Note: Descriptions are shown in the official language in which they were submitted.
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SPRAY BOOTH AND METHOD OF OPERATION
FIELD OF THE INVENTION
The invention relates generally to spray booths operated within a
building and providing a partially enclosed spray zone in which a worker can
spray
paint articles. For purposes of this specification, the term "paint" should be
understood as any liquid composition adapted to dry on an article to provide a
protective or decorative coating, including conventional paints and lacquers,
and the
process of "spray painting" involves spraying such varied compositions onto
article
surfaces.
DESCRIPTION OF THE PRIOR ART
Spray booths designed for use within a building are well known. In a
typical configuration, the spray booth has a metal housing that defines a
partially
enclosed spray zone in which a worker operates spray equipment. The spray
equipment commonly puts particulates, primarily small paint droplets, and
volatiles,
1 5 typically volatile organic compounds (VOCs), into the air within the
spray zone.
Such contaminants must be removed from the spray zone to avoid accumulation of
potentially explosive concentrations and to avoid deleteriously affecting a
worker's
health. To that end, an airflow producer is commonly used to draw contaminated
airflows from the spray zone through an air intake port into a plenum within
the
housing. The air intake port is commonly a large rectangular opening formed in
a
vertical housing wall that faces the spray zone. A particulate filter,
commonly
consisting of a metal framework that retains large rectangular filter pads, is
mounted
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to the air intake port to remove droplets of paint from incoming air flows.
The
filtered airflow is then directed to a discharge port coupled by ventilating
conduits,
separate and distinct from spray booth itself, to points external to the
building. In this
process, fresh air is drawn from the interior of the building to replace the
contaminated air removed from the spray zone.
Fresh air is ultimately drawn from outside the building to replace
contaminated air discharged to the outdoors. Equipment may be provided to
enhance
inflow of replacement air from outside and may heat incoming airflows during
winter
months. In summer, the building's air conditioning system may cool incoming
fresh
1 0 air. Since the entire volume of a building may be replaced several
times daily, the
attendant cost of heating or cooling fresh incoming airflows can be
formidable, and
such inefficient operation has been accepted for decades.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a method of controlling air
1 5 quality associated with a spray booth operated within a building. As in
the prior art,
the spray booth comprises a housing that defines a partially enclosed spray
zone in
which a worker operates spray equipment. As in the prior art, the spray
equipment
puts particulates and volatiles from paint compositions into the air of the
spray zone.
A flow of air is drawn from the spray zone and directed along a predetermined
path
20 extending from an air intake port to a return air port that discharges
air into the spray
zone. A filter assembly removes particulates from the airflow, and the
particulate¨
filtered airflow is thereafter passed through a filter assembly adapted to
remove
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volatiles. A minor portion of the airflow downstream from the particulate
filter
assembly is discharged from the housing to points external to the building. A
major
portion of the airflow downstream of the volatiles filter assembly is
discharged
through the return air port back into the spray zone.
A practical inexpensive filter for removing volatiles from spray booth
airflows will typically be unable to remove all volatiles. The invention
consequently
requires a minor portion of the airflow drawn from the spray zone to be
expelled from
the building. This causes fresh air to be drawn from the interior of the
building into
the spray zone in direct proportion to the extent of the minor airflow,
effectively
diluting the concentration of volatiles in the spray zone to acceptable
levels. For
purposes of this specification a "minor airflow portion" should be understood
as less
than 50% of the airflow drawn from the spray zone. A "major airflow portion"
should be understood as more than 50% of the airflow drawn from the spray
zone.
Observing such limits, the replacement air drawn from interior of the building
and
ultimately drawn from outside the building is effectively reduced by at least
50%
from prior practices. This reduces the cost for heating and cooling air drawn
from
outside the building to very roughly half of that experienced with the prior
art
practices described above. How much air must be discharged from a spray
operation
to avoid exceeding a lower explosive limit or to reduce the concentration of
toxic
volatiles can be determined separately for each spray composition used.
