Is air colder once it has been blown by a fan?

[Tempus]

Yes case airflow is important. But it seems to me that when I stick my hand in front of a fan, no matter how hot the air being blown is, it seems very cool. Does it get colder when it is blown?

- Tempus fugit donec vestrum relictus tripudium. Autem amor praeterea magis pretium.

 

[peteb]

Nope,

The factor to consider in airflow is that the air passing over your hand is constantly refreshed. This affects 2 things...

Most importantly for your hand it affects evaporation. Your body cools itself by moistening the skin, air next to your skin absorbs this moisure, but rapidly approaches the point where it will not absorb further moisture. By using a fan to move new air, the air against your skin is constantly refreshed and can absorb moisture faster. The evaporation of moisture absorbs heat with it into the air from you - cooling you down. This is the basis of sweating.

The whole process is called windchill. It affects any object that has a moisture content, or has moisture on it's surface. This is why windy days feel colder than still days, even though the weather forecast says the temperatures are the same.

The second effect is to do with heat transfer directly (which is for our heatsinks, since they do not usually have any moisture on them). If you had a heatsink with no fan, in still air the air around the sink would heat up. As the temerature of the air rises, it cannot rise above that of the heatsink, and the rate of heat absorbtion slows, until the the two materials, heatsink and air are at approximately similar temps. By moving air with a fan, you present a new source of cooler air to the heatsink. The difference in temperatures between the air and metal is higher, and therefore the rate of heat transfer is greater => heatsink gets colder and we can run our cpus faster ;o)

Hopefully this is understandable? It's not the most elegant description...

So, in summary air is not colder, but it has greater cooling capability as you are exposed to more air. Kind of like an artifical way of raising surface area.

Pete.

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email for application details<P ID="edit"><FONT SIZE=-1><EM>Edited by peteb on 04/22/01 03:29 PM.</EM></FONT></P>

 

[Crashman]

No, and I spent three hours trying to explain the concept that peteb laid out to a group of morons and they still didn't understand!

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[peteb]

lied or laid? Significant difference

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[Crashman]

Thanks

Cast not thine pearls before the swine

 

[Tempus]

Well I understand.

- Tempus fugit donec vestrum relictus tripudium. Autem amor praeterea magis pretium.

 

[Guest]

Not absolutely certain on this one but I am fairly certain that the second part of your description is actually what Windchill is rather than the first part.

Windchill is more to do with convection rather than evaporative heat loss.

 

[igottaknife]

You're right. There is another point to be made about the evaporative cooling. I think I remember a subject in thermodynamics that describes all matter (solid, liquid, gas, plasma, BE condensate, whatever...) as always in thermal flux. Evaporation is a phase change much like boiling. Heat flows throughout the matter and in some cases, like sweat, boils away. It only happens at the surface and only in very minute amounts. Also that effect Tempus explained is a form of refrigeration.

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[peteb]

I'm pretty certain that windchill is something that affects moist objects through evaporation. I'll do some more research and post back later.

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[Guest]

Which explains why air conditioning feels a lot better than just running a fan on a hot day.

 

[jlbigguy]

To nitpick, air blown through a fan exits a bit warmer then when it entered the fan. Depending on the size of the fan, the motor can give off quite a bit of heat, which is carried away by the airflow (unless the motor is to the side of the fan, and the fan is belt driven).

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[dryfly]

To add to big guy (please don't read this if you don't like physics, cuz I am about to give a physics lesson!) the fan is doing "work" on the air. About 200 years ago a guy whos name (i think) was Lord Bard discovered that work and heat (temperature) are directly related. When you do work on something you heat it up. Just like when you make a drink in the blender with ice. Turn on the blender and the ice first gets chopped up, and then, if you keep going you will notice that the ice actually melts. THis is not just because of the blades, you are doing "work" on the drink so you warm it up. The fan will also do "work" on the air, so you warm it up, although I agree the amount of increase in air temperature could probably not be measured.

You can try the drink experiment at home with a blender and your favorite "mix" of drinking liquids with ice. Good luck with your "experiments"!!!!

Now you know why I love physics!!!! So many wonderful "experiments" to do!!!!!

Later

PS. Just want to let you know I don't want to suggest that you should do this experiment with alcohol. I don't drink and I think drinking is a waste of time and money. I would much rather buy a volcano II than a bottle of rum, for example. I say this cuz I realize lots of young people read this board. (By young I mean anyone who is 40 yrs or less.)

<font color=blue>You don't know what it is, but it's there. Like a splinter in your mind.</font color=blue><P ID="edit"><FONT SIZE=-1><EM>Edited by dryfly on 04/28/01 12:29 PM.</EM></FONT></P>

 

[peteb]

<<Now you know why I love physics!!!! So many wonderful "experiments" to do!!!!!>>

Especially if they involve drinking!!!

Also, if the air pressure rises, which it will on the output side of the fan, then the air will heat due to compression. Fortunately there should be a roughly equal opposite effect on the intake side as a low pressure causes air to cool _slightly_

Net, the air probably does rise slightly in effective temperature, however the enhanced cooling effects of increased airflow greatly outweigh this.

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[dryfly]

Pete: Good point about the air pressure, forgot about that. That is the way jumbo airliners air condition the cabins, by using a small engine to compress air, then cool it, the allow it to expand again. The expansion causes the cooling effect.


<font color=blue>You don't know what it is, but it's there. Like a splinter in your mind.</font color=blue>

 

[Tempus]

I thought the AC came from the knobs above your head where the lights are that blow air.

- Tempus fugit donec vestrum relictus tripudium. Autem amor praeterea magis pretium.

 

[ejsmith2]

Goodus Jobbus. I'm think the food they serve is what goes down the intakes during takeoff; fast, cheap, easy.

 

[Guest]

You got it dryfly! When you compress something, it releases heat like crazy! so when you compress and release outside the airplane, it cools down. Then you pipe the cooled air back in, and viola! chilled air. I think thats how it works. It may work the other way around. I'll check

Aklein


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[ejsmith2]

No, it really works that way =)

What they do is use 'bleed' air from the exhaust of the jet, just behind the flame holders. It's hot as hell, runs down a channel, and turns a small compressor (which has a ram intake). The compressed fresh air runs down a heat exchanger and is piped into the cabin, which allows for the slight pressurization of the cabin during flight. With the smaller Learjets, the compressor is chucked for a simple ram intake, which has a throttling valve to compress/cool the air.

This is the same way a diesel engine, and liquid oxygen plant works too.....