Dave Finkelnburg on sun 25 nov 01
Hi all!
DISCLAIMER: I am not a Licensed Professional Engineer and am not acting
as one. If you want or need a stamped design, go to an experienced,
qualified professional engineer.
Rod at RedIron Studios in Vancouver, British Columbia, Canada, wondered
about how much air should move through his spray booth.
For good spray booth design the face velocity of the booth should be
between 100 and 500 feet per minute.
This is per my copy of "Industrial Ventilation," which is a manual of
recommended practice published by the American Conference of Governmental
Industrial Hygienists. I think you will find basically the same information
in the American Society of Heating, Refrigeration, and Air Conditioning
Engineers' book, ASHRAE Fundamentals.
The 100 to 500 FPM velocity is called "capture velocity" or "face
velocity." It's the velocity of air at the "face" of the spray booth. The
lower the velocity, the smaller the glaze particles the air stream is
capable of capturing.
In general, smaller hoods need higher face velocities.
You can calculate the amount of air flow you need to pull through your
hood. It's equal to the open area at the face of the hood times the air
velocity you want to move across that open area and into the hood.
Keep in mind that any fan you buy will have a rated capacity, in Cubic
Feet per Minute (CFM) here in the U.S. (liters/minute in the more civilized
world) and that capacity will be affected by how long the exhaust duct is
out of the booth and how dirty the filters are inside the booth. In fact,
as the filters plug the air flow will drop toward zero!
The best fan for a spray booth is something like a squirrel cage fan.
Axial flow fans (look like a light weight airplane or boat propeller)
are quiet but much less able to pull air as the filters plug off. With that
type of fan it's a good idea to oversize the fan, and keep the filters
clean.
Finally, the operator should ALWAYS wear a properly fitted, adequately
filtered respirator while spraying glaze, even in a spray booth. With the
air turbulence around you, there is always some overspray which the operator
is exposed to. The most harmful glaze particles are the smallest ones,
those less than one micron in diameter!
I hope some of this is helpful.
Dave Finkelnburg in Idaho watching birds shake the snow off the
lilac branches as they come to the feeder
WHC228@AOL.COM on mon 26 nov 01
Dave
The best of the spray booths that I have found are the kind with a water
filtration system. They do not get plugged. The problem with filters that are
sized small enough to take those sub-micronic particles out of the air is
that they plug up too fast. Water type spray booths take out the offending
particulate and do not plug up.
A good example of the kind that I mention can be found in both DeVillbus and
Binks spray equipment catalogues. There are detailed drawings and the
specifications on the types of blowers Etc. If you are clever you can use
that information to make your own spray booth. The engineering has been done
for you.
I have used a spray booth that is of that form for nearly twenty years and it
works great. I have been inspected by the Department of Environmental
Resources several times and passed their inspections as well as the OSHA
inspections.
The tubeaxiel fans that they use seem to be the best for moving a lot of air
and having the kind of suction necessary to do that kind of job.
Those kinds of fans can be gotten from the GRAINGER catalogue.
I have seen far too many booths that just dump the air outside of the
building, only to have the exhaust air come in another window. I ever know of
a situation where the air is exhausted from a pot shop directly at a
neighbors open window.
The sad thing about not watching out for this kind of hazard is that it could
be years before the damage is discovered. Lungs just can't be replaced.
Bill Campbell
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