Patent Publication Number: US-2016238243-A1

Title: Burner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure claims benefit of priority under 35 U.S.C. 119(a)-(d) to an Italian Patent Application No. 102015902330519 filed on Feb. 16, 2015, which is incorporated by reference herein. 
     FIELD OF TECHNOLOGY 
     The present invention relates to a burner for a gas heater. 
     BACKGROUND 
     Gas heaters of the prior art comprise a combustion chamber with a heat exchanger, a burner either connected to or inside the combustion chamber for producing heat by combusting a mixture of fuel gas and combustion supporting air within the combustion chamber, a feeding pipe for feeding the gas and air mixture to the burner, as well as a frame for supporting and connecting the burner to the combustion chamber. 
     More specifically, the burners of the prior art comprise a combustion membrane having: an inner surface in flow communication with the feeding pipe or with a mixing and/or distribution chamber of the fuel gas mixture, and a diffuser layer, which forms an outer surface (or combustion surface) of the membrane, facing the combustion chamber, and the mixture is conveyed through the combustion membrane into the combustion chamber where the combustion occurs, in the form of a flame pattern on the combustion surface. 
     The diffuser layer consists of a wall with a plurality of through openings. According to the type of combustion membrane, such a wall may be a perforated sheet, a fabric, a mesh (for brevity, meshes and fabrics will be indicated hereinafter by the term “meshes”) or a porous or perforated wall of ceramic material. 
     Furthermore, a distributor may be provided upstream of the diffuser layer (with reference to the flow direction of the gas-air mixture) with the aim of distributing the mixture in a desired manner towards the diffuser wall. The known distributors are generally made as walls with a plurality of through openings, e.g. made of a perforated sheet, and which may form an “inner” layer of the combustion membrane or, alternatively, a component spaced apart from the combustion membrane. 
     The heat produced by the combustion is carried by means of the hot combustion gases (by convection) and by means of thermal radiation to the heat exchanger for heating a fluid, e.g. water, which is then conveyed to a user, e.g. to a heating system of an industrial process, of living spaces or the like and/or of sanitary water. 
     For a targeted, safe use of the heater, it is desirable to control and be able to vary in a controlled manner the heating power of the burner and the local and total flow rate of the fuel mixture through the combustion membrane. 
     Indeed, the flow rate of the fuel mixture affects flame temperature, combustion membrane temperature, overall thermal power, and flame stability, but regretfully even the onset of undesired instability phenomena and detachment of the flame from the combustion surface. 
     In addition to the local flow rate of the fuel mixture, flame stability also depends on the local temperature of the combustion membrane, which local temperature is, in turn, affected by geometry and weight distribution in the combustion membrane and in the supporting frame. For this reason, combustion membranes are often made of metal meshes or fabrics which facilitate a tridimensional, double-curvature shaping. 
     While the perforated sheet may be easily folded about a single axis (or about multiple parallel axes), a shaping with curvature about several non-parallel axes results in a stretching on the plane of the sheet with uncontrollable, unrepeatable deformations of the holes and possible breakage of the sheet “bridges” between adjacent holes. 
     On the other hand, metal fiber meshes are expensive and dimensionally poorly stable, and therefore are often used in combination with a supporting metal mesh which further increases the burner cost. Moreover, the properties (permeability, thickness, density) of the metal fiber meshes have a statistic distribution with high standard deviation. This makes it difficult to control the parameters which affect the combustion and results in uncertainties in the ignition, flame detection, fuel flow load and specific thermal power of the burner. 
     A very common type of burner has a flat metal frame with an elongated opening with two parallel rectilinear longitudinal edges and two semi-circular end edges, to which a combustion membrane is fixed with a diffuser layer made of metal mesh having a “cannoli” or semi-cigar shape with a central portion which is cylinder-segment shaped and two end portions which are spherical-segment-shaped. 
     This type of known burner shows the advantages and disadvantages of the prior art in an exemplary manner: the use of metal meshes to facilitate the formation of complex tridimensional shapes while keeping the perforation or porosity features of the mesh nearly unchanged, but at the expense of higher cost and greater statistic variability of the properties (weight distribution, permeability, mechanical strength and thermal expansion) of the metal mesh with respect to sheets and porous ceramic layers. 
     SUMMARY 
     Therefore, it is the object of the present invention to provide a burner with a tridimensional geometry with features such as to avoid at least some of the drawbacks of the prior art. 
     It is another particular object of the invention to better conciliate the needs of better controlling the combustion parameters and manufacturing the combustion membrane at low costs. 
