Description

Technical Field

[1 ] The invention relates to gas premix burners that have a woven, knitted or braided burner deck comprising metal fibers. Such gas premix burner can e.g. be used in boilers or in instantaneous water heaters.

Background Art

[2] Gas premix burners with a knitted or woven fabric comprising metal fibers as burner deck positioned on a perforated plate or woven screen (a woven wire mesh) which is acting as gas distribution plate are known. It is a benefit of such burners that the burner deck (e.g. a knitted or woven fabric) can freely expand when hot, while the perforated plate or the woven wire mesh is remaining sufficiently cool. Such burners are e.g. known from US4657506 and WO2004/092647.

Disclosure of Invention

[3] The primary object of the invention is to provide an improved gas premix burner.

[4] A first aspect of the invention is a gas premix burner comprising:

- a perforated plate, a woven wire mesh or an expanded metal sheet;

- a woven, knitted or braided burner deck comprising metal fibers, supported by the perforated plate, woven wire mesh or expanded metal sheet. The burner deck is the surface on which the combustion of the premix gas occurs after the premix gas has flown through it.

The woven, knitted or braided burner deck comprises at least a zone with a high density of at least 1250 g/dm ³ .

The zone with a high density includes preferably at least 25%, more preferably at least 30%, more preferably at least 40%, even more preferably at least 70%, of the surface of the burner deck.

In a preferred embodiment; the zone of a high density covers the complete burner deck. [5] Preferably, the zone with a high density has a density of at least 1350 g/dm ³ , more preferably of at least 1400 g/dm ³ , more preferably of at least 1450 g/dm ³ , more preferably of at least 1500 g/dm ³ , more preferably of at least 1750 g/dm ³ , even more preferably of at least 2000 g/dm ³ . And preferably below 3500 g/dm ³ , more preferably below 2500 g/dm ³ .

[6] The value of the density for a burner deck can be set by compressing a fabric to a specific thickness for use as burner deck.

[7] Preferably, the zone with a high density is not connected via metal bonds to the perforated plate, woven wire mesh or expanded metal sheet supporting the woven, knitted or braided burner deck.

[8] It is known that boilers in which heat is generated by a burner can show thermo acoustical instabilities. The result is noise that can be very irritating. In gas premix burners, air is fed by a fan and mixed with combustible gas, e.g. by means of a venturi, and introduced in a premixing chamber after which the premix of gas and air is combusted after flowing through a porous burner deck. The hot flue gas transfers its thermal energy to a fluid in a heat exchanger after which the flue gas is evacuated through a chimney. The combination of parts of the boiler results in it that noise is generated, e.g. by the gas flow through the fan. The presence of the flame can amplify any noise that is present, from a level that the noise is not audible up to levels that are very annoying. Noise is a standing wave. The flame is not constant over time. The short term fluctuations in the flame can coincide with the frequency of the noise resulting in amplification of the standing waves (and consequently of the noise). This process is called thermo-acoustic instability. The burner needs to be operated over a certain load range and also in a range of the air to gas ratio. This creates a large range of possible conditions of operation of the boiler, that each need to be sufficiently silent in operation, meaning that acoustic instabilities should be sufficiently low over the full range of modulation of the burner. The interactions between the different parameters are believed to be extremely complex and not understood. A known solution in the use of mufflers in the boilers, however this is an expensive solution.

[9] Surprisingly, the gas premix burners of the invention have shown to have substantially less thermo acoustic instabilities than prior art gas premix burners.

[10] The use of knitted burner decks is preferred, because it allows

manufacturing of burners with a more complex double-curved burner deck shape. The knitted burner deck can be using spun yarns comprising metal fibers of discrete length, using metal multifilament yarns, or using metal monofilaments.

[1 1 ] In a preferred embodiment, the woven, knitted or braided burner deck comprises a zone or zones with a density less than the density of the zone with high density.

Preferably the zone or zones with density less than the density of the zone with high density has a density lower than 1 100 g/dm ³ , preferably lower than 1000 g/dm ³ , but preferably higher than 800 g/dm ³ , more preferably higher than 900 g/dm ³ .

Preferably the zone or zones with density less than the density of the zone with high density cover at least 20%, more preferably at least 30%, even more preferably at least 40% of the surface of the burner deck.

