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Reducing vibration and exhaust sound
2

Reducing vibration and exhaust sound

Reducing vibration and exhaust sound

(OP)
Dear All,

I am a newbie to the acoustics and ducting engineering.
I have a task given to solve the issue of vibration and sound generated by a kitchen exhaust duct.

The kitchen is in the ground floor and two exhaust hoods laid up-to 5th floor vertically of the building.
In the 5th floor there is an office adjacent to the duct and their complaint is about
1) Heavy noise generated by the blower of the duct
2) The vibration caused by it
3) The smell coming out of hoods ( thinking of increasing the height of the duct and having two exhaust vents )

I am searching for anti vibration pads to be fixed between the blower and the platform on which it is situated.

Could please throw some light how to proceed for solving this issues. Any specific criteria is there to search for the right AV pads. Are Vibration isolators and Anti vibration pads both are same ?

I browsed through some threads with key words "vibration isolators etc., I could not understand the intricacies.

Any basic steps in approaching to fix this vibration and noise generation in the ducts are greatly helpful.

Thanks,

RE: Reducing vibration and exhaust sound

Install a flexible duct above the fan to disconnect the fan vibration from the duct work.
Spray duct exterior with a visco-damping material to damp duct vibration.
Do you need to consider fire code requirements?

Ted

RE: Reducing vibration and exhaust sound

Where is the "blower" relative to the 5th floor occupants?
Regarding Item 2, " The vibration caused by it " . I foresee issues with how the blower is mounted, and how the ducting is mounted.

The noise control booklet published by the Swedish workers protection fund was adapted by B+K, apparently re-named NOSE control, and is available on their website.

https://www.bksv.com/media/doc/bn1299.pdf

It has lots of practical information about complications and resolution of sound and vibration issues with rotating machinery.

I personally would insist on knowing the frequency(s) of the troublesome sound and vibrations.

https://danratherjournalist.org/about-dan/controve...

RE: Reducing vibration and exhaust sound

Quote:

What's the frequency, Kenneth
Hilariously appropriate (the title, not the cirumstances).

btw, I tried to flag you (Tmoose) on this thread: thread407-454228: mounting plate leveling/slope



=====================================
(2B)+(2B)' ?

RE: Reducing vibration and exhaust sound

Hi EPete. Thanks for that MOST interestng "mounting plate leveling" link.

RE: Reducing vibration and exhaust sound

(OP)
Thank you all for the kind replies.

The flexible duct option we have explored some time back. But the quoted price what we got for flexible duct was way too high compared to fixed duct made of GI. Is it so or should we take more quotes.

The picture of the two blowers of capacities 5 HP (1400 rpm)and 7.5 HP (1420 rpm) with the mounting platform is attached with this post. The platform is situated is adjacent to the office window.

We are planning to construct civil platform replacing the MS one.
Could please let me know how to calculate the frequency(s) of the troublesome sound and vibrations.

Should we use combination of anti vibration pads and vibration isolators OR vibration pads are enough ?

Thanks,




RE: Reducing vibration and exhaust sound

"I personally would insist on knowing the frequency(s) of the troublesome sound and vibrations." - TMoose (that's me!)

" Could please let me know how to calculate the frequency(s) of the troublesome sound and vibrations. " - vij36 , the OP.

I don't know how to calculate the troublesome sound and vibrations.
As best I know they must be determined from sound and vibration measurements processed with a spectrum analyzer.

Something like this //might// work for the sound problem.
https://itunes.apple.com/us/app/spectrum-analyzer-...

Something like this //might?/ work for the vibration.
https://www.youtube.com/watch?v=A-AnOwhJpOE

Is the building air conditioned?
The building wall looks to be poured concrete.
If that window could be kept permanently closed and a couple of panes of heavy plate glass installed with an airspace between them it might reduce the noise and smell significantly with no other changes.
https://www.acousticalsurfaces.com/acoustic_window...

RE: Reducing vibration and exhaust sound

I can't imagine that this installation is in the USA, since there are so many things wrong!

Exhaust Fan Vibration Mounts:
https://www.bing.com/search?q=exhaust+fan+vibratio...

Exhaust Fan Silencers:
https://www.bing.com/images/search?q=exhaust%20fan...

A vibration mount provide isolation of vibrations transmitted from fan/ducts to support structure. Vibration mounts are typically fastened between fan base and support base and inserted into the duct supports. Most catalogs for vibration mounts offer static deflection values and simple sizing graph that is on very rigid support structure. The diamond plate on light structural frame is NOT rigid. Most small fans would have fabric expansion joint at fan inlet and discharge that help reduce vibrations into the ducts. There is none in the photo. Vibration short circuits are any rigid support (including rigid conduits) between vibrating fan or duct that connect to the wall and floor deck. I see several in the photo. The fan case and ducting appears to have thin thermal insulation, but not acoustical; there is a difference. There are many other issues, but the noisy office would be best controlled by moving fan to the roof or down to 1st floor level (2nd choice). Perhaps then the "customer" could enjoy a walk on the Balcony!

Walt

RE: Reducing vibration and exhaust sound

(OP)

Quote (tmoose)

If that window could be kept permanently closed and a couple of panes of heavy plate glass installed with an airspace between them it might reduce the noise and smell significantly with no other changes.

We will explore this option for reducing the noise. i have to ask if the users of the office are ok with this permanently closed window.

Quote (strong)

The fan case and ducting appears to have thin thermal insulation, but not acoustical; there is a difference
Could you point me to some standard acoustic insulation.

