COMBUSTION MEMBRANE FOR A GAS BURNER

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 which forms 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 a 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 associated causes detailed among the aforesaid negative phenomena and the damaging effects for a satisfactory combustion have been widely described in the technical literature.

Attempts have been made to respond to the described needs by making an outer side of the combustion membrane in 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.

The metal fabrics and metal mesh most suitable for making combustion membranes are made of multi-fiber threads or yarn with metal fibers having a diameter less than 50 micrometers such as to ensure the “covering” function of the combustion membrane but also the function of well-distributed porosity and thermal insulation by virtue of a certain thickness of the fabric or mesh due to the increased quantity of individual metal fibers.

One of the problems not adequately solved to date in burners with combustion membrane made of fabric or metal mesh relates to the deforming and preserving behavior of a deforming layer of the combustion membrane. In addition to requiring the actual combustion membrane, all burners of this type require a more or less sturdy outer structure (outer metal mesh or outer perforated sheet metal) according to the three-dimensional shape and size of the combustion surface required by the application. But often, in the presence of an outer structure for supporting the combustion membrane, in the areas extending between the mechanical connection positions between the fabric or the metal mesh and the support structure, the mesh or fabric optimized for combustion performance is not true to the desired local shape, which ideally would be the one of a semi-rigid membrane and not a freely flexible one.

Therefore, it is the object of the present invention to provide a new and innovative combustion surface and combustion membrane for gas burners and a gas burner having features such as to avoid 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 combustible gas is conveyed, and an outer side on which combustion of the combustible gas occurs once it has crossed through the combustion membrane, said combustion membrane comprising a base fabric having two opposite fabric surfaces, which form a combustion surface exposed on the outer side and an inner surface facing the inner side, respectively, in which:

• • the base fabric forms a braid of metal threads comprising warp threads and weft threads transverse to the warp threads, said fabric being made on a loom (unlike mesh which is to be considered excluded from the definition of “fabric”), and the warp and weft threads are each a bundle of a plurality of metal fibers having a fiber thickness less than 50 micrometers,
  • the combustion membrane further comprises a plurality of monofilament metal threads having a monofilament thickness greater than 100 micrometers, directly integrated in the base fabric by loom weaving so as to stiffen the combustion membrane.

This reconciles, in a new and advantageous manner, the need to arrange the largest number possible of fibers having as reduced thickness as possible and distributed uniformly or sequentially (in order to ensure the porosity, thermal insulation and thermal inertia of the combustion membrane) with the contrasting need to give the combustion membrane greater rigidity and a certain ability to preserve shape in the base fabric plane.

In order to better understand the invention and appreciate the advantages thereof, some non-limiting embodiments thereof will be described below 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. 3A is an enlarged and diagrammatic sectional view of a combustion membrane according to an embodiment of the invention, also showing an optional additional support layer,

FIGS. 4A and 4B are views of the two sides of a metal fabric of a combustion membrane according to an embodiment of the invention,

FIG. 4C shows an enlarged detail of FIG. 4A where monofilament threads arranged according to a weaving pattern are highlighted,

FIG. 4D shows an enlarged detail of FIG. 4B where monofilament threads arranged according to a further different weaving pattern with respect to FIG. 4C are highlighted,

FIG. 5A shows a first side of a metal fabric of a combustion membrane according to a further embodiment of the invention,

FIG. 5B shows an enlarged detail of FIG. 5A where monofilament threads arranged according to a weaving pattern are highlighted,

FIG. 5C shows an enlarged detail of a second side of the fabric in FIG. 5A where monofilament threads arranged according to a further different weaving pattern with respect to FIG. 5B are highlighted,

FIG. 6 shows a multi-fiber metal thread bound by a water soluble binding thread,

FIG. 7 shows a wavy or “crimped” multi-fiber metal thread bound by a water soluble binding thread,

FIG. 8 shows a twisted and “hairy” (so-called hairy spun yarn) multi-fiber metal thread of the metal fabric according to embodiments.

FIG. 9 diagrammatically depicts a monofilament thread about which a multi-fiber thread helically extends.

FIG. 10 shows a mesh combustion membrane according to a further aspect of the invention.

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 generating heat by combustion of combustible gas and combustion air,
  • a feeding system 3 for feeding gas or the mixture of 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, when provided, 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) for controlling 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 burner 2 and adapted to provide an electrical ionization signal varying as a function of a combustion condition of 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 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 air) into burner 2,
  • a tubular combustion membrane 14, e.g., cylindrical in shape, and coaxial 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 gas 13 or the gas-air mixture to pass from the inside of burner 2 to an outer side 17 of the combustion membrane 14 where the combustion occurs (combustion area 8).

