EP4310395A1

EP4310395A1 - Combustion membrane for a gas burner made with continuous fiber threads

Info

Publication number: EP4310395A1
Application number: EP23185731.9A
Authority: EP
Inventors: Massimo Gilioli; Andrea Casoni; Alberto PEZZIN
Original assignee: Beckett Thermal Solutions SRL
Current assignee: Beckett Thermal Solutions SRL
Priority date: 2022-07-20
Filing date: 2023-07-17
Publication date: 2024-01-24
Also published as: CN117433016A; IT202200003048U1; US20240027064A1

Abstract

A combustion membrane (14) for a gas burner (2) comprises a fabric or mesh (21) of interlaced metal wires (22), having two opposite interlacing surfaces (19, 20) forming a combustion surface (19) and an inner surface (20) of the fabric/mesh (21), respectively, wherein the metal wires (22) are formed by continuous metal fibers (22') which form a yarn (22) of mass by length in the range from 0.8 g/m to 1.4 g/m.

Description

The present invention relates to a combustion membrane for a burner, in particular for a completely or partially premixed burner, such as 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 face the combustion chamber,

where the combustible gas or the mixture of combustible gas and combustion air (hereafter in the description, the term "gas" denotes both a "combustible gas" and a "mixture of combustible gas and combustion air") is conveyed through the combustion membrane on the outer side of which the combustion occurs, in the form of a flame pattern on the combustion surface.
Moreover, a distributor can be provided upstream of the diffuser layer (with reference to the flow direction of the gas) in order 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 metal, and can form an "inner" layer of the combustion membrane or alternatively a component spaced apart from the combustion membrane.
The heat generated by the combustion is directed by the hot combustion gases (convection) and by heat radiation to a heat exchanger for heating a fluid, e.g., water, which is then conveyed to a utility, such as a heating system of an industrial process, living spaces, or the like, and/or sanitary water.
For desirable and satisfactory use of the burner and the combustion system, it is desirable, on the one hand, to vary the heating power of the burner and the gas flow rate through the combustion membrane in a controlled manner, and on the other hand, to ensure an operation that is as safe, silent and long-lasting as possible.
In order to better meet the aforesaid needs, it is necessary to reduce or prevent some phenomena which can 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 the combustion membrane temperature,
a highly uneven flow velocity distribution of the gas across the combustion membrane,
a low or reduced thermal insulation function of the combustion membrane or a single layer of the combustion membrane during burner operation.

These undesirable phenomena cause high combustion noise, limited burner resistance to high temperatures, damage to the structure of the burner itself, in particular to sheet metal parts of the combustion membrane, as well as the occurrence of uncontrollable flame phenomena.
The causal connections between the aforesaid adverse phenomena and the detrimental effects thereof on a satisfactory combustion have been extensively described in the technical and patent literature concerning gas burners, and are not repeated here for brevity.
In order to reduce or suppress some or all of the listed adverse phenomena, it is known to provide 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 operating (gas flow) 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 the combustion performed by such burners.
Attempts have been made to respond to the described needs by making an outer side of the combustion membrane of metal fabric or metal mesh in order to achieve a desired effect of thermal insulation of the combustion membrane and thermal protection of burner portions upstream of the combustion membrane, and in order to achieve a better distribution of the gas permeability of the combustion membrane and finally in order to achieve better flame stability.
However, the attempts to make metal meshes and fabrics from metal yarns in the desired thickness, permeability, and structure configurations have proved to be difficult to implement, by means of weaving looms or by means of the available knitting machines, this being the reason why the characteristics of metal fabrics and metal meshes for combustion membranes available to date are considerably limited and dictated by technological constraints of the industrial weaving and knitting technology, and no experimental weaving or knitting, crafted with more freely definable interlacing and/or yarn structure characteristics, appear to have been attempted.
Therefore, it is the object of the present invention to provide a new and innovative combustion surface made of mesh or fabric, as well as combustion membrane for gas burners and a gas burner, having features such as to obviate at least some of the drawbacks of the prior art.
These and other objects are achieved by 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 a combustible gas is conveyed, and an outer side, on which the combustion of the combustible gas occurs after it has crossed the combustion membrane, said combustion membrane comprising a fabric or mesh of interlaced metal wires, having two opposite fabric surfaces forming a combustion surface exposed on the outer side and an inner surface facing the inner side, respectively, where the metal wires (22) are formed by continuous metal fibers (22') which form a yarn (22) with mass by length in the range from 0.8 g/m to 1.4 g/m.
Advantageously, the continuous metal fibers (22') are not mutually twisted and form said untwisted yarn (22).
With a further advantage, the individual metal fibers (22') each have a length substantially corresponding to the length of the yarn (22) formed therewith.
Preferably, the metal fibers (22') have a waviness which gives an additional extension to the yarn (22) in a direction transverse to a longitudinal direction thereof.
Nevertheless, due to the extension continuity of the fibers along the entire yarn and, by giving the yarn an ability to bulge diametrically, it is possible to make advantageously "coarse" fabrics and meshes which inherently exhibit low fiber density per individual yarn, as well as a lower number of threads per unit area of the combustion membrane, 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.
The ability of the yarn to bulge is further improved by the waviness or crimping of the fibers, as well as by the fact that the fibers extend next to on another so that they can remain "separate" instead of being twisted together.
In order to better understand the invention and appreciate the advantages thereof, a description is provided below of some non-limiting exemplary embodiments, with reference to the accompanying drawings, in which:

figure 1 is a diagrammatic view of a gas combustion system, for example for a boiler, with a burner provided with a combustion membrane,
figures 2 and 3 are perspective and sectional views of an exemplary burner, provided with a combustion membrane,
figure 3A is an enlarged and diagrammatic section view of a combustion membrane according to an embodiment of the invention,
figure 4 shows a burner with a combustion membrane according to an embodiment,
figure 5 shows a detail of a metal yarn bound with a bonding thread according to an embodiment,
figures 5.1 and 5.2 show further details of a metal yarn of a combustion membrane, according to embodiments.

Detailed description of the combustion system 1

With reference to figure 1, a gas combustion system 1, e.g., for a boiler, comprises:

a burner 2 for producing heat by combustion of combustible gas and combustion air,
a feeding system 3 for feeding the combustible gas or mixture of gas and combustion air to the burner 2, said feeding system 3 comprising a gas control device 4 for controlling a flow of the combustible gas (e.g., 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 an air gate valve) to control a flow of combustion 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 varying 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 (figures 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 air) into the burner 2,
a tubular combustion membrane 14, e.g., cylindrical in shape, and coaxial 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 gas 13 or the gas-air mixture to pass from inside the burner 2 to an outer side 17 of the combustion membrane 14 where the combustion occurs (combustion area 8).

A tubular silencing accessory (without reference numeral) is also shown in the burner 2 in figure 3, 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 wall 11 in flow communication with the inlet passage 12, as well as a perforation for the gas 13 or the gas-air mixture to pass from inside the burner 2 to an outer side 17 of the combustion membrane 14 where the combustion occurs (combustion area 8).
Similarly to 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 has an inner side 18 to which a combustible gas 13 is conveyed and an outer side 17 on which the combustion of the combustible gas 13 occurs after it has crossed the combustion membrane 14, said combustion membrane 14 comprising a fabric or a mesh, indicated as a whole by reference numeral 21, of interlaced metal wires 22 having two opposite interlacing surfaces 19, 20 forming a combustion surface 19 exposed on the outer side 17 and an inner surface 20 facing the inner side 18, respectively, where the metal wires (22) are formed by continuous metal fibers (22') which form a yarn (22) with mass by length in the range from 0.8 g/m to 1.4 g/m.
Advantageously, the continuous metal fibers (22') are not mutually twisted and form said untwisted yarn (22).
With a further advantage, the individual metal fibers (22') each have a length substantially corresponding to the length of the yarn (22) formed therewith.
Preferably, the metal fibers (22') have a waviness which gives an additional extension to the yarn (22) in a direction transverse to a longitudinal direction thereof.
The continuity of the fibers 22' and the waviness of the yarn 22 by mechanically crimping the entire yarn 22 or the individual fibers 22' makes the yarn 22 advantageously "bulky," and forms an improved alternative to a twisted yarn obtained by conventional spinning techniques (short fibers twisted together, the so-called "twisted spun yarn"), which, when applied to the metal yarns and fibers considered here, exhibits considerable complications of the spinning and weaving process.
The fabric/mesh 21 is advantageously supported by and in contact with a support layer 38, e.g., a perforated sheet metal or a support metal net, 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.
The combustion membrane 14 can thus be a single-layer structure (including only the fabric/mesh 21) or a multilayer structure (containing at least the fabric/mesh 21 and the support layer 38 (Figures 3, 3A).
The fabric/mesh 21 can only consist of a fabric made with warp and weft threads by means of a weaving loom, thus excluding meshes made by interlacing a thread from a continuous coil.
Similarly, the fabric/mesh 21 can only consist of a mesh made by interlacing a thread 22 from a continuous coil, thus excluding fabrics made with warp and weft threads using a weaving loom.

