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Author Topic: Fans Are So Shallow, I can see right through them !!  (Read 7142 times)

Offline neilep

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Dearest Lovelies,


Fans are great...they keep you cool and make a pleasant whirring  sound.

Like most fans..the blade portion is what 90% blade  ? and 10% gap ?...so how come I can see right through it when the blade is spinning ?

With such a small gap one would think that the solidity would still be there but just slightly faded in colour....but no !..it's very transparent indeed...and the faster it goes the less blade you can see, the more transparent it becomes.


..why's that then ?


 

paul.fr

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« Reply #1 on: 19/04/2007 14:49:55 »
Dearest Lovelies,


Fans are great...they keep you cool and make a pleasant whirring  sound.

Like most fans..the blade portion is what 90% blade  ? and 10% gap ?...so how come I can see right through it when the blade is spinning ?

With such a small gap one would think that the solidity would still be there but just slightly faded in colour....but no !..it's very transparent indeed...and the faster it goes the less blade you can see, the more transparent it becomes.


..why's that then ?


not an answer, but you do realsie that fans do not actually cool your room. don't you?
 

Offline neilep

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« Reply #2 on: 19/04/2007 15:07:21 »
Dearest Lovelies,


Fans are great...they keep you cool and make a pleasant whirring  sound.

Like most fans..the blade portion is what 90% blade  ? and 10% gap ?...so how come I can see right through it when the blade is spinning ?

With such a small gap one would think that the solidity would still be there but just slightly faded in colour....but no !..it's very transparent indeed...and the faster it goes the less blade you can see, the more transparent it becomes.


..why's that then ?


not an answer, but you do realsie that fans do not actually cool your room. don't you?

They do when you hang ice cubes on a piece of string down in front of them...el cheapo air con !!


yep, I gather they jusy move the air around a bit  without reducing the temperature.

So how come they ' feel' cooler when you get that blast of air on your back ?
 

Offline ukmicky

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« Reply #3 on: 19/04/2007 15:16:26 »
Not sure but could it be that they are removing the surface layer of heat from your body through your sweat.

Does that makes sense
 

Offline eric l

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« Reply #4 on: 19/04/2007 15:16:53 »
  • The fact that you do not see the rotor spinning is an other case of what is illustrated by a test everyone must have seen in physics class :  a circular disk is divided in seven or 14 sectors, each showing an other colour of the rainbow.  When you make the disk spin, you do not see the different colours but just gray.  So in this case you do not see the different wings but you do see what shows up repeatedly behind them
  • You can see the fan spinning if you use a stroboscope.  If the frequency of the flashes is synchronized with the speed of your fan, it will look as if the fan stands still, if the frequency is marginally lower, you'll see the fan turning forwards, if the frequency is higher it will turn backwards.  A single TL-tube will have the same effect, but since you can not change its frequency, you would have to change the speed of the fan.  (Twin tube ornaments will have the tubes arranged in such a way that one is on while the other is off and vice versa, so no stroboscopic effect.)

And concerning that other point :  you feel colder because the blast of air helps your transpiration evaporate, which requires energy.
 

Offline Batroost

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« Reply #5 on: 19/04/2007 17:58:40 »
Remember that you are hotter than your surroundings and are continually losing heat through the surface of your skin - not just through evaporation of sweat but also through thermal radiation and natural convection. If you were the same temperature as your surroundings it would usually imply that you'd died....

You don't actually feel temperature - your body responds to the rate at which you are losing heat, which is a good surrogate measurement except when something in your environment artificially boosts or reduces the rate of heat loss. So, for example, if you stand outdoors on a cold day you will feel much colder if there is a wind blowing as this gives 'forced convection' a more efficient process; continually moving cold air across your skin and removing heat. This is the idea of 'wind chill' you sometimes hear mentioned on weather forecasts. Though no matter how strong the wind, it won't cool anything down to a lower temperature than that of the 'still' air.

A fan is just a mini-wind. By moving cold air across your skin it is causing 'forced convection' and taking away heat faster than 'natural convection'.
 

Offline Batroost

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« Reply #6 on: 19/04/2007 18:07:30 »
I just thought of another example....

