Abstract:
An external mix air assisted spray nozzle assembly having a nozzle body with pressurized liquid and air passages, an air cap having a central opening in coaxial relation to a liquid discharge orifice of the nozzle body for discharging an annular air stream in surrounding relation to liquid discharging from the discharge orifice, and the air cap has a pair of opposed thin walled cylindrical members projecting in opposed relation to each other for directing pressurized air to atomize and shape the discharging liquid spray pattern, while minimizing undesirable build up of solids on the terminal ends of the projecting cylindrical members.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/325,669, filed Apr. 19, 2010, which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to spray nozzle assemblies, and more particularly, to external mix air atomizing spray nozzle assemblies in which a discharging liquid flow stream is atomized and formed into the desired spray pattern by pressurized air externally of the liquid discharge orifice. 
       BACKGROUND OF THE INVENTION 
       [0003]    External mix air atomizing spray nozzles are known for their ability to generate fine liquid particle spray patterns and control liquid particle size and spray distribution by pressurized air, substantially independent of liquid flow rate. Such spray nozzle assemblies typically include a liquid spray tip through which the liquid flow stream is directed and an air cap mounted in surrounding relation to the liquid spray tip for directing pressurized air streams that interact with the liquid flow stream discharging from the spray tip to further break down the liquid into particles and to direct the particles into the desired spray pattern. Such air assisted spray nozzles commonly are used in industry for directing highly viscous coatings onto various products. 
         [0004]    By virtue of the turbulence that can be created as a result of the intermixing pressurized liquid and air streams discharging from the spray nozzle assembly, randomly directed fine liquid particles can contact and accumulated on externally exposed faces of the liquid spray tip and air cap, referred to as bearding, which can quickly impede the discharge of the liquid and air flow streams and prevent the necessary uniform application of the coating materials. In some cases, such accumulations can occur within very short periods of operation, necessitating frequent shut-down of the production line in order to clean the nozzle assemblies. Repeated interruption in the spray operation significantly affects efficiency of the processing system. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0005]    It has been determined that high pressure air streams discharging from the discharge orifices of such air caps along adjacent air cap surface areas create low pressure zones which tend to entrain fine particles and draw them into contact with the air cap in a manner that accelerates particle accumulation and bearding. Furthermore, it has been determined that higher pressurized air flow streams increase fine particle breakdown and accentuate bearding. 
         [0006]    It is an object of the present invention to provide an improved external mix air assisted spray nozzle assembly adapted for more efficiently spraying highly viscous materials. 
         [0007]    Another object is to provide an external mix air assisted spray nozzle assembly as characterized above which has a design that substantially reduces or eliminates undesirable build up of sprayed material on externally exposed faces of the liquid spray tip and air cap. 
         [0008]    Still another object is to provide an external mix air atomizing spray nozzle assembly having an air cap which substantially reduces low pressure zones about pressurized air discharge orifices of the air cap, and hence, further minimizes fine liquid particle build-up about the air discharge orifices. 
         [0009]    Yet another object is to provide an external mix air assisted spray nozzle assembly of the foregoing type which is operable at lower air pressures that further reduce the fine particle liquid generation and build-up on external surfaces of the air cap. 
         [0010]    A further object is to provide such a spray nozzle assembly that is relatively simple in construction and which lends itself to economical manufacture and usage. 
         [0011]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a longitudinal section of an illustrative external mix air assisted spray nozzle assembly in accordance with the invention, taken axially through the spray nozzle assembly; 
           [0013]      FIG. 2  is an enlarged fragmentary section of the spray tip and air cap of the illustrated spray nozzle assembly; 
           [0014]      FIG. 3  is an end view of the nozzle body of the illustrated spray nozzle assembly; 
           [0015]      FIG. 4  is a front perspective of the air cap of the illustrated spray nozzle; 
           [0016]      FIG. 5  is a front end view of the air cap shown in  FIG. 4 ; 
           [0017]      FIG. 6  is a side view of the air cap shown in  FIG. 4 ; 
           [0018]      FIG. 7  is a rear perspective of the air cap shown in  FIG. 4 ; 
           [0019]      FIG. 8  is a longitudinal section of an alternative embodiment of external mix air assisted spray nozzle assembly in accordance with the invention; and 
           [0020]      FIG. 9  is a front perspective of the air cap of the spray nozzle assembly shown in  FIG. 8 . 