However,
the inventor has noted that choosing the minor airflow to be about 10% of the
total
incoming airflow and the major airflow to be about 90% of the total incoming
airflow
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is appropriate for most applications. The energy savings derived from such
operation
can be very significant.
Other aspects of the invention will be apparent from drawings and
description relating to preferred embodiments of the invention.
DESCRIPTION OF THE DRAWINGS
The invention will be better understood with reference to drawings, in
which:
Fig. 1 is a schematic representation of a preferred embodiment of a
spray booth embodying the invention;
Fig. 2 is a front view of the spray booth;
Fig. 3 is a top view of the spray booth;
Fig. 4 is a side view of the spray booth;
.Fig. 5 is a perspective view of the spray booth;
Fig. 6 is a perspective view of a filter assembly that removes VOCs
from airflows;
Fig. 7 is a side view of the VOC filter assembly;
Fig. 8 is a view of the filter assembly from above;
Fig. 9 is a cross-sectional view along lines 9-9 of fig. 7; and,
Fig. 10 is a cross-sectional view in a central horizontal plane of the
filtering assembly.
DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is made to figs. 1-5 that show a spray booth 10 with a
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housing 12 constructed largely of sheet metal. The housing 12 defines a
partially
enclosed spray zone 14 closed at its sides, top, bottom and back but having an
open
forward face to allow access by a worker 16 (shown in fig. 1). A portable
spray gun
18 (shown in fig. 1) may be used in the spray zone 14 to paint articles (not
shown).
Partial enclosure of the spray zone 14 is sufficient to contain air
contaminated with
particulates and volatiles for removal with airflows. Lighting fixtures 19
(apparent in
fig. 1,3,5) are mounted to the housing 12 to illuminate the spray zone 14.
Installation and use of such lighting in spray booths is entirely conventional
and will
not be discussed further.
As most apparent in fig. 1, the housing 12 defines an airflow path
(defined largely by air plenums and ports) that extends from an air intake
port 20 to a
return air port 22. Airflows associated with the housing are indicated with
arrows in
fig. 1. A main airflow producer (fan unit) 24 draws an airflow from the spray
zone
14 through the air intake port 20 into an air intake plenum 26 formed in a
lower
portion of the housing 12. The air intake port 20 is a large rectangular
opening
formed, as in the prior art, in a vertical housing wall defining a rear
surface of the
spray zone 14. A filter assembly 28 is mounted in the air intake port 20 to
remove
particulates from incoming airflows. The particulate filter assembly 28 is a
conventional paint arrestor grid with replaceable rectangular sheets of
filtering
material mounted in a metal framework, as known in prior art spray booths.
The airflow is drawn from the air intake plenum 26 through a filter
assembly 30 adapted to remove VOCs and into an intermediate plenum 32 located
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physically above and sequentially downstream of the air intake plenum 26. The
VOC
filter assembly 30 is located downstream of the particulate filter assembly 28
so that
paint particulates do not accumulate to a significant degree in the VOC filter
assembly 30 and impair its operation. The main airflow producer 24 is
operatively
mounted within the intermediate plenum 32 and propels the airflow through a
return
air duct 33 toward a return air plenum 34 immediately above the spray zone 14.
A
secondary airflow producer (fan unit) 36 is mounted on the return air duct 33
and
coupled to the duct 33 via a discharge port 37 (apparent in figs. 3 and 5
where the
airflow producer 36 has been removed). The secondary airflow producer 36
diverts a
minor portion of the airflow along the flow path to a discharge duct 38
leading to
points external to the building in which the spray booth 10 is operated.
In a typical application, the main airflow producer 24 might be
selected to draw 7000 cubic feet of air per minute through the air intake port
20. The
secondary airflow producer 36 might be selected to divert a minor portion of
that
airflow, about 700 cubic feet per minute, for discharge from the building.
