From: John De Armond
Subject: Re: Restaurant Kitchen Vent Noise
Date: Sat, 04 Dec 1999 21:18:38 EST
Newsgroups: sci.engr.heat-vent-ac,alt.sci.physics.acoustics

Owning a restaurant or two, I thought I'd throw my 2.5 cents worth
(inflation) in.  My experience with exhaust fans is that they are
not very noisy unless a) they are out of balance from grease buildup
and/or b) they have bad bearings.  I high volume restaurant can
build up enough gunk on the fan to cause noise in a week or so. 
Very few independent restaurant owners/managers deal with this
problem until something breaks or the fan quits working.  Bearing
noise is common because the major brands of fans use ordinary ball
bearings to support the vertical fan shaft (belt drive).  These are
not designed to handle the thrust and quickly become noisy
preliminary to failure.  I've solved that problem with my fans by
installing Timken tapered bearings on my fans.  Five+ years and no
noise or failure.

Second comment.  I don't have my NFPA standard at hand at the moment
but in general, the fan is NOT required to be welded to the duct. 
The hood and duct is required to be welded or otherwise attached
together in an approved manner.  The fan typically just sits on the
grease gutter.  This allows the fan to easily be dismounted for
cleaning.  It is certainly possible to isolate the fan from the duct
if that becomes necessary.

In reply to the UK writer, the reason the code requires the duct to
be so secure is that the grease can build up an inch thick or more
in a poorly maintained duct and when that catches on fire and the
fire is driven by the high velocity air going to the fan, the heat
is intense enough to melt (not just distort) thin ductwork.  As an
ex-volunteer fireman, I've seen just how quickly a grease hood fire
can torch a building.  While generally opposed to mandatory building
codes, this is one area where I think the NFPA standard is a little
lax.  I have a foot clearance between my hood vent pipes and
anything flammable.  I stopped at a foot only because major
structural changes would have been required to go further.  The
ducts are insulated with high temperature insulation.

John