"Burner"

The present invention relates to a burner for a combustion unit, for example in gas boilers. The combustion units of the prior art comprise a combustion chamber with a heat exchanger, a burner connected to the combustion chamber for heat production by means of the combustion of a mixture of combustible gas and combustion air inside the combustion chamber, as well as a feed duct for feeding the burner with the mixture of gas and air.

The burner comprises:

- a frame of gasproof material, connectable to the combustion chamber and supporting a diffusion layer of a gas permeable material, and

- a combustion surface (the so-called combustion head) formed by a diffusion layer portion intended to be facing in the combustion chamber and surrounded by the frame, so that a mixture of gas conveyed through the diffusion layer may be combusted in the form of a flame pattern on the combustion surface.

In solutions known to inventors (some of which may be public, while others are in-house knowledge not published by inventors), the diffusion layer is made of a material resistant to very high temperatures in order to withstand the temperatures generated by the combustion while the frame is made of a material less resistant to high temperatures due to cost reasons and to frame optimization reasons for the support and connection function.

In a solution known to the inventors (Figure 6), the combustion surface and the frame are made in a single piece of sheet metal resistant to high temperatures, perforated at the combustion surface and gasproof at the frame. This solution is disadvantageous due to the formation of fracture caused by high thermal gradients between very hot areas and relatively cold areas.

Similar breaks have also been observed in a further solution known to the inventors (Figure 7) in which the perforated metal sheet combustion surface and the gasproof metal sheet frame have been assembled, e.g. by means of welding points.

In order to obviate breaks due to thermal stresses, it is known to the inventors to place on the perforated metal sheet a flexible porous layer (Figures 8, 9), for example a metal fiber fabric or mesh acting as a combustion surface and distancing the combustion area from the metal sheet, at the cost of having to provide and assemble an additional and expensive component. The inventors have found that, in order to ensure a correct combustion of the burner under variable operating conditions, the outer perimeter or boundary of the combustion surface (for example, of a porous layer applied on a perforated metal sheet) should be as concentrated as possible along a continuous boundary line, non-jagged and without interruptions nor gradual transition areas.

Outside the combustion surface delimited by the boundary line, the material of the burner (frame area) should be gasproof, and therefore without holes and without micro-openings communicating the gas mixture feeding space with the combustion chamber.

Inside the combustion surface delimited by the boundary line, the material of the burner (diffusion areas) should be perforated or porous in order to allow the passage of the mixture of gas from the feeding space, through the combustion surface, into the combustion chamber. A realization of the boundary line between the permeable combustion surface and the adjacent gasproof frame area, as neat as possible and without local irregularities, minimizes transitional and boundary fluid-dynamic disturbances, which tend to alter the orientation of the gas mixture flow (ideally orthogonal to the combustion surface) near the boundary line (Figures 2, 3).

Conversely, an irregular and jagged boundary line favors local fuel flows more oriented in directions tangent to the combustion surface, causing local overheatings of the burner and a worsening of the combustion (Figures 4, 5).

On the basis of economic considerations, it is attempted to achieve a saving by designing burner components as small and as light as possible, in particular the combustion surface, which must withstand very high temperatures and must be manufactured so as to ensure a uniform porosity or permeability, and is, therefore, particularly expensive.

In the prior solutions, known to inventors, in order to accomplish the aforesaid gasproofing of the frame area in a repeatable manner and save by reducing the size of the single components, the junction between the diffusion layer forming the combustion surface and the frame is performed in the combustion surface or along the boundary line thereof with the gasproof frame area.

Such mechanical junction is performed, for example, by means of continuous welding, continuous rolling, brazing, or spot welding, with long, and therefore expensive, processing times, in the range of tens of seconds or minutes.

It is therefore the object of the present invention to provide a burner and a method for the manufacturing thereof, having such features as to obviate at least some of the drawbacks of the prior art.

