Saturday, 3 August 2013

How fans do and do not cool

It's now air conditioner season. We have one at home but it is just in one room and so far we have hardly used it. We have an electric fan, which seems to work. 

There are three ways of looking at how fans work. There's the science-free way, the way with a little science, and the heavy science way. 

The unscientific way of looking at it is that the fan is sending out cold air. The air feels cool, and the fan makes you feel colder so this view has some logic. 

If a little science is applied, it's tempting to think that the fan is not sending out cool air, and that it is simply cooling by helping the moisture on our bodies to evaporate by blowing the air away from next to our skin. This view will, quite correctly, note that fans are not going to make the room cooler, because the fan motor is generating friction and electrical resistance is heating up the coils and there is generally an increase in entropy all round. 

While an air conditioner will cool down the room by pumping heat outside, a fan will, if anything, make the room warmer by bringing in electricity, looking at a closed thermodynamic system. 

But this is ignoring Bernoulli. One way of looking at the Bernoulli effect, recently mentioned on an episode of the BBC's In Our Time talking about the completely irrelevant subject of cosmic rays, is by blowing on your hand. If you open your mouth and breathe, it feels warm. If you purse your lips, it feels cool. The reason it feels cool when you purse your lips is because the air is at a lower temperature. The reason it is at a lower temperature is because it is at a lower pressure. Since temperature relates to the amount of heat, and lower pressure means that the gas is spread out over a larger volume, so the temperature drops. The reason for the pressure drop is that the air is moving faster.

Another way of looking at it is what happens on narrow streets at Matsumoto Bon Bon. This is an annual festival where a few thousand people dance around the streets for three or four hours. Some of the streets are wide dual carriageways and some narrow one-way streets. What usually happens, as the evening goes on and more of the beer carted around behind each group gets drunk, is that the procession ceases to have a uniform density of people, and instead has some very crowded areas where nobody is moving, and some completely empty sections. The narrow streets always become empty, so the dancers have to run along these to catch up with the next group, often having been dancing on the spot before. What is happening is that the narrow street is making people go faster, and because they are going faster there is more space between each person and the density has gone down. Exactly the same thing happens between the atoms in a gas. This, incidentally, is what keeps planes in the air. 

It also means that the air coming out of a fan is actually colder, since it is moving faster and has lower pressure. But unless you keep the whole house at a low pressure, the overall temperature is not going to stay down. The fan will only cool you down if you are standing in front of it. Otherwise, it should be switched off. 
(fan graphic taken from: without permission)