By Earl Hunsinger
Have you ever turned on a fan to cool down a room? While an electric fan can cool down a person, it will heat up a room (okay, probably not by much, but it won't make it cooler).
First, let's consider why standing in front of a fan can cool you down if you're hot. A fan helps for two reasons. Assuming that the room is below the temperature of your body (usually 98.6 degrees Fahrenheit), the fan will cool you down for the same reason that you blow on a cup of hot coffee before drinking it. In technical terms, this is because of the second law of thermodynamics. The Entropysite website provides a more technical explanation of this second law of thermodynamics.
Basically, temperatures tend to even out. In practical terms everyone understands this; a hot cup of coffee or a plate of hot food left on the table will become cold if it is not consumed immediately. Its temperature and the temperature of the room tend to even out, with the food becoming cooler and the room becoming hotter (only slightly, unless it is a very large plate of food). This wouldn't happen if the room was the same temperature as the food; it would be faster if the room was very cold. The transfer would take place slower if the food was in an insulated container. In contrast, it happens faster if we blow on the food. This is because as the air immediately surrounding the food heats up, the heat transfer slows; when we blow on it (or place it in front of a fan), we replace this warmed up air with cooler air. This is the same reason that your car's radiator has a fan, and why the car will overheat if this fan fails. Just like the plate of food or the car's radiator, standing in front of a fan will cool us down because the warm air next to our skin will be replaced with cooler air from the room.
The other reason why standing in front of a fan cools a person down is known as evaporative cooling. This works even if the air is as hot as we are, and doesn't work as well if the room is very humid, which is why hot humid weather is more uncomfortable than hot dry weather. When we get hot, most of us perspire. The evaporation of this perspiration cools us off. You might compare this to boiling water. In order to boil water, heat must be added. The same thing applies any time liquid water (such as perspiration) changes to water vapor. In the case of our perspiration, the heat comes from our skin, thus lowering the temperature of our skin and cooling us off. The more humid the air is, the slower our perspiration evaporates.
These two reasons are why a fan will cool us down, but what about our room? Unless we use the fan to pull in cooler air from some other place, such as outside air in the evening, the only thing the fan is doing is pushing around air of the same temperature. While this air circulation may be beneficial for other reasons, it will not change the temperature of the room. Could you use ice to cool down ice? Could you use boiling water to heat up boiling water? In the same way 80 degree air from one side of the room will not cool down the 80 degree air, or the 80 degree furniture, on the other side of the room. The only way a fan circulating air in a closed room would have any cooling effect would be if we sprayed water in front of it, allowing the water to evaporate. Of course, this would also increase the humidity, which is usually undesirable.
Since the electric motor used in an electric fan gets hot when it runs, the fan will actually heat up the room. Unless the motor is very large, this heating effect is very slight. It is usually offset by the cool feeling we get when we stand in front of the fan, allowing it to cool us off, even if it doesn't cool off the room.