EP4089328A1

EP4089328A1 - A combustion membrane for a gas burner

Info

Publication number: EP4089328A1
Application number: EP22171830.7A
Authority: EP
Inventors: Massimo Gilioli
Original assignee: Beckett Thermal Solutions SRL
Current assignee: Beckett Thermal Solutions SRL
Priority date: 2021-05-10
Filing date: 2022-05-05
Publication date: 2022-11-16
Also published as: IT202100011888A1

Abstract

A combustion membrane (14, 14') for a gas burner (2) comprises a layer made of perforated metal sheet (23), in which perforation holes (24) of an individual perforated metal sheet (25) of the layer of perforated sheet (23) have a ratio (D/S) between an equivalent diameter (D) of the hole (24) and the thickness (S) of the individual sheet (25) which is greater than or equal to 1 (D/S ≥ 1),
wherein the layer made of perforated metal sheet (23) comprises a plurality of said individual perforated metal sheets (25) overlapping in a pile of sheets and with said holes (24) overlapping and aligned to form channels (26) for the passage of the combustible gas (13) through the pile of sheets, wherein said channels (26) have a ratio (D1/S1) between an equivalent diameter (D1) of the channel (26) and the thickness (S1) of the pile of sheets which is less than 0.7 (D1/S1 < 0.7).

Description

The present invention relates to a combustion membrane for a gas 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.
Gas combustion systems comprise:

a burner, which is connectable to a combustion chamber of a boiler or another application, for the production of heat by combustion of combustible gas and combustion air inside the combustion chamber,
a feeding system for feeding combustible gas and combustion air, or a premixed mixture of gas and air, to the burner,
an ignition system, for example an ignition electrode, for igniting the combustion,
a flame presence sensor adapted to provide a flame signal which can be associated with the combustion conditions, for example excess air,
an electronic control unit, connected to the feeding system, the ignition system and the flame presence sensor, and adapted to control the ignition system and the feeding system depending on an operating command or program and depending on the flame signals.

The feeding system usually comprises a fan, driven by an electric motor, for the suction and conveying of a flow of combustion air, as well as a solenoid valve for controlling a flow of combustible gas.
It is known to conduct the gas and air flows separately in the combustion zone of the burner or to premix the gas and air flows upstream of the burner and feed a single pre-mixed gas and air flow to the burner.
The known ignition systems comprise, for example, an ignition electrode which can be electrically fed to generate a combustion ignition spark.
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, facing the combustion chamber,

in which the gas and air mixture 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.
The diffuser layer conventionally consists of a perforated metal sheet (highly heat resistant steel) with or without an additional outer layer of fabric or metal mesh or of porous ceramic.
For a desirable and satisfactory use of the burner and combustion system, on the one hand it is desirable to vary the heating power of the burner and the flow rate of combustible gas or mixture of combustible gas and combustion air flow rate in a controlled manner through the combustion membrane, and on the other hand, to ensure a safe and controllable operation of the burner, in particular a controllable and safe combustion without the risk of explosion or flashback also when combustible gases with high speed combustion are used.
Motivated by the need for energy efficiency, sustainable energy storage, an increasing need for thermal energy and the need for a clean combustion which does not generate polluting substances, there is an increasing will to replace hydrocarbon gases by hydrogen (H₂) in multiple combustion applications, among which also in burners with a combustion membrane.
However, hydrogen has a much higher combustion speed than the other commonly used combustible gases, other conditions being equal. With the combustion membranes of the known art, such high combustion speed considerably accelerates the advancement of the flame in the direction opposite to the flow of gas through the combustion membrane, thus resulting in a flashback of the flame in the burner and in the impossibility to modulate the thermal input of the burner to low values with relatively low gas flow speed.
Therefore, the need is felt to modify and improve the burners of the known art so as to make them usable with hydrogen and at the same time, allow a modulation of the burner power by regulating the flow rate of the hydrogen gas flow (also at low flow speeds) and obviate the risk of flashbacks.
The employment of perforated metal sheet as diffuser layer requires an affordable perforation process of the sheet and with speed adapted to mass industrial production. This is currently possible only by means of a perforation process by mechanical press or die punching (cutting, punching holes) which however has intrinsic dimensional limits, that is the individual holes may be obtained with a hole diameter/sheet thickness ratio ≥ 1 by mechanical punching. In other words, the mechanical punching is not adapted to make perforations in which the diameter of the individual hole is significantly less than the thickness of the metal sheet.
On the other hand, according to the flashback theory of combustion at a hole or passage channel - the so-called "light back theory" - the flashback phenomenon pertains to the flow boundary velocity gradient and:

