Patent Publication Number: US-2023147789-A1

Title: Combustion membrane for a gas burner

Description:
The present application claims the benefit of priority to Italian Patent Application No. 102021000026435, filed on Oct. 14, 2021, the entire contents of which are hereby incorporated by reference. 
     The present invention relates to a combustion membrane for a burner, in particular for a completely or partially premixed burner, for example for boilers, swimming pool heaters, hot air generators, or ovens for industrial processes. 
     The burners of the prior art comprise a combustion membrane having:
         an inner surface in flow communication with the feeding system,   a diffuser layer forming an outer surface (or combustion surface) of the membrane, intended to be facing the combustion chamber,       

     in which the combustible gas or the mixture of combustible gas and combustion supporting air (hereafter in the description, the term “gas” denotes both a “combustible gas” and a “mixture of combustible gas and combustion supporting air”) is conveyed through the combustion membrane at the outer side of which combustion takes place, in the form of a flame pattern on the combustion surface. 
     Furthermore, a distributor may be provided upstream of the diffuser layer (with reference to the flow direction of the gas) to distribute the gas in the desired manner towards the combustion membrane. The known distributors are generally made as walls with a plurality of through openings, for example made of perforated sheet, and may form an “inner” layer of the combustion membrane or alternatively, a component which is spaced apart from the combustion membrane. 
     The heat generated by the combustion is directed by means of the hot combustion gases (convection) and by means of heat radiation to a heat exchanger to heat a fluid, e.g., water, which is then conveyed to a utility, for example a heating system of an industrial process, residential environments or the like and/or domestic water. 
     For desirable and satisfactory use of the burner and combustion system, it is desirable, on the one hand, to be able to vary the heating power of the burner and gas flow rate through the combustion membrane in a controlled manner and, on the other hand, to ensure the safest, quietest and longest-lasting operation possible. 
     To meet the aforesaid requirements in an increasingly satisfactory manner, it is necessary to reduce or prevent some phenomena which may occur during a non optimal combustion process, including:
         a localized or extensive detachment of the flame from the combustion surface,   a localized or extensive overheating of the combustion membrane,   a highly uneven distribution of combustion membrane temperature,   a highly uneven distribution of gas flow velocity across the combustion membrane,   a low or reduced thermal insulation function of the combustion membrane or a single combustion membrane layer during burner operation.       

     These undesirable phenomena cause high combustion noise, limited burner resistance to high temperatures, damage to the burner structure itself, in particular to sheet parts of the combustion membrane, as well as the occurrence of phenomena of lack of flame control. 
     The causal connections between the aforesaid negative phenomena and their detrimental effects on satisfactory combustion have been extensively described in detail in the technical and patent literature concerning gas burners, and are not repeated here for the sake of brevity. 
     To reduce or suppress some or all of the listed negative phenomena, it is known to equip gas burners with accessory structures, e.g., inserts or diaphragms, to locally bias the inert masses of the burner and the fluid dynamic conditions of the gas flow and, thus, the fluid dynamic and mechanical behavior of the burner. 
     These noise reduction accessories must be optimized on a case-by-case basis for the fluid dynamic, mechanical, dimensional, and combustion conditions of the individual burner model, and their efficacy is often limited to undesirably narrow (gas flow) operating ranges. 
     Therefore, the need is felt for additional means and strategies to improve gas burners, particularly premixed or partially premixed gas burners, and to further optimize combustion performed by means of such burners. 
     Attempts have been made to respond to the described requirements by making an outer side of the combustion membrane of metal fabric or metal mesh to achieve a desired thermal insulation effect of the combustion membrane and thermal protection of portions of the burner upstream of the combustion membrane, and to achieve a better distribution of the gas permeability of the combustion membrane and finally to achieve better flame stability. 
     However, the attempts to make meshes and metal fabrics from metal yarns in the desired thickness, permeability and structure configurations have proved difficult, by means of available weaving looms or by means of the available knitting machines, which is why the characteristics of metal fabrics and metal meshes for combustion membranes available to date are considerably limited and dictated by the technological constraints of industrial weaving and knitting technology, and no experimental weaves or knits, crafted with more freely definable interlacing and/or yarn structure characteristics appear to have been experimented. An example is the use for metal fabrics and metal meshes for combustion membranes of only yarns with parallel fibers as smooth and long as the yarn itself, which, according to the inventors, excessively limits the possibilities of defining the functional characteristics of the combustion membrane fabric or mesh in a more targeted fashion. 
     It is the object of the present invention to provide a new and innovative combustion surface made of mesh or fabric, and combustion membrane for gas burners and a gas burner, having features such to avoid at least some of the drawbacks of the prior art. 
     These and other objects are achieved by means of a combustion membrane for a gas burner according to claim  1 . Some advantageous embodiments are the subject of the dependent claims. 
     According to an aspect of the invention, a combustion membrane for a gas burner has an inner side, to which combustible gas is conveyed, and an outer side, on which combustion of the combustible gas occurs after it has crossed through the combustion membrane, said combustion membrane comprising a fabric or mesh of interlaced metal threads, said fabric or mesh having two opposite interlacing surfaces, which respectively form a combustion surface exposed on the outer side and an inner surface facing towards the inner side, wherein the metal threads are formed by twisted metal fibers to form a spun yarn and:
         the individual metal fibers are shorter than the yarn formed therefrom, and free ends of the metal fibers protrude divergently from the yarn along its longitudinal extension and make the yarn hairy, or   the metal thread is a yarn of mass per length in the range from 0.8 g/m to 1.4 g/m.       

