Silence a Return grille

[HVACctrl]

One client has a mechanical room right next to the reception area of an office. There are three 5-ton AHUs raised about 3 feet up inside the raised double doors facing the reception area and a return grille (about 7'W x 2'H) underneath the double doors. The return makes a lot of noise and the client wants to make it quieter.

He considered building a "bench" to put in front of and/or over the grill (the built-in bench would have to have openings to allow air in). Outside the reception office, there is another, similar-sized return grille on a wall at a 90° angle.

Is there any type of attenuator that could be built into a bench or any other means to silence the return air some? I thought about finding a way to have more air suck in from the other return opening which would be outside the reception area or even cutting a third return opening or larger return opening at the outer return. Any other ideas?

Thanks,

Ed

 

[darkvibe]

Is it fan noise or air velocity noise that is causing the problem?

 

[HVACctrl]

I'm not sure. I would guess air velocity but perhaps fan noise as well. Anything in particular I should be listening for?

Ed

 

[CESSNA1]

HVACCTRL: If it is air noise you can reduce it by making a larger opening in the door to reduce the face velocity and perhaps add some sound insulating material. If you just add a bench and do not change the size of the opening you will probabaly have little effect on the noise. If it is mechanical (ie fan ) noise then attenuation with sound proofing is probably your best bet. Your client could also relocate the reception area away from the room.

What is the current face velocity on the opening?

Regards
Dave

 

[HVACctrl]

I don't know the face velocity, but I suppose I could find out. The client asked the question regarding the noise, but it is really not a part of the scope of the work we are doing- which is a pretty small job.

How do I tell if its air velocity noise or mechanical? Is there a difference in pitch? If there is a combination (which I would guess), would the silencer still be worth it?

Cutting a larger hole is not an option. The return is not in the door, but beneath the raised doors. It is in concrete wall and cutting the hole any bigger would mean cutting into the doors. We could look into opening the other return that is in the hallway and outside of the reception area. We could also look into dampering the return so more comes from the hallway grille and not the reception area.

I'm still not 100% sure how to tell if the noise is mechanical or air velocity in nature.

Thanks for the help.

Ed

 

[CESSNA1]

ED: Trying to describe noises verbally is going to be good. Noise in the intake/exhaust grilles is typially a "shhhhhhhhhhhh" or "whoosing" type sound. Mechanical noise can be a "booming" type noise or a "rumble" or a "banging" or "grating/grinding" type noise. As it is rotary equipment there may also be a "beat" with it. It may be you have both since the grille will allow the mechanical noises to pass. Actually some "slight" air noise is usually desirarble because it adds a soothing background noise that is pleasant to people.

Regards
Dave

 

[cme]

measure the grille and look at the nc level for a similar grille ...... nc 25 to 30 is what want

is there a volume damper behind the grille?

 

[HVACctrl]

If I get back out there, I'll try to get a better feel for the sound.

Could be a volume damper. I'd have to look into it. If I go back out there I'll check for that.

I've done NC calcs on VAV boxes in the past, but its been a while and I remember it being a painstaking process and pretty particular to the installation conditions, etc....

The biggest problem is that we were doing a small room load survey to determine air volumes required for each room. That part is pretty much done. We really don't have the time in the job to go much farther on the return air noise.

I just thought that if it was a quick and easy fix, I would recommend something to the owner and my boss wouldn't mind.

Ed

 

[willard3]

snip
I just thought that if it was a quick and easy fix, I would recommend something to the owner and my boss wouldn't mind.
snip

If there were a quick and easy fix, someone would have done it already.

If you want to control the noise, you are going to have to spend some Engineering time finding out what is causing the noise and spend further Engineering time finding out how to attenuate it.

 

[berkshire]

HVACctrl (Mechanical)
Have you tried removing the grille and seeing what the sound level change is?
We had a job where the grille whistled, it was 50% open area. We changed the grill for one of 75% open area and the noise went away.
B.E.

 

[HVACctrl]

willard3,
Yeah, I gues you might be right. It wasn't a problem before because the space was configured differently and the noise wasn't near anyone. now its an issue. I was hoping I could find an easy fix.

berkshire,
Maybe I need to suggest this or look into it myself. Sounds like a great idea!

Thanks everyone!

Ed

 

[ChasBean1]

Sorry for the late reply, but there's something in your initial givens that leads me to believe that it's NOT an air velocity issue. You said there are three 5-ton AHUs. I'm guessing that the three total about 6,000 cfm which based on the return grill dimensions is a pretty low inlet velocity... I'd probably lean toward sound lining of the return duct and mixing chamber, and checking the fan bearings for damage and greasing them...

 

[HVACctrl]

ChasBean1,
Thanks for the ideas! I'll look at the units to see what the overall air volumes are. I'll alos try to get a more firm handle on the grille sizes.

There is really no return duct to speak of. There are just two holes cut in the walls of the room (and grilles added) and the air sucks right in through the grilles into the mechanical room. There is one grille on one wall (inside the reception area) and one on a wall 90 deg away that is right outside the door entering the reception area.

Thanks again. If any new info comes up, I'll post it.

Ed

http://www.engineerboards.com

 

[HVACctrl]

What kind of inlet velocity range is acceptable in general? How about for velocity noise issues?

Thanks,

Ed

http://www.engineerboards.com

 

[ChasBean1]

There's no one number but I'd try to keep it below about 1,000 fpm. The register manufacturer should have noise criteria ratings for different velocities.

 

[HVACctrl]

Good point on the mfg.
I think I swagged about 500-600 fpm out out of it. This includes both the 7' x 2', the 5' x 2' (on corner wall) and assuming 50% open area.

Thanks again!

Ed

http://www.engineerboards.com

 

[VEEKRISH]

3 5.0 TR Unit should be delivering around 6,000 CFM. If they are Chilled water Units, then the grille area of 14 sq ft (even accounting for 50 % free area) should leave you with a Extract Air Velocity of about 850 FPM - good enough and should not create a noise issue. Probably the noise is from the fan itself (Bearings ?). In which case increasing the Return Air size isn't going to help any. Check if the bearings are OK, balance the fan and if possible provide some acoustic insultaion within the AHU Room (use about 1 inch Fibreglass covered with Glass Fibre Cloth and Perforated aluminum sheet) to further dampen the noise. If the noise still persists, then you must look at Sound attenuators (with splitters).

 

[Jabba007]

Think about this too. If there is a direct line from the fan opening thru the grill, it'll be REAL tough to attenuate that fan noise without reconfiguring the ductwork.

I have had that bit me before. Elbows help with noise a lot. If you can see the fan opening thru the grille, or thru the 7'x2' opening with the grille removed... IMO you're screwed.

Jabba

 

[acmech13]

the noise you re hearing is most likely caused by the fact that there is no return air duct. You are saying the return grill on the wall is connected to your return bax the unit sits on. you must keep all returns open on every hvac system. your machines won't work right if they are starved of air flow.