Radiant burner

Abstract

A radiant burner comprises a body defining a premixing chamber and a combustion chamber. The premixing chamber is separated from the combustion chamber by at least one radiant burner plate (2) which has multiple levels of burner surface. The combustion chamber is further limited by a first radiant screen (4). The radiant burner further comprises a second radiant screen (3) in the combustion chamber. The second radiant screen is spaced from, but near and parallel to the radiant burner plate(s), such that this second radiant screen acts as an extended burner surface and also heats up said at least one radiant burner plate when in use.

Description

Technical Field

[0001] The present invention relates to radiant burners comprising a radiant burner plate and a screen.Background Art

[0002] Radiant burners comprising a radiant burner plate and a screen are known e.g. from US4799879 or EP0539278. The screen together with the radiant burner plate provides the radiative output of the burner, which averages at levels around 50% efficiency. In the past the radiative output of the burners has been increased by modification of the radiant burner plate from a radiant burner plate with rows of through holes or perforations serving to channel the mixture of air and combustion agent from the rear of the plate to the radiating face, to a radiant burner plate wherein the through holes or perforations are arranged in what is nowadays called honeycomb pattern as described in e.g. US4,569,657 or US4,799,879. This or similar modifications of the radiant burner plate increased the temperature level and consequently also the radiative output of the burner. On the other hand, these honeycomb-like patterns are creating local overheating of the burner plate on the places where the flames are, and also cause poor temperature uniformity and relative low average burner surface temperature and thus lower energy efficiency. These local high temperatures define therefore also the limitation of the use of such through hole or perforation patterns, and also define the limitation on the amount of radiation energy which can be obtained with such systems.

[0003] Another way of achieving higher radiative output was proposed in e.g. US 3,847,536 which uses two radiative screens above the radiant burner plate. Also this modification of the radiant burner caused local overheating of the radiant burner plates in the middle of the radiant burner, which urged the skilled person to lower inputs which resulted in lower (local) temperatures of the radiant burner plate for prolonging the life time of the radiant burner. In document US 3 291 188, a burner member comprises a block of refractory material defining a multiplicity of small bore passages extending in generally parallel relation to one another from a first boundary surface of said block toward a second boundary surface thereof, said passages adapted to conduct a combustible gas mixture from said first boundary surface toward said second boundary surface for combustion adjacent to said second boundary surface, at least some of said passages opening into said second boundary surface, said second boundary surface defining therein a plurality of indentations each having a peripheral wall substantially longer than the bore diameter of any one of said passages, means for at least some of said passages for conducting at least some of said combustible gas mixture away from said passages at an angle to the axes of said passages and through said peripheral wall into an adjacent indentation, and a radiation and gas pervious metallic matting overlying at least a portion of said second boundary surface and extending outwardly from said second boundary surface beyond the region of burning of said gas mixture to thereby cause a layer of exhaust gases to stagnate in close proximity to said second boundary surface.

[0004] However, still further enhanced efficiency of the radiant burners is desired.Disclosure of Invention

[0005] An aspect of the claimed invention provides a radiant burner which comprises a body defining a premixing chamber and a combustion chamber. The premixing chamber is separated from the combustion chamber by at least one radiant burner plate which has multiple levels of burner surface. The combustion chamber is further limited by a first radiant screen. The radiant burner further comprises a second radiant screen in the combustion chamber. The second radiant screen is spaced from, but near and parallel to the radiant burner plate(s), such that this second radiant screen acts as an extended burner surface and also heats up said at least one radiant burner plates by back radiation when in use. According to the invention, the second radiant screen is an arrangement of parallel spaced round rods or square bars. In a preferred embodiment, first and second radiant screens are produced from highly heat resistant materials such as ceramics, especially aluminium or zirconium oxide, aluminium titanate, silicon oxide, corundum or mullite, silicon carbide, silicon nitride or metal infiltrated ceramics, such as silicon-infiltrated silicon carbide. Alternatively, the radiant screens can also be fabricated from heat-resistant materials of other nature such as e.g. materials which contain more than 50% by weight of a metal silicide, such as molybdenum disilicide (MoSi 2 ) or tungsten disilicide (WSi 2 ). In another preferred embodiment, the radiant screens are fabricated from highly heat resistant steel grades, such as high level stainless steel grades like Kanthal APM or APMT, different grades of FeCrAl alloy designed for high temperature corrosion, Chrome / Nickel steel grades like Avesta 253 MA, 153 MA, Inconel 601, Incoloy 800HT, Incoloy MA956.

