Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to improvements in paint
spraying booths.
Paint spray booths are necessary to contain and
control the paint spray used in spraying individual
items. Usually the individual items may be suspended
from a conveyor which passes through the paint spray
booth allowing an operator to spray each item. A
considerable amount of spray is wasted as over-spray and
must be removed by the booth filtering system ~o that it
does not contaminate the atmosphere. There are varying
regulations concerning the rate of removal of the spray
and the degree of allowable contamination of the
atmosphere. These regulations differ from country to
country and from state to state.
Usually, one wall of the paint spray booth behind the
object to be sprayed in the direction of spraying
comprises impaction style filter panels or impingement
style paint arrestor panels. Exhaust means are used to
provide an air current through the paint spray booth and
through the filter panels or arrestor panels to tend to
exhaust paint spray from the booth via the panels. Any
spray not entrapped by the panels may be undesirably
vented to the outside atmosphere.
Various problems have been experienced in providing
efficient units and in maintaining the efficiency. The
question of maintaining the efficiency is particularly
troublesome due to the fact that paint spray tends to
build up on the filters and clog the filters. This
creates an unacceptable working atmosphere for the
workers.
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It has been found that generally optimum conditions
enforce dimensions of the paint spraying booth within very
narrow limits. Thus, it is found that the object to be
sprayed should be located roughly three feet in front of
the filter or arrestor wall and that any space behind the
wall should be at least three feet to allow for even
distribution of exhaust air.
It has also been found that paint impingement
arrestor panels are often to be preferred over impaction
style filter panels since paint arrestor panels
comprising spaced vertical foraminous sheets are not so
subject to clogging as are conventional filters. These
paint arrestor panels present surfaces which are coated
by over-spray and provide tortuous parts through which
any residual over-spray must travel. Thus, the over-
spray falls on various surfaces of the sheets allowing
maximum opportunity to coat the sheets. There is also an
opportunity for fall-out of paint spray. Such paint
arrestor panels are described in U.S. patent No. 2,583,489
to Meyer issued January 22nd 1952, U.S. patent No.
3,744,222 to Delao issued July 10th 1973, and U.S. patent
No. 4,321,064 to Vargo issued March 23rd 1982. These
paint arrestors are especially useful in that they stay
rigid even when wet and heavy with paint spray and hence
do not tend to sag out of place. They are also regarded
as at least as efficient as the more conventional filters
which are used which must necessarily be of coarse pore
size to mitigate clogging.
Conventional coarse impaction style filters have come
under scrutiny in various jusisdictions as to their
potential fire risks and additional problems have been
encourtered in providing filters or arrestor panels which
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comply , with various regulations for example, in Canada
it is necessary to comply with the Underwriters
Laboratories and the Underwriters Laboratories of Canada
Regulations andd with the regulations of the National Fire
Protection Association, Handbook 33, and, in Ontario, with
the Industrial Safety Branch regulations of the MInistry
of Labour.
The conventional coarse impaction style filters are
sometimes backed by finer filters. Thus U.S. patent No.
4,237,780 shows a coarse filter as a wall of a paint
spraying booth with a space behind it and then a fine
filter. In that case the -filters are made from wood
chips which are later used for fuel. Such arrangements
of wood chips are perhaps theoretically advantageous where
wood chips are in plentiful supply and can be used for
fuel, but for safety reasons may not be used at least in
Canada a convenient quickly interchangable unit locatable
in a wall framework and quickly interchangable when full
is normally desirable. Attempts have been made to
produce easily interchangeable units as for example in
U.S. patent No. 2,730,033 to Mellor issued January 10th
1956, U.S. No. 4,009,012 to ~Ieffler issued February 22nd
1977, U.S. No. 4,113,454 to Cvacho issued September 12th
1978, and U.S. No. 4,493,718 to Schweizer issued January
15th 1985. Of these U.S. No. 2,730,033 discloses a
coarse cheese cloth filter which may be changed
frequently backed by a finer fiberglass filter which is
not subject to so many changes. The space between these
two filters is not a dead space for the fall-out of
particles to the ground since all particles passing
through the cheese cloth filter will fall onto the
fiberglass filter and the various of safety regulations in
regard to combustion must be considered at least in
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relation to some of these previously suggested filters.
U.S. paten-t No. 4,113,454 discloses first and second
vertical filters for the space therebetween but each of
these filters comprise a panel the full size of the wall
5 of the spray chamber. If the spray chamber is large,
they will present difficulty when replacement was needed.
U.S. patent No. 4,009,012 discloses a wall having plural
openings for the location of filter elements of the wall.
There is no particular discussion of a construction of
10 the filter elements but rather their fastenings into the
framework. U.S. patent No. 4,493,718 is concerned with
the desirability of making a conventional filter rigid by
supporting it on a wire grid. The filter is supported on
the grid by prongs projecting from the grid onto which
15 the filter secured. It may be possible that such a
filter may sag on the prongs when it is heavy and wet with
paint spray. If such sagging were extreme it might be
possible that unfiltered spray would pass beyond the
filter wall at least at the top edge thereof where
20 sagging has taken place.
An attempt has now been made to design filter units
to make up a wall of a paint spray booth, which filter
units are quickly and easily interchangable, and which
mitigate clogging by paint spray and sagging or otherwise
25 deforming under weight of filtered materials.
Thus according to the invention is provided a system
for mitigating over-spray during paint spraying, the
system comprising a wall in front of which objects are
paint sprayed, the wall having paint spray arrestor
30 panels interchangably located in a wall frame work, and
exhaust means to provide an air current from front to
rear through the arrestor panels;
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6885
a spray trap located behind each of at least some of
the arrestor panels, the spray trap comprising:
a filter spaced from the arrestor panel by a duct,
the cross section of which conforms to the periphery of
an arrestor panel, and which, at one end, has means for
attachment to the wall frame work and, at the other end,
is closed by the filter.