A tubular silencing accessory (without a reference numeral) is also shown in burner 2 in FIG. 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 non-tubular or non-cylindrical in 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 gas 13 or the gas-air mixture to pass from the inside of 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, a perforated distributor wall can be positioned in order to distribute the combustible gas 13 in a desired manner towards the combustion membrane 14 in burner 2, upstream of the combustion membrane 14 (with reference to the flow direction of the combustible gas 13) and spaced apart therefrom.

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 once it has crossed the combustion membrane 14, said combustion membrane 14 comprising a base fabric 21 having two opposite fabric surfaces 19, 20 which form a combustion surface 19 exposed on the outer side 17 and an inner surface 20 facing the inner side 18, respectively, where the base fabric 21 forms a metal thread braid 22 comprising multi-fiber warp threads 28 and multi-fiber weft threads 29 transverse to the multi-fiber warp threads 28, said base fabric 21 being made on a loom (unlike mesh which is to be considered excluded from the definition of “fabric”) and the multi-fiber warp 28 and weft 29 threads each comprise a bundle of a plurality of metal fibers 22′ having a fiber thickness less than 50 micrometers.

According to the invention, the combustion membrane 14 further comprises a plurality of monofilament metal threads 25 having a monofilament thickness greater than 100 micrometers, directly integrated in the base fabric 21 by loom weaving so as to stiffen the combustion membrane 14.

This reconciles, in a new and advantageous manner, the need to arrange the greatest number possible of fibers 22′ having as reduced thickness as possible and distributed uniformly or sequentially (in order to ensure the porosity, thermal insulation and thermal inertia of the combustion membrane 14) with the contrasting need to give the combustion membrane 14 greater rigidity and a certain ability of shape preservation in the base fabric plane.

According to embodiments, the monofilament threads 25 have a flexural strength which is 50% or 75% or 100% greater than the flexural strength of the warp threads 22 and weft threads 22 of the base fabric 21.

In the press shaping of the combustion membrane 14, in order to reach the movements set by the press, the monofilament threads 25 (having greater cross section than the individual fibers of the multi-fiber threads) undergo greater deformations and can reach the yield point limit, thus obtaining an irreversible plastic deformation component. On the other hand, the same greater cross section of the monofilament threads 25 increases the resistance to elastic bending thereof, and thus further contributes to preserving the shape set by the press.

According to an embodiment (FIG. 9), a multi-fiber thread (configured, for example, as described with reference to the multi-fiber threads 22) helically extends about one or more or each of the monofilament threads 25. The monofilament thread 25 and the multi-fiber thread can be twisted together or the second can be wound about the first.

According to an embodiment, the monofilament metal threads 25 create a braid of monofilament warp threads 26 and monofilament weft threads 27 transverse to the monofilament warp threads 26.

The combustion membrane 14 is a single-layer structure encompassing, by loom weaving, both the base fabric 21 and the braid of monofilament threads 25.

The monofilament threads 25 can extend each directly along and bordering in contact with a corresponding multi-fiber warp thread 28 or multi-fiber weft thread 29, respectively, of the base fabric 21.

The monofilament threads 25 can extend each exactly according to the weaving pattern of the multi-fiber warp thread 28 or the multi-fiber weft thread 29 of the base fabric 21 with which it is associated.

The monofilament warp threads 26 can be positioned at each warp pitch of the base fabric 21 (FIG. 5 C) or preferably, at a multiple of warp pitches of the base fabric 21 (FIGS. 4C, 4D, 5A), or advantageously, at every second warp pitch of the base fabric 21 (FIGS. 4C, 5B).

Similarly, the monofilament weft threads 27 can be positioned at each weft pitch of the base fabric 21 (FIG. 5 C) or preferably, at a multiple of weft pitches of the base fabric 21 (FIGS. 4C, 4D, 5A), or advantageously, at every second weft pitch of the base fabric 21 (FIGS. 4C, 5B).

According to an embodiment, the monofilament weft threads 27 are woven into the base fabric 21 by a dedicated monofilament feeder different from the feeder of the multi-fiber weft thread 29 of the base fabric 21. This allows controlling the weaving on an industrial scale, ensuring the quality thereof and using standard weaving components.

The monofilament thread 25 has a thickness transverse to the longitudinal extension, or diameter, thereof in the range from (greater than) 100 micrometers to 250 micrometers, preferably from 160 micrometers to 250 micrometers, e.g., 200 micrometers, depending on the acceptable density of monofilament threads 25 and the rigidity and plastic deformability of the combustion membrane 14 required.