with a hole diameter/length ratio = 1, the speed profile of the flow of gas outlet from the hole is not completely parabolic, resulting in a flattened combustion speed profile susceptible to axial flashback through the hole,
with a hole diameter/length ratio << 1, the speed profile of the flow of gas outlet from the hole is completely parabolic, the occurrence of flashback is not promoted.

Accordingly, the flashback phenomenon may be controlled or excluded according to the combustion speed of the gas used only by means of a high flow speed through the hole and/or increasing the length of the hole with respect to the diameter thereof.
When using hydrogen as combustible gas, there is a need for a hole diameter/sheet thickness ratio << 1 to avoid the flashback.
In the current art, the need for a low cost mass production of perforated sheets for combustion membranes, with the related dimensional limitations of the holes, and the need for a hole diameter/sheet thickness ratio << 1 to prevent the flashback in the use of hydrogen as combustible gas, cannot be reconciled.
It therefore is the object of the present invention to propose a new combustion membrane comprising perforated metal sheet and a new burner, having such features as to achieve one or more of the following:

allow the use of hydrogen as combustible gas,
reduce the risk of flashback also in the presence of gas with increased combustion speeds,
reduce the risk of flashback also in the presence of relatively low combustible gas flow,
allow the manufacturing of the membrane with a perforation with a hole diameter/length ratio at contained costs.

At least part of the objects of the invention are achieved by means of a combustion membrane for a gas burner adapted to the combustion of gases having an increased combustion speed, for example hydrogen gas, and operated with or without pre-mixing the combustion gas, according to claim 1. The dependent claims relate to advantageous and preferred embodiments.
According to an aspect of the invention, a combustion membrane for a gas burner forms an inner side to which a combustible gas is conveyed and an outer side on which the combustion of the combustible gas occurs after the crossing of the combustion membrane thereby, in which the combustion membrane comprises a layer made of perforated metal sheet, in which perforation holes of an individual sheet have a ratio between an equivalent diameter D of the hole and length S of the hole corresponding to thickness S of the individual sheet D/S which is greater than or equal to 1 (D/S ≥ 1), characterized in that the layer made of perforated metal sheet comprises a plurality of said individual metal sheets overlapping in a pile of sheets and with said holes overlapping and aligned to form channels for the passage of the gas through the pile of sheets, in which said channels have a ratio D1/S1 between an equivalent diameter D1 of the channel and length S1 of the channel corresponding to thickness S1 of the pile of sheets which is less than 0.7 (D1/S1 < 0.7).
As is generally known, the equivalent diameter is defined as the diameter which would have a circular section with the same ratio between perimeter P and cross section area A, that is to say D = 4 A / P for the hole of the individual sheet and D1 = 4 A1 / P1 for the channel of the pile of sheets.
In the fluid-dynamic case herein applicable, the effective perimeter is the one lapped by the fluid. The equivalent diameter for a duct having circular section by definition corresponds to the geometrical diameter.
By virtue of the configuration of the layer of perforated sheet as pile of a plurality of individual layers of sheet, the perforation may be obtained with an affordable mechanical press perforation process on the individual sheet, and regulating the ratio D1/S1 between the equivalent diameter D1 and length S1 of the channel may be adjusted to prevent the flashback by means of the selection of the thickness and number of individual sheets to be overlapped in the pile of sheets.
The membrane structure further allows a simplification of the production and management and storage process of the perforated sheets or sheets to be perforated because different combustion membranes with different thicknesses of the layer of perforated sheet and with different diameter/length ratios of the perforation channels may be made with a single type of starting sheet having a single thickness (preferably thin), also for combustion applications with gas or gas mixtures not having very high combustion speeds.
The object of the invention is further achieved by a gas burner, in particular a partially or completely premixed gas burner, having the aforesaid combustion membrane.
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:

figure 1 is a diagrammatic view of a gas combustion system, for example for a boiler, with a burner provided with a combustion membrane according to an embodiment of the invention,
figures 2 and 3 are perspective and sectional views of an exemplary burner, provided with a combustion membrane according to an embodiment,
figures 4, 5 and 6 are exploded and lateral perspective views of an exemplary burner, provided with a combustion membrane according to a further embodiment,
figure 7 is a sectional view of the combustion membrane according to an embodiment of the invention,
figure 8 is an enlarged view of a detail in figure 7,
figure 9 is a perspective view of a combustion membrane in a manufacturing step, according to an embodiment,
figure 10 shows a detail of the combustion membrane in figure 8.

Detailed description of the combustion system 1

With reference to figure 1, a gas combustion system 1, for example for a boiler, comprises:

a burner 2 for producing heat by means of combustion of combustible gas and combustion air,
a feeding system 3 for feeding the combustible gas and combustion air to 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 an air control device 5 (for example, 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 air,
an electric ignition device 6 for igniting the combustion, for example an ignition electrode adapted to generate a spark,
optionally, a flame sensor 7 arranged at a combustion area 8 of burner 2 and adapted to provide a varying flame signal which varies as a function of a combustion condition of burner 2,
an electronic control unit 9 connected to the feeding system 3, with the ignition device 6 and optionally with the flame 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 optionally, depending on the flame 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 13) of combustible gas and combustion air into burner 2,
a tubular or cylindrical combustion membrane 14, and coaxial with respect to a longitudinal axis 15 of 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 or gas and air mixture 13 from inside burner 2 to an outer side 17 of the combustion membrane 14 where the combustion occurs (combustion area 8).

The burner 2 in figure 3 further has a tubular silencing accessory (without reference numeral), which is optional.
According to a further embodiment (figures 4, 5, 6), the gas burner 2 comprises:

a support frame or housing 18 forming, for example a lateral wall in the form of a frame 19 and a bottom wall 20, one of which forms one or more inlet passages 12 for the introduction of combustible gas or the gas-air mixture 13 into burner 2,
a substantially flat combustion membrane 14', for example planar or curved or convex, and having a peripheral edge 21 connected to the support housing/frame 18, in particular to the lateral wall 19, in flow communication with the inlet passage 12, as well as a perforation for the passage of the gas and air mixture 13 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 burner 2, upstream of the combustion membrane 14, 14' (with reference to the flow direction of the combustible gas 13) and spaced apart therefrom, a perforated distributor wall 21 can be positioned in order to distribute the combustible gas 13 in a desired manner towards the combustion membrane 14, 14' (figure 4).

Detailed description of the combustion membrane 14,14'

