambient temp is cooler, passing more of it over something with higher than ambient temp would allow for more air-object heat transfer interaction.

That is true; for "inanimate" hot objects (like computer processors) is is, I think, the only effect.

I suppose the relative effectiveness of that vs cooling by evaporation for human bodies depends on temperature, humidity and various other factors.

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3 hours ago, Sensei said:

 Fans are widely used to cool down electronic equipment and devices e.g. CPU.. which has more than 70-80 C during operation.. There is no (significant) evaporation from surface of CPU (as it has no more water than is initially in air).. still fan is cooling it.. 

Because the ambient air at ~ 20ºC is replacing the hot air, and the cooler air gets heated.

 

 

3 hours ago, dstebbins said:

And isn't kinetic energy basically just a fancy word for "heat?"

The additional KE of air from a fan is minimal — the bulk is moving a few m/s, while the random motion speed is hundred times larger.

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3 hours ago, Scott of the Antares said:

I mistakenly thought it was because if the ambient temp is cooler, passing more of it over something with higher than ambient temp would allow for more air-object heat transfer interaction. Therefore more air movement means higher capacity for cooling.

How does this work then if the ambient temp is above body temp for example? Would a fan not work as intended in this situation?

Why do you feel that is a mistake? This conduction/convection is part of the process, the other part being evaporation cooling, which also is increased by convection.

If the ambient is above body temperature only the evaporation cooling is effective....so relative humidity is critical.

Fans will increase the temperature of the room overall, all things being equal. If, say the walls were cooler than ambient, it might cool the room. 

Edited by J.C.MacSwell

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9 minutes ago, J.C.MacSwell said:

Why do you feel that is a mistake

I thought this was how it worked but Strange mentioned evaporation only. I now see that both are involved; today is a good day as I have corroborated what I thought to be true and Strange has taught me something new as well:) thanks guys.

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On the same topic, you can buy portable air conditioning units, that work like a standard air conditioning unit, except that instead of dumping the heat to the outside atmosphere, like in a fixed AC unit, they dump it by evaporating water. 

They are the most awful excuse for AC that you will ever come across. Yes, they do produce cool air. But the price is a massive increase in humidity. So unless you are standing right in front of it, you actually feel far hotter and more uncomfortable, as soon as it's switched on.

I used to play snooker in a very large snooker hall, and on hot nights, they would turn one on. You could tell within minutes that they had done so, because you would start to sweat twice as much as before. Just one unit in a huge hall was enough to make everyone suffer. Needless to say, as soon as we worked out what was happening, we complained loud and clear, but the management took some convincing that a unit that was blowing cool air was making everyone sweaty. 

They're still on sale now. Terrible things.

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1 hour ago, mistermack said:

On the same topic, you can buy portable air conditioning units, that work like a standard air conditioning unit, except that instead of dumping the heat to the outside atmosphere, like in a fixed AC unit, they dump it by evaporating water. 

They are the most awful excuse for AC that you will ever come across. Yes, they do produce cool air. But the price is a massive increase in humidity. So unless you are standing right in front of it, you actually feel far hotter and more uncomfortable, as soon as it's switched on.

I used to play snooker in a very large snooker hall, and on hot nights, they would turn one on. You could tell within minutes that they had done so, because you would start to sweat twice as much as before. Just one unit in a huge hall was enough to make everyone suffer. Needless to say, as soon as we worked out what was happening, we complained loud and clear, but the management took some convincing that a unit that was blowing cool air was making everyone sweaty. 

They're still on sale now. Terrible things.

The key to a swamp cooler is to keep the windows open so you get constant moving air, pretty much the opposite of what you do with regular AC. And if you couldn't feel the cooler air movement in your snooker hall, their fan wasn't big enough. You're right about the overall humidity, but the breeze should feel even cooler against your skin. I grew up with a swamp cooler that would practically knock you down if you stood in the upstairs hall where the fan came in. 

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32 minutes ago, Phi for All said:

The key to a swamp cooler is to keep the windows open so you get constant moving air, pretty much the opposite of what you do with regular AC. And if you couldn't feel the cooler air movement in your snooker hall, their fan wasn't big enough. You're right about the overall humidity, but the breeze should feel even cooler against your skin. I grew up with a swamp cooler that would practically knock you down if you stood in the upstairs hall where the fan came in. 

No, it wasn't in that class at all. If you stood three or four feet in front of it, it was quite nice, but anything more and you couldn't really feel anything. And yet, it changed the humidity very noticeably throughout the entire hall, which had about 12 to 15 full size snooker tables. 

I would imagine, without doing any research, that it wouldn't be so bad in an environment that was already close to 100 percent humidity. Can you actually raise the humidity, if it is already at maximum? If not, then you wouldn't notice any extra humidity from it, but would get the benefit of the cooling. But in a dry atmosphere, the difference would be striking, as we experienced. 

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17 minutes ago, mistermack said:

I would imagine, without doing any research, that it wouldn't be so bad in an environment that was already close to 100 percent humidity. 

Evaporative coolers aren't as efficient when the humidity gets past 50%. Remember, the air cools as the water in it evaporates, so in high humidity there's more water and less evaporation going on. The air coming out of the unit would be the same temperature as the air coming in without evaporation.

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10 minutes ago, mistermack said:

No, it wasn't in that class at all. If you stood three or four feet in front of it, it was quite nice, but anything more and you couldn't really feel anything. And yet, it changed the humidity very noticeably throughout the entire hall, which had about 12 to 15 full size snooker tables. 

I would imagine, without doing any research, that it wouldn't be so bad in an environment that was already close to 100 percent humidity. Can you actually raise the humidity, if it is already at maximum? If not, then you wouldn't notice any extra humidity from it, but would get the benefit of the cooling. But in a dry atmosphere, the difference would be striking, as we experienced. 

The problem is you won't get the cooling at 100% humidity unless the water is cooler than ambient...and if so why not circulate though a heat exchanger/radiator to both cool and dehumidify?

A fan should work better than using ambient temperature water.

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39 minutes ago, mistermack said:

No, it wasn't in that class at all. If you stood three or four feet in front of it, it was quite nice, but anything more and you couldn't really feel anything. And yet, it changed the humidity very noticeably throughout the entire hall, which had about 12 to 15 full size snooker tables.

Swamp coolers are more about moving air than about temperature. Heating and air conditioning are all about affecting BTUs, but swamp coolers are all about air movement and units of cubic feet per minute. Without a proper blower, you're just adding humidity. And as I mentioned, many places with a swamp cooler make the mistake of closing the windows. 

Edit to add: A high volume fan would be a bad thing for a snooker hall. Not everyone appreciates a cool breeze on their balls.

Edited by Phi for All
additional thought

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I just did a little search, and there are two different classes of portable machine. One is the evaporative cooler, or swamp cooler, and the other is a so-called portable AC unit, which is actually a compressor type like a wall or window unit, but it's on wheels, and has one or two pipes that have to be fitted to a window, so they're not really portable in the true sense. 

Neither get a very good rating, from the journalists who test them. 

The swamp coolers seem to be best for dry areas, with low relative humidity, as has been pointed out by others. The portable AC units have various drawbacks, including not being very effective, not being very portable, and being noisy and expensive to run. 

I reckon the future will be in cooled blankets and even cooled clothes. Far more efficient than cooling an entire room.

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