Description of Surface Profile Features of the Base Fabric 21

According to an aspect of the invention, both fabric surfaces 19, 20 form ribs 23 in high relief alternating with valleys 24 in low relief, and both the ribs 23 and the valleys 24 have an extension, in at least one direction in the plane of the base fabric 21, greater than the space occupied by at least three consecutive warp threads in the weft direction and greater than the space occupied by at least three consecutive weft threads in the warp direction.

Due to the ribs 23 in high relief alternating with the valleys 24 in low relief, the metal base fabric 21 of the combustion membrane 14 achieves a technical effect of discrete, repetitive but not continuous spacer, and thickness of the fabric itself not completely filled with metal material, which improves the thermal insulation capacity and allows a gas distribution through the metal fabric not only in the direction orthogonal to the plane of the fabric, but also in the plane of the fabric 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 gas 13 across the combustion membrane 14.

According to an embodiment, at the ribs 23, at least one of the fabric surfaces 19, 20 forms one or more floats 30 (i.e., passages of a multi-fiber weft thread 29 over several consecutive multi-fiber warp threads 28, or passages of a multi-fiber warp thread 28 over several consecutive multi-fiber weft threads 29).

According to an embodiment, at the valleys 24, at least one of the fabric surfaces 19, 20 forms areas free from floats or with floats shorter than the floats in the ribs 23 of the same fabric surface (where “shorter” means “passages of one multi-fiber warp/weft thread over a smaller number of consecutive multi-fiber warp/weft threads than the floats in the ribs 23”).

Description of Permeability Features of Fabric 21

The base fabric 21 is permeable to gases and has localized first areas 31 with low permeability alternating with localized second areas 32 with higher permeability than the first areas 31.

According to an embodiment, both the first areas 31 and the second areas 32 have an extension, in at least one direction on the plane of the base fabric 21, greater than the space occupied by at least three consecutive multi-fiber warp threads 28 in the weft direction and greater than the space occupied by at least three consecutive multi-fiber weft threads 29 in the warp direction.

According to an embodiment, at the first areas 31, the base fabric 21 forms one or more floats 30, while at the second areas 32, the base fabric 21 forms areas free from floats or with floats shorter than the floats 30 in the first areas 31 (i.e., passages of one multi-layer warp/weft thread over a smaller number of consecutive multi-layer warp/weft threads than the floats 30 in the first areas 31).

According to an embodiment, at the floats 30 of the first areas 31, the metal threads 22 forming said floats 30 are locally enlarged with respect to a width of the metal threads 22 at the second areas 32.

For example, the difference in gas permeability between the first areas 31 and the second areas 32 is visible and verifiable against the light as a difference in light transmission through the base fabric 21.

The first localized areas 31 with low permeability alternating with the second localized areas 32 with higher permeability than the first localized areas 31 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.

Description of an Example Embodiment of the Base Fabric 21

According to an embodiment (FIGS. 5A, 5B, 5C), the ribs 23 and the valleys 24 define a repetitive pattern of first rows 33, preferably straight, inclined with respect to the weft and warp directions in a first direction, and second rows 34, preferably straight, inclined with respect to the weft and warp directions in a second direction transverse to the first direction, where said first rows 33 and second rows 34 intersect, thus delimiting rhombus-shaped areas 35, where the two diagonals of the rhombus-shaped area 35 (the segments joining the opposite vertices of the rhombus) are parallel to the weft and warp directions of the base fabric 21.

Further advantageously, each rhombus-shaped area 35 is crossed by at least 1 or 2, but preferably by more than two, monofilament warp threads 26 and by at least 1 or 2, but preferably by more than two, monofilament weft threads 27.

The shape of the base fabric 21 thus configured (independently of the monofilament threads) has proven to be surprisingly advantageous with reference to the features of porosity, thermal insulation, deformability in various three-dimensional shapes, and fabrication by industrial weaving.

The braid of larger monofilament threads 25 and the multiple presence thereof in each rhombus-shaped area 35 provides further stiffening and capacity to preserve a three-dimensional deforming status, which is desirable for this type of combustion membrane 14.

In the burner, the combustion membrane 14 can, but not necessarily must, be supported by and in contact with an additional support layer 38, e.g., a perforated layer or a metal support mesh, arranged on the inner side 18 of the combustion membrane 14.

Description of the Multi-Fiber Metal Thread 22

According to an embodiment, the multi-fiber metal threads 22 comprise bundles of metal fibers, e.g., unspun, or bundles of parallel or braided or spun or twisted metal fibers, e.g., of the “long fiber filament” or “short fiber filament” type.