[12] A burner deck with zones of different densities can be obtained by

different levels of compression of different zones of the fabric that is used for the burner deck.

[13] Embodiments with zones with different densities have shown to provide synergistic benefits, in that the presence of such zones of the burner deck with lower density than the density of the zone with high density further reduces acoustic instabilities.

[14] In a preferred embodiment, one or more sections of the burner deck of the gas premix burner are double curved; and the zone or zones of the burner deck with density less than the density of the zone with high density, comprise at least part of, and preferably in full, the one or more sections of the burner deck that are double curved. Where a surface is at a point on it double curved, there is at that point no direction in which the radius of curvature at that point is infinite. As an example, a cylindrical burner is a burner that has a single curved surface. A sphere is an object that is double curved over its full surface.

[15] This preferred embodiment allows easy production of double curved

burners according to the invention. In zones in which the fabric that will form the burner deck is less compressed, it can more easily be deformed, allowing draping and mounting the fabric on the supporting perforated plate, woven wire mesh or expanded metal sheet, while obtaining synergistic benefits of less thermo-acoustic instabilities and the benefits of using a fabric as burner deck.

[16] The use of knitted burner decks is preferred as knitted fabrics allow more easily setting different levels of density by different levels of compression of the knitted fabric that will be used as burner deck.

[17] In a preferred embodiment of the invention, the zone with a high density does not cover points of the burner deck that have a smallest radius of curvature of less than 5 mm, preferably of less than 8 mm.

[18] Geometrically, at each point of the burner deck, many radii of curvature can be defined; each of them is associated with a particular cut according to a plane containing the normal line to the burner deck at the point under consideration. The intersection of this plane with the burner deck results in a trajectory. The radius of curvature is the radius of the circle in the intersecting plane, which osculates to second order the trajectory at the point under consideration. Out of all these possible planes, containing the normal line through the point under consideration, with associated trajectories and radii of curvature, the smallest radius can be determined for each position of the burner deck.

[19] In a preferred embodiment of the invention, the zone or zones of the

burner deck with density less than the density of the zone with high density comprise the circumference of the burner deck. Such burners have shown better results. [20] In a preferred embodiment of the invention, the burner deck comprises a zone with a density lower than 900 g/dm ³ , preferably lower than 750 g/dm ³ . Preferably the burner comprises an ionization electrode and /or an ignition electrode, and a zone with a density lower than 900 g/dm ³ (and preferably lower than 750 g/dm ³ ) is provided at the location of the ionization electrode and/or at the location of the ignition electrode.

[21 ] Preferably, the zone with a density lower than 900 g/dm ³ (and preferably lower than 750 g/dm ³ ) covers less than 20%, more preferably less than 10%, of the surface of the burner deck.

[22] Such embodiments have specific synergistic benefits:

- When an ignition pen is installed at such a zone, ignition of the burner is reliably facilitated, eliminating problems of bad, late or noisy ignition.

- When an ionization pen is installed at such a zone, ionization current measurement by means of the ionization pen can be used in a broad load range of the burner as a reliable indication of the air to gas ratio of the gas premix burner and hence as input for the modulation of the air to gas ratio supplied to the gas premix burner. Improved modulation contributes to the avoidance of acoustical instabilities, as the burner can be better controlled to avoid falling into a range of operation in which acoustical instabilities could occur.

[23] In a preferred embodiment of the invention, the burner deck has over its full surface a constant density.

[24] Preferably, the burner deck has a mass per unit of area larger than 1000 g/m ² , preferably larger than 2000 g/m ² and preferably smaller than 2750 g/m ² . Examples of fabrics that can be used for the burner deck are knitted fabrics with a specific weight of 1250 g/m ² or 1400 g/m ² or 2400 g/m ² .

[25] In a preferred embodiment, the burner deck has over its full surface a constant mass per unit of area [26] In preferred embodiments, the burner deck is not over its full surface bonded to the perforated plate, woven wire mesh or expanded metal sheet supporting the burner deck.

[27] In preferred embodiments of the invention, the burner deck is bonded

locally, e.g. via spot or line welding, to the perforated plate, woven wire mesh or expanded metal sheet supporting the burner deck.