Quote (strong)

but the noisy office would be best controlled by moving fan to the roof or down to 1st floor level (2nd choice). Perhaps then the "customer" could enjoy a walk on the Balcony!
will propose this to the client and ..
Thank you all for the replies.

Meanwhile could please throw some light on the below "anti vibration cork slab". Will it be a good alternate to civil platform.

Link

RE: Reducing vibration and exhaust sound

There are a lot of acoustical lagging materials, configurations, and products with different cost, ease of installation, and availability:

https://www.bing.com/search?q=acoustical+lagging&a...

I would not purchase the cork sheets for this outdoor application, but if you had it and wanted to "try it" then good luck! There is no technical data about the cork.

Walt

RE: Reducing vibration and exhaust sound

(OP)
Dear All,
Yesterday I have met a vendor and he showed pics of vibration isolator saying that they are suitable.

Have attached with this post.

One question from my boss is that is the exhaust noise is due to blower or duct ?

How to know this ? Please guide.

Thanks,

RE: Reducing vibration and exhaust sound

Quote:

One question from my boss is that is the exhaust noise is due to blower or duct ?

How to know this
Knowing the frequencies of interest may help as Tmoose said. Something like blade pass frequency with harmonics would tend to point toward the fan. Something like broadband noise would tend to point toward the duct. In the end it may not be black and white but you'll know a lot more if you know the frequencies.

My advice to anyone who might want to determine the spectral content of noise - if you have an Android phone, get this free App (no ads, no fee, no invasive permissions other than obviously-required recording permissions). I am not affiliated with the app but am thoroughly impressed:

Spectroid
https://play.google.com/store/apps/details?id=org....


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(2B)+(2B)' ?

RE: Reducing vibration and exhaust sound

"Dear All,
Yesterday I have met a vendor and he showed pics of vibration isolator saying that they are suitable."

>>>>>> IF <<<<<<< the "vibration" is the result of the fan's running vibration is being passed from the fan frame to the support frame below and then into the building, then using isolation mounts between the fan frame and the support frame has something to offer. The isolators MUST be selected with at least slightly engineered stiffness. The weight of the fan is NOT ENOUGH.

If the "vibration" is the result of ay of the different possible types of fan's running vibration being passed via the duct into the building wall, then isolators under the fan frame will not likely help at all.

For my money there REALLY need to be some detailed vibration and sound measurements done to get this right without dozens of failed attempts.

RE: Reducing vibration and exhaust sound

How long since the grease filter and fan blades were last cleaned?

RE: Reducing vibration and exhaust sound

(OP)

Quote (electricpete)


Have installed the app and measured the noise near the blower motor and near the exhaust vent. Pic attached

Can please let me know how to analyze this.

Quote (btrueblood)

How long since the grease filter and fan blades were last cleaned?
Records indicate this was done a year back. What is the ideal frequency of this manintenance ?

RE: Reducing vibration and exhaust sound

Cleaning the grease filters? Daily if possible, weekly otherwise. Fan blades should be inspected for grease buildup at each filter cleaning interval, and cleaned if grease is visible.

RE: Reducing vibration and exhaust sound

Quote:

Have installed the app and measured the noise near the blower motor and near the exhaust vent. Pic attached
Thanks for posting that. We have on left hand side sound from near the blower and on the right hand side sound from near exhaust vent.

For the benefit of others, the top half is a spectrum (log scale frequency) and the bottom half is a representation of how the spectrum has evolved over time (bright colors are high magnitudes) – the horizontal axis is the same frequency axis as the spectrum above, the vertical axis is time (newest time on top). I call it a waterfall plot.

On the top half of the spectrum, the red curve represents “peak hold” spectrum – the highest magnitude at each frequency over the recording interval (or since last reset). The Yellow curve represents the “instantaneous” spectrum, or more precisely the latest single spectrum computed when the screenshot was taken.

Looking at the red curves, the 4 highest peaks are the same on the left as they are on the right:
173hz (labeled on left)
~400hz
~ 600hz
1184hz (labeled on the right)

We can see in the waterfall plot on the bottom that at least three of these frequencies are fairly persistent (bright yellow lines), all except for 400hz.

There is also a pattern of lighter bands below 173hz. Looks like maybe 70hz, 95hz, 115hz very rough guesses. Would have liked to get a better estimate of those frequencies.

I’m not sure what to make of it. 173hz and below is pretty low. If I had to guess the offensive noise is associated with the higher frequencies. You could bring a tone generator app on your phone and play the candidate frequencies to see how well they match the annoying sound.

What is the running speed of the motor? Is there a belt? If so, another question is what is the running speed of the fan. Is there a vfd involved? Do you have any idea how many blades the fan has?


=====================================
(2B)+(2B)' ?

RE: Reducing vibration and exhaust sound

(OP)

Quote (electricpete)

What is the running speed of the motor? Is there a belt? If so, another question is what is the running speed of the fan. Is there a vfd involved? Do you have any idea how many blades the fan has?

Running speed of both motors (5 HP and 7.5 HP ) 1440 RPM
There is no belt. Impeller connected directly to motor shaft
Motor speed and impeller speed both are same
Each motor has Single blade
There is no VFD

Quote (electricpete)

You could bring a tone generator app on your phone and play the candidate frequencies to see how well they match the annoying sound.

I will do a trial and update you.

Also one vendor suggesting Electrostatic precipitation for smoke and odor control.

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