     By virtue of the use of “large” fibers and/or “large” threads, which are heavy in themselves and diametrically coarse or “puffy” due to their hairiness, it is possible to make similarly “coarse” or “heavy” fabrics and knits which inherently have a lower thread count density per unit area and thus a higher and desired gas permeability, also in the presence of greater thickness (and thus greater thermal insulation properties) and/or greater mass (and thus greater thermal inertia), than the “light” or “thin” fabrics of the prior art. 
    
    
     
       In order to better understand the invention and appreciate the advantages thereof, a description is provided below of certain non-limiting exemplary embodiments, with reference to the accompanying drawings, in which: 
         FIG.  1    is a diagrammatic view of a gas combustion system, for example for a boiler, with a burner provided with a combustion membrane, 
         FIGS.  2  and  3    are perspective and sectional views of an exemplary burner, provided with a combustion membrane, 
         FIG.  3 A  is an enlarged and diagrammatic section view of a combustion membrane according to an embodiment of the invention, 
         FIG.  4    shows a burner with a combustion membrane according to an embodiment, 
         FIG.  5    shows a detail of a metal thread bound with a binding thread according to an embodiment, 
         FIG.  6    shows a twisted and “hairy” metal thread (a so-called “hairy spun yarn”) of the metal fabric or thread mesh according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE COMBUSTION SYSTEM  1   
     With reference to  FIG.  1   , a gas combustion system  1 , e.g., for a boiler, comprises: 
     a burner  2  for producing heat by means of combustion of combustible gas and combustion supporting air, 
     a feeding system  3  for feeding the combustible gas or mixture of combustible gas and combustion supporting air to the burner  2 , said feeding system  3  comprising a gas control device  4  for controlling a flow of the combustible gas (for example, an electrically controllable gas valve or gas conveying means or gas suction means) and, if provided, an air control device  5  (e.g., air conveying means or air suction means, an electric fan, a radial fan, an air valve or gate air valve) to control a flow of combustion supporting air, 
     an electric ignition device  6  for igniting the combustion, e.g., an ignition electrode adapted to generate a spark, 
     possibly, an ionization sensor  7  arranged at a combustion area  8  of the burner  2  and adapted to provide an electrical ionization signal which varies as a function of a combustion condition of the burner  2 , 
     an electronic control unit  9  connected to the feeding system  3 , the ignition device  6  and the ionization sensor  7 , the electronic control unit  9  having a combustion control module  10  adapted to control the ignition device  6  and the feeding system  3  depending on an operating program and user commands and depending on the ionization signal, 
     Detailed Description of the Burner  2   
     According to an embodiment ( FIGS.  2 ,  3   ), the gas burner  2  comprises:
         a support wall  11  forming one or more inlet passages  12  for the introduction (of the mixture) of combustible gas  13  (and combustion supporting air) into the burner  2 ,   a tubular combustion membrane  14 , e.g., cylindrical, and coaxial with respect to a longitudinal axis  15  of the burner  2  and having a first end connected to the support wall  11  in flow communication with the inlet passage  12 , a second end closed by a closing wall  16 , and a perforation for the passage of the gas  13  and of the air mixture from inside the burner  2  to an outer side  17  of the combustion membrane  14  where the combustion occurs (combustion area  8 ).       