It is a particular object of the invention to better reconcile the need to make a neat and regular boundary line between the combustion surface and the frame area, with the need to simplify and economize on the manufacturing of the burner. This and other objects are achieved by means of a gas burner suitable for a combustion unit of the type having:

- a first housing part (combustion housing) internally delimiting a combustion space,

- a second housing part (gas inlet housing) internally delimiting a gas feeding space,

- a burner interposed between the first housing part and the second housing part,

wherein said burner comprises:

- a frame of gasproof material having an outer peripheral portion connectable to at least one of the first and second housing parts,

- a diffusion layer of a gas permeable material, said diffusion layer forming a combustion surface surrounded by the frame along a boundary line between a gas permeable diffusion area and a gasproof peripheral area,

wherein the combustion surface is intended to be directed in the combustion space so that a mixture of gas conveyed through the diffusion layer may be combusted in the form of a flame pattern on the combustion surface,

characterized in that the frame comprises:

- a front frame formed in a single piece of sheet metal forming a front opening delimited along the boundary line,

- a rear frame formed in a single piece of sheet metal different from the front frame and forming a rear opening overlapping with the front opening,

wherein the diffusion layer is sandwiched and clamped between said front and rear frames which are connected to one another in a plurality of discrete connection positions spaced from one another and placed externally to the diffusion layer.

The construction of the frame as a double metal sheet layer construction allows to perform the connection between the frame and the diffusion layer by means of a sandwich connection with a more or less strong clamping force, by virtue of a connection between the front frame and the rear frame distant from the boundary line and without local deformation or direct welding operations between the metal sheet and the diffusion layer along the boundary line.

This allows to use techniques for connecting two metal sheets only, at connection positions distant from one another and therefore executable at the same time, with a saving of burner assembly costs and time, which not only compensates, but exceeds the higher cost of the material for the double metal sheet of the frame.

Furthermore, the absence of localized weldings or deformations at the boundary line between the combustion surface and the surrounding gasproof area of the frame, allows for a neater construction without any local alterations of the boundary line and, therefore, an improvement in combustion.

In order to better understand the invention and appreciate the advantages thereof, a description of a number of non-limiting exemplary embodiments of the burner and the manufacturing method is provided below, with reference to the accompanying Figures, in which:

- Figure 1 is a front perspective view of a flat burner for a combustion unit;

- Figures 2 and 3 are perspective and sectional views showing the ideal direction of a gas flow through a diffusion layer of the burner in Figure 1 ;

- Figures 4 and 5 are perspective and sectional views showing alterations in the direction of the gas flow through a diffusion layer of a burner with an irregular boundary line between the diffusion area and a surrounding frame area;

- Figures 6 to 9 are schematic sectional views of prior burners known to the inventors,

- Figures 10 to 17 are schematic sectional views of burners according to embodiments of the invention,

- Figures 18 and 19 are schematic sectional views of combustion units equipped with a burner according to an embodiment of the invention,

- Figure 20 is a schematic sectional view of a burner according to a further embodiment.

With reference to Figures 18, 19, a combustion unit 1 comprises a first housing part 2 (combustion housing) internally delimiting a combustion space 3, a second housing part 4 (gas inlet housing) internally delimiting a gas feeding space 5, and a burner 6 interposed between the first housing part 2 and the second housing part 4 or connected to at least one thereof, for example by means of the interposition of gaskets 7.

The burner 6 comprises a frame 8 having an outer peripheral portion 9 connectable to at least one of the first 2 and second 4 housing parts, and a diffusion layer 10 of a gas permeable material. The diffusion layer 10 forms a combustion surface 1 1 surrounded by the frame 8 along a boundary line 12 between a gas permeable diffusion area 13 (formed by the diffusion layer 10) and a gasproof peripheral area 14 (formed by the frame 8). A burner 6 mounted in the combustion unit, the combustion surface 1 1 being directed in the combustion space 3 so that a mixture of gas conveyed through the diffusion layer 10 may be combusted in the form of a flame pattern on the combustion surface 1 1 .

According to an aspect of the invention, the frame 8 comprises a front frame 15 formed in a single piece of sheet metal forming a front opening 16 delimited along the aforesaid boundary line 12, and a rear frame 17 formed in a single piece of sheet metal different from the front frame 15 and forming one or more rear openings 18 overlapping with the front opening 16 of the front frame 15.

The diffusion layer 10 is sandwiched and clamped between said front 15 and rear frames 17 which are connected to one another in a plurality of discrete connection positions 19 spaced from one another and placed externally to the diffusion layer 10.