     It is a further particular object of the invention to provide a burner of the cannoli-shaped type which is improved in terms of manufacturing, cost and combustion property repeatability. 
     These and other objects are achieved by means of a gas burner for a heater according to claim  1 . The dependent claims relate to advantageous embodiments. 
     In accordance with an exemplary embodiment of the invention, a gas burner for a heater comprises: 
     a planar, metal frame portion with an elongated opening defined by two opposite longitudinal edges and two transversal end edges, 
     a combustion membrane in the form of a tridimensional shell, connected to the metal frame along the longitudinal and transversal end edges, said combustion membrane having: 
     a diffuser layer forming an outer combustion surface made of a perforated metal sheet, said surface extending with a single curvature out of the plane of the frame portion between the two longitudinal edges, 
     two closure portions made of metal sheet which extend from the transversal end edges out of the plane of the frame portion and which close otherwise free end spaces between the combustion layer and the frame portion. 
     This allows to simplify the manufacturing of the tridimensional membrane by virtue of an easier forming of the perforated sheet of the diffuser layer by means of simple folding (about a single axis), e.g. in a press, and of the use of simple sheet “plug” portions for closing the end zones of the burner, which in the prior art required to form spherical caps of metal mesh. 
     The construction of the combustion surface made of a perforated metal sheet and its single curvature shape, thus without hole distortion, result in highly accurate, constant and repeatable properties (permeability, weight distribution, thermal capacity, thermal conductivity, mechanical strength, thermal expansion), which allow the combustion parameters to be better controlled. 
     The construction of the combustion surface and of the entire tridimensional combustion membrane of metal sheet obviates the need to use expensive materials, such as metal meshes or porous ceramic materials, and facilitates the connection of the combustion membrane made of metal sheet to the frame made of metal sheet. 
     In the scope of the present description and of the claims, the term “single curvature” means a curvature or a folding of a sheet, layer or tridimensional curved plane about a single axis or about multiple parallel axes, such as not to result in a distortion of the curved plane with respect to the initially flat shape. The term “single curvature” itself does not exclude the presence of flat zones (infinite curvature radius) which can be formed in combination, e.g. in an alternating manner, with curved zones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the invention and appreciate the advantages thereof, a description of some embodiments of the burner of the invention will be provided below by way of non-limiting example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a top view of a burner according to an embodiment of the invention, 
         FIG. 2  is a perspective view of the burner in  FIG. 1 , 
         FIG. 3  is a partial front view of the burner in  FIG. 1 , 
         FIG. 4  is an exploded view of a burner according to a further embodiment, 
         FIG. 5  is a view of the burner in  FIG. 4  in an assembled configuration, 
         FIG. 6  shows a frame with two closure end portions of a combustion membrane according to an embodiment, 
         FIG. 7  is a cross-section view of a burner according to a further embodiment, 
         FIG. 8  shows various embodiments of a diffuser layer of the burner seen in cross section, 
         FIG. 9  shows embodiments of the burner seen in longitudinal section. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the figures, a gas burner  1  for heaters comprises a planar metal frame portion  2  with an elongated opening  3  (e.g. extended along a longitudinal axis  4 ) defined by two longitudinal edges  5  extending in the direction of a longitudinal axis  4  and mutually opposite and facing each other, and by two end edges  6  which are transversal to the longitudinal axis  4  and also mutually opposite and facing each other. 
     Burner  1  further comprises a combustion membrane  7  in the form of a tridimensional shell, connected to the frame portion  2  along the longitudinal edges  5  and transversal edges  6 . The combustion membrane  7  has a diffuser layer  8  which forms an outer combustion surface  9  made of a perforated metal sheet, and which extends between the two longitudinal edges  5  with a single curvature out of the plane  10  of the frame portion  2 . In other words, the diffuser layer  8  made of a perforated metal sheet is connected to the two longitudinal edges  5  and extends from the longitudinal edges as an arched bridge with single curvature over opening  3 . 
     The combustion membrane  7  further has two closure portions  11  made of metal sheet which extend from the transversal edges  6  out of the plane  10  of the frame portion  2  and which close (otherwise free) end areas  12  between the diffuser layer  8  and the transversal edges  6  of the frame portion  2 . 
     This allows the manufacturing and forming of the tridimensional membrane  7  as a tridimensional shell or box to be simplified by virtue of an easier forming of the perforated sheet of the diffuser layer  8  by means of simple folding (about a single axis), e.g. in a press, and by virtue of the use of simple sheet “plug” portions for closing the end areas of burner  1 , which in the prior art required to form spherical caps of metal mesh. 