Thus a
major portion of the airflow, 6300 cubic feet per minute, is directed to the
return air
port 22 and back into the spray zone 14. Discharging the minor airflow portion
to the
outdoors effectively causes 700 cubic feet per minute of airflow to be drawn
from the
interior of the building, and ultimately from outdoors, to introduce fresh air
into the
spray zone 14. Since the major portion of the airflow is discharged back into
the
spray zone 14, heating and cooling costs are very significantly reduced.
It should be noted that the diverted minor airflow portion might be
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drawn from the air intake plenum 26 or any other point in the flow path
downstream
of the particulate filter assembly 28. The arrangement illustrated is
preferred since it
reduces discharge of potentially toxic volatiles from the building and makes
location
of a discharge outlet less critical. It is not required, however, to achieve
the energy
savings associated with the invention. The minor diverted airflow might also
be
achieved by providing an appropriate physical branch in the flow path
downstream of
the particulate filter assembly 28 and leading to the discharge duct 38.
However, use
of the secondary airflow producer 36 to divert a very specific volume of the
main
airflow per minute produces far more reliable results.
The filter assembly 28 tends to release particulate filtering material
into the airflow. The large cross-sectional dimensions of the return air
plenum 34,
which extends over much of the ceiling structure of the spray zone 14, slow
the major
airflow portion before discharge through the return air port 22. The return
air port 22
is a rectangular opening with cross-sectional dimensions corresponding to
those of
the return air plenum 34. The slowed airflow portion is passed through a
particulate
filter 40 mounted in the return air port 22. The filter assembly 40 is
preferably a
conventional particulate arrestor grid similar to that mounted in the air
intake port 20
but oriented horizontal. Diffusion and slowing of airflow in the return air
plenum 34
facilitates trapping of entrained filter particulates at the filter assembly
40 before
discharge of air back to the spray zone 14.
Details of the construction of the VOC filter assembly 30 are apparent
in figs. 6-10. The VOC filter assembly 30 includes a generally cylindrical
structure
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42 roughly 18 inches in diameter and 24 inches in length. The structure 42
includes a
pair of generally cylindrical wire mesh screens 44, 46 mounted concentrically
about
the central lengthwise axis of the cylindrical structure 42 and defining a
central,
lengthwise flow passage 47 with a diameter of about 11 inches. The inner and
outer
screens 44, 46 are dimensioned and spaced to define a generally cylindrical
cavity 48
(indicated in fig. 9) with a radial depth of about 1 inch. The cavity 48 is
open at an
upper end of the cylindrical structure 48 to receive pellets 50 of filtering
material
(indicated in fig. 10). The pellets 50 comprise conventional activated carbon
but any
pelletized gas phase removal media known or yet to be developed might be
substituted. The cylindrical structure 42 includes a high efficiency
particulate air
(HEPA) filter 52 formed as a corrugated cylindrical sleeve and located about
the
outer mesh screen 42. The filter assembly 28 at the air intake port 20 removes
large
particulates entrained with incoming airflows but the HEPA filter 52 ensures
that fine
paint droplets are removed to avoid contaminating the activated-carbon pellets
50.
The lower end of the cylindrical structure 42 is closed with a lower cap
54. The lower cap 54 is essentially a circular disk with an upwardly directed
circumferential flange 56 that extends around the periphery of the cap 54. The
flange
56 assists in centering the lower end of the cylindrical structure 42 relative
to the
lower cap 54, and during assembly, the lower ends of the two mesh screens 44,
46
and the HEPA filter 52 are glued to the upper face of the cap 54. This
arrangement
closes the lower end of the cavity 48 against loss of activated-carbon pellets
50 and
also closes the lower end of the cylindrical structure 42, particularly its
central flow
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passage 47, against upward airflows that are not VOC¨filtered.
An annular upper cap 58 is seated on the upper end of the cylindrical
structure 42. The cap 58 closes the upper end of the cylindrical structure 42
but has a
central circular opening 60, with a diameter of roughly 11 inches, that
registers with
the vertical flow passage 47 to allow upward discharge of filtered airflows.
