Need help reducing noise/vib from hvac unit

[royono]

Hello all,
New to the forum and not an engineer, but work with them! I hope it's ok to have joined the forum and to ask this question here. If not please let me know.
I have recently installed a package heat pump unit on the roof of my building. Of course I was aware that there would be a noise issue, just not sure how much.
The roof is constructed of 3.25" x 14" glu-lam beams on about 6' centers, sheathed with 4.5" thick S.I.P.S. panels (3.5" EPS foam core with .5" OSB board glued to either side)
I built level curbs directly above two of the beams rising about 6" off the roof deck. I spanned between these two curbs with two beams constructed of three pressure treated 2x6s glued and screwed together. The package unit sits directly on these wooden beams.
My layman's intention was to try to transfer the vibration more directly to the beams as opposed to the rigid panel roof deck.
The reality is that the noise level inside is unacceptable. Bad on start up and shut down, and a constant strumming while the compressor is running, not so bad when it's just the fan motor running.
I have been researching vibration dampening mounts but thought it best to get some professional opinions before redesigning the mounting.
Would it help to to install some isolators between the beams and roof? Unit and beams? Both? What type? Should I build the beams of a different material?
Thank you in advance for any help you may be able to provide. My apologies if this is not the appropriate place to ask these questions.
Jonathan

 

[Tmoose]

When the unit is running, can you feel vibration in the curb in your finger tip? Does the adjacent roof feel the same.
My general rule is - if a panel larger than a mouse pad has "feelable" levels of vibration at frequencies much over 30 Hz/1800 rpm it is likely broadcasting sound like a loud speaker.

I'm guessing isolation mounts with an inch or 2 static deflection will be needed to quiet the roof.

Then there is the question of vibration being transmitted via pipes, ducts or electrical connections, and if there are air handlers, noisy airflow.

ASHRAE Handbooks have lots of practical information about this tricky subject

 

[royono]

Tmoose,
Thank you for your reply.
Yes the vibration is noticeable, through your fingertips and the bottoms of your feet!
I have used flexible ductwork at the return and a flexible coupling in the supply. The electric is connected with plastic flexible conduit.
The unit weighs about 300 lbs.
Would there be any additional benefit to modifying the beams? Stiffer or more flexible? Heavier? I will need to change them anyway to gain the space necessary for the isolators. Wood or steel? Concrete? (Additional mass?)
Is there a preferred number of mounting points and locations? I have seen some discussion about things mounted from 3 points? Being a rectangular unit the four corners make the most sense to me, but if it helped I could change the mounting locations.
Thanks again!
Jonathan

 

[GregLocock]

re 4 mounting points vs 3. In theory you might be better off with three, but practicality is far more important. I'd aim at a suspension frequency of 10 hz or less.

Using isolators will give much better results than beefing the structure up, unless you use tons of concrete.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IRstuff]

A system is only as good as its weakness link. So, beefing up at one point might simply transfer the perturbation to a different point, which may be equally weak. Greg's approach is probably the more relevant, given that it's unlikely that you can retrofit for a concrete pad. Note, however, isolators have their own idiosyncrasies and pitfalls. Conventional vertical isolators will need to be specified for the appropriate frequency under load. Also, standard vertical isolators may produce excessive sway or rocking in the unit, due to mismatching of the isolators. Custom designed isolators that are focussed at the CG will minimize the rocking, but will require an engineer to properly design the isolation system.

Note also, excessive vibration may be indicative of manufacturing defects or even design flaws. You should find out whether this level of vibration is typical to this type of unit.


TTFN
faq731-376

 

[Tmoose]

http://www.mason-industries.com/masonind/_doc/pdf/ashlec.pdf

 

[Strong]

Here is a product that may be suitable:

http://0vib.com/products-page/seicon-hvac-isolator/

I have not used this product or sell it. Buyer beware and be sure to correctly size any vibration mounts! Do not use the simple formulas pr charts in the back of a vibration mont catalog. They are based on a rigid foundation, and your roof is not. It is best to have a consultant (like me) make sound & vibration measuremnts and recommend the correct modification.

Walt