The combustion membrane 14, 14' forms an inner side 22 to which a combustible gas is conveyed and an outer side 17 on which the combustion of the combustible gas occurs after the crossing of the combustion membrane 14, 14'.
The combustion membrane 14, 14' comprises a layer made of perforated metal sheet 23, in which the perforation holes 24 of an individual sheet 25 have a ratio D/S between an equivalent diameter D of hole 24 and the length S of hole 24 corresponding to a thickness S of the individual sheet 25, which is greater than or equal to 1 (D/S ≥ 1).
Layer 23 made of perforated metal sheet comprises a plurality of said individual metal sheets 25 overlapping in a pile of sheets and with said holes 24 overlapping and aligned to form channels 26 for the passage of gas 13 through the pile of sheets, in which said channels 26 have a ratio D1/S1 between an equivalent diameter D1 of channel 26 and the length S1 of channel 26 corresponding to thickness S1 of the pile of sheets which is less than 0.7 (D1/S1 < 0.7).
Advantageously, the holes 24 overlapping and aligned to form said channel 26 all have a substantially identical perimeter shape and the equivalent diameter D of the holes 24 is equal to the equivalent diameter D1 of channel 26.
According to an embodiment, all the individual metal sheets 25 of layer 23 have the same thickness S.
This allows the manufacturing and storage of starting and semi-finished sheets to be rationalized.
According to an alternative embodiment, the individual metal sheets 25 of layer 23 have different thicknesses S, preferably exactly two different thicknesses S.
This allows increasing the obtainable number and "resolution" of total thicknesses of layer 23, and therefore the "setting resolution" of the diameter/length ratio of the channels 26, while in any case keeping low the number of types of sheet and semi-finished products to be managed during manufacturing.
According to an embodiment, the intrados surfaces 27 of the holes 24 of the overlapping individual sheets 25 belonging to a same channel 26 are arranged flush between one another so as to give channel 26 a shape which substantially is without steps.
According to an alternative embodiment, the intrados surfaces 27 of the holes 24 of the overlapping individual sheets 25 belonging to a same channel 26 are offset in direction transverse to the length of channel 26 in an offset range between zero to 1/5 of the equivalent diameter D1 of channel 26, preferably from zero to 1/10 of the equivalent diameter D1 of channel 26.
This reconciles the needs for an industrial manufacturing with acceptable tolerances and a three-dimensional bending of the perforated sheets 25 and/or of the whole layer 23 of perforated sheets after the perforation process, with the need for a sufficiently uniform shape of channel 26 to meet the fluid-dynamic and thermodynamic design needs of burner 1.
According to embodiments, the holes 24 and the channels 26 preferably have a preferably concave circular or slot shape (view in direction of the flow of gas), that is to say without inversion of sign of curvature and without protrusions towards the inside of hole 24 or channel 26.
Advantageously, the layer of perforated sheet 23 may comprise from 2 to 10 overlapping individual perforated sheets 25.
Thickness S of the individual perforated sheets 25 may be in the range from 0.3 mm to 2 mm, preferably in the range from 0.5 mm to 2 mm, even more preferably in the range from 0.5 mm to 1 mm, preferably 0.6mm.
According to an embodiment, the ratio D/S between the equivalent diameter D and the length S of hole 24 is in the range of 1 to 2 (1 ≤ D/S ≤ 2), while ratio D1/S1 between the equivalent diameter D1 and the length S1 of channel 26 is less than 0.6, preferably in the range from 0.6 to 0.01, even more preferably in the range from 0.45 to 0.05.
According to an embodiment, the perforated sheets 25 are made by mechanical perforation, for example punching, of sheets not yet permanently fastened to one another, preferably by means of mechanical punching of individual sheets.
After the perforation of the individual sheets 25, the individual sheets 25 are:

individually bent and then stacked and connected to one another to form the layer of perforated sheet 23, or alternatively,
stacked and connected to one another and then bent together to form the layer of perforated sheet 23.

The bending of the perforated sheets 25 is carried out, for example by calendering or deformation by press.
The connection to one another of the perforated sheets 25 of the layer of perforated sheet 23 advantageously occurs by welding, for example by means of temporary weld points 28 prior to the bending and/or by permanent means of weld beads 29 and/or permanent weld points 28.
Advantageously, the connection to one another of the perforated sheets 25 of the layer of perforated sheet 23 comprises one or more welds arranged at, or in the vicinity of, initially free edges of the perforated sheets 25.
According to an embodiment, the combustion membrane 14 has a tubular shape, for example cylindrical or frustoconical, and the layer of perforated sheet 23 is made by bending and welding individual perforated sheets 25 to form a plurality of layers of individual tubular perforated sheet spaced apart from one another, and then mutually (telescopically) inserting said layers of individual tubular perforated sheet to form the layer of tubular perforated sheet 23.
Alternatively, the combustion membrane 14 has a tubular shape, for example cylindrical or frustoconical, and the layer of perforated sheet 23 is made by bending and/or calendering in a "C" shape and overlapping individual perforated sheets 25, possibly connected to one another by means of one or more temporary welds, to form the layer of perforated sheet 23 in the form of open channel with two free opposite longitudinal edges 30 (figures 9, 10), and by means of successive matching and joining by means of welding the two opposite longitudinal edges 30 to form the layer of tubular perforated sheet 23.
According to an embodiment, for a correct alignment of the holes 24 of the individual sheets 25 in the position of the corresponding channels 26 of the layer of perforated sheet 23, during the fastening of the individual perforated sheets 25 to one another and/or during the bending of the whole pile of overlapping individual sheets 25, outer reference edges of the individual perforated sheets 25 are positioned and/or offset by means of a positioning mask/frame (not shown) and/or by means of one or more reference notches 31 (figure 10) formed at the outer reference edges.
According to an embodiment, the individual perforated sheet or sheets 25 arranged on the inner side 22 of the combustion membrane 14 may be made of a different metal, for example less heat-resistant and less costly (because less exposed to the increased combustion temperatures), of a metal of the individual perforated sheet(s) 25 arranged or exposed on the outer side 17 of the combustion membrane 14.
According to an embodiment similar to the one in figures 7, 8, the layer of perforated sheet 23 of a burner, for example cylindrical or planar flat or curved flat, comprises 4 individual layers of perforated sheet 25, each one having thickness S of 0.6 mm (overall thickness S1 = 2.4 mm), with holes 24 having diameter D of 0.8 mm. The maximum outer diameter of the layer of perforated sheet 23 in cylindrical configuration is, for example 70 mm.
Advantageously, if the layer of perforated sheet 23 has a tubular or cylindrical shape, the longitudinal edges 30 thereof (figure 10) are connected to one another by means of a weld bead passing through all the layers of individual sheet and having trapeze cross section, preferably with lateral sides 32 having a step/s.
According to an embodiment, the combustion membrane 1 may further comprise an outer permeable layer 33 (only partially shown in figures 7, 8) without metal sheet, and which covers at least a perforation area of the layer of perforated sheet 23 on the outer side 17 of the combustion membrane 14, 14'.
The outer permeable layer 33 may comprise one or more:

