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Author Topic: Reducing fan noise by removing the fingerguard/grill  (Read 18937 times)
W9IQ
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« Reply #30 on: April 05, 2017, 12:41:58 PM »

Quote
When a piece of gear requires 'X' number of cubic feet of air for proper cooling and the size of the fan is limited to a specific size then the only option is to increase the number of blades, change angle of the blades and or increase the speed of the fan rotor.

...or reduce the system impedance.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
M0HCN
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« Reply #31 on: April 06, 2017, 07:03:46 AM »

... Or increase the system impedance and lower the volumetric flow....
Blowers are usually quieter then fans, so if designing from scratch a heatsink with a large surface area of closely spaced fins can be a reasonable way to go if you can generate sufficient pressure without putting the blades into aerodynamic stall.

... Or lower the thermal resistance between the die and the heatsink....
This directly lets you run the heatsink hotter, which means you have more delta T available and need less air, sometimes fitting a bigger power device really helps here as they usually have lower Rjc.

End of the day is is just doing good design, and trading things off the way that suits your requirements, being able to make that tradeoff is one of the reasons I homebrew.

73 Dan.
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K8AXW
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« Reply #32 on: April 07, 2017, 08:55:27 PM »

Glenn:  Please explain system "impedance" to me as used in this case.
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KI7AQJ
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« Reply #33 on: April 08, 2017, 12:58:38 AM »

Try making some dust filters from some Scotchbrite cleaning pads, and the noise levels go down considerably, BUT REMEMBER TO CLEAN THE FILTERS!!!! I started making filters from Scotchbrite or Beartex for lab equipment ages ago, and it works for linear amps too. Just remember to wash the filters often. Simple soap & water works fine. After 4 or 5 years make some new filters.
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W9IQ
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« Reply #34 on: April 08, 2017, 04:14:17 AM »

Allen,

When engineering a forced air cooling system, there are two first order variables placed into the equation:  the heat generated by the device and the maximum temperature rise allowed.

The required fan CFM at sea level can be estimated as:

   CFM = 3.16 * Watts of Heat Generated / ΔT

   where ΔT is the maximum desired temperature rise and CFM is the cubic feet per minute of the fan

Note that this is only an estimate. If the equipment is to be operated at higher elevation levels for example, then the airflow must be increased as cooling is not determined by airflow but by the mass of air. Thus the reduced air density at elevation requires an increased CFM.

Then the system impedance must be estimated or measured. To measure the system impedance, a manometer is typically used while the required CFM is flowing. A simple example of this is:



This can be thought of as "back pressure" on the fan. A particular fan can only produce a given CFM under a given system impedance. Thus the engineer must consult the charts of the fan manufacturer to select an appropriate fan for the job. In high system impedance situations, placing fans in series (push / pull) is often more effective than a single higher CFM fan. In low impedance systems, parallel fans are also a design option. The number of fans can also be affected by system reliability goals.

The reduction of system impedance can be quite intuitive (e.g. straighter paths, larger exit vents, etc.) and can also become quite esoteric (shaped inlet and outlet ports, heat stacking, etc.).  By constructing a simple manometer, the hobbyist can experiment with techniques to lower the system impedance and directly confirm the effectiveness. Any meaningful reduction would allow the fan speed to be reduced or a lower CFM (quieter) fan to be used (see formulas below and note about pressurized systems).

Since a well designed product will consider the altitude variations of where the product might be used, it is often possible to reduce the CFM or speed of the fan when the product will be consistently operated closer to sea level. There are some relationships that can help estimate the effects of such changes:

    1.) CFM is directly related to speed ratio. Cut the speed in half and the CFM drops to one half.
    2.) Pressure is directly related to the square of the speed ratio.
    3.) Noise reduction of the fan is related to speed by approximately: ΔdB = 50 * log10(New speed / original speed)

So if you drop the fan speed in half, the CFM drops by 50%, the noise drops by 15 dB, and the temperature rise will double - assuming constant heat generation inside the device.

Some designs, particularly those with closely spaced cooling fins, require a minimum system impedance in order to properly cool the device. If the system impedance is too low, the air currents tend to flow around the closely spaced fins and thereby not adequately cool the device. By pressurizing the enclosure, the density of the air is increased thereby increasing the cooling capacity at the expense of a (typically) noisier fan.

- Glenn W9IQ
 
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K8AXW
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« Reply #35 on: April 08, 2017, 09:00:32 PM »

Thank you very much for the detailed explanation of "impedance" as  used in air flow/cooling.  This is the first time I've ever heard that word used under these circumstances! 

Live and learn.....until you no longer live!

Al
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KM1H
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« Reply #36 on: April 15, 2017, 05:27:48 PM »

I did what KD0REQ originally said on my al811h amp and it did help.  Once I find some suitable 3 inch diameter flexible hose of some type, I want to relocate the fan inside some kind of noise cancelling box.  My goal is to make the amp virtually silent.

How do you plan to eliminate the sound of forced air? Even exhausting outside is not that silent at the amp but at least you can hear it slow down from lack of lubrication.
Also the resistance of the hose requires a higher CFM.
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AC7CW
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« Reply #37 on: April 15, 2017, 06:01:43 PM »

I did what KD0REQ originally said on my al811h amp and it did help.  Once I find some suitable 3 inch diameter flexible hose of some type, I want to relocate the fan inside some kind of noise cancelling box.  My goal is to make the amp virtually silent.

How do you plan to eliminate the sound of forced air? Even exhausting outside is not that silent at the amp but at least you can hear it slow down from lack of lubrication.
Also the resistance of the hose requires a higher CFM.

The fan could be placed outside. That eliminates the noise from the fan motor and the air exiting the unit but not the air entering the unit... and you could reverse the fan direction for heating of the shack... actually the entry air could be piped from outside as well to further eliminate noise.
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K8AXW
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« Reply #38 on: April 15, 2017, 07:17:05 PM »

The noise of the air entering the device, being of a lower pressure and more volume is usually quieter than the pressurize air leaving the device.

If the blower is located in another room or outside, you should hear very little air flow noise.
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W9IQ
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« Reply #39 on: April 15, 2017, 07:41:50 PM »

The "noise" of air occurs when there is an impedance change. Reduce impedance changes, and you reduce the noise.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KM1H
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« Reply #40 on: April 23, 2017, 12:43:52 PM »

The "noise" of air occurs when there is an impedance change. Reduce impedance changes, and you reduce the noise.

- Glenn W9IQ

Pretty hard/impractical/expensive with restrictive tube sockets, anode fins, and air hoses in the way.

The Henry 8K was designed to be remotely located and the controller was in the shack. Anyone who has been in a facility with a 3CX3000 knows what the cooling sounds like.

Carl
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W9IQ
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« Reply #41 on: April 23, 2017, 09:55:57 PM »

As noise abatement had apparently not been a design priority on these products and given their engineered age, I would estimate the first 3 to 8 dB of reduction to be quite simple and inexpensive. After that, the real engineering begins.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N3QE
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« Reply #42 on: April 24, 2017, 03:53:45 AM »

As noise abatement had apparently not been a design priority on these products and given their engineered age, I would estimate the first 3 to 8 dB of reduction to be quite simple and inexpensive. After that, the real engineering begins.

They're over $1000, but the Bose aviation noise-canceling headsets are amazingly effective: https://www.amazon.com/Bose-Aviation-Headset-Bluetooth-Cable/dp/B010FTYIUS

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KC4ZGP
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« Reply #43 on: April 24, 2017, 05:30:12 AM »


Want less noise? Wear earplugs.

T.V. too loud? Move further away.

Glad to have helped.
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