The
upper cap 58 also has a circumferential flange 62 that extends downward from
around
the periphery of the cap 58. The flange 62 is dimensioned to locate closely
about the
HEPA filter 52 to center the upper end of the cylindrical structure 42
relative to the
upper cap 58.
A vertical rod 64, aligned with the central lengthwise axis of the
cylindrical structure 42 and threaded at both ends, allows the caps 54, 58 to
be drawn
toward one another with threaded fasteners to grip the cylindrical structure
42 and
also to mount the VOC filter assembly 30 to the housing 12. The lower end of
the
rod 64 extends through a central vertical clearance hole in the lower cap 54
and
carries a lower nut 66 that can be threaded upward against the bottom face of
the
lower cap 54. The upper end of the rod 64 extends centrally through the
opening 60
of the upper cap 56. A U-shaped horizontal bracket 68 with a length of 15
inches is
located above and marginally spaced from the upper cap 58. The upper end of
the
rod 64 extends through a central clearance hole in the bracket 68 and carries
a nut 70
that can be threaded downward against the upper face of the bracket 68.
Rotating the
nuts 66, 70 effectively tightens the upper and lower caps against the ends of
the
cylindrical structure 42, securing the filter assembly 30 in its operative
orientation. In
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its operative orientation, the filter assembly 30 receives airflows radially
through its
HEPA filter 52 and activated¨carbon pellets 50 and discharges the filtered
flows
upward along its central passage 47 and through the central opening 60 of the
upper
cap 58.
How the VOC filter assembly 30 is installed in the housing 12 will be
most apparent from fig. 1 which provides a schematic cross-section in which
dimensions of the components of the VOC filter assembly 30 and surrounding
mounting structure are exaggerated and minor details of construction are
omitted. A
thin horizontal metal plate 72 separates the air intake plenum 26 and the
intermediate
plenum 32. The separator plate 72 has a circular opening 74 with an 11-inch
diameter
in which the VOC filter assembly 30 is installed. A worker can access the air
intake
plenum 26 by partially disassembling the particulate filter assembly 28 or
alternatively entering through a removable access panel (not illustrated)
mounted to
the housing 12. The worker can then orient the VOC filter assembly 30 as
apparent
in fig. 1 with the opening 60 of the upper cap 58 registered with the opening
74 of the
separator plate 72 and with the rod 64 extending vertically through the
opening 74.
Another worker can access the intermediate plenum 32 by removing access panels
76
to install the bracket 68 on the rod 64 and mount the upper nut 70 on the rod
64. The
nut 70 can then be rotated to draw the filter assembly 30 upward until the
upper cap
58 firmly engages the lower face of the separator plate 72. The caps 54, 58
are
simultaneously drawn tight about the upper and lower ends of the cylindrical
structure
42.
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Spent pellets 50 can be replaced periodically. To that end, the filter
assembly 30 is removed from the housing 12 by reversing the installation steps
described above. With the upper cap 58 removed, the filter assembly 30 is
inverted to
discharge the pellets 50 from the cavity 48. The filter assembly 30 can then
be
restored to its operative orientation, and fresh pellets can be poured into
the open
upper end of the cavity 48. The filter assembly 30 can then be mounted once
again to
the separator plate 72. In practice, the HEPA filter 52 is unlikely to require
replacement as often as spent activated-carbon pellets 50. If replacement is
required,
the filter assembly 30 may be removed as described above, and delivered to a
filter
supplier for replacement.
Only a single VOC filter assembly 30 has been shown. In practice,
with an intake air flow of roughly 7000 cubic feet of air, three such VOC
filter
assemblies would be appropriate. The separator plate 72 may be provided with
additional circular openings to accommodate the additional filter assemblies.
It will be appreciated that particular embodiments of the invention
have been described and that modifications may be made therein, beyond those
already suggested, without departing from the scope of claims.
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