I once spent some time inside a shutdown nuclear reactor. Although shutdown it was still sitting at about 65 degrees Celsius (150 F). This was done in a fully enclosed air-cooled suit - a bit like a space-suit. The cooling effect came from blowing cold air around inside (and then out of) the suit. If the cooling had only come from enhanced evaporation of sweat I think I might have suffered from severe dehydrated...
 

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« Reply #7 on: 20/04/2007 01:23:19 »
imagine yourself as the kid from the old readybrek advert, you have that red glow around you. that would be your body heat.

if you place the fan facing you, it will blow this heat away and cool you down by cooling your sweat and/or creating a cooler current flow over you.

also, lots of people leave fans running in a room "to cool the room", this can actually heat the room depending on what setting you have the fan on. the heat generated by the motor can put more heat into the room than the fan removes by circulating the air.

i have this problem at work sometimes when the aircon. goes down in the server room, they want to put fans in there to cool it down when all it is actually doing is moving the warm air around not cooling it down. as i demonstrated with a good thermometer.
 

Offline neilep

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« Reply #8 on: 20/04/2007 14:42:22 »
  • The fact that you do not see the rotor spinning is an other case of what is illustrated by a test everyone must have seen in physics class :  a circular disk is divided in seven or 14 sectors, each showing an other colour of the rainbow.  When you make the disk spin, you do not see the different colours but just gray.  So in this case you do not see the different wings but you do see what shows up repeatedly behind them
  • You can see the fan spinning if you use a stroboscope.  If the frequency of the flashes is synchronized with the speed of your fan, it will look as if the fan stands still, if the frequency is marginally lower, you'll see the fan turning forwards, if the frequency is higher it will turn backwards.  A single TL-tube will have the same effect, but since you can not change its frequency, you would have to change the speed of the fan.  (Twin tube ornaments will have the tubes arranged in such a way that one is on while the other is off and vice versa, so no stroboscopic effect.)

And concerning that other point :  you feel colder because the blast of air helps your transpiration evaporate, which requires energy.


ERIC SIR !!..........as always I thank you for your very informative  detailed response which helps this silly sheepy understand the nature of fans.

Thank you
 

Offline neilep

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« Reply #9 on: 20/04/2007 14:44:26 »
Remember that you are hotter than your surroundings and are continually losing heat through the surface of your skin - not just through evaporation of sweat but also through thermal radiation and natural convection. If you were the same temperature as your surroundings it would usually imply that you'd died....

You don't actually feel temperature - your body responds to the rate at which you are losing heat, which is a good surrogate measurement except when something in your environment artificially boosts or reduces the rate of heat loss. So, for example, if you stand outdoors on a cold day you will feel much colder if there is a wind blowing as this gives 'forced convection' a more efficient process; continually moving cold air across your skin and removing heat. This is the idea of 'wind chill' you sometimes hear mentioned on weather forecasts. Though no matter how strong the wind, it won't cool anything down to a lower temperature than that of the 'still' air.

A fan is just a mini-wind. By moving cold air across your skin it is causing 'forced convection' and taking away heat faster than 'natural convection'.

EXCELLENT..and THANK YOU Batroost!!  ....this is really interesting and I appreciate your informative reply....you have explained it very well indeed.
 

Offline neilep

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« Reply #10 on: 20/04/2007 14:48:19 »
imagine yourself as the kid from the old readybrek advert, you have that red glow around you. that would be your body heat.

if you place the fan facing you, it will blow this heat away and cool you down by cooling your sweat and/or creating a cooler current flow over you.

also, lots of people leave fans running in a room "to cool the room", this can actually heat the room depending on what setting you have the fan on. the heat generated by the motor can put more heat into the room than the fan removes by circulating the air.

i have this problem at work sometimes when the aircon. goes down in the server room, they want to put fans in there to cool it down when all it is actually doing is moving the warm air around not cooling it down. as i demonstrated with a good thermometer.

THANK YOU PAUL.......this is amazing that the heat of the fan (dependent on the setting) can in fact serve to counteract it's very purpose !!......does this really apply to household fans though ?...I've never  really noticed them getting too hot !
 

Offline neilep

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« Reply #11 on: 20/04/2007 14:52:27 »
I just thought of another example....

