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Grandpa -

I am asking you this question because it occurred to me that ships must have a lot of electric fans aboard. The question is: why do electric fans get so dirty? It seems to me that they attract more dirt than any other household object. This is an honest question and I hope you won't consider it to be beneath your notice (but I'm sure you know that the best scientists have considered nothing to be beneath their notice).

- Bertie

Sent from AllExperts.com at 14 Apr 2001  (Grandpa is an expert on this site - some examples come from here.)  (I haved learned since, that Bertie is a man who lives in Australia.  He uses lots of fans in his work.)

 

Bertie -

What an interesting question.  Probably, at first, one would think that a fan would not get as dirty as something standing still, because it would whip the dust off as fast as it got on.

You can imagine the dust particles in the air - you've seen them when there is a beam of light across the room in front of you.  They just meander around - so small that gravity does not overcome their tendency to wander with the currents of air.

When a fan blade hits these particles, it's fast enough for some of them to stick to the blade.  Some dust particles are very dry and wouldn't be likely to stick, but others have a little oil or something with them, and they are a little sticky.  Because the blades of the fan are moving, they hit lots of these particles.  If you had two identical fans, one running and the other standing still, the running fan would get the dirtiest.  (Which fly swatter would have the most flies stuck to it - one that was being swished very fast - or one standing still?)

You will also notice, however, that the tops of things get dusty - (Check the top of the refrigerator.)  You know we dust the tops of tables - but not the bottoms.  On the fan that is not running, you will notice that the very tops of blades will still get dusty, but the running fan will get dusty pretty much all over the sides of the blades facing in the turning direction - the backs of the blades won't be so much.  Because of the currents that are formed, the dust will take some characteristic shape on the blades.  Probably the leading edges will get the most dust, and the trailing edges the least.

I had to think about this to answer it - because I've never thought about it before.  Of course, I had seen the dirty fans, but hadn't noted that they were much more dirty than other things in the same room.  Your question made me ponder.

Fun question.

- Grandpa

(Hey - I just took a look at a fan we had put away.  Luckily, it was still dirty.  It was obvious that the dirt was not only on the blades of the fan, but all over it.  I think this is because the fan is moving lots of air across all these surfaces, and therefore lots of dust has a chance to collect there.  Also, I think, even dry dust collects, because all this air has some oil, etc., and that collects on the fan - then the dry dust can stick to that.)

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