ForumsThe TavernHow Fans Really Work: Fans Can Actually Heat Your Drinks Up

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Reton8
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Reton8
3,173 posts
King

I hadn't given this much thought before, but recently on the web I learned that fans cool you off due to convective heat loss. The fan, by blowing air on you, makes it easier for sweat to evaporate from your body and thus cool you (source: http://science.howstuffworks.com/question22.htm). The fan also increases conduction on you (Newton's Law of Cooling & some elaboration on this forum) .

In other words, as long as the room is below your body temperature, the fan brings fresh cooler air molecules to your body more quickly, which cool you a bit and more quickly than without a fan.
Also, as long as the rooms humidity is not at 100% the fan brings more fresh air molecules to your body more quickly, which allow more sweat to evaporate form your body more quickly and thus cool you (this can happen regardless of the temperature of the room and air molecules. Here humidity is key.)

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So fans don't cool off a room (granted the objects in the room are the same temperature as the air molecules being blown on them, which should be the case). The fan isn't making the temperature of the room drop . The fan is blowing air molecules on you that are the same temperature as the room itself. Only you are cooled and not the room itself.

Well, I used to think putting my ice cooled drinks in the air flow of the fan would keep them cool longer, but this fan stuff got me thinking. If I have a fan in a room that is a closed environment (meaning the fan is not bringing in cold air from outside or another room and the fan is not blowing hot air out of the room), and I have a cold drink in that same room, and I then put that drink into the wind flow of the fan, I should be heating my drink up more quickly and not keeping it cool longer (as long as the temperature of the room is higher than that of the drink).

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Think about it this way. The drink is sitting in a medium which is a mixture of air. If no air is blowing on the drink then the air surrounding the drink and the drink itself will heat up the drink to a certain temperature. The drink will also cool down the air to a certain temperature. An equilibrium is reached. The drink is now sitting in air that is "spent" or air that can no longer change the temperature of the drink because both are the same temperature. Thus, the drink stops heating up.

Now, if you blow air on the drink, the drink will not end up sitting in a pocket of air that is "spent" or the same temperature of the drink. Instead, new higher temperature air molecules will be continually blown onto the drink until the drink heats up to the temperature of the room itself.

(Technically, even with out the fan, convection should cause a stream of new air molecules to surround the drink as the air molecules that are cooled by the drink should sink and warmer air molecules should replace them. But what the fan does is increase the speed of the convection current and thus increase the speed at which the drink is heated.)

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So, fans can actually heat up your drink.

  • 8 Replies
Gamer_Cale
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Gamer_Cale
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Nomad

Only you are cooled and not the room itself.


Not if you position the fan to blow hot air out or draw cold air in depending on the temperature difference from the room to outside.
Reton8
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Reton8
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King

Not if you position the fan to blow hot air out or draw cold air in depending on the temperature difference from the room to outside.


If I have a fan in a room that is a closed environment (meaning the fan is not bringing in cold air from outside or another room and the fan is not blowing hot air out of the room)


Already covered it lol. But I should have said this is the case I'm using for basically the whole post.
xeano321
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xeano321
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Farmer

In the middle of winter, a pipe had broke in the wall of my house and soaked the carpet in the hallway. My Dad sopped up most of the water, then took a fan and turned it on high and pointed it down the hallway. My bedroom being at the end of the hallway, was noticeably colder then the rest of the house, and the air the fan was blowing down the hallway was warmer then the rest of the house.

It still hurts my head, but I had to freeze for two days before the carpet dried. xP

Reton8
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Reton8
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King

My bedroom being at the end of the hallway, was noticeably colder then the rest of the house, and the air the fan was blowing down the hallway was warmer then the rest of the house.


Pardon me if I misunderstood you, I can't tell if you are saying that I'm incorrect, correct, or just making a statement lol but...

There are a few things that could have been happening. Without any measurements being taken it's hard to pinpoint exactly the cause.

One, I have a feeling this isn't the case, but your room may just be better at receiving cold are from a central air conditioning unit, if you have one, causing your room to be more cold than the rest of the house. But, the effect may come and go and this time it happened to correlate with the fan and thus it seemed this way. My room tends to be extremely warm if the door is closed and another room in my house extremely cold.

Two, the fan may have cause an area of low pressure in your hallway causing warm air from your room to move into the hallway and thus your room was cooled. Even with the door closed I am assuming there is enough of some sort of crack somewhere around the door to allow air to move through even when closed. If I close the door in my bathroom and turn on the bathroom fan I will notice air being "sucked" into the room, through the cracks between the door and the door frame. So even a small bathroom fan can cause a decent amount of air to flow (pressure change).

Three, the soaked carpet may have had a cooling effect on your room. Water has a high specific heat and therefore doesn't change temperature quickly (or "easily" in a sense), especially when compared with other matter such as metal which typically will warm and cool quickly. The effect of cooling from the soaked carpet may have been greater from the effect of warming from the air blown by the fan.

Four, remember that the fan from the air can be room temperature, but still produce a cooling effect on you, as the fan is increasing the rate of evaporation of sweat on your skin. Also, the air coming form the fan may have been higher in temperature more near the fan and then cooled by the time it reached your room (the air traveling over the soaked carpet).

Five, the burst pipe may have gotten the walls or ceiling wet also contributing to the cooling effect in your room.

But, I have no real idea, I wasn't there lol

But I couldn't see how it would be otherwise with a cold drink (below room temperature) being placed in a closed room with a fan (no air being blown in from outside, no air being blown out). The air flow on the can should more quickly raise the temperature of the can. And think of this, you probably use a fan most often when it is warmer outside than it is indoors or when both are at nearly the same temperature. Also, even with a door open the rest of the house (or space) should probably be about the same in temperature as the room the cold drink is in. So most likely when a fan is on and a cold drink is in it's air flow, the drink is being warmed more quickly and not being kept cool longer.

Hmmm, but I'm not an expert here. It would be interesting to see if someone could prove otherwise or at least confirm this for me.
Kasic
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Kasic
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Jester

I wasn't aware the functioning of fans was such a mystery to everyone.

Reton8
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Reton8
3,173 posts
King

I wasn't aware the functioning of fans was such a mystery to everyone.


I'm not saying it's a mystery, but the thought of a fan typically associated with cooling actually heating something might be unique. On top off possibly someone thinking that leaving a fan on in a room might cool the room temperature down over an extended period isn't true. I mean maybe you already knew it, but just googleing "how fans cool you down" or some sort of search, most sites that state how it's done do in a fun-fact trivia sort of way.

Obviously most people know how a fan cools things, it blows air on it. It was the details of what's happening that I found interesting. I didn't realize that overall room temperature did not decrease when using a fan. I also found it interesting to think of a fan as an object that would heat something up, instead of cool it down. I wasn't trying to insult anyone's intelligence, just share a fun fact I didn't think was too well known. (But I suppose it could be boring or maybe more people think of fans as something that heats stuff up than I thought).
Kasic
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Kasic
5,557 posts
Jester

I didn't realize that overall room temperature did not decrease when using a fan.


You're adding energy to the room. Friction of air on the blades, the internal motor, etc. In a closed environment, a fan is a heat source. Now, if you use it right you can lower the temperature of a room, by either blowing hot air out/cold air in, but by itself a fan is really just pushing air around. The evaporative cooling is assisted by the increased air flow is all.
MacII
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MacII
1,315 posts
Shepherd

Anyone mentioned air circulation yet?

increased air flow is all


Bingo.
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