     The construction of the combustion surface  9  made of a perforated metal sheet and its single curvature shape, thus without distortion of the holes  13 , result in highly accurate, constant and repeatable properties (permeability, weight distribution, thermal capacity, thermal conductivity, mechanical strength, thermal expansion), which allow the combustion parameters to be better controlled. 
     The construction of the combustion surface  9  and of the entire tridimensional combustion membrane  7 , including the closure portions  11 , of metal sheet avoids the need to use expensive materials, such as metal meshes or porous ceramic materials, and facilitates the connection of the combustion membrane  7  made of metal sheet to the metal sheet frame  2 . 
     According to an embodiment, the diffuser layer  8  with the combustion surface  9  made of metal sheet has a sectional shape on a section plane transversal to the longitudinal axis  4  in the form of: 
     arc ( FIG. 8 a   ), 
     circular arc ( FIG. 8 b   ), 
     circular arc tapering or flattened in the direction of the frame ( FIG. 8 c   ), 
     arc composed of a plurality of curved lengths with different curvature radiuses ( FIG. 8 c   ), 
     arc composed of one or more curved lengths and one or more alternating straight lengths ( FIGS. 8 d, e   ), 
     arc composed of multiple rectilinear lengths which are mutually inclined ( FIG. 8 h   ), 
     arc with a flattened or rectilinear apical length ( FIG. 8 d   ), 
     arc with two opposite flattened or rectilinear side lengths ( FIGS. 8 e, f   ), 
     arc with two opposite flattened or rectilinear side lengths and a flattened or rectilinear apical length and two curved rounded lengths arranged between the side lengths and the apical length ( FIG. 8 g   ), 
     wavy ( FIG. 8 i   ). 
     In an exemplary embodiment, the diffuser layer  8  with the combustion surface  9  has a constant cross section along the longitudinal extension of the combustion surface  9 , i.e. along the longitudinal axis  4 , similar to an extruded shape. 
     Various possible tridimensional shapes of the combustion surface  9  made of metal sheet are thus possible: 
     a half pipe, in which the opening angle a of the profile of the combustion surface may vary according to the embodiment within reasonable limits, e.g. 180°+/−30° ( FIG. 8 k   ), 
     a circular half pipe, 
     a half pipe tapering or flattened in the direction of frame  2 , 
     or more generally a shape with constant cross section along the longitudinal axis  4 , in which such a cross section may consist of: 
     one or more curved lengths and one or more straight lengths in an alternating manner, 
     a plurality of curved lengths with different curvature radiuses, 
     an arc with a flattened or rectilinear apical length, 
     an arc with two opposite flattened or rectilinear side lengths, 
     a wavy shape. 
     According to an embodiment, the closure portions  11  have a wall  14  which is substantially flat and transversal to the plane  10  of the frame portion  2  and a folded edge  15  which connects the flat wall  14  to the diffuser layer  8 . 
     This facilitates the manufacturing of the closure portions  11  by means of a press and ensures the continuity of the combustion membrane  7  even in the presence of thermal expansions. 
     The folded edge  15  of the closure portion  11  may have an arched shape which is substantially compatible with the arched shape of the diffuser layer  8 . 
     The closure portions  11  may each form a wall  14  which is substantially flat and perpendicular to the plane  10  of the frame portion  2  ( FIGS. 2, 5, 6 ). Alternatively, the flat walls  14  may be inclined with respect to the plane  10  of the frame portion  2  towards opening  3  (in a converging manner,  FIG. 9 a   ) or away from opening  3  (in a diverging manner,  FIG. 9 b   ). 
     The closure portions  11  and/or the transversal edges  6  may comprise one or more folds or stiffening ribs, e.g. bulges, which could extend from frame  2  through the transversal edge  6  up into the closure portion  11  ( FIG. 6 ). 
     Thereby, the longitudinal thermal expansions of the diffuser membrane  7  can be prevented from spreading irreversibly, thus deforming the closure portions  11  and creating unplanned gas passage zones. 
     According to an embodiment, at least one of the closure portions  11  and the diffuser layer  8  is formed together with the frame portion  2  in a single piece of metal sheet ( FIGS. 4, 6 ). It is particularly advantageous to form at least one or both of the closure portions  11  together with the frame portion  2  in a single piece of metal sheet, e.g. by folding the closure portions  11  out of the plane  10  of frame  2  along the transversal edges  6  of opening  3  ( FIGS. 4, 6 ). 
     This saves material and connection costs and increases the tightness of the connection between the combustion membrane  7  and the frame  2 . 