       
    
    
       [0021]    While the invention is susceptible of various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Referring now more particularly to  FIG. 1  of the drawings, there is shown an illustrative external mix spray nozzle assembly  10  in accordance with the invention. The illustrated spray nozzle assembly  10  includes a nozzle body  11 , a liquid spray tip  12  at the discharge end thereof, an air cap  14  mounted in surrounding relation to the discharge end of the nozzle body  11  by a retaining ring  16 , and a control module  17  at an end opposite the spray tip  12  for controlling the liquid spray discharge from the spray nozzle assembly. The basic structure and mode of operation of the spray nozzle assembly are known in the art, for example, as shown in U.S. Pat. No. 5,707,010. 
         [0023]    The illustrated nozzle body  11 , as depicted in  FIGS. 1 and 3 , has an axial liquid flow passage  18  and a plurality of radial fluid passages. The radial passages include a liquid inlet port  19  for connection to a supply liquid to be sprayed and communicating with the liquid flow passage  18 , an atomizing air inlet port  20  radially offset from the liquid inlet port  19  ( FIG. 1 ) for connection to a pressurized air source or other pressurized fluid for assisting in atomization of the liquid to be sprayed, and a fan air inlet port  21  also for connection to a pressurized air source for assisting in direction and form of the discharging liquid spray. 
         [0024]    For controlling liquid flow and discharge through the liquid passage  18 , the control module  17  may be one of a plurality of standardized spray control modules or accessories that can be quickly and easily interchangeably mounted on the nozzle body  11  for enabling more versatile use of the spray nozzle assembly for particular spray applications. The illustrated control module  14  includes a body member  40  that carries a shut-off valve needle  41  of a conventional type for reciprocating movement with respect to the spray tip  12 . The valve needle  41  has a piston assembly  42  at its opposite end which is biased in a valve closing position by a spring  44  retained within a cap  45  threadedly engaged with an upstream end of the body  40 . The body  40  has a downstream relatively small diameter cylindrical hub portion  46  which carries an O-ring  47  that is removably positionable within an upstream cylindrical bore  48  of the nozzle body  11  with a threadless union. For releasably securing the control module  14  in the mounted position, a retainer ring  50  is provided which threadably engages an upstream threaded hub portion  51  of the nozzle body  11 . 
         [0025]    During operation, for axially moving the valve needle  41  to an open position (to the left as viewed in  FIG. 1 ) against the force of the spring  44 , control drive air or some other fluid is supplied via an inlet port  54  of the module into a cylinder adjacent a forward side of the movable piston  42 . As is known in the art, the control fluid, i.e., compressed air, may be controlled externally, such as by solenoid actuated valves, for controlling sequential opening of the valve needle  41 . 
         [0026]    The spray tip  12  in this case has a forwardly extending nose portion  60  which defines a liquid discharge orifice and which extends into and through a central opening  61  of the air cap  14  for defining an annular air discharge orifice  62  through which atomizing air directed to the atomizing air inlet  20  discharges ( FIG. 2 ). The atomizing air inlet  20  in this case communicates with a longitudinal passage  64  in the nozzle body, which in turn communicates with an annular passage  65  defined between the spray tip  12  and nozzle body  11 , which in turn communicates with a plurality of longitudinal passageways  66  through the spray tip  12 , and in turn through a downstream conical passageway  68  that communicates with the annular discharge orifice  62 . The air cap  14  further has opposed longitudinal passages  70  which communicate with respective angled passages  71  through which fan air directed from the fan air inlet  21  discharges to assist in forming of the discharge spray pattern. The fan air in this case communicates from the fan air inlet  21  through a longitudinal passage  72  in the nozzle body  11 , an annular chamber  74  between the spray tip  12  and nozzle body  11 , and the longitudinal and angled air cap passages  70 , 71 . 