The multi-fiber metal threads 22 can be at least or only initially bound by a binder, e.g., water-soluble or non-soluble binding thread 37, e.g., PVA or polyester, or by a water-soluble or non-soluble binder adhesive, e.g., polymeric.

According to an embodiment, fabric 21 is a “heavy” or “thick” fabric, meaning like a fabric with a weight per area of fabric equal to or greater than 1.3 kg/m², e.g., in the range from 1.3 kg/m² to 1.6 kg/m², preferably 1.3 kg/m².

Alternatively, with economic advantage, fabric 21 is a “semi-heavy” or “semi-thick” fabric, meaning like a fabric with a weight per area of fabric in the range from 1.2 kg/m² to 1.3 kg/m², preferably 1.26 Kg/m² to 1.28 kg/m².

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., of 1 g/m.

Advantageously, the metal thread 22 consists of fibers 22′ with diameter in the range from 30 micrometers to less than 50 micrometers, e.g., of about 40 micrometers.

According to an embodiment, the material of the metal threads 22 or metal fibers 22′ can be, e.g., a ferritic steel, or a FeCrAl alloy, e.g., doped with Yttrium, Hafnium, Zirconium.

The metal thread 22 can be, e.g., a Y, Hf, Zr doped FeCrAl alloy yarn, weighing 1 g/m, and consists of fibers having a diameter of 40 micrometers, untwisted, possibly crimped (wavy), held back by a binding thread 37, possibly PVA or polyester binding thread, 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

• • According to a further embodiment, the material of metal threads or metal fibers can be, e.g., a ferritic steel, or a FeCrAl alloy, e.g., additionally containing Yttrium, Hafnium, Zirconium.

The multi-fiber metal thread 22 can be, e.g., a Y, Hf, Zr doped FeCrAl alloy yarn, weighing 1 g/m and composed of fibers 40 micrometers in diameter, spun, e.g., with 30 to 150 twists per meter, possibly with fiber ends divergently protruding from the yarn (“hairy”), with fibers shorter than the yarn, e.g., with fiber length in the range from 7 cm to 30 cm, not necessarily but possibly held back by a binding thread 37, possibly made of PVA or polyester, and having, for example, the same “doped” composition as shown in the table above.

Description of the Monofilament Thread 25

According to an embodiment, the monofilament thread 25 can be, e.g., a non-doped or a Y, Hf, Zr doped FeCrAl alloy thread 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

For example, the material of the monofilament metal threads 25 can be a ferritic steel, or a FeCrAl alloy, e.g., additionally containing Yttrium, Hafnium, Zirconium.

Description of the Combustion Membrane 14 with Mesh 121

According to a further aspect of the invention (FIG. 9), a combustion membrane (14) for a gas burner (2) 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 once it has crossed the combustion membrane (14), and comprises a base mesh (121) (unlike the above-described fabric made on a loom) having two opposite mesh surfaces (19, 20) which form a combustion surface (19) exposed on the outer side (17) and an inner surface (20) facing the inner side (18), respectively, where the base mesh (121) forms a braid of one or more multi-fiber metal threads (22) each comprising a bundle of a plurality of metal fibers (22′) having a fiber thickness less than 50 micrometers, where the combustion membrane (14) further comprises a plurality of monofilament metal threads (25) having a monofilament thickness greater than 100 micrometers, directly inserted into the base mesh (121) so as to stiffen the combustion membrane (14).

REFERENCE NUMERALS IN DRAWINGS AND DESCRIPTION

• • gas combustion system 1
  • burner 2
  • feeding system 3
  • gas control device 4
  • air control device 5
  • electric ignition device 6
  • ionization sensor 7
  • combustion area 8
  • electronic control unit 9
  • combustion control module 10
  • support wall 11
  • inlet passage 12
  • combustible gas 13
  • combustion membrane 14
  • longitudinal axis 15
  • closing wall 16
  • outer side 17
  • inner side 18
  • combustion surface 19
  • inner surface 20
  • base fabric
  • metal threads 22
  • metal fibers 22′
  • ribs 23
  • valleys 24
  • monofilament threads 25
  • monofilament warp threads 26
  • monofilament weft threads 27
  • multi-fiber warp thread 28
  • multi-fiber weft thread 29
  • float 30
  • first areas with increased permeability 31
  • second areas with increased permeability 32
  • first rows 33
  • second rows 34
  • rhombus-shaped area 35
  • warp direction 36_O
  • weft direction 36_T
  • binding thread 37
  • support layer 38
  • base mech 121