The distance of the filter from the arrestor panel
may be such as to allow some settling out of any
over-spray which still remains in the air current passing
through the arrestor panel.
It is desirable that the space behind the filter wall
of a spray booth be as small as possible to save space
resources for other purposes. However, in practice, it
has been found that a distance of at least three feet is
required to allow for settling out of any remaining over-
spray. When a secondary filter according to the invention
is provided, a suitable spacing of the secondary filter
from the arrestor panel may be about six inches and this
distance may be comprised within the three feet necessary
behind the arrestor panel.
An embodiment of the invention will now be described
with reference to the accompanying drawings in which:
Figure 1 shows an illustrative view of a spray booth
incorporating an embodiment of the present invention.
Figure 2 shows a section of a booth such as that of
Figure 1, in use;
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Figure 3 is an exploded view of a filter unit
embodying the invention; and
Figure 4 i8 a section of an arrestor panel.
A spray booth 10 includes a filter wall 12 including
arrestor panels 14. Behind the filter wall 12 is a rear
compartment 18. Objects 20 are conveyed in front of the
wall 12 by means of' a conveyor 22. Exhaust vent 16
contains an exhaust fan 24 and draws air through the wall
12 from an inlet 26 which is shown in the roof of the
spray booth but may, in fact, be anywhere suitable. The
front wall of the spray booth is not shown, and there
must, of course, be a door into the booth at any suitable
point.
Each filter unit 1~ of the wall 12 is shown in
exploded view in Figure,2'together with a fragment of the
wall framework. The unit 14 consists of an arrestor
panel 28 which may be of any suitable form but
conventionally comprises vertical sheets of board with
circular holes 30 therein. Suitably there are three
spaced layers of board 33 having holes 30 offset in
relation to holes 30 in the next adjacent board. Thus
the over-spray under the influence of the exhaust fan 24
tends to travel in the direction of the arrows shown in
Figure 4 tending to coat both rear and front interior
surfaces of the arrestor panel. The arrestor panels are
set in apertures of a wall frame work 34 of a size and
shape to accept them. They may be secured by clips or
may be of a sufficiently tight fit to stand freely in the
apertures.
Behind each arrestor panel a spray trap 36 may be
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secured to the wall frame 34. The spray trap 36 may
comprise a short length of duct or tube having similar
cross-section as the plane of the arrestor panel.
Suitably, a flange 38 extending outwardly from the
5 forward surface of the duct 36 may be used to connect the
duct to the frame work. The rear of the duct 36 may be
provided with an inwardly extending flange 40 which
serves to retain filter 42 which is of the same
peripheral dimensions and shape as the arrestor panel 28.
The filter 42 is suitably formed to mitigate any
tendency to sag in the filter material. This may be
achieved by utilizing a wire frame 44 to maintain the
shape of the filter 42. The wire frame 44 may suitably
comprise a peripheral frame with at least one
15 strengthening cross-wire in either of the vertical or
horizontal directions. The actual filter material may
comprise a front sheet of filter material and a rear
sheet of filter material with the wire frame 44 sandwiched
therebetween. The sheets of filter material may be
20 connected, for example, by sealers at the top and bottom
edges so that the wire frame 44 may hold them in position
agai.nst sagging. In particular the wire frame 44 holds
the top edges from sagging below the top edge of the frame
44. Thus a gasket of filler material may be formed
25 around the edges of the frame 44 especially when the side
edges of the sheets are also sealed about the grid. One
of these sheets is suitably polyester material and the
other sheet may be fiberglass material. However9 this is
a matter of choice.
In operation, an operator 50 using a spray gun 52 may
spray an object 20 being conveyed by the conveyor 22.
Over-spray will be conveyed by air current entering the
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spray booth l0 at inlet 26 and being directed through the
filter wall 12 under the influence of exhaust fan 24 in
exhaust vent 16. The over-spray will first pass through
the arrestor panels and will coat the front surface
thereof and interior surfaces with wet paint spray.
Since the holes 30 of the arrestor panels are of an
appreciable size, paint build-up on these panels can be
considerable without clogging. However, as these panels
are of board or other semi-rigid material, there is
little tendency to sagging or breaking to damage the
integrity of the paint arresting system.
When any over-spray which is not entrapped by the
arrestor panel reaches the space within the duct 36, in
front of the filter 42, there may be a tendency for wet
or dry paint particles to settle out in this space. This
will fall into the duct 36 and can later be removed. Any
still remaining, paint contaminant will then pass into
filter 42. The loading of spray into th;s filter 42 is
quite slight in comparison with the total amount of over-
spray initially produced. Even so, there may be somebuild-up in weight of entrapped spray in this filter.
Therefore, precautions are taken in the provision of the
wire grid to maintain the shape of filter 42. Moreover,
the edges of this filter 42 which are sealed or otherwise
connected about the wire grid 44 which may act to firmly
locate the filter 42 within the duct 36, flange 40
serving to support the filter 42 from the rear.
If there is any over-spray still remaining in the air
current, after it has passed through arrestor panel 28,
the space within duct 36 and filter 42, it may settle out
in the remaining rear compartment 18 of the spray boothO
~owever it has been found from tests made on a primary
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arrestor panel and a secondary filter trap according to
the invention, the efficiency may be about 75% in the
arrestor panel and more than 95% of the remaining spray.
Thus it may be calculated that when using a system
embodying the invention, at a very high average density
of over-spray generated while painting and calculated at
0.73 grains per cubic foot of exhaust air, the efficiency
factors of the filters would provide a stack emission
rate of less than 0.01 grains per cubic foot of exhaust
air.
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