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Dust Collector - Loud!

Dust Collector - Loud!

Dust Collector - Loud!

(OP)
We have a dust collector installation (~10,000 cfm against 8" e.s.p.). The unit is ducted return back to the space, with a 5' silencer on the return.

The problem is that the system has a low frequency rumble that literally shakes the whole room, you can feel deep in your chest.

Anyone out there have any advice on retrofits that could be tried to reduce the low frequency rumble? The sound rating on the unit (w/silencer) is 77dBA, but A-scale weighing largely ignores low frequency sound.

RE: Dust Collector - Loud!

Pls contact me and I will inform you about the Spiral Fan, low noise at low frequency and very high efficiency

RE: Dust Collector - Loud!

If the noise is radiating from large flat surfaces of the collector or the duct, you could stiffen or blanket the surfaces.

If the noise is actually unsteady flow in the return duct, you could return it outside...

... or you could try erecting a "temporary" building around the silencer.  You are of course trying to turn the building itself into a larger silencer.  Instead of a discrete exit, it could have porous walls, e.g. faced with perf metal or screen on both sides and stuffed with f/g batting.

... or you could ask a silencer manufacturer for help.

Did it always rumble, or is this new behavior?

Mike Halloran
Pembroke Pines, FL, USA

RE: Dust Collector - Loud!

(OP)
Thanks Mike,

The noise is actually returned noise to the space. The collector itself is not very loud.

We've attempted to exhaust the return air outside, and this removes much of the low frequency sound, however we'd like to save this as a last resort as 10,000 cfm makes for quite a negative in the space.

RE: Dust Collector - Loud!

It is probable that the fan in the system is running in stall.  This would be the first check to make.  The stall frequency is either 2/3 or 1.5x running speed.

RE: Dust Collector - Loud!

(OP)
From the low frequency rumble I was thinking that the fan could be in stall.

I'd prefer not to slow the fan down, and lose duct conveying velocity, could anyone see a problem with speeding the fan up?

RE: Dust Collector - Loud!

If the fan is "in stall", _would_ speeding it up increase the flow?

I don't know; I'm asking.

I.e. is it like torque converter stall, or like axial flow compressor stall?

Mike Halloran
Pembroke Pines, FL, USA

RE: Dust Collector - Loud!

(OP)
Increasing speed would increase the flow, but it might take the fan out of the 'stall' region of the fan curve.

I can't slow the fan down very much without losing the velocity I need for dust conveying.

Right now my main duct has a velocity of 3815 fpm, minimum conveying velocity for sawdust is 3800 fpm, I thought we might try to spin the fan slower for 3000 fpm and see what the effects were, but would prefer to have more velocity.

RE: Dust Collector - Loud!

Changing the fan speed will not change the relative position of the operating point on the characteristic IE the fan will still be in stall.  Additionally it is possible that you could overstress the fan by speeding it up and likely overload the motor.

Your options are:
1. Reduce the system resistance.  If it is a pulse jet filter this may be possible by increasing the pulse frequency.  Perhaps you can reset the DP on the control unit.
2.  If the fan is fitted with variable inlet vanes try closing them slightly.  Although this in theory reduces flow it sometimes can increase flow if it takes the fan out of the stall region.  But could get rid of your pulsation problem.
3.  If the motor has some power margin consult the fan supplier to see if they can "tip" the impeller.  Alternatively they may be able to provide a higher pressure impeller.
4.  If none of the above can be achieved you will have to replace the fan with a fan that produces higher pressure so that the operating point lies to the right of peak pressure on the fan characteristic curve.

Try to verify that stall is the problem before making major changes (although it is certainly the most likely cause).  Try to get a copy of the fan curves so you can check the operating point.

Fred

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