Abstract:
A spray nozzle includes a tubular channel extending along a longitudinal axis, a slot formed by two approximately plane surfaces converging in a direction of the channel and located on either side of a plane including the longitudinal axis of the channel, and a dome connecting the channel and the slot wherein a length of the dome represents less than 50% of a largest transverse dimension of the channel and a plane cross section of the dome is symmetric and it is defined by at least two different circular arcs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a spray nozzle for liquid, in particular for coating liquid under high pressure. Furthermore, the invention relates to a device for spraying liquid, in particular for coating liquid under high pressure, comprising such a nozzle. 
     2. Brief Description of the Related Art 
     A device for spraying liquid or sprayer, either of the manual type or of the automatic type, generally comprises a spray nozzle, sometimes several, which is(are) mounted at the downstream end of the sprayer. The terms “upstream” and “downstream” herein refer to the direction of flow of the liquid in the sprayer. The term “upstream” denotes elements located on the side of the sprayer where the liquid to be sprayed arrives from a supply source. The term “downstream” denotes elements located on the side of the sprayer where the liquid is sprayed in droplets. 
     Such a sprayer may, for example, be intended for spraying coating liquids such as waterborne or solvent-based paints. To produce the spraying of the liquid in droplets, the sprayer is connected, by means of one of more tube(s), to a pump designed to put the liquid under high pressure, for example 70 bars. The spraying is carried out at the downstream end of the nozzle, which has a geometry determined depending on the desired shape for the jet of droplets of the sprayed liquid. 
     To the aim of shaping the jet of sprayed liquid into a “fan”, usually called a “flat” spray, a nozzle such as that illustrated by  FIGS. 1 and 2  is known from the prior art. As  FIG. 1  shows, the nozzle  1  comprises a body  2  which defines, on the upstream side, a chamber  3  through which the liquid arrives and, on the downstream side, a channel  4  for conveying the liquid from the chamber  3  through to the outlet of the nozzle  1 . The chamber  3  and the channel  4  extend along a longitudinal axis X 1 -X′ 1  of the nozzle  1 . Downstream of the channel  4 , the nozzle  1  comprises a slot  6  intended to shape the liquid jet into a flat spray. As  FIGS. 1 and 2  show, the slot  6  is formed by two surfaces  61  and  62  which are plane, which converge in the direction of the channel  4 , and which are positioned on either side of a plane P 6  including axis X 1 -X′ 1 . The one-eyed bottom of the channel  4  obtained in the body  2  before milling the slot  6  has the form of a hollow dome  5 , which is herein referred to by the word “dome”. The dome  5  connects the channel  4  and the slot  6 . 
     In nozzles of the prior art, the dome  5  has the shape of an ogive or triangular arch or a hemispherical shape, the length of which approximately equals the diameter of the channel  4 . As  FIG. 2  shows, the intersection of the dome  5  with the surfaces  61  and  62  of the slot  6  defines an outlet orifice  7  of the nozzle  1  in the overall shape of a flattened ellipse. 
     When the nozzle  1  sprays a liquid under high pressure, for example 70 bars, the geometry of the orifice  7  shapes the jet into a cone with an elliptical cross section. With the nozzle  1 , the flow rate of the sprayed liquid is not uniformly distributed in this elliptical cross section. On the contrary, it has higher concentrations towards the distant edges of the ellipse. In the field of spraying coating liquids, this type of distribution of liquid is called the “tails effect”. It has been observed that the more rounded the edges of the ellipse are, the larger are the “tails” in the flow of liquid. 
     The “tails effect” has the drawback of leading to asymmetric wear of the nozzle  1 , by overwearing down the edges of the orifice  7 . The more abrasive the sprayed liquid is, the greater this wear is. This wear increases the “tails effect” and therefore leads to a reduction in the quality of the spraying. In addition, it reduces the service life of the nozzle  1 , even when the material of the body  2  has a high hardness. 
     GB-A-1 312 052 describes a flat spray nozzle comprising a discontinuity at the junction between the channel and the dome connecting the channel and the slot of the nozzle. However, the nozzle of GB-A-1 312 052 does not permit to significantly decrease the “tails effect” so as to get a sufficient spraying quality. 