metal fiber fabric or
metal fiber mesh or
sintered metal fiber panel or
sintered ceramic fiber panel or
ceramic and silicon carbide porous composite material or
solid and heat-resistant open-cell spongy material.

In the context of the present description, fabric means a textile structure with an intertwining of threads or fibers, for example knitted (knitted fabric) or manufactured on a loom by intertwining warp threads with weft threads (fabric) according to a determined order and criterion. More specifically, a textile structure is intended as a substantially two-dimensional extension (flat or curved) in space and with a very reduced thickness with respect to the two-dimensional extension.
The term "combustible gas 13" denotes:

combustible gases intended for combustion together with primary combustion air, conveyed to the combustion region 8 through burner 2 but not necessarily together with the combustible gas, or
combustible gases intended for combustion together with primary combustion air, conveyed to the combustion region 8 from outside burner 2, or
a complete or partial premixture of combustible gas and combustion air fed into burner 2.

Claims

A combustion membrane (14, 14') for a gas burner (2), forming an inner side (22) to which a combustible gas (13) is conveyed and an outer side (17) on which the combustion of the combustible gas (13) occurs after the crossing of the combustion membrane (14, 14'), said combustion membrane (14, 14') comprising a layer made of perforated metal sheet (23), wherein perforation holes (24) of an individual perforated metal sheet (25) of the layer of perforated sheet (23) have a ratio (D/S) between an equivalent diameter (D) of the hole (24) and the thickness (S) of the individual sheet (25) which is greater than or equal to 1 (D/S ≥ 1), wherein the layer made of perforated metal sheet (23) comprises a plurality of said individual perforated metal sheets (25) overlapping in a pile of sheets and with said holes (24) overlapping and aligned to form channels (26) for the passage of the combustible gas (13) through the pile of sheets, wherein said channels (26) have a ratio (D1/S1) between an equivalent diameter (D1) of the channel (26) and the thickness (S1) of the pile of sheets which is less than 0.7 (D1/S1 < 0.7).
A combustion membrane (14, 14') according to claim 1, wherein the overlapping and aligned holes (24) to form a same channel (26) have all a substantially identical perimeter shape and the equivalent diameter (D) of the holes (24) is equal to the equivalent diameter (D1) of the channel (26).
A combustion membrane (14, 14') according to claim 1 or 2, wherein all the single perforated metal sheets (25) of the layer of perforated sheet (23) have the same thickness (S).
A combustion membrane (14, 14') according to claim 1 or 2, wherein the single perforated metal sheets (25) of the layer of metal sheet (23) have different thicknesses (S) or have exactly two different thicknesses (S).
A combustion membrane (14, 14') according to any one of the preceding claims, wherein the intrados surfaces (27) of the holes (24) of the overlapping single perforated metal sheets (25) belonging to a same channel (26) are arranged flush between one another so as to give the channel (26) a shape which substantially is without steps.
A combustion membrane (14, 14') according to any one of claims 1 to 4, wherein the intrados surfaces (27) of the holes (24) of the overlapping single perforated metal sheets (25) belonging to a same channel (26) are offset in a transverse direction to the length of the channel (26) in an offset range between zero to 1/5 of the equivalent diameter (D1) of the channel (26), preferably from zero to 1/10 of the equivalent diameter (D1) of the channel (26).
A combustion membrane (14, 14') according to any one of the preceding claims, wherein:
- the holes (24) and the channels (26) have a concave circular or slot shape without inversion of sign of curvature and without protrusions towards the inside of the hole (24) or of the channel (26), and/or

