Silence a Return grille
Silence a Return grille
(OP)
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
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





RE: Silence a Return grille
RE: Silence a Return grille
Ed
RE: Silence a Return grille
What is the current face velocity on the opening?
Regards
Dave
RE: Silence a Return grille
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
RE: Silence a Return grille
Regards
Dave
RE: Silence a Return grille
is there a volume damper behind the grille?
RE: Silence a Return grille
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
RE: Silence a Return grille
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.
RE: Silence a Return grille
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.
RE: Silence a Return grille
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
RE: Silence a Return grille
RE: Silence a Return grille
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
www.engineerboards.com
RE: Silence a Return grille
Thanks,
Ed
www.engineerboards.com
RE: Silence a Return grille
RE: Silence a Return grille
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
www.engineerboards.com
RE: Silence a Return grille
RE: Silence a Return grille
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
RE: Silence a Return grille