I once spent some time inside a shutdown nuclear reactor. Although shutdown it was still sitting at about 65 degrees Celsius (150 F). This was done in a fully enclosed air-cooled suit - a bit like a space-suit. The cooling effect came from blowing cold air around inside (and then out of) the suit. If the cooling had only come from enhanced evaporation of sweat I think I might have suffered from severe dehydrated...

woooo !!...so you looked like one of those scientist type peeps who you see on X Files and stuff ?......Was it a drill or a genuine emergency ?
 

Offline Batroost

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« Reply #12 on: 20/04/2007 17:16:12 »
Quote
Was it a drill or a genuine emergency ?

No just a routine inspection. Actually the really uncomfortable bit was the 'simulator'. They electrically heat this to 70 celsius, you go in (a long way) in your suits then they 'simulate' a problem by turning off your air! To get out you have to take the helmet off and the first breath's a doozy!!!!
 

Offline DrDick

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« Reply #13 on: 20/04/2007 18:48:16 »
Actually, "wind chill" is a misnomer.  Wind just intensifies the feeling of the air.  If the air around you is cooler than you are, it will feel cool.  However, if the air is hotter, it will feel that much hotter.  I experienced this first hand when I lived in Arizona and rode a motorcycle in the summer.  The air is hot, and the wind feels very hot.  It can actually feel so hot it's painful.  The leathers really help there.  Even though they look hot, they actually help you feel cool.  Also, because you do sweat inside them, the wind blowing through the leathers helps to evaporate the sweat and cool you down (as long as you're moving - getting stuck in traffic on a hot day wearing black leathers is NOT fun).

Dick
 

Offline Batroost

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« Reply #14 on: 20/04/2007 19:11:11 »
Nice illustration. Gets called "wind chill" over here (UK)... but with global warming, who knows?
 

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« Reply #15 on: 21/04/2007 01:11:16 »
THANK YOU PAUL.......this is amazing that the heat of the fan (dependent on the setting) can in fact serve to counteract it's very purpose !!......does this really apply to household fans though ?...I've never  really noticed them getting too hot !

Hi Neil, yes household fans! you may not have noticed them getting warm but the heat generated from the fan put more heat in to the room. Try it, put a good thermometer in a room, turn the fan on and close the door. i guarantee (no money back) the temperature will not go down, but may well go up.
 

lyner

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« Reply #16 on: 21/04/2007 10:44:30 »
See also:
www.thenakedscientists.com/forum/index.php?topic=7324.0

The effect of 'transparency' is related to the lag in response of our light receptors.
The other topic (above)   involves the same phenomenon. The receptors on your retina average out the light arriving on them over about 1/25 of a second.
 You see a mixture of the scene behind the fan and a blurred version of the whole fan. If there is, overall, a gap of 10% between the blades, the transparency will be about 10%, which is easily enough to see what's behind.
Detecting 'flickering' motion can not have been a priority for our ancestors, so we evolved acuity and colour vision at the expense of that. Our peripheral vision is , actually, more sensitive to flicker / movements - which makes sense, if you are  to avoid being sneaked up on in the jungle.  You will notice that fluorescent tubes and TV displays may appear to flicker when they are at the edge of your vision but don't flicker when you look straight at them.
 

Offline BillJx

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« Reply #17 on: 23/04/2007 19:30:44 »

Hi Neil, yes household fans! you may not have noticed them getting warm but the heat generated from the fan put more heat in to the room. Try it, put a good thermometer in a room, turn the fan on and close the door. i guarantee (no money back) the temperature will not go down, but may well go up.

My simplified explanation: The air temperature represents the average kinetic energy of the air molecules.  Adding energy by plugging in a fan can't lower the total energy in the room.  If ice is being melted or water is being evaporated, then that's a different situation because energy gets "stored" in the transition from solid to liquid or liquid to gas.  So in that case, the temperature can go down even though no energy has been removed from the room.

BTW a refrigerator with the door open will also warm the room.  A refrigerator moves energy from inside the fridge to outside it, but produces some heat in the process of doing the work.  In general, all the energy "consumed" by an appliance will eventually be dissipated as heat, whether the appliance is a fan, a fridge or an electric heater.
 

paul.fr

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« Reply #18 on: 23/04/2007 19:36:44 »
Nice and simple ;) thanks, Bill
 

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