High velocity ductwork? 1

[remp]

Hi,

I am refitting an old industrial packaging hall into an office.
The old packaging hall had a VAV system installaed over 20 years ago (pretty forward thinking for then. We have stripped out all the old VAV boxes because they did not work and the controls were not working. However I would love to salvage the ductwork.
It is all circular ductwork up to the VAV boxes and the calculated duct air velocity is approx. 12m/s. Is this too high for an office installation. After the VAV box the air flow rate is down to normal low velicity.
Will I have noise problems with the high velocity in the circular duct.
All the bend are clean sweeps and the take offs are at
45 deg.

 

[willard3]

Approx 2300 feet/min (for the metric challenged) is pretty fast and noisy unless the system is 20,000 cfm or above in a 48" dia duct.

 

[marcoh]

Is the system noisy currently? If not then noise (obviously) would not be an issue maintaining the higher velocities.

One option may be to reduce the supply air temperature (ie new coils would be required, maybe grilles as well) which would reduce the air flow.

 

[SylvestreW]

Here's something I wrote up a while ago for a project with similar details:

"Flow with velocities greater than 2000 feet per minute generates self-noise when the air stream becomes turbulent. This typically occurs in regions where the air is forced to turn (i.e. bends), where there are changes in duct cross-sections or branches & take-offs.
This type of noise is known as “regenerative flow noise” and is primarily a function of the air speed and duct diameter.

For a duct with a nominal cross-section area of four (4) sq.ft, and a velocity of 3000 fpm, the regenerative noise due to a 90o elbow is
Frequency (Hz) 63 125 250 500 1000 2000 4000 8000
Regen Noise(dB)86 74 67 60 53 <40 -- --

As shown, the noise is predominantly low frequency. In general, the noise levels can be expected to drop by approximately 8 dB for each reduction of 1000 fpm in velocity, and approximately 4 dB for each halving of the cross-sectional area of the duct.
The values presented are for a “worst-case” 90deg elbow. Typically one would expect regenerative noise due to changes in duct area, take-offs, smooth bends etc. to be less than shown above."

The above #s are based on ASHRAE calcs. Now, that may give you an idea of things, but there's a lot of other things to consider.

Other things to consider:
1) The noise from the AHU itself. The noise propagating down the duct can often be as/more significant.
2) Noise issues all depends on where the high velocity ducts traverse. If they traverse sensitive office spaces, then you'll likley not meet the NC criteria/recommendations. IF you have the ducts traverse the corridors/bathrooms/etc, then likely the NC criteria will be higher in those spaces and noise less of an issue.
3)There's 2 issues with duct noise, i)breakout noise and ii)end noise. You may have either/both issues to deal with.

I hope that helps. There's really no such thing as "too high", it just means additional mitigation will be necessary to adequately address the issue.

-
Syl.

 

[ChrisConley]

Very useful post on high velocity ductn noise Syl. Enjoy the star!