How does dust stick to a fast moving fan?
xanadu
Posts: 3,347
I was cleaning my fan blades and I always wonder how they get so dirty. To me it is strange that dust can accumulate on something moving so fast because I imagine the fan and dust as solid objects bouncing off each other. I Google it, and see a bunch of answers but that caused further confusion.
Most are hung up on the fact once you get a thin layer going, 'it is easier for more to stick'. That doesn't seem right to me, you'd think the centrifugal force would cause a larger dust structure to stress and fall off.
Then there's the static issue, which I get to certain point, but it seems like there is more to it after the static dissipates.
Some dust clumps seem like they were glued on, could it be humidity is a factor?
Someone needs to invent a dustless fan that doesn't use filters
Most are hung up on the fact once you get a thin layer going, 'it is easier for more to stick'. That doesn't seem right to me, you'd think the centrifugal force would cause a larger dust structure to stress and fall off.
Then there's the static issue, which I get to certain point, but it seems like there is more to it after the static dissipates.
Some dust clumps seem like they were glued on, could it be humidity is a factor?
Someone needs to invent a dustless fan that doesn't use filters
Comments
-Phil
A site note: if static IS the reason Its kinda Freeky that ya have STATIC near Computer parts ...... Ekk!
My bare metal attic fan blades are squeaky clean; but I'm sure that if they were plastic, they'd be thoroughly caked by now. OTOH, I've seen metal exhaust fans for paint booths whose blades get caked, but that may be more from chemical adhesion. Also, the painted metal fan blades in my shop heater have accumulated a little dust, but the paint insulates the dust from the actual metallic surface, preventing discharge.
-Phil
If you short a Van de Graff from Ball to GND . does the ball have any potental ?
I don't think it would ..... same with a ( all ) metal fan and fan case ..
easy to test ....
( aquadaq paint and a fan.)
I don't quite understand the static relationship between metal and plastic. I know metal can build up a charge but like Phil said if it's grounded how would static be a factor there.
The boundary layer is talked about a lot, how there is no velocity at the surface, that might be why I'm confused I've never heard of that, good stuff.
I can use a Propeller to control the power of and monitor the RPM of the fans. I guess the fans all need to have the same airflow, speed, environment and surface area which can't be too hard to pull off.
Fan 1 - Control experiment/stock fan
Fan 2 - Antistatic sprayed fan
Fan 3 - Fan with rough sanded surface
Fan 4 - Glass rod spinning same RPM
Fan 5 - Metal rod spinning same RPM
That should provide some results to compare hopefully. I'm thinking run it for a month and check the blades in 48 hour intervals with a basic microscope and also digital camera and make a time lapse or cool comparison at least.
Intersting, my attic has no fan but I'm going to acuire a metal fan for this test. I'm wondering why there are no metal ATX fans because static seems to be one of the largest contributing factors here.
Heat-sinks are metal but they don't move so I can see why they would accumulate dust. I did a couple continuity tests from heat-sink to ground and it seems like there is not path to ground for most CPUs. I read about a 'dust free' spinning heat-sink and wonder if metal is that much better why not just make a metal ATX fan...
"If you short a Van de Graff from Ball to GND . does the ball have any potental ? I don't think it would ..... same with a ( all ) metal fan and fan case .." - Look at Charging by induction ...
... an electroscope for example. Bring a charged object near an electroscope and the leaves on the electroscope move apart, yet no charge has actually transferred between the charged object and the electroscope, yet the leaves still repel. This is because induced charge at one end of the electroscope creates an imbalance.... say the charged object has a negative charge, then the induced charge at the top of the electroscope will become positive ; likewise the resulting charge at the bottom of the electroscope will be negative, causing the leaves to repel. Since the charge was induced and there was no transfer of charge, then the difference in charge between top and bottom is just displacement within essentially a single, electrically conductive piece of metal.
The same thing can happen with a metal fan, even though the center of the blades are grounded through the shaft of the motor and to the mains, the ends as they rotate can develop a static charge against the air... not much of a charge, but still present. ...and since the fan is grounded, it does dissipate the charge quickly, making it less likely to collect dust, but not an impossibility.
BTW) Attached is a simple 15 minute electroscope ... instead of foil leafs I used two aluminum balls attached to a very fine 'horse hair' copper wire and an old planters peanuts jar.... lid insert replaced with a circular piece of plastic.
Right. For a dust mite, electrostatic attraction is a lot stronger than centrifugal force, gravity, or the earth's magnetic field. And once tiny particles are stuck to each other, other factors come into play...like molecular diffusion, in which matter seeks a lower energy state. That's why you have to change from hard wax to soft wax if the snow isn't newly fallen.
Yes, indeed. When looking at phenomena of this sort, it's important to get down to perhaps even the nanoscopic level. For example, if your fan blades are made of aluminum, you might need to take into account that the aluminum metal is actually coated with aluminum oxide, which is an electrical insulator. Metallic aluminium is very reactive with atmospheric oxygen so a thin passivation layer about 4 nm thick forms on its surface exposed to air. So this could be a player in the electrostatic charge build-up. For iron, there is a good chance you've got iron oxides, which probably don't conduct electricity very well when dry.
Just like the Earth has a boundary layer so does the surface of the fan? Essentially no matter what material you use at the surface of the fan you're basically dumping dust onto it by sucking the air over it even though it is moving?
If that is true it seems the thing that would keep the least dust overall is a smooth flat surface (which wouldn't push much air) and with the least resistance to ground. I guess that sums up why computer fans are the way they air (are).
lol, we also need to study how much dust sticks to an empty chair...
C.W.
Next up. A few year back I was talking to an insurance claims investigator. He told that one big problem was all the dust settling on the radiator grills at the back of refridgerators which has a high probability of catching fire when it accumulates too much. That is, your own old skin kills you.
However perhaps normal dust particles are on average uncharged, or have a sign that depends on other factors (passing thunderstorms!).
Also once a blade has a layer of dust the friction between this dust layer and the air is going to determine future charge distribution (and the stickiness of the layer will be v. important too).
I've noticed when cleaning computer fans that a strong smell of ozone can sometimes be present - indicating active static generation,perhaps related to the spinning blades carrying significant static charge - or perhaps due to the mains voltages in the switching power supply. I've never seen a metal bladed computer cooling fan though - perhaps it would be a good idea?
And then there is the issue of velocity....
If you ride a motorcycle in the rain, you get wetter, faster than if you are sitting on a motorcycle at a stop sign in the rain. The fact that you are in motion allows for deeper penetration of droplets and you actually accumulate more rain by moving through it. Impact may cause dust to stick.
I have no idea why the mentioned attic fan stays dust free. I seems that would be an anomaly as all the fans I look at are dirty. After all, everything gets dirty with use and age -- unless you have a clean up gnome dwelling in your attic.
I occasionally bring the leaf blower in and blow out everything electronic, and the fridge. The fridge to improve efficiency, didn't realize that it caused fires, yikes!
Need to check the clothes dryer outlet too.
BTW, it's best to do this when wifey's away.
The effect applies to any situation where particles in an air stream have to make a turn. For example it applies to tubing that brings air samples in to a detector. There is a size-dependent loss per unit length as particles plate the inside of the tube. And, for lowest loss, avoid generic plastic. If plastic is used, it is specially treated to make it electrically conductive.