I know a fan cools off a person by allowing the perspiration on his skin to evaporate faster, thereby cooling him off. But does a fan do anything to actually lower the temperature in a room? Again, I can see how it could cool off a room that had good ventilation, by blowing hotter air outside. But what about a closed room? Would a fan actually maybe make it hotter by causing the air to move more, thereby increasing the kinetic energy?
Yes, it would marginally raise the Temp, but it could still feel cooler, if you had the breeze on you.
So if you aren’t in the room, the temp will go up? Should you shut the fan off when you leave the closed room to keep the room cooler?
If the room is perfectly insulated, the fan cannot remove heat away from the room. The energy used to turn the fan eventually turns into heat (friction at the motor, friction between fan and air, friction between air and wall, etc.). So overall, the temperature will rise.
In reality, if the room is cooler than the outside air, the fan will raise the temperature even more by stirring up the air and letting more of it come in contact with the warm windows and walls. If the room is warmer than the outside air, similarly, the fan will decrease the temperature.
Also, if the sun is shining into the room, heat is being “produced” (converted from light to heat) on the wall or floor. In this case, turning on the fan may serve to cool those surfaces and transfer the heat to other walls and windows, which may facilitate removal of the heat from the room.
So, usually, if there is no outside air, and nobody in the room, turn the fan off. Is that the answer you were looking for?
Put the fan down, back away slowly and no one will get hurt. The FFSA (Federal Fan Safely Agency) will be here shortly to make sure we are all safe.
A minor comment here. Even in the absence of evaporation, a fan will cool a person off simply through the wind-chill effect.
That is, the body heat of the person warms up the air around them. If the air is moving, it blows away this insulating layer of air, replacing it with cooler air. The cooler air more readily absorbs more heat from the person, etc.
This assumes, of course, that the ambient temperature of the air is below body temp. I’ve found that I can usually tell when it gets around 100 degrees (F) outside; at this point, a breeze feels hot on the skin rather than cool.
So, if there is a fan in the room, is there any circumstance where it could be used to lower the temperature of the room?
Scr4 said that if the room is warmer than the outside air, the fan will decrease the temperature. But decrease it how much? Decrease it only to the point that the temperature equals that of the outside air? Or decrease it even further?
Yes, if the room is much warmer than the outside, you would direct the fan against the least insulated area, it would blow hot air to cool surface, cooling the air. But why you would not simply open the window & blow the cool air in, is beyond me.
But it wouldn’t be a cool surface would it? It’s my understanding that as long as you’re in equilibrium, the temperature on the inside of the wall would be equal to the temperature in the room, the temperature on outside surface would be equal to the temperature outside, and the temperature in the wall would vary according to the materials. I could be wrong (as usual).
Come to think of it, if what I said before was correct, in those situations, you should put your fan outside and blow air over the outside surface of the wall.
Don’t forget that the fan is consuming electrical energy (converting it to mechanical energy, and moving the air). This heats up the room directly- feel the motor of a fan (not the spinning part :)) and you’ll feel that it’s warm- this is dumping some heat into the room. If you’ve got a perfectly insulated room (no heat flow in or out), eventually the fan itself will heat the room to an infinite temperature. A human being dumps more heat into a room than an average fan, though. The fan heat could more than make up for the any cooling it did by blowing slightly warmer air to a cool wall (cooled from outside temperatures). You’re probably better off leaving the fan off when you’re not inside, unless it really is blowing air to a cool wall. If the fan is actually blowing cool air from outside (a fan in a window sill is good for that), you’d leave it on.
Arjuna34
That would be true only if the wall was perfectly insulated. If the wall was not perfectly insulated, and the room was warmer than the outside, there would be a constant heat flow from the inside air to the wall, then through the wall, then from the outside surface of the wall to the outside air. For each step you need a temperature gradient for heat flow to occur. The difference may be very small - if the room was at 90 degrees and it is 70 outside, maybe the inside surface of the wall would be 89 degrees and outside surface at 71 degrees. If you had a wall made of Aluminum, the inside and outside surfaces may be more like 81 degrees and 79 degrees respectively.
By the way, what did you mean by ‘equilibrium’? The above scenario can be an equilibrium, but it requires a heat source inside the room to maintain the temperature gradient. If there was no heat source inside the room, the equilibrium will be reached when the inside temperature is the same as the outside temperature.
SCR4:
I suppose ‘steady’ is a more accurate way to describe what I mean. If you have two heat resevoirs and a process between them your themodynamic properites in space will initially be varying with time (e.g.) the temp at point x in the wall will be a function of time. This variation with time will eventually tend to zero, when you have a steady process. Yes, you need a temperature gradient, but I think the reason for the difference between the ambient and surface temperature will be due to the temperature gradient across the thermal boundary layer.
You can’t have a descrete change in temperature except for at time=0. At least I can’t conceive of a discrete change on a theoretical level. On a practical level, I can say that the outer surface of my oven is fucking hot when the oven’s been on for a while. The only way I can reconcile this fact theoretically is that there must be a very steep temperature gradient across the extremely thin thermal boundary layer.