     The burner  2  in  FIG.  3    further shows a tubular silencing accessory (without reference numeral), which is optional and could be reduced in size or completely eliminated. 
     According to a further embodiment, the combustion membrane  14  can be substantially flat, e.g., planar or curved or convex, or however of non-tubular or non-cylindrical shape, and having a peripheral edge connected to the support housing wall  11  in flow communication with the inlet passage  12 , as well as a perforation for the passage of the gas  13  or of the gas-air mixture from inside burner  2  to an outer side  17  of the combustion membrane  14  where the combustion occurs (combustion area  8 ). 
     In analogy with prior solutions with conventional combustion membranes, according to an embodiment, in the burner  2 , upstream of the combustion membrane  14  (with reference to the flow direction of the combustible gas  13 ) and spaced apart therefrom, a perforated distributor wall can be positioned in order to distribute the combustible gas  13  in a desired manner towards the combustion membrane  14 . 
     Detailed Description of the Combustion Membrane  14   
     The combustion membrane  14  having an inner side  18  to which a combustible gas  13  is conveyed and an outer side  17  on which combustion of the combustible gas  13  occurs after it has crossed through the combustion membrane  14 , said combustion membrane  14  comprising a fabric or mesh, indicated as a whole by reference numeral  21 , of interlaced metal threads  22 , having two opposite interlacing surfaces  19 ,  20 , which respectively form a combustion surface  19  exposed on the outer side  17  and an inner surface  20  facing towards the inner side  18 , wherein the metal threads  22  are formed by metal fibers  22 ′ twisted to form a spun yarn, and:
         the individual metal fibers  22 ′ are shorter than the yarn  22  formed therefrom, and free ends  22  of the metal fibers  22 ′ protrude divergently from the yarn  22  along its longitudinal extension and make the yarn  22  hairy, or   the metal thread  22  is a yarn  22  of mass per length in the range from 0.8 g/m to 1.4 g/m.       

     By virtue of the use of “large” fibers and/or “large” threads, which are heavy in themselves and diametrically coarse or “puffy” due to their hairiness, it is possible to make similarly “coarse” or “heavy” fabrics and knits which inherently have a lower thread count density per unit area and thus a higher and desired gas permeability, also in the presence of greater thickness (and thus greater thermal insulation properties) and/or greater mass (and thus greater thermal inertia), than the “light” or “thin” fabrics of the prior art. 
     The fabric/mesh  21  is advantageously supported by and in contact with a support layer  38 , e.g., a perforated sheet or wire mesh support, arranged on the inner side  18  of the combustion membrane  14  and forming part of the combustion membrane  14  itself or forming only a support structure for the combustion membrane  14 . 
     Thus, the combustion membrane  14  can be a single-layer structure (including only the fabric/mesh  21 ) or a multilayer structure (containing at least fabric/mesh  21  and the support layer  38  ( FIGS.  3 ,  3 A ). 
     The fabric/mesh  21  can only consist of a fabric made from warp and weft threads by means of a weaving loom, thus excluding meshes made by interlacing a continuous coil thread. 
     Similarly, the fabric/mesh  21  can only consist of a mesh made by interlacing a continuous coil thread, thus excluding fabrics made with warp and weft threads using a weaving loom. 
     Detailed Description of the Metal Thread  22   
     According to an embodiment, the metal threads  22  comprise bundles of metal fibers  22 ′, e.g., interlaced, spun or twisted, e.g., of the long fiber filament or short fiber filament type. 
     The metal threads  22  can be at least or only initially bonded by means of a binder, e.g., water-soluble or non-soluble bonding thread  37 , e.g., PVA or polyester, or by means of a water-soluble or non-soluble bonding adhesive, e.g., polymeric. 
     According to an embodiment, the metal threads  22  can be chosen in the group of so-called “Staple Spun Yarn,” “Folded Yarn,” “Plied Yarn,” “Doubled Yarn” as defined, for example, in “Fundamentals of Yarn Technology” © 2003, CRC Press LLC, Chapter 1.2.1, Table 1.1. 
     Furthermore, in this description, “Plied Yarn” is specifically understood to indicate a yarn consisting of two or more separate subyarns twisted together. 
     The subyarns, in turn, can each consist of two or more tertiary yarns twisted together, respectively, forming a so-called “multi-folded yarn.” 
     According to an embodiment, the metal threads  22  are not of the “LONG FILAMENT” type. 
     Advantageously, the fabric/mesh  21  can be a “heavy” or “coarse” fabric or mesh, i.e., having a weight per area either of fabric equal to or greater than 1.3 kg/m 2  or in the range from 1.3 kg/m 2  to 1.6 kg/m 2 . 
     Advantageously, the metal thread  22  is a yarn of weight per length in the range from 0.8 g/m to 1.4 g/m, advantageously from 0.9 g/m to 1.1 g/m, e.g., 1 g/m. 
     Advantageously, the metal thread  22  consists of fibers with diameters in the range from 30 micrometers to 50 micrometers, e.g., approximately 40 micrometers. 
     The “big” fibers  22 ′ and “big” threads  22  allow economical and industrially advantageous manufacture of “coarse” fabrics which are not excessively gas impermeable. 
     According to an embodiment, the material of the metal threads  22  or metal fibers  22 ′ can be, for example, a ferritic steel, or a FeCrAl alloy, e.g., doped by means of Yttrium, Hafnium, Zirconium. 
     The metal thread  22  may be, for example, a Y, Hf, Zr doped FeCrAl alloy yarn, weighing 1 g/m and composed of fibers 40 micrometers in diameter, i.e., spun yarn, e.g., with 30 to 150 twists per meter, possibly with fiber ends  22 ′ protruding divergently from the yarn  22  so as to be hairy (“hairy yarn”), with fibers  22 ′ shorter than the yarn  22  itself, e.g., with fiber lengths in the range of 7 cm to 30 cm, not necessarily but possibly restrained by means of a binding thread  37 , possibly made of PVA or polyester, and having, for example, the same “doped” composition. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 C 
                 Mn 
                 Si 
                 Al 
                 Cu 
                 Cr 
                 Y 
                 Hf 
                 Zr 
                 P 
                 S 
                 Ti 
                 N 
                 Ni 
                 Fe 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Min. 
                   