     SUMMARY OF THE INVENTION 
     The present invention aims in particular to solve these drawbacks by proposing a spray nozzle with a longer service life and enabling a flat jet to be produced with a relatively uniform distribution of liquid, and therefore to improve the spraying quality. 
     To this aim, the subject-matter of the invention is a spray nozzle for liquid, in particular for coating liquid under high pressure, comprising:
         a tubular channel for channelling said liquid, said channel extending along a longitudinal axis;   a slot for shaping a jet of liquid coming from said channel, said slot being formed by two approximately plane surfaces, said surfaces converging in the direction of said channel and being positioned on either side of a plane comprising said longitudinal axis of said channel; and   a dome connecting said channel and said slot;       

     The length of said dome, measured parallel to said longitudinal axis, represents less than 50%, preferably between 20% and 45%, of the largest transverse dimension of said channel, measured in a plane that is orthogonal to said longitudinal axis. 
     Said dome has a plane cross section that is symmetric relative to said longitudinal axis. Said plane cross section is defined by at least two circle arcs which extend between a downstream end portion of said channel and said longitudinal axis and which have different radii and centers located on the side of said channel. 
     According to other advantageous, but optional, features of the invention, taken in isolation or in any technically feasible combination:
         said plane cross section is defined by three circle arcs which extend between said downstream end portion of said channel and said longitudinal axis and which have different radii and centers located on the side of said channel;   a first circle arc close to said downstream end portion of said channel has a radius less than half of the smallest dimension of said channel, measured in a plane orthogonal to said longitudinal axis, the circle arc the furthest from said downstream end portion of said channel having a radius greater than half the largest dimension of said channel, measured in a plane orthogonal to said longitudinal axis, and each other circle arc having a radius of a size greater than the radius of the preceding circle arc and less than the radius of the following circle arc;   said first circle arc is tangent to said downstream end portion of said channel and each other circle arc is tangent to the preceding circle arc;   said channel has a cylindrical form with a circular base, said dome has a rotational symmetry about said longitudinal axis of said channel;   the dimensions of said channel, measured in a plane that is orthogonal to said longitudinal axis, are between 0.1 mm and 1.8 mm;   said slot has a height of between 0.2 mm and 2 mm and a width of between 0.02 mm and 1.6 mm;   said surfaces form an angle of between 10° and 110° between them;   said nozzle is made of a material having a hardness greater than 50 on the Rockwell C scale, this material being able to be selected from the group comprising the tungsten carbides and the ceramics.       

     Furthermore, the subject-matter of the invention is a device for spraying liquid, in particular coating liquid under high pressure, characterized in that it comprises a nozzle such as disclosed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be well understood, and its advantages will become apparent, in the light of the following description, provided only by way of non limiting example and with reference to the annexed drawings, in which: 
         FIG. 1  is a cross section of a spray nozzle of the prior art; 
         FIG. 2  is a partial view from above, at a larger scale, of the spray nozzle illustrated in  FIG. 1 ; 
         FIG. 3  is a cross section similar to that of  FIG. 1  of a spray nozzle according to a first embodiment of the invention; 
         FIG. 4  is a view similar to that of  FIG. 2  of the nozzle illustrated in  FIG. 3 ; 
         FIG. 5  is a partial cross section, at a larger scale, along the line V-V of  FIG. 4 ; 
         FIG. 6  is a view similar to that of  FIG. 4  of a spray nozzle according to a second embodiment of the invention; 
         FIG. 7  is a view similar to that of  FIG. 5  of the nozzle partially illustrated in  FIG. 6 ; and 
         FIG. 8  is a perspective view of a spraying device according to the invention comprising a nozzle according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As  FIG. 3  shows, the nozzle  101  comprises a body  102  which defines, on the upstream side, a chamber  103  through which the liquid arrives and, on the downstream side, a channel  104  for conveying the liquid from the chamber  103  through to the outlet of the nozzle  101 . The direction of flow of the fluid through the nozzle  101  is represented by an arrow F, which then allows to notice the upstream and downstream sides of the nozzle  101 . 