Detailed description of the metal wire 22

The metal wires (22') are formed by metal fibers (22') forming a yarn (22) of mass per length in the range from 0.8 g/m to 1.4 g/m.
According to an embodiment, the continuous metal fibers (22') are not mutually twisted and form said untwisted yarn (22).
According to an embodiment, the individual metal fibers (22') each have a length substantially corresponding to the length of the yarn (22) formed therewith.
According to an embodiment, the metal fibers (22') have a waviness which gives an additional extension to the yarn (22) in a direction transverse to a longitudinal direction thereof.
The waviness is a waviness with less wavelength and less amplitude than the wavelength and amplitude of a knitting or weaving waviness of the fabric or mesh (21).
According to an embodiment, said waviness is formed over the entire length of the yarn (22).
According to an embodiment, the continuous metal fibers (22') of the same yarn (30) are detached from one another, at least in stretches, along more than 50%, or more than 22%, of the overall length of the yarn (22), thus making the yarn (22) bulged.
According to an embodiment, the metal fibers (22') have a circular cross-section.
According to an embodiment, the fabric/mesh (21) has a mass per area either equal to or greater than 1.3 kg/m², or in the range from 1.2 kg/m² to 1.5 kg/m².
According to an embodiment, the metal wires 22 comprise bundles of metal fibers 22' of the "non-twisted long fiber filament" type, but advantageously locally wavy, preferably over substantially the entire length of the wire 22.
The metal wires 22 can be at least or only initially bound by means of a binder, e.g., a water-soluble or non-soluble bonding thread 37, e.g., made of PVA or polyester, or by means of a water-soluble or non-soluble bonding adhesive, e.g., polymeric.
According to an embodiment, the metal wires 22 can be selected from the group of so-called long or continuous filament crimped yarns as defined, for example, in "Fundamentals of Yarn Technology" © 2003, CRC Press LLC, Chapter 1.2.1, Table 1.1.
In order to ensure the substantially continuous extension of both the yarn 22 and the individual fibers 22', as well as the manufacture and weaving or knitting thereof, the yarn and/or fibers can have knotted connecting stitches, preferably narrow.
According to an embodiment, the metal wires 22 are of the "long filament" type.
Advantageously, the fabric/mesh 21 is a "heavy" or "coarse" fabric or mesh, i.e., having a weight per area of fabric either equal to or greater than 1.2 kg/m² or in the range from 1.2 kg/m² to 1.5 kg/m², preferably of 1.3 kg/m².
Advantageously, the metal wire 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., of 1 g/m.
Advantageously, the metal wire 22 consists of fibers with diameter in the range from 30 micrometers to 50 micrometers, e.g., of about 40 micrometers.
The "big" fibers 22' and the "big" wires 22 allow for an economical and industrially advantageous manufacture of "coarse" fabrics/meshes 21 which are not excessively impermeable.
According to an embodiment, the material of the metal wires 22 or metal fibers 22' can be, for example, a ferritic steel, or an FeCrAl alloy, e.g., doped with Yttrium, Hafnium, Zirconium.
The metal wire 22 can be, for example, a yarn made of FeCrAl alloy doped with Y, Hf, Zr, weighing 1 g/m and consisting of fibers with a diameter of 40 micrometer, either wavy or crimped, such that it is less one-dimensional and more transversely bulged. The wire 22 is possibly held by a bonding thread 37, e.g., made of PVA or polyester, and having, for example, the following "doped" composition:

C Mn Si Al Cu Cr Y Hf Zr P S Ti N Ni Fe

Min. 5.5 19 0.03 0.03 0.03 rest

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 embodiment of the invention, both interlacing surfaces 19, 20 form high-relief ribs 23 alternating with low-relief valleys 24, and both the ribs 23 and the valleys 24 have an extension, in at least one direction in the plane of the fabric/mesh 21, three times greater, preferably four times greater than the thickness of the metal wires 22.
By virtue of the high-relief ribs 23 alternating with the low-relief valleys 24, the metal fabric/mesh 21 of the combustion membrane 14 achieves a technical effect of a discrete, repetitive but not continuous spacer, and a thickness of the fabric/mesh itself 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 obviates an overheating of the combustion membrane 14, improves the thermal insulation of the combustion membrane 14, reduces the risk of flame detachments, and improves the flow velocity distribution of the gas 13 across the combustion membrane 14.

Description of permeability characteristics of the fabric/mesh 21

The fabric/mesh 21 is permeable to gases and has localized first areas 26 with low permeability alternating 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, three times greater, preferably four times greater than the thickness of the metal wire 22.
For example, the difference in gas permeability between the first areas 26 and the second areas 27 is visible and verifiable against the light as a difference in light transmission through the fabric/mesh 21.
The first localized areas 26 with low permeability alternating with the second localized areas 27 with higher permeability than the first localized areas 26 proved to be advantageous with reference to a reduction in the risk of flame detachments and with reference to a better flow velocity distribution of the gas across the combustion membrane 14.