[0006] The radiant burner plate is preferably made of a ceramic material with high temperature resistance, and excellent mechanical and thermodynamic properties such as e.g. cordierite or zirconia; partially stabilised zirconia (PSZ), alumina, silicon carbides or other high level technical ceramics. Height difference in between two levels of burner surface of the radiant burner plate is from 1 to 20 mm. More preferably, from 1 to 10 mm. Even more preferably, from 2 to 7 mm. Most preferably 5 mm.

[0007] The radiant burner plate has multiple levels of burner surfaces. These multiple levels are arranged in rows and are alternating per one row of through holes / perforations on the radiant burner plate. An example of such burner plate can be found in Figure 1, or alternatives in Figures 2 and 3. These types of burner plates, as such, provide less emissivity compared to ceramic tiles with honeycomb or similar perforation patterns. This is due to the multiple level burner surface, wherein the lower levels of the burner surface of the radiant burner plates provide a higher radiative output because the sides of the rows also heat up and provide an additional radiative output, but the highest level of burner surface does not have such additional radiative output. So the overall radiative output, and therefore also the energy efficiency, of such multilevel radiant burner plate as such, is lower than honeycomb-like perforations in the radiant burner plate.

[0008] However, although radiant burner plates are used which as such have a lower radiative output, it was surprisingly observed that by the use of such a second radiant screen near the radiant burner plates, the radiative output of the radiant burner plates can be increased without leading to local overheating of the burner plates, as this would result in early failure of the radiant burner plates. This might be explained, without pretending to be scientifically correct, by the fact that the back radiation of the second radiant screen on the radiant multilevel burner plates is the highest on the highest level of the burner surface as this is closest to the second radiant screen. This highest level thereby also heats up more than the lower levels of the burner surface, which are at a bigger distance from this second radiant screen. As these lower levels in the burner surface of the radiant burner plates were already at higher temperatures by the effect of the flames heating up the surface surrounding the cavity wherein the perforations open, the overall effect of the present invention is that the different levels in the burner surface of the radiant burner plates are at the same temperature when in use. Stated otherwise, a greater temperature uniformity of the burner surface of the radiant burner plate is attained. The person skilled in the art will understand that this greater temperature uniformity combined with the plurality of radiant screens results in a significant higher energy efficiency of the complete radiant burner. According to the invention, the distance between the second radiant screen and the highest level of burner surface of the at least one radiant burner plates is between 3 and 50 mm. More preferably, the distance between the second radiant screen and highest level of the radiant burner plate is between 5 and 30 mm, even more preferably between 10 and 25 mm, most preferably between 15 and 20 mm. In a preferred embodiment, the second radiant screen is positioned such that the second radiant screen follows the direction of the rows of the highest level of burner surface of the radiant burner plate.

[0009] The first radiant screen is preferably a metal grid. In another preferred embodiment, the first radiant screen is an arrangement of parallel spaced round rods or square bars. In a further preferred embodiment, the first and second radiant screens are arranged in the same direction. In an alternative preferred embodiment, the first and second radiant screens are arranged in shifted angles with respect to one another. More preferably, the first and second radiant screens are at a 90° angle.

[0010] A further observed advantage of the present invention is a lower level of emissions of byproducts of combustion, such as Nitrogen Oxides or Carbon Monoxide, which is probably due to the second radiant screen which acts as an extended burner surface and provides a more complete combustion of the gas-air mixture.

[0011] Another aspect of the claimed invention provides a radiant burner with at least one further radiant screen in the combustion chamber.Brief Description of Figures in the Drawings

[0012] Example embodiments of the invention are described hereinafter with reference to the accompanying drawings in which Figures 1 to 3 show a cross section of example embodiments of radiant burner plates used in the present invention. Figure 4 shows an example embodiment of the present invention, with cut out for better view of the build up of the radiant burner. Figure 5 shows a side view of the example radiant burner of figure 4, also with cut out for better view of the build up of the radiant burner. Figure 6 shows an alternative example embodiment of the present invention. Figure 7 shows a side view of the example radiant burner of figure 6. Mode(s) for Carrying Out the Invention

[0013] Example embodiments of the present invention will now be described with reference to Figures 1 to 7.

