cooling by fan?

[Comandante]

somebody told me that a decent room fan, eg 40cm one, does not cool air in the room and at first thought this seemed logical and i would've said the same thing, but the question itself made me question it twice. is it really true that a fan does nothing more to air besides putting it in fast motion?

 

I did a quick search on google and I an answer popped up on howstuffworks; http://science.howstuffworks.com/question22.htm

 

They seem to be saying the same thing. But this is what still makes me curious;

 

If we ignore for now that the fan generates heat, let us assume it's just a very efficient fan that doesn't generate considerable amount of heat and all it does is circulate air in the room.

 

Now, considering the shape of the fan you can see it was designed in a way that takes the air from the back and speeds it up blowing it to the front. What I don't clearly understand is how and why this is happening. Is the fan body generating area of low pressure in the spinning part such that the atmosphere is pushing the air through the fan - IF SO; doesn't this then decrease the temperature of the air as pressure and temperature are related - and to what extent? OR; is the fan body simply carving through the air in circular motion such that the air is only 'mechanically' moved from the back to the front - just quickly. I've never read much about aerodynamics so I'm just guessing for that part, but I'm a bit more familiar with temperature/pressure relationship so that's what makes me curious.

 

Anyone got a good answer?

[YT2095]

all it does is circulate the air, it doesn`t cool it at all, the only reason it Seems to is by evaporation.

 

put a thermometer in front of a fan and it won`t change temp reading, put the thermometer bulb in a damp cloth and measure it, it Will have cooled.

 

if you want to get Really technical, a fan will actually Increase a rooms temp!

it creates collision of air molecules heating them slightly

[insane_alien]

yeah, i second YT. it will actually heat up the air (not really noticable in a room with an average fan). I done an experiment on this back in high school where there is a small box with a fan in it (the motor is outside the insulated box to stop it heating it up) and if you turn it on and watch the temperature, it rises.

 

although, if you scaled it up to a 5m by 5m by 5m room your looking at a 4m diameter fan spinning at mach 2 at its edges.

[Comandante]

hmm, so you are saying that fan generates no pressure change? or are you saying that it does generate low pressure but this does not affect temperature at all?

 

i'll try the thermometer later on, but let's just say the thermometer can't detect such small amounts of temperature change? how would you know?

[swansont]

You must ask yourself where the energy would have gone. The fan may produce larger pressure at the outlet, but that does not propogate through the room. Ultimately you have a steady-state pressure and a fixed volume, and are dumping the excess heat of the fan motor into the room as well as the energy of the fan blades (as YT noted). What you are actually doing is hastening the process of equilibrating the hot things to the (rising) ambient temperature. You need to actively pump the energy out to another reservoir in order to cool the whole volume.

[Comandante]

swansont, you're referring to the room as a whole or just part of it as I am? Because if referring to the room as a whole of course I think the total amount of heat energy would be constant (or in a constant rise you could say due to heat generated by the fan and other devices) provided good isolation, but what I'm curious about, perhaps didn't stress it properly earlier, is the air coming out from the fan blades at the front and say few meters ahead.

 

My idea was that the air would come out slightly cooler (the heat energy from the air being absorbed by the blades perhaps? then later being emitted back into the room etc -> flow of energy) - ignoring the idea that the air would Seem to be cooler because you would feel cooler (speaking of this I can say I have some understanding, it has to do with conductivity of heat, eg. you're in a room with tiles on one side and wood on the other, they are both of exactly the same temperature, but of course you would feel the tiles to be much colder to your feet as they drain more heat). Continuing on what I was saying, the slightly cooler air would then either absorb the heat off a person's body and eventually mix with the rest of the air in the room. That being only a scenario I envisioned, I still seek the answer to my main question; Does the air cool down as the pressure is reduced around the spinning area and would this be measurable?

[mysolipsis]

the position of the fan may make a difference as well. for instance if you have a fan placed on the ceiling, then this will force the air from the upper part of the room down, hence pushing the air from the bottom of the room up (force air convection). now even though there isn't a drastic difference in temp, in theory the air at the top of the room should be warmer (heat rises) therefore the air at the feet (which should be cooler) pushes up and in a sense cools the ambient air around the upper part of our body, which most people seem to notice more. what do you think? ya? or na?

