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Controlled Environments - explained
When someone mentions
a “controlled environment” most people think of a cleanroom
or operating room. In actuality, there are 15 general
categories of controlled environments1 and many
subcategories under these.
One of the most
prevalent types of controlled environments, probably right
down the street from you at an automotive dealer, is the
downdraft paint spray booth. This device is a fairly recent
invention for the automotive industry and grew out of the
base-coat/clear-coat technology of automotive painting.
In prior years, cars
were painted with successive coats of enamel to get that
mirror finish. If any particles or debris were embedded in
the wet paint on the car and caused a raised bump or a
“fish-eye,” the paint technician just took some sandpaper,
removed it and repainted the spot. Then it was up to you to
apply car wax every 6 months or so to keep the paint
protected and looking new.
This all changed
with base-coat/clear-coat technology. Enamel is still
applied but a final coat of polyurethane is sprayed to
protect the finish. Unlike enamel, polyurethane cannot be
spot touched as easily as enamel and requires a complete
recoating of the surface of the spot (hood, fender, etc.).
With this process
came the need for higher levels of filtration along with
better technology than the cross-draft style paint spray
booth. Taking a cue from cleanroom technology, downdraft
paint spray booths have a set of prefilters to remove larger
particles and ceiling diffusion filters that move air in a
laminar flow from the ceiling down over the car and out
through a floor grid. The ceiling filter is usually a
complete filter mat firmly clamped in place that filters out
5 micrometer particles and larger (the thickness of the
polyurethane layer) and diffuses the air down through the
booth at approximately 100 feet per minute2. Most ceiling
filters are designed to operate approximately 2000 hours
before being changed.
The floor of the
booth is made up of a grid system that allows air to pass
down and through a set of filters that capture the paint
overspray. Overspray filters can be fiberglass, synthetic or
paper paint arrestor media. Although the amount of paint
overspray varies with different types of paints and spray
guns, most achieve only about 50% transfer efficiency; hence
there is a large amount of paint that is captured by the
overspray filters. In a high volume paint shop, these
filters can be changed as often as every two days.
Finally, most booths
are designed with a furnace to heat the air and rapidly dry
the painted surface, passing the air back through prefilters
and diffusion filters to maintain air that is free of
finish-damaging particles. The entire process can be
completed in about 1½ hours, allowing the booth to handle
some 5- 6 cars per day and maximize the operational
efficiency of the automobile body shop.
This is another
example of a highly technical process that changed an entire
industry. None of this would be possible without the
increased level of filtration provided by the air filtration
industry.
References:
1
NAFA Guide to Air Filtration, 3rd
Edition, Chapter 9, p. 9.10 - 9.12
2 NFPA 33
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