[28] In preferred embodiments of the invention, the burner deck is bonded to the perforated plate, woven wire mesh or expanded metal sheet at edge zones of the burner deck, and preferably only bonded at the edge zones of the burner deck.

In a preferred embodiment, the burner deck is soft welded over at least part of its surface to the perforated plate, woven wire mesh or expanded metal sheet. Preferably the soft welding is performed over at least 50% of surface of the burner deck, more preferably over at least 75% of its surface, and even more preferably substantially over its full surface or over its full surface. Preferably the soft welding is performed (e.g. by means of capacitor discharge welding) such that when pulling the woven, knitted or braided burner deck from the perforated plate, woven wire mesh or expanded metal sheet, the soft welded bonds between the woven, knitted or braided burner deck and the perforated plate, woven wire mesh or expanded metal sheet are broken rather than that breakage in the woven, knitted or braided burner deck occurs. The test method to determine that the burner deck is soft welded, is pulling in peel-off mode: an edge portion of the burder deck is removed from the perforated plate, woven wire mesh or expanded metal sheet, and folded over 180°. Pulling the burner deck is then done by hand or using pliers, wherein the pulling force is exerted parallel with the perforated plate, woven wire mesh or expanded metal sheet, in a direction of 180° to the burner deck. In pulling, the force builds up until the burner deck is progressively peeled off from the supporting perforated plate, woven wire mesh or expanded metal sheet leaving no metal fibers of the burner deck on the supporting perforated plate, woven wire mesh or expanded metal sheet (indicating that soft welding occurred); or until progressively destroying the burner deck at least partly wherein metal fibers of the burner deck remain attached to the supporting perforated plate, woven wire mesh or expanded metal sheet (indicating that no soft welding occurred). Within the limits of the described "pulling in peel-off mode" the conclusion whether or not the burner deck is soft- welded to the supporting perforated plate, woven wire mesh or expanded metal sheet is independent of further parameters.

Such embodiments have shown further improvement in the reduction of thermo-acoustical instabilities.

[30] In embodiments in which the burner deck is over part of its surface or over its complete surface bonded via soft welding to the perforated plate, woven wire mesh or expanded metal sheet, the benefits of using a woven, knitted or braided burner deck comprising metal fibers are maintained. The benefits are that when the burner is in use the woven, knitted or braided burner deck can freely expand; and the perforated plate, the woven wire mesh or the expanded metal sheet remains sufficiently cool.

[31 ] Preferably the woven, knitted or braided burner deck comprises or

consists out of spun yarns, which comprise metal fibers of discrete length.

[32] In a preferred embodiment, the woven, knitted or braided burner deck comprises yarns comprising or consisting out of metal filaments. With filament is meant a fiber of virtually infinite length. The yarns comprising metal filaments can be metal multifilament yarns or can be metal monofilament yarns.

[33] In a preferred embodiment, the burner deck is one layer of a woven,

knitted or braided fabric, placed on the perforated plate, woven wire mesh or expanded metal sheet.

[34] In a preferred embodiment, the burner deck is knitted, woven or braided using yarns comprising or consisting out of a plurality of metal filaments or metal staple fibers in the cross section, or using yarns consisting out of metal monofilaments.

[35] In a preferred embodiment, the surface of the woven, knitted or braided burner deck at the other side than the side of the perforated plate, woven wire mesh or expanded metal sheet is not covered by another metallic object, such that the surface of the woven, knitted or braided burner deck is, when the burner is in use, the surface on which combustion takes place.

[36] Examples of preferred metal fibers are stainless steel fibers. A specifically preferred range of stainless steel fibers are chromium and aluminium comprising stainless steel fibers as in DIN 1 .4767, e.g. as are known under the trademark FeCrAlloy.

[37] Preferred are metal fibers with equivalent diameter of less than 50 μιτι, more preferably of less than 40 μιτι. With equivalent diameter of a fiber is meant the diameter of a circle with the same surface area as the cross sectional area of that fiber.