     The chamber  103  and the channel  104  extend along a longitudinal axis X 101 -X′ 101  of the nozzle  101 . In the example of  FIG. 3 , the channel  104  has the overall form of a cylinder with an axis X 101 -X′ 101  and a circular base of diameter D 104 . Downstream of the channel  104 , the nozzle  101  comprises a slot  106  intended to shape the liquid jet into a flat spray. As  FIGS. 3 and 4  show, the slot  106  is formed by two surfaces  161  and  162  which are plane, which converge in the direction of the channel  104 , and which are positioned on either side of a plane P 106  comprising the axis X 101 -X′ 101 . 
     The nozzle  101  furthermore comprises a dome  105  connecting the channel  104  and the slot  106 . “Dome” denotes the one-eyed base of the channel  104 , which is obtained in the body  102  before milling the slot  106 . “Connecting” means bringing into fluid communication. 
     The length L 105  of the dome  105 , measured parallel to the longitudinal axis X 101 -X′ 101 , here represents 25% of the diameter D 104  of the channel  104 . In practice, the length L 105  of the dome  105  represents less than 50%, preferably between 20% and 45%, of the diameter D 104  of the channel  104 . In other words, the dome  105  has a short or flattened shape in relation to the dome  5  of the nozzle  1  of the prior art illustrated in  FIG. 1 . 
     The tubular channel of the nozzle subject-matter of the invention may, as a variant, be prismatic in shape or be in a cylindrical shape with a non-circular base, for example an elliptical base. In this case too, the length of the dome is less than 50%, preferably between 20% and 45%, of the largest transverse dimension of the channel, measured in a plane orthogonal to the longitudinal axis of the nozzle. 
     The dome  105  has rotational symmetry around the axis X 101 -X′ 101 . A cross section of the dome  105  through a plane containing the axis X 101 -X′ 101 , for example through the plane P 106 , is defined by two circle arcs C 1051  and C 1052  that extend between a downstream end portion  1041  of the channel  104  and the axis X 101 -X′ 101 . The circle arcs C 1051  and C 1052  have the respective radii R 1051  and R 1052  and respective centres O 1051  and O 1052  located on the side of the channel  104 , i.e. opposite the downstream portion of slot  106 . 
     On the right-hand part of  FIG. 5 , the two arcs C′ 1051  and C′ 1052  extend between the end portion  1041  of the channel  104  and the axis X 101 -X′ 101 , symmetrically with the arcs C 1051  and C 1052  relative to the axis X 101 -X′ 101 . The radius R 1051  is relatively small compared with the radius R 1052 . Thus, the radius R 1051  is less than half the diameter D 104 , which represents both the smallest and the largest transverse dimension of the channel  104 . “Transverse” denotes a dimension measured in a plane orthogonal to the longitudinal axis X 101 -X′ 101 . Conversely, the radius R 1052  is greater than half the diameter D 104  of the channel  104 . 
     In addition, the circle arc C 1051  is tangent to the end portion  1041  of the channel  104  and the circle arc C 1052  is tangent to the circle arc C 1051 . Thus the arcs C 1051  and C 1052  are joined in a continuous manner and without any singularity. By symmetry, the geometry of the arcs C′ 1051  and C′ 1052  is identical to that of the arcs C 1051  and C 1052 . The dome  105  thus has a shape that is overall trapezoidal or a shape of half a convex lens. 
     The shape of the dome  105  may be comprised of more than two circle arcs joined to each other. In such a case, the radius of the circle arc closest to the downstream end portion of the channel is less than half the largest transverse dimension of the channel, the radius of the circle arc furthest from the downstream end portion of the channel is greater than half the largest transverse dimension of the channel; and each other circle arc has a radius of a size greater than the radius of the preceding circle arc and less than the radius of the following circle arc. In addition, in such a case, each circle arc is tangent to the preceding circle arc. 
     In the example of  FIGS. 3 to 5 , the slot  106  has a height H 106  of around 0.55 mm and a width l 106  of around 0.12 mm. The height H 106  is considered in the direction defined by the intersection of the plane P 106  with the plane of  FIG. 4  and the width l 106  is measured in the plane of  FIG. 4  and perpendicular to the plane P 106 . In practice, the height H 106  may be between 0.2 mm and 2 mm and the width l 106  may be between 0.02 mm and 1.6 mm. As  FIG. 5  shows, the surfaces  161  and  162  form an angle α of around 30° between them. In practice, the angle α may be between 10° and 110°. 
