Would like to get some views on this one. My son, a toddler, is asleep in his cot this evening, and it's quite a warm evening.
I think that it'll be wise to put a small fan on close to his bedside, however my wife's argument is that, given that it is effectively a sealed room (apart from cracks in door frame etc), the fan itself will only serve to heat the air up.
I think that she is crazy. Thoughts?
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The fan will heat the air but only by a fraction of a degree
She's correct, it will heat the room up. But it's similar to saying that emptying red dye into the ocean will make the seas red.
The point is, it moves air rapidly which improves evaporation of sweat from the skin, therefore leaving the person cooler than they would've been without a fan. The difficulty is that they need to be in the air flow for that to have any effect - and if he's in a cot and it's making him too cold he has no way of turning the fan off, so he would be stuck in the middle of the airflow all night.
So in conclusion - yes, it does make the room minutely warmer, but if he's catching some of the airflow he'll be cooler than if there wasn't a fan in there at all.
I'd get a timer set at the plug as well if I was you.. 15 minutes on, 15 off sort of thing, then he's not gonna get too cold or too warm, or at least it'll help find a balance.
The fan in itself does not cool down the air. As it is, you're inputting kinetic energy into the molecules of "air" (for simplification) and the more energy they have, the hotter they are. E = kT per particle (E is energy, k is Boltzmann's constant, T is temperature), so the little bit of extra kinetic energy you put in will increase T by a tiny bit. As the particles are so small, the energy jump is so small that the temperature increase is negligible.
Think about a computer fan. Your CPU cooler works by pushing air over the heatsink (which is radiating energy), the air heats up and is carried away from the area. In this way you can move all that hot air out of the case and your heatsink will eventually cool down. The fan continually supplies the heatsink with air at a lower energy which, when it comes into contact with the heatsink, lowers the sink's energy and raises the air's in an attempt to get to equilibrium. Where is this going? Well, consider the point when you can't get any more heat off the heatsink. When does this happen? At equilibrium - that is, when the heatsink and the air are at the same temperature. Unless you're artificially cooling the air as it enters the system, this will be room temperature. Thus, you can't actually cool something down to below ambient with just a fan. It's why when you're building a computer in a hot country, you really need to consider the effects when summer gives you an extra 20 degrees.
The effects are basically the same for fans in rooms. There are two positive effects they have:
1) They improve the rate of convection in the room - hot air rising, cold air falling. Convection currents can form in a warm room (though they're imperceptible to us), creating circulating streams of warming and cooling air. A fan will help speed this up and should mean that the air is "recycled" over the thing you're pointing it at. In theory (I'm guessing) this should mean that the rate of heat transfer over the thing you want to "cool" increases. This is like a heatsink, but again you could only cool things down to ambient temperature.
Critically this means that you don't necessarily need to point the fan at the cot, so long as the air is moving.
2) The upshot of the above is the "wind chill" factor. When you go outside on a summery day, it might be 25 degrees outside. If there's a wind blowing, you feel colder but the temperature of the air hasn't changed. As has been mentioned, the wind helps your sweat evaporate faster which is how your body sheds extra heat.
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Endothermic thermoregulation (sp?)
The body attempts to cool itself by producing sweat, which in contact with air produces an endothermic reaction, as it draws energy from the skin in order to evapourate and transfers to the air.
By adding a fan into the mix, you're increasing the rate of air flowing over the skin, thus keeping a constant differential and enhanced cooling.
Yes the fan will heat the room due to friction on the blades etc, etc, but the increase is negligible compared to the energy transfer from the skin to the air
Thusly in a colder atmosphere with a high wind flow, and water on the skin, hypothermia is likely to take hold, due to the increased heat loss
Last edited by TAKTAK; 24-05-2010 at 11:03 PM. Reason: Fixed ;)
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TAKTAK.co.ukOriginally Posted by razer121
The mysterious localised fan-death phenomenon, surely!Cookie for whoever gets it first!
Andeh13 (25-05-2010)
Actually not an endothermic reaction, which is a chemical reaction that requires heat to maintain it (or the reacting chemicals absorb heat, so they feel cold. The opposite is an exothermic\ reaction that gives out heat (the reaction of methane with oxygen for example) However the water (sweat) the body produces does not react with the air. However it does take energy from the skin (heat enerfy) to evaporate the water. This energy is the latent heat of evaporation.
Consider a puddle of water. That will evaportae but requires the input of heat to give a water molecule sufficient energy to leave the liquid phase and enter the gaseous phase. This energy is the latent heat of evaporation, and the water left behind will ose heat (and therefore the temperature will fall) as the water molecules evaporate. The same effect is noticed if you spill alcoho; on your hand - alcohol is more volatile than water (it evaporates more quickly) so your hand feels cold.
As a coroolary - you will get a worse byrn from steam at 100 degrees C than you will from water boiling at 100 degreesc. Although they are at the same teperature, the steam contains more energy (the latent heat of evaporation).
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TAKTAK (24-05-2010)
Fixed, with my imbecilic parts left in
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Not at all - it's how we learn - I've posted incorrect stuff in my time and people have kindly (or occasionally not so kindlyFixed, with my imbecilic parts left in
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To add to the useful replies, evaporation rate is also governed by the gradient of humidity - this is, evaporating into dry air is going to occur at a faster rate than into humid air. If there is no air movement then water molecules in the air directly above the skin will only move away slowly, at more or less normal diffusion rates. This means you raise the local humidity above the evaporation site and evaporation slows down.
What a fan/the wind does is enable that local humid air to disperse much more quickly than usual, dropping the local humidity over the skin and increasing evaporation rate, which takes energy out of the skin and cools it.
Windmills do not work that way. GOODNIGHT
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Good job MSIC lives in London/Herts rather than Korea, that's all I can say.
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