- the layer of perforated sheet (23) comprises from 2 to 10 overlapping individual perforated sheets (25), and/or

- the thickness (S) of the individual perforated metal sheets (25) is in the range from 0.3 mm to 2 mm or in the range from 0.5 mm to 1 mm, or is 0.6 mm.
A combustion membrane (14, 14') according to any one of the preceding claims, wherein the ratio D/S between the equivalent diameter D and the length S of the hole 24 is in the range of 1 to 2 (1 ≤ D/S ≤ 2), while the ratio D1/S1 between the equivalent diameter D1 and the length S1 of the channel 26 is less than 0.6, preferably in the range from 0.6 to 0.01, even more preferably in the range from 0.45 to 0.05.
A combustion membrane (14, 14') according to any one of the preceding claims, wherein the individual perforated sheet or sheets (25) arranged on the inner side (22) of the combustion membrane (14) are made of a metal other than a metal of the individual perforated sheet or sheets (25) arranged on the outer side (17) of the combustion membrane (14).
A combustion membrane (14, 14') according to any one of the preceding claims, wherein the combustion membrane (1) further comprises an outer permeable layer (33), without metal sheet, and which covers at least a perforation area of the layer of perforated sheet (23) on the outer side (17) of the combustion membrane (14, 14'),
said outer permeable layer (33) is selected from the group of material consisting of:
metal fiber fabric,

metal fiber mesh,

sintered metal fiber panel,

sintered ceramic fiber panel,

ceramic and silicon carbide porous composite material,

solid and heat-resistant open-cell spongy material.
A gas burner (2), in particular for the combustion of hydrogen, comprising a combustion membrane (14, 14') according to one of the preceding claims.
A method for making a combustion membrane (14, 14') according to any one of claims 1 to 10, wherein:
- the individual perforated metal sheets (25) are made by punching sheets not yet permanently fastened to one another,

- after the perforation of the individual metal sheets (25), the individual perforated metal sheets (25) are:
- individually shaped and then stacked and connected to one another to form the layer of perforated sheet (23) or,

- stacked and connected to one another and then bent together to form the layer of perforated sheet (23).
A method according to claim 12, wherein:
- the shaping of the individual perforated metal sheets (25) is carried out by calendering or deformation by press,

- the connection to one another of the individual perforated metal sheets (25) of the layer of perforated sheet (23) occurs by welding.
A method according to claim 12 or 13, wherein the connection to one another of the individual perforated metal sheets (25) of the layer of perforated sheet (23) comprises one or more welds arranged at initially free edges of the perforated sheets (25).
A method according to one of claims 12 to 14, wherein the combustion membrane (14) has a tubular shape and the layer of perforated sheet (23) is made by:
- bending and welding individual perforated sheets (25) to form a plurality of layers of individual tubular perforated sheet spaced apart from one another, and

- successive mutual insertion of said layers of individual tubular perforated sheet to form the tubular layer of perforated sheet (23).
A method according to one of claims 12 to 14, wherein the combustion membrane (14) has a tubular shape and the layer of perforated sheet (23) is made by:
- bending or calendering in a "C" shape and overlapping individual perforated metal sheets (25) to form the layer of perforated sheet (23) in the shape of open channel with two free opposite longitudinal edges (30), and

- then matching and joining by means of welding the two opposite longitudinal edges (30) to form the tubular layer of perforated sheet (23).