     The channel  104  has a length L 104 , measured along the axis X 101 -X′ 101 , of around 1.1 mm. In practice, the length L 104  may be between 0.4 mm and 3.5 mm. Moreover, the diameter D 104  of the channel  104  has a value of around 0.55 mm and may in practice be between 0.1 mm and 1.8 mm. 
     As  FIG. 4  shows, the intersection of the “flattened” or “short” dome  105  with the plane surfaces  161  and  162  that form the slot  106  defines an outlet orifice  107  that is approximately rectangular in shape with rounded corners. To the extent that the surfaces  161  and  162  are symmetric in relation to the plane P 106  and the dome  105  has a symmetry with the axis X 101 -X′ 101 , the orifice  107  has, in the elevation of  FIG. 4 , a symmetry by quadrants, the center of which is at the intersection of the plane P 106 , of the axis X 101 -X′ 101  and of the plane of  FIG. 4 . 
     The geometry and the dimensions of the nozzle  101 , in particular of its flattened dome  105 , define the approximately rectangular shape of the outlet orifice  107 . Such a nozzle enables to considerably reduce the “tails effect”, hence to render the liquid flow rate more uniform in the jet sprayed under, for example, 70 bars, or even under lower pressure, for example 40 bars. To the extent that this sprayed jet is more uniform, the quality of the spraying, hence of the application of this jet for example the coating of an object, is significantly improved. In addition, as the “tails effect” is reduced, the wearing of the edges of the outlet orifice  107  of a nozzle  101  according to the invention is greatly reduced, thereby increasing the service life of the nozzle  101 . 
     The description of  FIGS. 4 and 5  given above can be directly transposed to  FIGS. 6 and 7 , except the hereafter stated differences. An element of  FIG. 6  or  7  similar or corresponding to an element of  FIG. 4  or  5  gets the same numerical reference with prefix  2  replacing prefix  1 . 
     One thus defines a nozzle  201 , a longitudinal axis X 201 -X′ 201 , a body  202 , a channel  204  with a diameter D 204  and a radius R 204 , a dome  205  with a length L 205 , a slot  206  with an outlet orifice  207 , circle arcs C 2051  and C 2052  with respective radii R 2051  and R 2052  and respective centers O 2051  and O 2052  and a downstream end portion  2041 . 
     The nozzle  201  differs from the nozzle  101 , because the plane cross section of the dome  205  is defined by three circle arcs C 2051 , C 2052  and C 2053 , instead of two for the dome  105 . Alike the plane cross section of  FIG. 5 , the plane cross section of  FIG. 7  is symmetric with respect to longitudinal axis X 201 -X′ 201 . Thus, on the right-hand part of  FIG. 7 , three arcs C′ 2051 , C′ 2052  and C′ 2053  extend between the end portion  2041  of the channel  204  and axis X 201 -X′ 201 , symmetrically with the arcs C 2051 , C 2052  and C 2053  relative to the axis X 201 -X′ 201 . 
     The radius R 2053  is greater than the radius R 2052 , the latter being itself greater than the radius R 2051 . Furthermore, the radius R 2051  is less than the radius R 204  of the channel  204  and the radius R 2053  is greater than the radius R 204 . 
     The geometry and the dimensions of the nozzle  201 , in particular of its dome  205 , permit to further decrease the “tails effect”, hence to render more uniform the liquid flow rate in the sprayed jet, with respect to nozzle  101 . 
       FIG. 8  illustrates a sprayer  100  or device for spraying liquid, in particular coating liquid under high pressure, comprising a nozzle  101  according to the invention. The sprayer  100  therefore produces sprays of improved quality and relatively uniform. In addition, the sprayer  100  requires fewer operations for replacing the nozzle  101 . 
     To the aim of further increasing the service life of the nozzle  101 , the latter may be made of a material having a high hardness, which may be selected from the group comprising the tungsten carbides and the ceramics or any other material having a high hardness. A high hardness means a hardness greater than 50 on the Rockwell C scale.