Advantages of the invention

By using continuous fibers and "large" threads, which are diametrically coarse per se or "bulged", it is possible to make similarly "coarse" or "heavy" fabrics and meshes 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 thicknesses (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 high-relief ribs alternating with the low-relief valleys, the metal fabric/mesh of the combustion membrane achieves a technical effect of a discrete, repetitive but not continuous spacing, and a thickness of the fabric/mesh itself 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 obviates the overheating of the combustion membrane, improves the thermal insulation of the combustion membrane, reduces the risk of flame detachments, and improves the flow velocity distribution of the gas across the combustion membrane.
The first localized areas with low permeability alternating with the second localized areas with higher permeability than the first localized areas proved to be advantageous with reference to a reduction in the risk of flame detachments and with reference to a better flow velocity distribution of the gas across the combustion membrane.
Therefore, the individual aspects of the invention are not only individually significant to solve the problems of the prior art, but a combination thereof is further synergistic.

Claims

A combustion membrane (14) for a gas burner (2), said combustion membrane (14) having an inner side (18) to which a combustible gas (13) is conveyed and an outer side (17) on which the combustion of the combustible gas (13) occurs after it has crossed the combustion membrane (14), said combustion membrane (14) comprising a fabric or mesh (21) of interlaced metal wires (22), having two opposite interlacing surfaces (19, 20) forming a combustion surface (19) exposed on the outer side (17) and an inner surface (20) facing the inner side (18), respectively, wherein the metal wires (22) are formed by continuous metal fibers (22') which form a yarn (22) with mass by length in the range from 0.8 g/m to 1.4 g/m.
A combustion membrane (14) according to claim 1, wherein said continuous metal fibers (22') are not mutually twisted and form said untwisted yarn (22).
A combustion membrane (14) according to claim 1 or 2, wherein the individual metal fibers (22') each have a length substantially corresponding to the length of the yarn (22) formed therewith.
A combustion membrane (14) according to any one of the preceding claims, wherein the metal fibers (22') have a waviness which gives an additional extension to the yarn (22) in a direction transverse to a longitudinal direction thereof.
A combustion membrane (14) according to claim 4, wherein said waviness is a waviness with less wavelength and amplitude than the wavelength and amplitude of a knitting or weaving waviness of the fabric or mesh (21).
A combustion membrane (14) according to any one of the preceding claims, wherein said waviness is formed over the entire length of the yarn (22).
A combustion membrane (14) according to any one of the preceding claims, wherein the continuous metal fibers (22') of the same yarn (22) are detached from one another, at least in stretches, along more than 30%, or more than 50%, of the length of the yarn (22), thus making the yarn (22) bulged.
A combustion membrane (14) according to any one of the preceding claims, wherein the metal fibers (22') have a circular cross-section.
A combustion membrane (14) according to any one of the preceding claims, wherein the fabric/mesh (21) has a weight by area either equal to or greater than 1.2 kg/m², or in the range from 1.2 kg/m² to 1.5 kg/m², or of 1.3 kg/m².
A combustion membrane (14) according to claim 1, wherein:
said continuous metal fibers (22') are not mutually twisted and form said untwisted yarn (22),

the individual metal fibers (22') each have a length substantially corresponding to the length of the yarn (22) formed therewith,

the metal fibers (22') have a waviness which gives an additional extension to the yarn (22) in a direction transverse to a longitudinal direction thereof, said waviness being a waviness with less wavelength and less amplitude than the wavelength and amplitude of a knitting or weaving waviness of the fabric or mesh (21),

said waviness is formed over the entire length of the yarn (22),

the metal fibers (22') have a circular cross-section.
A combustion membrane (14) according to any one of the preceding claims, wherein the metal wire (22) consists of fibers (22') having a diameter in the range from 30 micrometers to 50 micrometers, or having a diameter of 40 micrometers, OR
wherein the material of the metal wires (22) is an FeCrAl alloy doped with Yttrium, Hafnium, Zirconium.
A combustion membrane (14) according to any one of the preceding claims, wherein both interlacing surfaces (19, 20) form high-relief ribs (23) alternating with low-relief valleys (24), and both the ribs (23) and the valleys (24) have an extension, in at least one direction in the plane of the fabric/mesh (21), three times greater than the thickness of the metal wires (22), OR
wherein the fabric/mesh (21) has first localized areas (26) with low permeability, alternating with second localized areas (27) with higher permeability than the first areas (26), wherein both the first areas (26) and the second areas (27) have an extension, in at least one direction in the plane of the fabric/mesh (21), three times greater than the thickness of the metal wire (22).
A combustion membrane (14) according to any one of the preceding claims, wherein the fabric/mesh (21) is supported by and in contact with a support layer (38) arranged on the inner side (18) of the combustion membrane (14).
A combustion membrane (14) according to any one of the preceding claims, wherein the metal wires (22) are at least or only initially bound by means of a binder comprising a bonding thread (37) or a bonding adhesive or a water-soluble binder.
A gas burner (2) comprising a combustion membrane (14) according to any one of the preceding claims.