Fan Theory?

AshleyC

AshleyC

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Ok with these ever common days of hot stuffy weather i was just mulling over how fans work and how best to use them. Not having one handy i cant test some theories out so maybe someone out there can help.

Do fans suck in the air directly behind the blades and push it forwards or rather push the air in front of them. I was wondering that if you opened a window and placed a fan in front of it pointing inwards, would it be more likely to suck in fresh air from outside and therefore cause more a breeze throughout the flat (with a window open in another room). Or would it be better off opposite the window pointing at it where it would generate a flow that would push the hot stuffy air out of the window helping with a breeze throughout once again.
 
I think they push air forward, creating an area of higher pressure in front and an area of lower pressure behind that sucks the air forward from behind the fan.

But I could be talking nonsense.


Edited
I would think it would be best to have 2 fans and 2 Windows. Ideally a fan blowing inwards from a cooler area and another fan blowing out another window.
 
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We had many heated discussions about this when I was at work. We all had fans but were rarely allowed to open windows for fresh air as "It's draughty!" (Said in a singsong whiny voice.)
If you have a fan on but no ventilation, it create movement of air which aids perspiration which has a cooling effect, but the heat of the fan motor actually makes the room warmer.


I nearly sounded as though I knew what I was talking about then!
 
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Suck in the air behind and push it out the front. Don't believe me? Fill a room with smoke and place a fan at the door facing outwards and switch it on.(y)
 
i'll see if i have a spare flare to let off but i think the other half may dissaprove. The place is smelly enough with film chems as it is. I think the 2 fan method would be best though.
 
The fans definitely pull from behind and push out front. There's no point in using a fan to draw air in from an open window if the air outside is warmer than inside.
Fans only really work to force convection cooling, they need to blow air onto your skin to cause the moisture to evaporate and in doing so cool your skin.Hence ceiling fans in hot countries.

Regarding the energy used for the fan motor that is converted to heat vs the cooling effect of the fan - true, they might heat up the room by a very small amount but that would be very small compared to the effect of the convection cooling.

Humidifying cooling fans that need to filled with water are very effective but usually noisy and of course not only raise the humidity but also need filling up regularly. You can get them pretty cheap mind - around £100 or so though.
 
AshleyC said:
I was wondering that if you opened a window and placed a fan in front of it pointing inwards, would it be more likely to suck in fresh air from outside and therefore cause more a breeze throughout the flat (with a window open in another room). Or would it be better off opposite the window pointing at it where it would generate a flow that would push the hot stuffy air out of the window helping with a breeze throughout once again.
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In a single window room, placing the fan so as to block the opening will be very inefficient... you are essentially trying to pressurize the room with no place for the air to go.
In order to best cool the room in such a situation you want the fan to pull cooler room air up from the floor. This creates rotating airflow in the room, past the window where hot air can escape and cool air can enter.
However, that is only in terms of air temp. Usually it feels cooler when the air from the fan is blowing on you due to increased evaporation of perspiration.
 
nilagin said:
Suck in the air behind and push it out the front. Don't believe me? Fill a room with smoke and place a fan at the door facing outwards and switch it on.(y)
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I had an idea it did that also. I tested this theory out a while ago. After a curry, I made an unpleasant smell behind the fan cooling the missus. She got it full in the face, she was not very happy.
 
drounding said:
.Hence ceiling fans in hot countries.
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Got four in this house :-)
Last house had aircon in the bedroom but never got around to it here. :(
 
Depends which way they're turning.
 
viv1969 said:
Got four in this house :)
Last house had aircon in the bedroom but never got around to it here. :(
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Hope you haven't got any in rooms with gas fires fitted as that's not only illegal but dangerous as well .
 
the black fox said:
Hope you haven't got any in rooms with gas fires fitted as that's not only illegal but dangerous as well .
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The first part of that is incorrect, and the second part is also untrue if the fan is sited correctly, and the interaction of both is tested.
 
