Silencers with Turning Vanes on Axial Fan - System Effect? 5

[FanAirCo]

I am application sales engineer for a Fan Manufacturer. We sold a Fiberglass Fan for a Sewer Lift Station. Basic exhaust system to remove H2S in Deep Well. Fan is a 20" Dia Tubeaxial (No Vanes on Fan).

Problem: Contractor purchased and installed a 2 Silencer, one Inlet and one Discharge located directly, within 3 In. of Fan Inlet and Discharge Flange. Fan war reported deficient on Flow after being tested by a Certified Air Balance Co. We did not furnish silencers or provide any info to the silencer manufacturer.

Each Silencer is has a set of internal turning vanes. See Picture.

I don't know anything of these silencers, first time I have ever encountered turning vanes in a silencer. I doubt the Silencer Manufacturer designed the Stationary Silencer Vane Pitch, Camber, Length, Quantity of Turning Vanes, with respect or consideration of "Tuning the Silencer Vanes” specific to this 20" FRP Fan.

Anyone encounter a similar situation? Anyone care to comment as to what sort of system effect would or could occur if these Silencer Vanes were not designed specific for this Fan?

 

[KiwiMace]

I generally agree that these look ridiculous and can't imagine what the designer was expecting or hoping to achieve. This looks like it will tear off down the duct in a week, with the material likely saturated with sewer gas as well.

What on earth is upstream that needs acoustic treatment?

As a vendor, you don't really need to be able to prove or estimate any level of system effect in the missing performance of the fan, rather, you can indicate poor practice and let the contractor sort it out.

 

[MechEngNCPE]

Silencers placed in the wrong location, even without the turning vanes can produce drastic static pressure losses. Designers should consult with vendors such as vibro-acoustics when specifying any type of silencer.

The inlet and outlet effects of these turning vanes into the fan probably caused tremendous static pressure losses.

 

[FanAirCo]

Thanks for taking a look guys.

The Silencers are not a national brand per-se and the Manufacturer's Data sheets do not show any Resistance vs. Flow Curve, just an estimated loss of 1/2" SP at 5000 CFM. We are attempting to get a Delta P across each of the 2 installed silencers. I was told the Silencer Internals are of 316SS and were accepted by the city and engineer.

These Fans are designed, per the equipment schedule 3,000 ACFM at 1.5" SP at Std. Density, .075 Lbs/Ft3. The Fan Manufacturer has tested, built and shipped many hundreds of this size and type with no flow deficiency reported.

I agree, during submittal process, they should let the Fan Manufacturer or Sales office know they intend to make changes to the system and add silencers with vanes right before and after the Fan so we can run an engineering inquiry first, but that never happens.

This installation is in 18" x 18" Rectangular FRP Duct with The Silencers Being Integral Square to Round with Turning Vanes. The Distance again from Silencers to Fan Flanges is only about 5 inches away. I pulled an AMCA Pub 203 Field Performance Measurements Page 131, see attached .PDF, shows the Flow is Not Uniform in an 18" x 18" Duct till at least 20.3" to be 100% Effective. Again, silencers at about 5" away...

Thanks again!

 

[KiwiMace]

Although not best practice, it is really common for the silencers to be ganged onto the fan like that, but not silencers with vanes. Usually just a round silencer or at most with an axial core.

 

[willard3]

My first question would be: "why do you need fan silencers in a sewer lift station?"
Machine noise level in a lift station is likely much higher than the fan noise. There will certainly be pre-rotation from the turning vanes on the fan inlet decreasing fan capacity.

 

[ChasBean1]

A couple of extra notes:

1. Assuming you have the design conditions (3K cfm and 1.5” static) without the silencers, the 3,000 cfm will drop to about 2,600 cfm by adding them (est. based on fan laws).

2. I don’t have anything to back this up (hunch only), but air going through these silencers will have a clockwise vortex. If the fan likewise spins clockwise there may be less resistance. If it spins counterclockwise it would seem there would be more loss on each side of the fan.

3. Consider removing the inlet silencer as noted by Kiwi. It looks like the silencer baffles can be removed on the outlet as well without too much difficulty if all else fails…

 

[FanAirCo]

Yes, Chas you are correct, the loss with these silencers causes a deficiency of about 400 CFM. 2600 CFM is right on.

Of the 2 Silencers, one has the correct vortex relative to the rotation of the Axial Impeller, the other does not.

Unfortunately the wrong silencer can not be turned 180, as the Silencers have one end Round 20" Dia and the other Square 18" x 18". The internals of the silencers are removable.

We are doing some additional testing and fact finding this week
and I will post results when known.

I agree with some of the other comments, silencers probably not needed or very effective in this application. Silencers were an after thought / change order for someone. It's a City Critical Noise Requirement. Thankfully the silencers were not part of our scope. I'm waiting to see if they test for noise with out the silencer internals.

 

[cry22]

Agree with most here.
1. you should get Pressure drop across each silencer from the balancing guy.
2. you should have a scheduled total fan static which should be compared against actual.
If you sold a fan to overcome X SP and the actual SP is Y, you can pinpoint the problem.

last but not least - As willard noted - Try to work out some common sense into these folks to get rid of these ridiculous silencers in such a space.

PS: the exhaust fan location should be at the end of the exhaust duct run, not in the middle of exhaust duct.