Patent Publication Number: US-6663693-B2

Title: Duct cleaning for powder spray system

Description:
RELATED APPLICATION 
     This application claims the benefit of co-pending U.S. provisional patent application serial No. 60/327,260 filed on Oct. 5, 2001 for DUCT CLEANING WITH PULSE VALVE, the entire disclosure of which is fully incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates generally to apparatus and methods for removing powder from powder conveying elements in a powder coating spray system. More particularly, the invention relates to methods and apparatus for removing powder that has adhered to interior surfaces or collected within interior regions of a powder transfer duct by incorporating a pulse valve or other device for applying a high volume high pressure discharge of air into the duct to dislodge the powder. The invention may conveniently be used, for example, in quick color change systems. 
     BACKGROUND OF THE INVENTION 
     Powder coating spray systems for applying a powder coating to objects typically include a partially enclosed powder spray booth and one or more powder spray guns. Various spraying technologies have been developed including electrostatic and non-electrostatic spraying. Regardless of the spraying technology used, a substantial quantity of powder overspray is usually generated because not all of the sprayed powder adheres to the target surface. This overspray powder is then recovered typically using some form of a filtering system because the overspray powder tends to be airborne or easily entrained into an air stream. A common powder recovery system utilizes one or more cyclone separators to filter powder entrained in an air flow. 
     Overspray powder is entrained in an air flow produced by operation of the cyclone separator, with the entrained powder being transferred from the spray booth to the cyclone separator via a powder transfer duct. Because the powder is traveling at a fairly high speed through the duct, it tends to adhere to portions of the interior surface of the duct due to impact. Eddy current effects may also cause powder to collect in corners or other interior regions of the duct. This trapped powder must be cleaned out of the duct as part of a color change operation. 
     Compressed air injectors are known to be used within an air flow conduit to assist in the entrainment of powder in the air flow stream, such as is shown in U.S. Pat. No. 5,906,676 issued to Drummond. The injector however directs pressurized air generally parallel to the conduit walls in order to augment air flow, not for dislodging trapped powder within the duct. 
     Typically, to clean the duct for color change, a cover or access door to the duct must be opened and an air wand or other manual air spray device is used to blow the powder off the interior duct surfaces. These manual cleaning operations are unsuitable for automatic or quick color change applications. 
     The need exists therefore to provide method and apparatus for removing powder in a duct without requiring manual spraying operations to improve speed for a quick color change powder spraying system. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a pressurized volume of air is directed at an interior surface of a powder transfer duct to dislodge powder particles adhering thereto. In one embodiment of the invention, the duct is a passageway for powder that is entrained in a primary air stream from a powder spray booth to a powder collection device such as, for example, a cyclone separator. In this embodiment, the pressurized air is provided by a device that can be selectively activated to direct a blast of high pressure air into the duct interior. In one specific embodiment, the device is realized in the form of a pulse valve that has an outlet that opens to the duct interior and directs a pulse of pressurized air at a surface or region within the duct where powder tends to accumulate, either from eddy current effects or from impacting the interior surfaces. The pulse valve may be mounted, for example, to a wall of the duct. Pressurized air is supplied to the pulse valve from a source such as an accumulator connected thereto. The pulse valve may be manually actuated or actuated by other suitable control mechanisms. In accordance with another aspect of the invention, the device is activated while air is flowing through the duct, such as when the cyclone separator is operating. In this circumstance, the dislodged powder becomes entrained in the primary air stream and is discharged to the cyclone separator. 
     In accordance with the invention, a method for cleaning a powder transfer duct is also provided. In one embodiment of the invention, a pressurized volume of air is injected into the duct at a location so as to direct the pressurized air at an interior surface or region of the duct to dislodge powder that is trapped in the duct. By blowing off the powder while the duct has a primary air stream flowing therethrough, the dislodged powder is removed from the duct. 
     These and other aspects and advantages of the present invention will be readily appreciated and understood from the following detailed description of the invention in view of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in partial cross-section an elevation in schematic form of a powder application system that incorporates one embodiment of the present invention; and 
     FIG. 2 is a simplified longitudinal view into a air duct taken along the line  2 — 2  in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, the present invention is illustrated in a schematic manner incorporated into a powder coating application system generally designated with the numeral  10 . Although the present invention is described herein with respect to specific elements of a typical powder coating spray system, such description is intended to be exemplary in nature and should not be construed in a limiting sense. Those of ordinary skill in the art will readily understand and appreciate that the present invention may be incorporated into many different types of powder spray systems, including but not limited to electrostatic and non-electrostatic systems. The invention may be used to reduce the time it takes to effect a powder color change, hence the invention finds particular application in quick change powder application systems, however, the invention is not limited to use in quick color change systems. For example, the invention may be incorporated into any powder transfer duct that is difficult to reach for manual powder blow off, to name just one example. The invention finds particular application in combination with an air duct that interconnects a powder spray booth and a cyclone separator, but may also be used in any powder transfer duct in which dislodging or removing trapped powder material is a concern. 
     Referring to the drawings then, the powder coating application system  10  typically includes a powder spray booth  12 , a powder collection or recovery system  14  and a powder transfer duct  16  that interconnects the booth  12  and the collection system  14 . In this example, the powder collection system  14  is realized in the form of a cyclone separator  18 . An exhaust duct  20  connects the cyclone  18  to a blower (not shown) which produces a high flow volume of air from the booth  12  through the duct  16 . An inlet portion  16   a  of the duct  16  is in fluid communication with an inlet opening  22  to the cyclone  18 . A by-pass portion  16   b  of the duct  16  may be provided for use if required when the cyclone  18  is not being used. 