The embodiment of frame 8 as a double metal sheet layer construction allows to perform the connection between the frame 8 and the diffusion layer 10 by means of a sandwich connection with a clamping force achievable according to the material of the diffusion layer 10 and the shape and size thereof, by virtue of a connection between the front frame 15 and the rear frame 17 distant from the boundary line 12 and without local deformation or direct welding operations between the metal sheet and the diffusion layer 10 along the boundary line 12. This allows to use techniques for connecting two metal sheets only, at connection positions 19 distant from one another and therefore achievable at the same time, with a saving of burner 6 assembly costs and time, which not only compensates, but exceeds the greater cost of the material for the double metal sheet of frame 8.

Furthermore, the absence of localized weldings or deformations at the boundary line 12 between the combustion surface and the surrounding gasproof area 14 of frame 8, allows for a more neat and distinct realization of the boundary line 12 without any local alterations and, therefore, an improvement in combustion.

According to an embodiment, the front frame 15 and the rear frame 17 are both made of sheet metal having a lower resistance to high temperatures with respect to the material of the diffusion layer 10. In advantageous embodiments, the front frame 15 and the rear frame 17 are made of stainless steel sheet. The diffusion layer may be selected from the group comprising:

A) perforated sheet metal in high thermal resistance material, for example high temperature ferritic stainless steel,

B) high thermal resistance metal fiber fabric, for example FeCrAI alloys,

C) high thermal resistance metal fiber mesh, for example FeCrAI alloys,

D) high thermal resistance ceramic or sintered porous material, for example cordierite, FeCrAI alloys and NiCrFe alloys,

E) metal mesh or metal wire mesh,

- two-layer or multilayer structures consisting of two or more layers in the previously listed materials, e.g. A+B, A+C, A+D, B+C, B+D, C+D.

According to an embodiment, (preferably all) the discrete connection positions 19 between the front frame 15 and the rear frame 17 are formed on a same connection plane 20 at one or more flat metal sheet portions 21 of the front frame 15 and of the rear frame 17 parallel to said connection plane. Advantageously, the flat metal sheet portions 21 extend beyond the discrete connection positions 19 on all sides for at least 2mm, preferably for at least 5mm, even more preferably for at least 10mm.

Furthermore, the connection positions 19 may be at a minimum distance from the boundary line 12 of 2mm (in the case of spot welding) or of 5mm (in the case of mechanical joining by means of TOX). The currently preferred distance between the connection positions 19 and the boundary line is equal to or greater than 6mm, preferably in the range between 8mm and 10mm.

The making of the connection positions 19 in generally planar and extended metal sheet portions 21 allows the use of stamps, presses and dies to make mechanical connections between the front frame 15 and the rear frame 17, in a fast and repeatable manner. Likewise, the generally planar and extended metal sheet portions 21 also allow the simultaneous achievement of a plurality of welding points spaced from one another, which are faster, simpler and less expensive with respect to a continuous welding or a spot welding.

According to an embodiment, the discrete connection positions 19 are formed in the outer peripheral connection portion 9 of frame 8, i.e. in the portion of frame 8 intended to be interposed and clamped, for example by means of the interposition of one or more gaskets 7, between the first housing part 2 and the second housing part 4 of the combustion unit 1 .

Alternatively or additionally, the discrete connection positions 19 are formed in the gasproof peripheral area 14 of frame 8 and spaced (for example, more towards the inside) with respect to the outer peripheral connection portion 9 of frame 8 (Figures 14, 19).

In this description, the term "connection position" means "connection or point of connection without spatial limitation to a single point in the mathematical sense".

According to an embodiment, the discrete connection positions 19 between the front frame 15 and the rear frame 17 may comprise a mechanical shape connection, e.g. by means of seal clinching (tox), non-through riveting, non-through clipping, and/or a connection with material continuity, e.g. by means of welding points spaced from one another, preferably by at least 15mm. In the sandwich configuration with the diffusion layer 10 interposed between the two front 15 and rear frame 17 metal sheets, for fastening the two metal sheets with one another (in particular beyond the extension of the diffusion layer 10), a mechanical fastening method, in particular seal clinching (tox), was found to be much cheaper in the manufacturing step, with respect to other connection systems, in particular with respect to welding or screwing.

Advantageously, the connection positions 19 are spaced from one another by at least 20mm. This allows perform all the connections at the same time in a single step by means of a press or by means of a point welding machine.