The thing that is not obvious about "sucking" on the low pressure side is that the air comes from all directions and not from along the fan's axis,i.e. a lot of air is coming in from the sides rather than in-line behind. So if your fan is on a windowsill inside the room you are getting a mix of air from in the room and outside. If it is cooler outside it may be better to put the fan outside blowing in. But as others have said the main mode of cooling if the room air is at a similar temperature to the outside air is moisture evaporation from your skin so you will feel most effect from sitting close to the fan. Also remember that in a room that feels hot in the sun (e.g. south facing) a lot of the heat you feel is radiant heat from the walls/ceiling, while air movement will help you will be better off getting away from the radiant heat.
 
We've got a locked cupboard at work where the wines and spirits are stored, it's not big, maybe about the size of your average under stairs broom cupboard. Last year we had a new chiller built, and thanks to the quality work and planning we get foisted upon us by the boss, the chiller unit would not fit in the designated space - so it's no inside the aforementioned locked cupboard. As expected, the lock up is now sweltering to the point that stock is getting ruined - corks popping and so on, the crummy ventilation grilles being utterly useless in aiding circulation - the solution? I took a desk fan and cable tied it into one of the grilles as an extractor fan, and it works!
 
sirch said:
The thing that is not obvious about "sucking" on the low pressure side is that the air comes from all directions and not from along the fan's axis,i.e. a lot of air is coming in from the sides rather than in-line behind.
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Not unless the system is confined (i.e. completely enclosed room).
More typically, the only air passing through from off axis is due to rotor tip vortices. This is due to high pressure air at the tip of the blades having low pressure to the side, and even lower pressure behind the blade tips. This causes the airflow at the blade tips (and typically at the blade roots as well) to flow in a rotating pattern. Much like the airflow behind a vehicle.
 
We have a box fan that is almost exactly identical to this one, down to the switches. Only difference is the colour.
http://img.diytrade.com/cdimg/513630/6462230/0/1216954623/12_Box_Fan.jpg

So during hot evenings (like last night) I put the fan on the window opening (the type that pushes out from bottom and doesn't allow rain to get in) and pull outside cool air into the bedroom for 10-20 minutes. It really helps cooling the room.
 
Good grief, one day of sunshine and we get a 20 post discussion on fans. :D
 
nilagin said:
Suck in the air behind and push it out the front. Don't believe me? Fill a room with smoke and place a fan at the door facing outwards and switch it on.(y)
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I'll wait until the wife's cooking dinner and try that.
 
viv1969 said:
The first part of that is incorrect, and the second part is also untrue if the fan is sited correctly, and the interaction of both is tested.
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I was presuming possibly incorrectly that you were talking about ceiling fitted fans with lights ,in which case they are illegal and dangerous to
 
the black fox said:
I was presuming possibly incorrectly that you were talking about ceiling fitted fans with lights ,in which case they are illegal and dangerous to
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I was.
 
In that case Ruth if you have open living flame gas fires fitted as I stated they are illegal and dangerous in that combination .we had a gas fitter in a couple of years ago to work on our boiler , he spotted the fan/light and fire combo and promptly told us that he had to disconnect the gas supply to the gas fire till the fan/light was removed .which he did and came back a few days later to re.connect it once I had fitted a normal light fitting
 
My fan in that room was tested and signed off by an industry certified engineer.
The combination has been retested, and approved since.
There are regulations which must be met, but it is in no way illegal, nor dangerous.
 
Your life at the end of the day ,carbon monoxide takes no prisoners ,even argumentative women can die from it
 
the black fox said:
Your life at the end of the day ,carbon monoxide takes no prisoners ,even argumentative women can die from it
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Please, feel free to prove me wrong with the legislation rendering it illegal.
 
I'm thinking of PC cases where because fans are attached to the side or back of the tower, they suck air into the box to cool the components inside and then out via a vent at the front or sides. But on a day like today where it's now 30 C in the shade and about 28 C indoors, using the same method by sticking a fan inwards on a window ledge would probably only serve to suck in the hot air from outside!

EDIT: Actually, I've just realised that the fan inside a PC tower does the job of sucking the air OUT of the box, with fresh air coming IN through vents on the side or front of the tower. I think this hot sunshine has gone to my head.
 
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