[0014] Figures 1 to 3 show cross sections of example embodiments of radiant burner plates which might be used in the present invention. Figure 1 shows two levels of burner surface of the radiant burner plate 2, figures 2 and 3 show three levels of burner surface, in two alternative forms.

[0015] Figures 4 and 5 show an example embodiment of the present invention. The first radiant screen 4 is a highly heat resisting metal grid fabricated from highly heat resistant steel grades, such as high level stainless steel grades like Kanthal APM or APMT, different grades of FeCrAl alloy designed for high temperature corrosion, Chrome / Nickel steel grades like Avesta 253 MA, 153 MA, Inconel 601, Incoloy 800HT, Incoloy MA956. The second radiant screen 3 is made of a highly heat resisting ceramic material, in this example aluminium or zirconium oxide, aluminium titanate, silicon oxide, corundum or mullite, silicon carbide, silicon nitride or metal infiltrated ceramics, such as silicon-infiltrated silicon carbide with a silicon infiltration grade of 5 to 50 % or even more. Alternatively, the radiant screens can also be fabricated from heat-resistant materials of other nature such as e.g. materials which contain more than 50% by weight of a metal silicide, such as molybdenum disilicide (MoSi 2 ) or tungsten disilicide (WSi 2 ). The radiant burner plate 2 is made of a two level burner surface, ceramic tile made of cordierite or alternate thermodynamically suited ceramics as mentioned above.

[0016] Figures 6 and 7 show an alternative example embodiment of the present invention. The first and second radiant screens are made of highly heat resisting material, in this example a ceramic like aluminium or zirconium oxide, aluminium titanate, silicon oxide, corundum or mullite, silicon carbide, silicon nitride or metal infiltrated ceramics, such as silicon-infiltrated silicon carbide with a silicon infiltration grade of 5 to 50 % or even more. Alternatively, the radiant screens can also be fabricated from heat-resistant materials of other nature such as e.g. materials which contain more than 50% by weight of a metal silicide, such as molybdenum disilicide (MoSi 2 ) or tungsten disilicide (WSi 2 ). In this example this first and second radiant screens are arranged in directions which are 90° with respect to one another. The radiant burner plate 2 is made of a two level burner surface, ceramic tile made of cordierite.

[0017] Thus there has been described a new radiant burner 1 possessing great flexibility of use and which is capable of reaching temperatures of about 1300 °C with a considerable radiation factor increase of about 10 % compared to existing technology.

[0018] Because of their possible use at very high temperatures e.g. 1300°C and higher, their high energy efficiency and their long service life, the radiant burner of the present invention are particularly suitable for drying web materials at high web speeds. One preferred area of application is the drying of moving paper webs.

[0019] The new improved radiant burner comprises a body defining a premixing chamber and a combustion chamber. The premixing chamber is separated from the combustion chamber by at least one radiant burner plate which has multiple levels of burner surface. The combustion chamber is further limited by a first radiant screen. The radiant burner further comprises a second radiant screen in the combustion chamber. The second radiant screen is spaced from, but near the radiant burner plate(s), such that this second radiant screen acts as an extended burner surface and also heats up said at least one radiant burner plate when in use.

Claims

1. A radiant burner (1) comprising a body defining a premixing chamber and a combustion chamber, said premixing chamber being separated from the combustion chamber by at least one radiant burner plate (2) which has multiple levels of burner surface, said combustion chamber being further limited by a first radiant screen (4), characterised in that the multiple levels are arranged in rows and are alternating per one row of through holes on the radiant burner plate (2) and said at least one radiant burner plate (2) has a height difference in between 2 levels of burner surface of 1 to 20 mm; said radiant burner (1) further comprises a second radiant screen (3) in said combustion chamber, said second radiant screen (3) being spaced from, but near and parallel to said at least one radiant burner plate (2) and the distance between said second radiant screen (3) and the highest level of burner surface of the at least one radiant burner plate (2) being between 3 and 50 mm, such that said second radiant screen (3) acts as an extended burner surface and also heats up said at least one radiant burner plate (2) when in use, and in that said second radiant screen (3) is an arrangement of parallel spaced round rods or square bars.

2. A radiant burner (1) according to claim 1, wherein said first radiant screen (4) is a metal grid or an arrangement of parallel spaced round rods or square bars.

3. A radiant burner (1) according to any of the claims 1 to 2, wherein said at least one radiant burner plate (2) is a ceramic burner plate.

4. A radiant burner (1) according to claim 1, wherein said combustion chamber further comprises at least one further radiant screen.

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