March 27, 2008

Fans Heating Up Air

Stupid Question ™
March 9, 2000
By John Ruch
© 2000

Q: If heat is just molecular movement and fans stir up air molecules, why doesn’t a fan make you hotter?
—Sarah Pessin

A: It’s sort of like trying to knock over a wall with your pinkie—the energy’s there, but other forces are so strong it doesn’t matter.

As long as air is relatively cool and dry, the overheated human body can dump excess heat into it by convection and evaporation.

Convection is the way heat moves through a fluid. Heated areas become less dense due to increased molecular motion and rise; cool areas become denser due to decreased molecular motion and sink. As long as there’s a constant heat source, convection will continue in this rise-and-sink cycle.

Thus, cooler air near the human body will heat up and rise away, removing some heat. Cool air will sink to replace it, and the cycle continues until the body cools or the air gets too hot.

Sweating works even better because the evaporation of water into dry air removes a lot of heat.

A fan makes both these slow processes faster and more effective by moving air around. Circulating air around the body forces convection to take place and moves drier air to the body so evaporation can continue. (If the air isn’t cooler or drier that the body, the fan does nothing to make you more comfortable.)

It is true that a fan will increase the average temperature of a closed room by speeding up air molecules, by causing friction between the air and solid surfaces, and by giving off the heat generated by the motor.

However, this heating is insignificant compared to the large personal cooling effects.

Heating by moving molecules is especially weak because the fan moves air on a macroscopic level. While the fan does cause some individual air molecules to speed up, it mostly just moves a big chunk of air within which most molecules continue to bounce around at their usual average speed. Therefore, the heating effect is minimal.

The radiant heat from the fan’s motor is much stronger, and even it doesn’t matter compared to the large cooling effects. (In fact, there are electricity-free fans that run on an open alcohol or kerosene flame.) Even the heat gain caused by waving a handheld fan is somewhat less than the cooling effect.

On top of it all, the heating effects virtually disappear in a room with an open door or window, and thus a constant supply of fresh air.

To keep my physicist friends happy, I must also point out that technically, heat isn’t just molecular motion. It’s the internal energy of a substance.
It’s not identical with temperature, which is a measure of average kinetic energy (motion) of molecules. For example, a 2-pound stone and a 1-pound stone can be at the same temperature (average kinetic energy), but the bigger stone will take longer to cool down because it has more heat energy.

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