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noisy exhaust system

noisy exhaust system

noisy exhaust system

(OP)
I have a restaurant exhaust system which includes 2 hoods running off one fan.
The noise is unbearable. Since day one there has been a loud noise and vibration.
I changed the fan wheel, and motor.
I tried several different belt tensions and rpm adjustments.

Sometimes the system will run nice and quiet for an hour or two and then back to the noise.

Anyone ever encounter a similiar experience?

RE: noisy exhaust system

There are two major causes for vibration in fan equipment. Aerodynamic vibration pertains to the airflow. Mechanical vibration pertains to rotating components, fasteners, and structural supports.
Since you have already changed the fan RPM with little success your problem probably has to be mechanical in nature.
It appears that you have eliminated, thru trial & error, the possibility of loose fasteners or loose unbalanced wheel as the culprit of the noise.
Therefore, you are left with the number one reason why fans generally make noise other than incorrect RPM.
Too frequently, fans are mounted on supports that have a natural vibration frequency near that of the fan.  At this frequency, the structure will tend to continue to vibrate once it has been set in motion.
Also, the duct attachment to the support structure (i.e. steel, block etc) may be loose.
In addition, the fan must be securely supported.  You do not indicate the type / location of your fan (i.e. roof upblast, sidewall exhaust, or inline duct fan.
If you can visually follow the duct routing than you are in a position to see where there is not sufficient isolation material between duct and any adjacent structural support, piping, conduit, etc.
If your situation prevents the visually inspection of the duct routing than the resolution can be time and cost consuming.  Access must be made to examine the duct routing from inlet to outlet.

RE: noisy exhaust system

(OP)
The entire duct is up on the exterior of the roof, it is supported around every 6 feet or so with an L channel
welded to the duct but not attached to the roof.
I'll have a look and see if I can move anything?

RE: noisy exhaust system

IF THE ROOF SUPPORTS ARE NOT SECURELY ATTACHED TO THE BUILDING--ITS A GOOD BET THAT THE DUCT EVENTUALLY MOVES/TRAVELS EVER SO SLIGHT THAT IT RESTS AGAINST THE ROOF PENETRATION.  EVERY SUPPORT DOES NOT REQUIRE ATTACHMENT TO ROOF--THESE ACT AS GUIDES.  HOWEVER, YOU DO NEED ANCHOR(S) LOCATED PROPERLY
DEPENDING ON THE TYPE AND LOCATION OF YOUR EXHAUST FAN, IT MAY BE NECESSARY TO INSTALL ISOLATION JOINTS AT FAN SUPPORT AND/OR FLEX JOINTS AT FAN INLET & OUTLET.

RE: noisy exhaust system

I don't know what an isolation joint is, I do know that flex joints made of a fabric material do not meet NFPA 96.  One of the problems is that the material will break down and leak oil.  More serious is that the fabric will burn in case of fire, allowing burning grease to drip onto the roof.  Enough burning grease,then the fire department has to fight a fire in motion.  

Is this a new fan or used fan?  If new, contact the mfg for help.  If used, and the seller will not help you are on your own.  The fan wheel may have had weights to balance the wheel.  Those weights mey be missing. The new wheel may need weights, again contact the mfg for advice.

Some fans use rubber mounts between the fan and curb. Are they in good shape, not rotted or missing.  Are any bolts tightened evenly?

If used, was the fan clean?  Grease will cause vibration.
Turn the wheel in both directions,  do you feel any roughnes, scraping?

Pull the access cover off, turn the fan on and look for vibration.  Sometimes the supports are bent or broken.  I have used custom made shims to stop vibrations.  Think door shims or folded cardboard.  Once I even used a spray can jammed betweem the square base and rounded tub.  

Tip the fan, is the top of the duct flat? any high spots? is the van solid or does it wiggle? Is the duct made of 16 gage steel or 18 gage stainless?

Have you asked the company who cleans your exhaust system for advice?

Good luck, intermittant vibrations can be diffucult to find.

RE: noisy exhaust system

Upon turning on the blower does the sound occur immediatly, or only once the fans are up to operating speed? Based on my experience in the restaurant business, vent-hoods are pretty much big pieces of sheet-metal. What I would do to try and diagnose the problem would be to start pounding away on the sheet-metal hood and ducting with a rubber mallet to try and duplicate the offending sound. I'd suspect your ventilation equipment is acting like an immense speaker and amplifying the vibrations from the motor itself.

RE: noisy exhaust system

Is the Vibration still a concern?
You indicated that the vibration noise occurs after the exhaust has run for an hour or two.
It may be possible that the motor is a little undersized for the exhaust unit.
Or the motor is the incorrect selection for the exhaust.
Reason being,  once you start your exhaust, the motor is 'stone cold'.  After running for sometime, the motor will heat up,  if undersized will heat up more than usual.
Once it has, the bearing housing will expand, allowing slight movement for the bearing inside of its housing.  With such a large fan blade attached to the shaft, the slightest movement inside of the housing will exaggerate the vibration, and noise.
If you find that once you switch your exhaust off, come back the next day, and the process is the same as explained above,  i would indicate the motor is heating too much.
Just a thought, and perhaps your answer.
let me know.

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