[Comandante]

the position of the fan may make a difference as well. for instance if you have a fan placed on the ceiling, then this will force the air from the upper part of the room down, hence pushing the air from the bottom of the room up (force air convection). now even though there isn't a drastic difference in temp, in theory the air at the top of the room should be warmer (heat rises) therefore the air at the feet (which should be cooler) pushes up and in a sense cools the ambient air around the upper part of our body, which most people seem to notice more. what do you think? ya? or na?

 

there's some truth in that too. though the only reason how i see that could work is if you were seated in a special position relative to the fan, because as the cold air goes up the warm air will come down and so even if you were cooled by little cold air, you would get warmer again (and i would assume the air would very quickly get mixed anyway so this would only work for a short time). besides, you're sitting on the bottom of the room, not in the middle or anywhere else - meaning that you are ALREADY in the colder part, and considering that - why would you want to get the warm air down on you anyway? unless it was winter or something ..

 

so really, the only way I see how a fan can cool a person is by inducing faster rate of perspiration. what' im curious about tho is whether there is any "heat loss" or heat transfer from one area to another as a result of low pressure.. but seems like not many have the answer

[Rocket Man]

in a stagnant room, you're pumping heat out. that heat provides a small amount of convection. the air is normally cooler than you are so it absorbs heat as it passes (even when you ignore perspiration)

a fan will move the air faster so it's like turning on the pump to a radiator.

 

you can safely ignore the pressure cooling effect as the room is supposedly a closed system of roughly uniform pressure. boyles law cannot apply to this scenario nor will the fan remove any energy from the system.

[spikerz66]

if you want to get Really technical, a fan will actually Increase a rooms temp!

it creates collision of air molecules heating them slightly

 

hahaha so true

 

fans cool bc of evaporation thats al they do. ando you get a nice breeze!

[Comandante]

think i'll go and put a bucket of ice under my ceiling fan that will surely cool it down

[YT2095]

I doubt that will work in any significant way, it lacks the surface area to be of any great help.

[Comandante]

why would surface area matter when its all going to melt down quick enough anyway, actually i've seen a device they call 'air conditioner' or something, it has a fan blowing behind a box in which ice can be put. that one works ok. but to cool the room as hot as mine right now... a bucket of ice will be nothing! i might consider a barrel

[YT2095]

you still need surface area, these devices employ cooling fins, the Ice absorbs the heat Via them.

[spikerz66]

maybe hes talking about one of those REALLY BIG buckets

[Ndi]

Cars get cooled slightly via fans because they employ a metal chassis. The fans at full speed compress the air from the outside, raising temperature by a degree or two. This air is rubbing against the chassis on its way to the cabin, losing the excess heat to the car body (which is now cooler, being vented in motion). When it reaches the ventilation grids, it expands, getting cooler, thus having an actual temperature difference relative to the body of the car and the air going over the chassis.

 

This is not significant for fans, I just inserted the fans to be into the question, actually it's been implemented in older cars and uses the pressure of the air in front of the car ramming against the ventilation intake. There is a noticeable difference in temperature when going at highway speeds, when pressure is higher and the chassis is being ventilated.

 

Just background info.

[Comandante]

cooling fins on them? interesting. also, as a matter of fact, if u have cubes of ice in a bucket it's still a big surface area isn't it? i mean the cubes aren't laid down perfectly, the air can go all the way to the bottom of the bucket, through the gaps in the cubes, and to make it even better small holes can be made on the bottom of the bucket to let the water down into the next compartment. it's still a small bucket though, probably woudln't make much difference to the overall room temp, but i could add more ice again. and this is way impractical too if you ask me, but it's a valid way to cool the air without regular air conditioning system

 

Ndi, thanks for the nice engineering info there

[YT2095]

I was thinking more along the lines of getting a large Alu heat sink with many many fins on it then bolting on copper pipes to the underside of it and sinking them into the ice bucket, then blow the air through the alu cooling fins.

[Comandante]

I see, sounds like a more efficient method. I'll actually give it a go sometime.