According to an embodiment, the boundary line 12 is formed by an edge 22 of the front frame 15 or, exceptionally and less preferably, of the rear frame 17, which extends without interruption all around the combustion surface 1 1 and, therefore, around the diffusion area 13. Preferably, the front opening 16 and the rear opening 18 have the same shape and are exactly overlapped so that both openings delimit the diffusion area 13 along the boundary line 12 (Figures 1 1 , 12, 13).

The boundary line 12 may advantageously be a closed polygonal line, for example rectangular or square, a closed curved line, preferably without inversion of curvature, for example circular or oval, or a closed line consisting of rectilinear sections and curved sections.

According to an embodiment, the front frame 15 or the rear frame 17 extends continuously from the boundary line 12 to the outer peripheral portion 9 and forms both the boundary line 12 and the outer peripheral portion 9.

This obviates the risk of unwanted leaks between the gas feeding space 5 and the combustion space 3 externally to the combustion surface 1 1 .

In an embodiment (Figure 18), both the front 15 and rear frames 17 extend continuously from the boundary line 12 to the outer peripheral portion 9 and form both the boundary line 12 and the outer peripheral portion 9.

In an alternative embodiment (Figures 14, 19), one of the front 15 and rear frames 17 does not extend to the outer peripheral portion 9, only contributing to the fastening of the diffusion layer 10, but not also to the gasproof fastening of the burner 6 to the housing of the combustion unit 1 .

The outer peripheral portion 9 is preferably plane to facilitate a gasproof connection thereof to the housing 2, 4 of the combustion unit 1 .

In an embodiment (Figures 10, 1 1 , 12), the frame 8 forms a step 23 extended in discrete sections or continuously around the combustion surface 1 1 , between the outer peripheral portion 9 and the boundary line 12. The step 23 gives the burner 6 a better shape stability and a programmed thermal deformation direction. Advantageously, the step 23 is shaped so as to position the boundary line 12 and the combustion surface 1 1 with respect to the outer peripheral portion 9 in a gas mixture flow direction 24 towards the side of the combustion surface 1 1 . In this way, the burner 6 has a pre-deformation in the same direction of a thermal deformation due to a greater heating of the front side thereof (combustion side).

Preferably, the step 23 is complementarily formed in both the front 15 and rear frames 17. In accordance with a further embodiment, the boundary line 12 extends in a single boundary plane 29 which may be parallel to the plane of the outer peripheral portion 9.

The combustion surface 1 1 may substantially be plane or rounded towards the combustion side.

In an embodiment (Figures 10, 16, 17), the rear frame 17 forms a portion of perforated metal sheet 25 extended across the entire diffusion area 13 and which forms the aforesaid rear openings 18 in the form of a perforation distributed across the diffusion area 13. The portion of perforated metal sheet 25 may have the function of distributing the mixture of combustible gas as desired towards the diffusion layer 10.

The portion of perforated metal sheet 25 may have a shape complementary to the shape of the diffusion layer 10 and extend adjacent thereto or in direct contact therewith (Figure 10).

Alternatively, the portion of perforated metal sheet 25 may have a shape non-complementary with the shape of the diffusion layer 10, for example a rounding (convexity) in the opposite direction (Figure 17) or a rounding (convexity) with a different radius of curvature, for example greater than the radius of curvature of the diffusion layer 10 (Figures 16, 17).

In an embodiment (Figures 12, 13, 15, 16, 17), the diffusion layer 10 forms a bend 26 extended along the boundary line 12 constituting a separation line between the combustion surface 1 1 and an outer edge 27 of the diffusion layer 10 which is clamped between the front frame 15 and the rear frame 17.

In an alternative embodiment (Figures 10, 1 1 , 14), the diffusion layer 10 is substantially flat and extends without bending from the diffusion area 13 to a clamping interstice between the front frame 15 and the rear frame 17.

In a further embodiment (Figures 13, 18, 19), the frame 8 extends in a substantially plane manner from the boundary line 12 to the outer peripheral portion 9.

According to a further embodiment (Figure 15), the rear frame 17 forms a deflector 28 formed near or extending around the rear opening 18 and protruding in the diffusion area 13 upstream of the diffusion layer 10 (with reference to the gas mixture flow direction 24), so as to influence the distribution of the mixture of gas on the diffusion layer 10 as desired.

