How come a fan makes things cooler? I know how wind chill works, but what about things that aren't damp. Just noticed that the wall the fan was blowing against was cold. Surely it's being hit with air the same temp in the rest of the room, so how come it's cooler than the other walls?
Suz PS lovely day
Heat is transferred in three ways. Conduction, convection and radiation. A fan can only affect convection. If the air is colder than the wall then the fan will cool the wall. If the air is hotter than the wall then the fan will heat the wall up. If they are both the same temperature then there will be no change in still air or moving air.
The inner surface of the wall is probably warmer than the air in the room because it has the sun beating down on the outer surface. This implies that the other walls/floors/ceiling are cooler thus keeping the air temperature lower than the warm wall.
It's not a good idea for the cooler air from the fan to be 'bounced' off a warm wall, as it will gradually transfer heat from the wall to the air, which is not what you are trying to achieve.
I am very fortunate to live in a house which is always lovely and cool.
Rick
1) Because they make the sweat on your skin evaporate better, the evaporation produces cooling. 2) The moving air transfers heat from you at 37C to the air at I know how wind chill works, but what
Wind chill is not dependant on the humidity (as in (1) above) it's about the fact that moving air feels cooler/colder than still air as in (2) above.
Just noticed that the wall the fan was
The walls must have started out hotter or be heated from the other side?
Alternatively, are very damp.
Thermal conduction from skin to still air isnt very good, so heat flow from warm body to cooler air is slow. Fan improves the thermal conduction between the two.
Its used the same way in a computer, to increase heat transfer from hot heatsink to air.
There is also the increased dampness evaporation it causes, which has some cooling effect.
NT
Thanks for all the replies. I suppose the thing that was puzzling me in particular was the fact the fan was blowing room temp air at a wall ( no heat source on any of the walls, either inside or out ) and this wall was cooler than the other walls - which had the same air touching them, albeit at a much slower speed.
I guess the walls must have been warmer than the air to start off with, and I'm warmer than both, and it was the conductive nature of the material when I touched them that make them *seem* cooler than air in the first place. The air did seem a lot warmer than any of the walls, but it's a good insulator isn't it? The old string vest thing.
Suzanne
Dave and Neil made good points. There is a diference in heat and temperature though. The fan is sharing it out more equitably, also pressure changes in air allow adiabatic as well as the otherbatic(is it anabatic or katabatic?) influences small though they are to take place.
Temperature, pressure and thingummie bobs are interactive. And of course the heat exchange between wall and air and the action of the fan is changing air pressure and temperatures all over the place.
Quite subtle yet brilliantly complex.
Your perspicacity Suz is worthy of a cross post to uk.sci.weather the acme dicussion of this sort of thing for people who will actually discuss this sort of thing and nothing but this sort of thing on the net.
Soft buggers.
Also - if the wall were damp and drying out, it would not only feel cooler but be cooler as it gives up latent heat of evaporation.
Nick
Well about 90% wrong - not bad for Usenet.
Fans affect conduction and not convection. Convection is merely conduction to a boundary layer which, being hot, rises and pulls in new air.
If the air is cooler than the wall, the fan will cool it.
However the majority of coolness you get from a breeze is in termns of water evaporation. It may well be thet the walls are sufficiently more humid in an inside room full of sweaty people that air movement will coll via evpaoration.
About 3% wrong, not bad for usenet :)
Fans can assist convection too, and get significantly better cooling than just the fan alone. However, this is not the way the fan is usually used, and in most cases you won't see any benefit.
Utter twaddle. Conduction is the transfer of heat with no net movement of the particles concerned. Convection is the transfer of heat by movement of a fluid. This movement can be caused by thermal gradients in the fluid itself or can be assisted by mechanical means. In the latter case it is called Forced Convection. A fan has no impact on conduction at all which would take place whether the air was still or moving. It increases convection and hence increases the rate at which warmed air is moved away from the hot surface to be replaced by cold air which can then transfer more heat away.
Of course the heat is transferred to the fluid by conduction (and to a small extent by radiation) but it is the increase in convection created by the fan which increases the rate of heat transfer.
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