On 7/15/16 6:17 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
I often here of people replacing fans with quiter ones, but I suspect that
all they really do is reduce the airflow. I believe that most of the noise
one hears is the movement of the air. From what I have read, sleve bearing
fans make a bit less noise than ball bearings fans. However, although the
MTBF of both types is similar at 20 degree C, the MTBF of sleve bearing
fans decreases quite a bit with only a modest increase in temperature.
where I used to work, we obsessed about fan noise (motion picture
industry, you want *quiet* fans with lots of air), and in the course of
that we bought dozens of small fans of one kind or another.
First, there's huge variation among different fans, all of the same
general size and kind. there's some general principles, but ultimately,
you have to buy one of the fans and try it. You can have 10 different
4" cooling fans with the same airflow performance and there could be a
20dB difference in noise.
Larger fan diameter is quieter than smaller for the same total air flow
- the linear speed of the air has a lot of effect on the noise - in
residential and quiet office space, for HVAC, the guideline is to keep
the speed below 1000 Linear Feet Per Minute (LFPM in the data sheets).
It's actually pretty non linear for a variety of reasons: your hearing
isn't linear, the spectral properties of the "wind noise" change with
speed, etc. This also factors into things like shed vortices off the
blades interacting with the supporting struts, and so forth. Slower is
quieter.
- smaller fans have to turn faster, so the blade rate is higher, making
them noisier (those little 20mm fans that whine - there's not much
different between those and a small mechanical siren)
- blades and supports interact - there's a whole lore about number of
blades and number of struts and whether the struts should align on
intake and exhaust side, etc. This is basically all done by empiricism:
each mfr has their own "secret sauce" for how they choose this.
- blade design has an effect, but not trivially analyzeable. In general,
large pitch, slow turning is better, unless the blade is close to
stalling, etc.etc. funky notches in the trailing edge, winglets at the
end of the blade may or may not help.
- the single most important factor that changes the performance of a fan
in terms of air flow is how close the end of the blades are to the
shroud or hole in which it spins. The tighter the better. You can fool
with curvatures and inlets and outlets and lengths of ducts and all
that, but the real important one is blade to wall clearance. Crummy
bearings and manufacturing tolerances require large clearances which
lead to poor performance (noise and "flow vs power in")
- bathroom type fans (e.g. shaded pole AC motor with a plastic blade)
are deliberately made noisy.
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