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Be Careful With Fans

In recent months, I’ve felt the need, as one who has made a study of the laws of physics, to educate the general public and dispel myths that abound in society today.
Today, I’d like to talk about fans. This is a topic of great personal significance to me in that, growing up, my parents wouldn’t turn the air conditioning on unless the temperature inside the house got up into the 80’s (about 27-29 Celsius). Instead, we were told to just turn the fan on. Knowing what I know now, I can say that that wasn’t the best of ideas.
To find out why I say that, let’s look at a fan from the standpoint of thermodynamics*. When you turn a fan on, you bring in a steady flow of energy into whatever room the fan occupies. Friction guarantees that, given enough time, all of this energy will be turned into heat. What this means is that, unless the energy is allowed to escape, then it will just continue to build up, heating the room. The good news is that the electrical energy brought into the room can easily escape in one of two ways. First, the energy can escape in the form of warm, moving air escaping out through cracks and other leaks in the walls, floor, and ceiling. Second, the energy can escape as heat through any of the same places**.
The question then becomes, why are we talking about thermo dynamics? The main reason is because it tells us that a fan doesn’t cool off a room, but rather heats it up. To cool off a room is to take energy away from it, and as discussed above, a fan only adds energy. This raises the question then of how a fan actually works. To answer this question, we need to talk about how the human body works first.
The human body does not feel heat, rather it feels heat transfer. As evidence of this, I submit two examples. First, consider an oven. If an oven has been on for long enough, both the air inside and the metal racks will be at the same temperature. Why then is it that sticking your hand in the oven only feels uncomfortable until you touch the metal racks, at which point it becomes excruciating? The answer is that heat can transfer much faster from the metal into your hand than it can transfer from the air into your hand. Second, consider a classroom, or any room for that matter. Imagine a wooden desk with metal legs. So long as the desk has been in the room for a long enough time, the wood top and the metal legs are at the same temperature. However, when touched, the wood feels normal, but the metal feels cold. This happens for the same reason that the hot oven air is uncomfortable, but the rack is painful.
With that in mind, we’re now ready to talk about fans. Whenever you’re surrounded by air that is colder than your body temperature, you start to warm up the air around you. As the air around you begins to warm up, it starts to form a layer of warm air*** that serves to insulate your body such that you don’t lose as much heat, and as a result start to feel hotter. A fan helps in this situation because it helps to break up the insulating bubble surrounding you by pushing hot air away and bringing in cooler air. This is a phenomenon known as forced convection.
Now we come to even more bad news. Forced convection works both ways. Consider the following. With a room that has exterior walls, the walls heat up as the sun beats down on them. As they heat up, a layer of warm air forms along the wall. This layer of warm air actually helps to further insulate the wall, limiting the amount of heat energy that can come into the room. However, a fan can break up the layer on the wall just as easily as it can the layer surrounding a person. What this means is that the cooler air that is forced across the wall absorbs energy from the wall faster than the hot air that was there. In other words, the fan allows the room to heat up faster by allowing more heat to transfer through the walls. What all of this means, taken together, is that a fan is usually only a good idea if there is someone around to enjoy it. If no one is home, then it’s probably best to leave it off.
You may have noticed that the statements made in the last two sentences contained qualifying words such as ‘usually’ and ‘probably’. There is a good reason for this. A fan, if used in conjunction with some other form of climate control, can be of great benefit. Consider the following. In a large room with an air conditioning vent on one end of the room, cool air gets pumped into said end of the room, thereby cooling it down. However, this cooling effect will take time to work its way across the room. A well placed fan can speed up this process by pushing cold air from one side of the room, to the other, thereby helping to create more uniform temperature in the room. Consider another example of when it may be beneficial to use a fan in an unoccupied room. At present, the room that I call my bedroom is in a spot in the house such that the ventilation doesn’t reach it very well. In other words, during the winter time, the room doesn’t seem to get as much hot air as the other rooms. During the summer time, the same can be said about cold air. Thankfully, a carefully placed fan by the air vent helps to suck more air out of the vent, thereby bringing more cool air into the room. The fan then is kept on, not because it cools the room down, but because it helps bring in more cool air from the ac vent so that the room will be cooler in the afternoon or evening when I return to it.
So, to summarize, a fan only adds energy to a room, thereby cooling it. But, properly placed fan can help increase the effectiveness of a device, such as an air conditioner or swamp cooler, that actually can lower the temperature.

*Thermodynamics is defined as the study of energy and the movement thereof.
**Heat energy flows from where it is hot to where it is cold. This means that, if the room is colder than whatever is on the other side of the floor, ceiling, or walls, it will stay in the room.
***The technical term for this is a thermal boundary layer.

Comments

Aw, for a second there I thought (reading your blog) that you were going to point out that a fan can transfer lots MORE heat to you, if the air surrounding you is hot enough to counteract the effect of any surface moisture evaporation (the "caught in a wildfire" scenario == i.e. the convection oven principle).
Your points of convective flow near a hot wall are well taken, however.
Interesting point: if air in a room is heated, but allowed to escape, the equilibrium state is unchanged enthalpy in the room air (the heat input transforms to work done at lifting the atmosphere outside as gas escapes the room).

$20p/h

Todd W.

Experienced Math and Engineering Tutor

200+ hours
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