     The primary air stream PA produced by the cyclone  18  blower draws powder entrained air from the booth  12 , such as overspray powder produced during a spraying operation. The powder laden air exits the spray booth  12  into the duct  16  and enters into the cyclone inlet  22  through the duct inlet portion  16   a . The powder separates from the air in a known manner within the cyclone  18 . The separated powder falls to a hopper  24  where it may be collected or otherwise disposed of, and the exhaust air passes out through the exhaust duct  20 . 
     Due to the fairly high velocity of the powder particles within the primary air stream traveling through the duct  16 , powder may collect in the duct  16  either due to impacting an interior surface of the duct  16  or settling in a low air flow region within the duct  16  such as can occur due to eddy current effects. These trapped powder particles must be cleaned out of the duct  16  as part of a color change operation. In accordance with one aspect of the invention, a source of pressurized air is activated that directs a volume or blast of pressurized air into the duct  16  interior to dislodge the powder that is trapped in the duct. By supplying this pressurized air concurrent with operation of the cyclone  18 , the dislodged powder becomes entrained in the primary air stream and will pass into the cyclone  18 . 
     In the illustrated embodiment, the pressurized volume of air is blast or pulse injected into the duct  16  by, for example, a quick pressure release control device such as a pulse valve  30 . The control device or pulse valve  30  receives pressurized air from an accumulator  32  that is filled with shop air  34  or other source of pressurized air. The accumulator  32  stores a volume of pressurized air until such time that the valve  30  is actuated to clean the duct  16 . A control valve  34  may be used to refill the accumulator  32  after the pulse valve is closed following discharge of the accumulator  32  by actuation of the pulse valve  30 . Alternatively, the pulse valve  30  may be directly connected to shop air or other source of pressurized air. A suitable pulse valve, for example, is part RCA-25PX, available commercially from Goyen valve. 
     A control mechanism  36  is used to actuate the pulse valve  30 . In one embodiment, the control mechanism is realized in the form of a manually operated valve handle. Alternatively, the pulse valve  30  may be actuated by other known mechanism such as an electric or pneumatic valve actuator for example. Multiple actuations may be used as required. 
     In accordance with another aspect of the invention, the pulse valve is mounted to a wall  16   c  of the duct  16  (see particularly FIG.  2 ). The location of the pulse valve  30  may be selected so as to direct the pressurized air blast at one or more of the interior surfaces IS of the duct  16  on which powder tends to accumulate or interior regions of the duct  16  where powder collects. In the exemplary embodiment, the valve  30  is mounted on a wall such that the pressurized air is released in a direction that is generally transverse the longitudinal axis of the duct  16   c  and hence transverse the direction of primary air flow through the duct  16  (in FIG. 2 the primary air flow direction would be “into” the plane of the drawing). Other directional orientations and mountings of the valve  30  may be used as required to most effectively dislodge trapped powder from surfaces and regions within the duct  16  for a specific situation. More than one pulse valve  30  may be used depending on the number of entrapment areas that are present within the duct work  16 . For example, a second pulse valve (not shown) may be installed on the duct  16  or for example on the by-pass duct portion  16   b.    
     When the pulse valve  30  is actuated, a preferably high volume high pressure blast of air dislodges powder trapped within the duct  16 . The accumulator may be discharged by a series of high pressure shorter duration blasts or a single somewhat longer blast. Multiple discharges of the accumulator may also be used during a cleaning operation. The dislodged powder becomes entrained in the primary air stream generated by the cyclone  18  and transfers to the cyclone  18  or through the by-pass duct  16   b . Although the pulse valve  30  is preferably actuated during operation of the cyclone  18 , this may not be required in all instances as the air blast may simply dislodge the powder and relocate it to an interior volume where it becomes entrained in the overall air stream to the cyclone  18  after the cyclone  18  is started. 
     Although the preferred embodiment of the pressure release control device is realized in the form of a pulse valve  30 , such description is intended to be exemplary in nature. Other devices and techniques may be used to provide a blast of pressurized air volume into the duct interior to dislodge powder trapped therein, although the invention excludes the use of a manually operated air wand through an access door. The invention is directed to the concept of providing the pressurized air without the need to open the duct  16  for manual cleaning. The invention thus also contemplates the method embodied in the use of the pulse valve to dislodge powder trapped within the duct  16 , as well as a method of injecting a supply or blast of a pressurized volume of air into the duct  16  that is directed at an interior surface or region with the duct  16  whereat powder accumulates or adheres to the duct interior surfaces. The pressurized air is injected without opening the duct  16 , and may also be performed contemporaneously with operation of the cyclone separator to immediately exhaust the dislodged powder to the cyclone  18 . Multiple actuations of the pulse valve may also be used if required. 
     In an exemplary embodiment, a two gallon accumulator may be used to store air at a static pressure of about 90 psi (shop air pressure). A cyclone separator may produce a primary air flow in the transfer duct  16  of about 2500 ft/min. The valve may have a ¾″ outlet orifice. Each air blast can be fairly short in duration, such as for example, about a half second or so, or can be longer or shorter depending on the specific cleaning needs. These values are intended to be exemplary in nature and should not be construed in a limiting sense. Values may differ for different spraying systems as required. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.