                   
                   
                 5.5 
                   
                 19 
                 0.03 
                 0.05 
                 0.03 
                   
                   
                   
                   
                   
                 rest 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 or 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 0.03 
               
               
                 Max. 
                 0.04 
                 0.4 
                 0.5 
                 6.5 
                 0.03 
                 22 
                   
                   
                   
                 0.03 
                 0.03 
                 0.5 
                 0.02 
                 0.3 
               
               
                   
               
            
           
         
       
     
     Description of Surface Profile Characteristics of the Fabric/Mesh  21   
     According to an aspect of the invention, both interlacing surfaces  19 ,  20  form ribs  23  in high relief alternating with valleys  24  in low relief, and both the ribs  23  and valleys  24  have an extent, in at least one direction in the plane of the fabric/mesh  21  greater than three, preferably greater than four, times the thickness of the metal threads  22 . 
     By virtue of the ribs  23  in high relief alternating with the valleys  24  in low relief, the metal fabric/mesh  21  of the combustion membrane  14  achieves a technical effect of discrete, repetitive but not continuous spacer, and the thickness of the fabric/mesh itself is not completely filled with metal material, which improves the thermal insulation capacity and allows a gas distribution through the metal fabric/mesh not only in the direction orthogonal to the plane of the fabric/mesh but also in the plane of the fabric/mesh itself. 
     This avoids overheating of the combustion membrane  14 , improves the thermal insulation of the combustion membrane  14 , reduces the risk of flame detachment, and improves the distribution of gas flow velocity  13  through the combustion membrane  14 . 
     Description of Permeability Characteristics of the Fabric/Mesh  21   
     The fabric/mesh  21  is permeable to gas and has localized first areas  26  with reduced permeability alternated with localized second areas  27  with higher permeability than the first areas  26 . 
     According to an embodiment, said first areas  26  and second areas  27  have an extension, in at least one direction in the plane of the fabric/mesh  21 , greater than three times, preferably greater than four times the thickness of the metal thread  22 . 
     The difference in gas permeability between first areas  26  and second areas  27  is e.g. visible and verifiable against the light as a difference in light transmission through the fabric/mesh  21 . 
     The first localized areas  26  with reduced permeability alternating with the second localized areas  27  with higher permeability than the first localized areas  26  proved advantageous with reference to a reduction in the risk of flame detachment and with reference to a better distribution of gas flow velocity across the combustion membrane  14 . 
     Advantages of the Invention 
     By virtue of the use of “large” fibers and/or “large” threads, which are heavy in themselves and diametrically coarse or “puffy” due to their hairiness, it is possible to make similarly “coarse” or “heavy” fabrics and knits which inherently have a lower thread count density per unit area and thus a higher and desired gas permeability, also in the presence of greater thickness (and thus thermal insulation properties) and/or greater mass (and thus thermal inertia), than the “light” or “thin” fabrics of the prior art. 
     By virtue of the ribs in high relief alternating with the valleys in low relief, the metal fabric/mesh of the combustion membrane achieves a technical effect of discrete, repetitive but not continuous spacing, and the thickness of the fabric/mesh itself is not completely filled with metal material, which improves the thermal insulation capacity and allows a gas distribution through the metal fabric/mesh not only in the direction orthogonal to the plane of the fabric/mesh but also in the plane of the fabric/mesh itself. 
     This avoids overheating of the combustion membrane, improves the thermal insulation of the combustion membrane, reduces the risk of flame detachment, and improves the distribution of gas flow velocity through the combustion membrane. 
     The first localized areas with reduced permeability alternating with the second localized areas with higher permeability than the first localized areas proved advantageous with reference to a reduction in the risk of flame detachment and with reference to a better distribution of gas flow velocity across the combustion membrane. 
     Therefore, the individual aspects of the invention are not only individually significant in solving the problems of the prior art, but a combination thereof provides further synergy.