[38] Preferred metal fibers for use in the invention, e.g. stainless steel fibers, with an equivalent diameter less than 50 micrometer or less than 40 micrometer, e.g. less than 25 micrometer, can be obtained by a bundle drawing technique. This technique is disclosed e.g. in US-A-2050298, US- A-3277564 and in US-A-3394213. Metal wires are forming the starting material and are covered with a coating such as iron or copper. A bundle of the covered wires is subsequently enveloped in a metal pipe. Thereafter the thus enveloped pipe is reduced in diameter via subsequent wire drawing steps to come to a composite bundle with a smaller diameter. The subsequent wire drawing steps may or may not be alternated with an appropriate heat treatment to allow further drawing. Inside the composite bundle the initial wires have been transformed into thin fibers which are embedded separately in the matrix of the covering material. Such a bundle preferably comprises not more than 2000 fibers, e.g. between 500 and 1500 fibers. Once the desired final diameter has been obtained the covering material can be removed e.g. by solution in an adequate leaching agent or solvent. The result is a bundle of metal fibers.

[39] Alternatively metal fibers for use in the invention, such as stainless steel fibers, can be manufactured in a cost effective way by machining a thin plate material. Such a process is disclosed e.g. in US-A-4930199. A strip of a thin metal plate is the starting material. This strip is wound a number of times around a rotatably supported main shaft and is fixed thereto. The main shaft is rotated at constant speed in a direction opposite to that in which the plate material is wound. A cutter having an edge line extending perpendicularly to the axis of the main shaft is fed at constant speed. The cutter has a specific face angle parallel to the axis of the main shaft. The end surface of the plate material is cut by means of the cutter.

[40] Yet an alternative way of producing metal fibers for use in the invention is via extraction or extrusion from a melt of a metal or metal alloy.

[41 ] Another alternative way of producing metal fibers for use in the invention is machining fibers from a solid block of metal.

[42] Yarns, comprising or consisting out of metal fibers, for the production of the knitted fabric, the braided fabric or the woven fabric for use in the invention can e.g. be spun from stretch broken fibers (such as bundle drawn stretch broken fibers) and/or can e.g. be yarns made from shaved or machined fibers. The yarns can be plied yarns, e.g. two ply, three ply... Preferred fabrics made from metal fibers have a mass per unit of area of between 0.6 and 3 kg/m ² ; preferably between 0.7 and 3 kg/m ² , even more preferred between 1 .2 and 2.5 kg/m ² .

[43] The knitted fabric, the braided fabric or the woven fabric can also comprise metal monofilaments. The knitted fabric, the braided fabric or the woven fabric can e.g. be produced out of metal monofilaments.

[44] In a preferred embodiment, the knitted fabric, the braided fabric or the

woven fabric has a mass per unit of area between 0.6 and 1 .3 kg/m ² , more preferably between 0.6 and 0.9 kg/m ² .

[45] Preferably, the gas premix burner of the invention is suited for use in a boiler or water heater.

[46] The second aspect of the invention is a boiler or water heater comprising a gas burner as in the first aspect of the invention.

[47] Features of different embodiments and of different examples of the

invention may be combined while staying within the scope of the invention. Brief Description of Figures in the Drawings

[48] Figure 1 shows an example of a gas premix burner of the invention.

Figure 2 shows an example of an inventive burner with a double-curved burner deck.

Figures 3 and 4 show cross sections of the burner of figure 2.

Figure 5 shows the knitted fabric used for the burner deck of the burner of figure 2.

Figure 6 shows another example of an inventive burner with a double- curved burner deck.

Figures 7 and 8 show cross sections of the burner of figure 6. Mode(s) for Carrying Out the Invention

[49] Figure 1 shows an example of a gas premix burner 100 of the invention.

The gas premix burner 100 has a single-curved knitted burner deck. The knitted fabric consists out of spun stainless steel fiber yarns knitted into a fabric. The knitted burner deck 1 10 is supported by a perforated metal plate 130. The knitted burner deck 1 10 has two zones with different densities. A zone 140 with a high density and a zone with a lower density 160, at which an ignition pen 170 is mounted. In the same way a zone with a lower density can be foreseen at an ionization electrode. It is also possible to provide the burner with a burner deck of uniform density equal to the density of the zone 140 of high density of the burner 100 of figure 1 .

