Some notes when using 171mm fans.
o Their asset is not in their diameter
---- the cast alloy shroud is 171mm
---- the blade area is considerably less - below 150mm
---- hence the "shroud shrunk" 171x150mm versions
o Their asset is in depth & so static pressure
---- most are 51-55mm deep, some NMB are 38mm deep
Their cfm may not be impressive re size - 200-235cfm,
what is impressive is their ability to *maintain it*.
Thus...
o Whilst 2x 120x25mm fans can match 171x55mm in cfm
o It takes 4x 120x25mm fans to match 171x55mm in pressure
Integration issues must be considered:
o 171mm fans are built for 1) Life 2) Pressure 3) CFM
---- noise considerations are mute to economic buyers
o Whilst 40-120mm fans dB(A) scales with rpm, 171mm do not
---- 171mm at 100% duty 51dB(A), 50% cycle 35-43dB(A)
Noise is thus a fundamental concern when using big fans.
o Least - motor IC noise (DC switched to make fan spin)
o Notable - depth for static pressure increases noise
o Major - bearings for heavy rotor magnets
---- DC fans have stationary motor coils
---- motor IC switch the DC around at N frequency
---- very big magnets in rotor hub chase the DC
Bearings are your major concern for PC applications:
o PAPST / Comair Rotor = "Rough"
---- major component of noise up to 65% Duty
---- an increasingly loud rumble, noisy stainless bearings
o NMB = "Smooth"
---- hard to get hold off
---- 6820PL-05W-B10 for 24V & B10-B50 if bothered
The solution for silence afficinado's is voltage:
o Choose the 24V fans
o Then run them at 12V to get the noise & load down
Remember for big fans the wattage is non-linear:
o 24V fans often have a wide range 12-32V DC
---- note the range is as *your* fan however :-)
o 16V = 4.9W, 32V = 17.0W
---- so running at 12V is actually under 4W
---- pressure is good, major noise is bearings
An unfortunate note about bearing noise is that it is
rarely "white-noise". Most A/C units you hear are designed
so the predominant noise from a 300-400cfm+ fan is airflow.
Airflow noise is a whitenoise, favoured because human
perception can most easily mask that at the perception level.
PWM with big fans (big = depth) gets problematic:
o Bearing noise is high
o Slippage is high due to bearing & big magnets & static load
o PWM frequency that is 1-2Khz may be objectionable
---- however it will be quite efficient
o PWM frequency that is 10-20Khz may not be noticed
---- however it is often less efficient
Efficiency comes down to the fan's motor design & loading.
o So wattage need not be great if you run a 24V fan at 12V
o Run bigger fans at 60-70% anyway re Noise & Longevity
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