Science

Why / how does fanned air cool?

Please explain the physics of why it is possible to swelter in a hot room yet be cooled by the same air if fanned by it.
I suspect it's to do with the number and rate of air molecules hitting the body, but not sure how this affects things where they come into contact with clothing /. bedding.

Segilla Sun 11/05/08 16:34

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gen2
Sun 11/05/08
16:44

It is evaporation of sweat that causes the cooling.

In still air, the air in contact with the skin becomes saturated with water vapour and evaporation stops - so no cooling.

Fanning replaces this moist air with dryer air so that evapporation (and hence cooling) can continue.

Robb Phoenix
Sun 11/05/08
18:44

A fan doesn't cool it fans actually add heat to a room.If you have a perfectly insulated room and you put an electric fan in it, then the room will get warmer. All the electricity that is driving the fan turns directly into heat. Fans only move the air about to make it "feel"cooler.

A ceiling fan cools by creating a wind chill effect; it does not lower the room temperature. Wind chill effect makes you feel cooler by accelerating the evaporation of perspiration on your skin. It is the feeling you get when you open the window in a moving car.

Segilla
Sun 11/05/08
19:49

Question Author

Thanks. But how would you explain more rapid cooling of say. a hot iron rod that was fanned as opposed to one which was allowed to cool naturally?

Robb Phoenix
Sun 11/05/08
20:20

Fanning removes the hot air from directly above the rod and replaces it with cooler air from the environment, thus speeding up the cooling process in the same way a convection oven speeds up the heating process

Teddio
Sun 11/05/08
20:41

The Specific Latent Heat of Vaporisation of water is 2.2 Megajoules per kilogram of water. The evaporation process requires a lot of heat energy which is obtained via the skin. Try this: a hot air hand dryer feels cool when it blasts against your wet hands but feels uncomfortably hot when your hands are dry.

BillyBB
Mon 12/05/08
01:04

Moving air stream has a lower pressure than the surrounding air. If that air has expanded due to lower pressure it's temperature will actually drop. Slightly.

sp1214
Mon 12/05/08
19:55

When you put your head out of a moving car window it feels very cold in the breeze. In this instance you are replacing the movment of the car instead of the movement of the fan blades.

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