According to a further embodiment, the front frame 15 and rear frame 17 are connected to one another so as to form a pre-assembled self-supporting diffuser-frame unit, which may be transported, stored and mounted in the combustion unit 1 as a single piece.

The burner 6 may advantageously be manufactured by means of the following steps:

A1 ) forming a front frame 15 in a single piece of sheet metal with a front opening 16 delimited along a boundary line 12,

A2) forming a rear frame 17 in a single piece of sheet metal different from the front frame 15 and with one or more rear openings 18,

A3) forming at least one of the front 15 and rear frames 17 with an outer peripheral portion 9 connectable to a housing 2, 4 of a combustion unit 1 ,

B) providing a diffusion layer 10 of a gas permeable material with a combustion surface 1 1 and an outer edge 27 extending around the combustion surface 1 1 ,

C1 ) overlapping the front frame 15 with the rear frame 17 so that the front opening 16 overlaps said one or more rear openings 18,

C2) interposing and clamping the sandwich diffusion layer 10 between the overlapping front 15 and rear frames 17, so that the combustion surface 1 1 is surrounded by the front opening 16 along the boundary line 12 and the outer edge 27 is clamped between the front frame 15 and the rear frame 17,

C3) connecting the front 15 and rear frames 17 to one another in a plurality of discrete connection positions 19 spaced from one another and placed externally to the diffusion layer 10.

Advantageously, the steps C1 , C2 and C3 are performed by means of a (preferably single) pressing or welding operation, wherein the press or welding machine is equipped with seats for receiving the front frame 15 and the rear frame 17 and tools or welding heads to make the connection.

Furthermore, a pre-assembly (for example, by means of spot welding) of the diffusion layer 10 with only one of the front 15 and rear frames 17 before the step C1 described above is advantageous.

The combustion unit 1 may be manufactured by means of the following steps:

- preparing the first housing part 2,

- preparing the second housing part 4,

- fastening the burner 6 by means of the outer peripheral portion 9 thereof between the first housing part 2 and the second housing part 4.

Further method features have already been described with reference to the structure of the burner 6 and are not hereby repeated for brevity.

Figure 20 shows a burner according to an alternative embodiment in which the discrete connection positions 19 are not necessarily placed externally to the diffusion layer 10 but, conversely, the discrete connection positions 19 are placed in the overlapping area between the front frame 15, (the outer edge 27 of) the diffusion layer 10 and the rear frame 17, but always at a distance from the boundary line 12. This solution, while renouncing to the advantages of a connection between only two metal sheets and to the advantages of a connection which does not directly affect the diffusion layer 10, ensures a sure mutual positioning between the frame 8 and the diffusion layer 10, in addition to the unmovable fastening of the diffusion layer 10. Furthermore, the placement of the connection positions 19 in the sandwich area formed by the front frame 15, the diffusion layer 10 and the rear frame 17 allows to reduce the overall radial extension of the double frame 15, 17, in particular in the cases in which an outer peripheral portion 9 is desired (for the connection of the combustion unit 1 to the housing 2, 4) without the double frame and without connection positions 19, as shown, for example, in Figure 20.

Further technical and geometric features (of frames 8, 15, 17, of the diffusion layer 10, of the boundary line 12, and of the connections 19) previously described are also applicable to the embodiment of Figure 20, but not hereby repeated for brevity.

Likewise, step C3) of the manufacturing method may be modified so that the front 15 and rear frames 17 are connected to one another in a plurality of discrete connection positions 19 spaced from one another and placed in an overlapping area between the front frame 15, (the outer edge 27 of) the diffusion layer 10 and the rear frame 17, but always at a distance from the boundary line 12.

The solutions hitherto described therefore hypothesize:

- constructions in which all connection positions 19 are made externally to the diffusion layer 10 and no connection between the front frame 15 and the rear frame 17 is made at the diffusion layer 10,

- constructions in which all connection positions 19 are made in the overlapping area between the front frame 15, (the outer edge 27 of) the diffusion layer 10 and the rear frame 17 and no connection between the front frame 15 and the rear frame 17 is made externally to the diffusion layer 10,

- constructions in which one or more of the connection positions 19 are made in the overlapping area between the front frame 15, (the outer edge 27 of) the diffusion layer 10 and the rear frame 17 and one or more of the connection positions 19 are made externally to the diffusion layer 10.