     In this embodiment, the construction of the folded edge  15  may include a cut-out  20  of the sheet in the frame portion  2  at the ends of the longitudinal edges  5  in the corner zone with the transversal edges  6  ( FIGS. 4, 5 ). These cut-outs  20  may be advantageously sealed by means of localized welds  21  which create four irreversible fixing points between the diffuser layer  8  and the frame  2 . 
     Alternative fastenings between the closure portions  11  and the transversal edges  6  of the frame comprise, for example, welds, crimping, fitting, gluing or folding. 
     Alternatively or additionally, at least one of the closure portions  11  and the diffuser layer  8  is connected to the frame portion  2  by means of an elastic fit, preferably a snap fit. For this purpose, it is advantageous to form a stop groove  16  in at least one of the two components made of metal sheet and an edge  17  in the other one, which is adapted to be snapped fit or generally placed in the stop groove  16  ( FIG. 7 ). 
     Such a fitting could be completed later by means of spot or seam welding to make the connection irreversible. 
     The fixing between the combustion membrane  7  (in particular the diffuser layer  8 ) and the frame portion  2  may be made by means of a fixing edge  17  which protrudes from an outer periphery of the combustion membrane  7  (in particular from the diffuser layer  8 ) and which is fixed, e.g. welded, crimped, fitted, glued or sandwiched, to the corresponding edge (in particular the longitudinal edge  5 ) of the frame portion  2 . The fixing edge  17  may be made of the same metal sheet of the combustion membrane  7  (in particular of the diffuser layer  8 ), e.g. by bending a peripheral region thereof, but is superimposed on the frame portion  2  on the combustion side  18  or preferably on the feeding side  19  ( FIG. 4 ) and does not allow the passage of fuel mix or the formation of flame. Therefore, in the present description, the fixing edge  17  is not considered within the definition of “diffuser layer  8 ”. 
     Advantageously, the diffuser layer  8  can be inserted from the feeding side  19  into the opening  3  of the frame portion  2  to engage the fixing edge  17  and/or a stop groove  16  with a longitudinal edge  5  of the opening  3 , and place the end portions against the folded edges  15  of the closure portions  11  ( FIG. 4 ). 
     In accordance with a further embodiment, burner  1  comprises a distributor  22  for distributing the fuel mix in a desired manner towards the diffuser layer  8 . Distributor  22  comprises one or more walls made of metal sheet with a plurality of through openings  23  and may form an inner layer (feeding side  19 ) of the combustion membrane  7  or, alternatively, a component spaced apart from the diffuser layer  8 . 
     Distributor  22  may be connected to the frame portion  2  or to the diffuser layer  8  by means of an elastic fit, preferably a snap fit. For this purpose, it is advantageous to form a stop groove in at least one of the two components made of metal sheet and an edge in the other, which is adapted to be snap fit or generally placed in the stop groove. The snap fitting could be completed later by means of spot or seam welding to make the connection irreversible. 
     In an embodiment, burner  1  comprises one or more folded tabs, formed in at least one of or in all the components (frame  2 , diffuser layer  8 , closure portions  11  or distributor  22 ) and which engage one or more of said other components to block the burner assembly  1  in a permanent manner. Distributor  22  may also comprise a fixing edge  24  configured in a manner identical or similar to that described with reference to the fixing edge  17  of the diffuser layer  8 , and may be inserted from the feeding side  19  into the opening  3  of the frame portion  2  to engage the fixing edge  24  and/or a stop groove with a corresponding edge of the opening  3  or of the combustion membrane  7  ( FIG. 4 ). 
     According to a further embodiment, the distributor  22  and the diffuser layer  8  are formed together in a single piece of metal sheet. In this example, the metal sheet may be folded so as to form a double wall structure extending as an arc out of the plane  10  of the frame portion  2  and further forming a connection edge with the frame portion  2  ( FIG. 7 ). 
     In an exemplary embodiment, the longitudinal edges  5  are rectilinear and parallel to each other and parallel to the longitudinal axis of opening  4  and burner  1 . The transversal edges  6  are also rectilinear, parallel to each other and preferably perpendicular to the longitudinal edges  5 . Thereby, opening  3  has an elongated rectangular shape. The closure portions  11  are preferably free from holes or are gas-impermeable, even if in particular embodiments a perforation and flame formation could also be contemplated in the closure portions  11 . The entire combustion membrane  7  is preferably free from metal mesh or fabric. 
     Those skilled in the art can make further changes and variants all within the scope of protection defined by the claims in order to satisfy contingent, specific needs.