         [0027]    In accordance with the invention, the air cap angled fan air passageways are defined by tubular extensions of the air cap that minimize both fine particle accumulation around the fan air discharge orifices and pressurized air operating requirements. The illustrated air cap  14  has an upstream cylindrical side wall  78  which defines a transverse retention flange  79  and a smaller diameter forwardly extending cylindrical base  80  with a pair of ears or projections  81  extending forwardly from diametrically opposed sides of the base  80 . The projections  81  in this case are defined in part by opposed portions of the cylindrical base  80 , opposed tapered side walls  82 , and opposite inwardly extending recesses  84 . The tapered side walls  82  and recesses  84  further define a central air cap end face  84  in elevated relation to the recesses  84 , which in this case has a relatively small rectangular shape, through which the central air cap opening  61  communicates. 
         [0028]    The angled fan air passage defining tubular extensions of the air cap  14  in this case are tubular members  90  that communicate with the respective longitudinal air cap passageways  70  and extend in inwardly and forwardly directed relation to the air cap end face  84 . While the tubular members  90  in the illustrated embodiment are integrally formed with the air cap  14 , alternatively, separate tubular members may be fixedly mounted within the projections. The tubular members  90  preferably extend a distance from the air cap projections  81  corresponding at least to the diameter of the angled passageways  71  and have a radial wall thickness no greater than ¼ the diameter of the angled passageways  71 . In the illustrated embodiment, the tubular members  90  have a wall thickness of about ⅙ the diameter of the angled passageways  71 . 
         [0029]    In keeping with the invention, the angled passageways are oriented at a relatively steep angle to the discharging liquid flow stream for maximizing impingement and atomization of the discharging liquid at lower air pressures and air volume for further minimizing material buildup about the fan air discharge orifices. In the illustrated embodiment, the angled fan air passages  71  are oriented at an angle a of about 30° with respect to a line perpendicular to the axis of the discharging atomized liquid flow stream. Such relatively steep angle facilitates impingement of the discharging fan air flow stream while enabling lower air operating pressures and volume which otherwise can generate and disperse fine particles onto the exposed air cap surfaces. Such relatively steep impingement angle of the fan air streams, together with the relatively small surface areas about the fan air discharge orifices, effectively prevent particle buildup that can impede reliable operation of the spray nozzle assembly. Due to the small surface area about the fan air discharge orifices, however, in some applications the angled fan air passages  71  may be oriented at greater angles, up to 45°, with respect to the discharging liquid flow stream. 
         [0030]    Referring to  FIGS. 8 and 9 , there is shown an alternative embodiment of spray nozzle assembly in accordance with the invention, wherein items similar to those described above have been given similar reference numerals. In this case, the air cap tubular members  90 , which define the angled air passageway  71 , are formed with transverse V-shaped cuts  95  that extend in a direction perpendicular to the plane of the opposed air cap projections  85  and tubular members  90 . Such transversely oriented V-shaped cuts  95  enable the fan air to spread out into a larger jet pattern that softens the impact of the fan air on the discharging atomized liquid flow stream for further minimizing the generation and direction of fine liquid particles onto exposed surfaces of the air cap. 
         [0031]    From the foregoing, it can be seen that the present invention provides an improved external mix air atomizing spray nozzle assembly adapted for more efficient spraying of highly viscous liquid materials. The spray nozzle assembly substantially reduces or eliminates undesirable buildup of sprayed materials onto externally exposed faces of the liquid spray tip and air cap. It unexpectedly achieves such enhanced performance by minimizing surface areas and low pressure zones about the fan air discharge orifices which otherwise can entrain fine particles and draw them into contact with the air cap surfaces. The air cap further can be efficiently operated at lower air pressures and air volumes for further minimizing undesirable buildup of material on exposed surfaces of the air cap.