[50] Table I summarizes the results of trials with the burner of figure 1 . All trials have been performed with the same burner geometries (except for modifying the knitted burner deck as indicated in table I) and with a uniform density of the knitted burner deck. A B C

1400 1000 Thermo-acoustic instabilities present during normal operation

2400 1200 Thermo-acoustic instabilities present during normal operation

1400 1273 Thermo-acoustic instabilities only present during

start-up sequence.

2400 1500 Thermo-acoustic instabilities only present during

start-up sequence

1400 1750 Burner operates well (no thermo-acoustic

instabilities under all possible circumstances)

2400 2000 Burner operates well (no thermo-acoustic

instabilities under all possible circumstances)

Table I: Results for burner decks with a constant density A: Mass per unit of area of the knitted burner deck (g/m ² ); B: Density (g/dm ³ ) of knitted burner deck; C: Observation of thermo-acoustic (TA) instabilities 1 ] Figure 2 shows an example of a gas premix burner 200 according to the invention with a burner deck comprising double curved sections. The burner 200 comprises a knitted metal fiber yarn burner deck 210 supported by a woven metal wire mesh (not shown on figure 2) and a metal plate 235. Figures 3 and 4 show the cross sections of the burner 200 along lines Ill-Ill and IV-IV respectively. Figures 3 and 4 show the woven metal wire mesh 330, 430 supporting the knitted metal fiber yarn burner deck 310, 410 and the plate 335, 435 welded along the edges of the knitted metal fiber yarn burner deck 310, 410 to the knitted metal fiber yarn burner deck 310, 410. This welding operation creates a weld between the metal plate 335, 435 and the knitted metal fiber yarn burner deck 310, 410 and through the applied heat at the same locations also between the knitted metal fiber yarn burner deck 310, 410 and its supporting woven metal wire mesh 330, 430. Figure 5 shows the knitted metal fiber yarn fabric 510 that is used for the burner deck of the burner shown in figure 2. The fabric 510 shows sections with different density. A first section consists out of zones 541 of high density. A second section consists out of zones 551 with density less than the density of the zones 541 with high density. An optional zone 560 can be present with density lower than 900 g/dm ³ (e.g. a density of 875 g/dm ³ ), zone at which an ionization electrode and/or an ignition pen can be advantageously be installed.

Table II summarizes the results of trials performed on the burner shown in figures 2 - 5, compared to the same burner geometry and a prior art knitted burned deck.

Table II: Results for burner deck with different density levels A: Mass per unit of surface area of knitted burner deck (g/m ² ); B: Density (g/dm ³ ) of the zone with high density; C: Density (g/dm ³ ) of the zone with density less than the zone with high density; D: Observation of thermo-acoustic

(TA) instabilities.

[52] Figure 6 shows another example of a gas premix burner according to the invention with double-curved sections. The burner 600 comprises a knitted metal fiber yarn burner deck 610 supported by a woven metal wire mesh 630 and a metal plate 635. Figures 7 and 8 show the cross sections of the burner 600 along lines VII-VII and IV-IV respectively. Figures 7 and 8 show the woven metal wire mesh 730, 830 supporting the knitted metal fiber yarn burner deck 710, 810 and the plate 735, 835 welded along the edges of the knitted metal fiber yarn burner deck 710, 810 to the knitted metal fiber yarn burner deck 710, 810. This welding operation creates a weld between the metal plate 735, 835 and the knitted metal fiber yarn burner deck 310, 410 and through the applied heat at the same locations also between the knitted metal fiber yarn burner deck 310, 410 and its supporting woven metal wire mesh 730, 830.

The burner deck 610 has a central zone 642 where it is single curved and two end sections 652 where it is double curved. A knitted metal fiber fabric of 1400 g/m ² is used as burner deck.

In a first example of this burner, the density of the burner deck was constant over its complete surface, 1500 g/dm ³ .

In a second example of this burner, the density of the burner deck at the two double curved end sections 652 and at the transition into the single curved central zone 642 is 950 g/dm ³ . The density of the burner deck in the central zone is 1700 g/dm ³ .

In both examples of this burner, the selection of the burner deck resulted in improved thermo-acoustic behaviour of the burner compared to prior art burners of the same burner deck geometry.