Abstract:
A spray booth for use within a building has a metal housing that defines a partially enclosed spray zone in which a worker operates spray equipment. The spraying operation places particulates and volatiles into the air of the spray zone. A main airflow producer draws a contaminated airflow from the spray zone through a large air intake port into an air intake plenum within the housing. A conventional particulate filter mounted in the air intake port removes droplets of paint or other coating compositions from the incoming airflow. The airflow is then directed through a filter in the air intake plenum that removes most, but not all, volatiles. Most of the fully-filtered airflow is discharged back into the spray zone but a small portion is forced by an auxiliary airflow producer to points external to the building. This causes an equal flow of fresh replacement air to be drawn into the spray zone from the interior of the building and ultimately from outdoors. Volatiles in the spray zone are reduced to acceptable levels while heating and cooling costs necessitated by replacement air flows are significantly reduced.

Description:
FIELD OF THE INVENTION 
       [0001]    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 
       [0002]    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, 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&#39;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 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. 
         [0003]    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&#39;s air conditioning system may cool incoming fresh 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 
       [0004]    In one aspect, the invention provides a method of controlling air 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 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 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. 
         [0005]    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 is appropriate for most applications. The energy savings derived from such operation can be very significant. 
         [0006]    Other aspects of the invention will be apparent from drawings and description relating to preferred embodiments of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention will be better understood with reference to drawings, in which: 
           [0008]      FIG. 1  is a schematic representation of a preferred embodiment of a spray booth embodying the invention; 
           [0009]      FIG. 2  is a front view of the spray booth; 
           [0010]      FIG. 3  is a top view of the spray booth; 
           [0011]      FIG. 4  is a side view of the spray booth; 
           [0012]      FIG. 5  is a perspective view of the spray booth; 
           [0013]      FIG. 6  is a perspective view of a filter assembly that removes VOCs from airflows; 
           [0014]      FIG. 7  is a side view of the VOC filter assembly; 
           [0015]      FIG. 8  is a view of the filter assembly from above; 
           [0016]      FIG. 9  is a cross-sectional view along lines  9 - 9  of  FIG. 7 ; and, 
           [0017]      FIG. 10  is a cross-sectional view in a central horizontal plane of the filtering assembly. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    Reference is made to  FIGS. 1-5  that show a spray booth  10  with a 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. 
         [0019]    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. 
         [0020]    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 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. 
         [0021]    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. 
         [0022]    It should be noted that the diverted minor airflow portion might be 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. 
         [0023]    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 . 
         [0024]    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  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 . 
         [0025]    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 passage  47 , against upward airflows that are not VOC-filtered. 
         [0026]    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 . 
         [0027]    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 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 . 
         [0028]    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 . 
         [0029]    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. 
         [0030]    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. 
         [0031]    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.