Monday, 5 October 2015

Ceiling fans make rooms warmer? How to make the most of fans.

Like probably a lot of Australians, fans have been my savior in summer. However, I've seen people who aren't using them properly. In this piece, I'll explain how fans work and how to get the most out of them.

How fans work

The easiest way to get the best use of your fan is to know how it works. Fans aren't overly complicated and consist of three main sections; the motor, the blades and the supports which hold everything in it's proper place.

When a fan is switched on, power goes to the motor which starts to spin, causing the blades to turn. These blades push the air in the same way that oars in a kayak push water. It's this displaced air that helps us stay cool, but only in the right conditions.

The right conditions

If you're body is wet from any source of water, such as swimming, sweating, or simply splashing water on your face, then a fan is going to be good. Some people believe that the movement of the air is what cools you down, this is somewhat true, but probably not useful for an Australian summer.

Isn't the wind enough?

When we deal with convection, (the heating or cooling of something by a moving fluid, in this case air) faster moving air will heat or cool faster. This is where wind chill comes from.  However, physics (specifically, the Laws of Thermodynamics) says that two objects will try to reach the same temperature when in contact. If you take ice cream out of the fridge and poured water over it (Which I don't recommend doing. Not because it's dangerous, but because it's a waste of tasty ice cream), the water will always get colder and the ice cream will always get warmer. It can't go the other way.

Similarly, you can't use wind to cool yourself down if the wind is hotter than your skin. Google says that skin temperature is about 32 so if the air is hotter then that and you aren't wet from something, then it won't cool you down no matter how windy it is. In fact, the fan is probably making the room warmer.

Why does the fan make the room warmer?

Have you ever touched the center of a ceiling fan? After it's been on for a while? (preferably while it's off) You'll probably notice that it gets pretty warm. Unless you have a perfect motor fitted (which is practically impossible) a decent amount of the energy being used to turn the motor is being converted into heat. This heat then spreads out into the air, making it warmer. The faster the motor is spinning, the more heat you get and the warmer it gets.

What's the deal with water?

Fans themselves actually can't cool us down, but they help with a process called evaporative cooling. As the name implies, this is cooling that happens when water evaporates. When water turns into steam, it absorbs a lot of energy from it's surroundings. To put 'a lot' in context, it takes 5 times as much energy to turn 1 kg of water into steam than it does to raise it's temperature from 0C to 100C.

When this happens to water in sweat, a decent portion of that energy comes from the surface of your skin which cools you down. This is why we sweat in the first place, it's very important for making sure that we don't overheat. Fans help with this process by raising the temperature of the water drops to a point where they're more likely to evaporate.

So how do I get the most from my fan?

To be sure that fan is working for you and not against you, there are only two things you need to remember;
  1. Keep your skin moist - Water is essential for fans to do their job. If you aren't sweaty, but still want to cool off splash on a little water. Damp clothes are great for keeping cool under a fan without the icky feeling that comes from sweat. 
  2. Don't leave it on when no one is in the room - evaporative cooling only works on things that are damp, which for most houses should only be people. Leaving it on will result in the room being hotter when you come back.
Hopefully this advice helps us avoid melting this summer. 

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