The invention relates to a porous metal fiber plate. Such plates in which the fibers are sintered to one another, are used, amon other things, as filter media.

It is further known from the European patent 0 157 432 to us these fiber webs as a membrane for radiant surface combustio burners for gas mixtures, in as far as steel fibers containing C and Al are used to make them resistant against high temperatures

Since the porosity of these non-woven steels fiber webs, fibe mats or sintered fiber plates is not always perfectly homogenous a uniform transverse gas flow over the entire surface of the plat cannot always be guaranteed. For a number of applications, thi has turned out to be a drawback, e.g. for burner membranes and fo the gas-permeable support plates for fluid bed treatments in whic a controlled uniform flow is desired, linked with a small pressur drop across the thickness of the plate.

It is an object of the invention to avoid this disadvantage of t known gas-permeable metal fiber plates and thus to provide plat with a controlled uniform gas flow. According to the inventio this goal is achieved by providing a porous metal fiber plate i which a regular pattern of transverse holes or passages has be made which, all together, occupy an overall free passage area 5 % to 35 % of the total surface area of the plate, while ea hole has a surface area of between 0.03 and 10 mm ² . Thus the g flow is forced primarily through these holes. This feature i favourable i.a. in view of achieving a small pressure drop acro the plate.

Insofar as said plates need to be utilized at very high tempera¬ tures, the metal fibers that are used must be resistant to these temperatures. The equivalent fiber diameters may range between about 8 μm and 150 μm. With an equivalent fiber diameter is meant here the diameter of a fictive perfectly cylindrical fiber, the cross-section surface of which corresponds to the average cross- section surface of a real fiber which is not perfectly circular or even not circular at all. The thickness of the plate is preferably between 0.8 mm and 4 mm and the plate is sufficiently rigid and strong to resist the selected pressure drops at the desired poro¬ sities. Plate thicknesses of 1, 2 and 3 mm, for example, are sui¬ table. The porous plate therefore does not need any extra support near its bottom surface or its top surface (e.g. with a steel plate). Thus the bottom and top surfaces remain freely accessible.

It is another object of the invention to provide a gas burner device comprising a housing with supply means for the gas to be burned, a distribution element for the gas stream and a porous metal fiber plate as a burner membrane which enables a control- Table and uniform gas flow to the burner membrane exit surface and as a consequence a uniform burning process over the entire burner surface and with a low pressure drop in the gas flow crossing the membrane.

Yet a further object of the invention resides in the provision of a durable burner membrane wherein certain surface areas do not prematurely deteriorate due to overloading or overheating versus other areas, due to inhomogeneities in porosity thereby causing uncontrollable preferential gas glow paths and burning areas.

Another important object of the invention relates to the design of a porous metal fiber plate, usable as a burner membrane over an

enormously broad power range and which is therefor suitable f both surface radiant and blue flame modes.

A further object of the invention deals with the design of burn membrane plates which offer remarkably low CO and NO _x -emissions a high yields.

Yet another object of the invention concerns the design of a g burner with less constraints as to prefiltration of the inflowi gas stream.

The provision of a gas burning device with less danger for res nances occuring in the gas stream and hence for avoiding whistli effects emerging during operation is to be considered a furth object of the invention.

On the basis of several embodiments, further details will her after be explained. Additional solutions according to the inve tion for specific or partial problems or objectives and t characteristics of these solutions, as well as the advantages th entail, will also be made clear.

Figure 1 is a sketch of a porous plate with circular hol according to the invention. Figure 2 shows one possible way of assembling this plate in housing with supply means for the gas and transport tion and flow means for it through the plate. Figure 3 represents schematically a pipe-shaped device f passing the gas flow through. Figures 4 to 7 relate to top views of several alternative patterns of transverse passages to be arranged in t porous plates.

Figure 8 shows a cross-section of a gas burner device accordin to the invention in which an acoustic muffling layer i clamped between the burner membrane and the distributio element. Figure 9 presents a cross-section of a gas burner device in whic a number of muffling layers are included, possibly alon with empty interspaces.

The porous metal fiber plate 1 according to figure 1 comprise holes 2 spaced at regular distances p (pitch) from one another These holes are by preference cylindrical in shape and, in parti cular, circul r-cylindrical. By preference, the area of each hol 2 is the same and lies between 0.03 and 3 mm ² , though more prefe rably between 0.4 and 1.5 mm ² , respectively between 0.5 an 0.8 mm ² . As will be seen below, these dimensions are to be chose i.a. depending on the thickness of the plate 1, its porosity an the intended application. When the hole 2 thus has a circula cross-section, the diameter of each circle will be 0.8 mm for surface area of approximately 0.5 mm ² . The holes 2 are by prefe rence made with a punching operation since this assures a smoot cylinder wall. If so desired, holes can also be punched wit triangular, square, rectangular or other shapes. The holes ma also be made with laser beams. Thus, in principle, very smal holes with a diameter of at least 0.2 mm are possible for thi plates.

Figures 4 to 7 illustrate other preferred shapes of passages slots of different shapes and their regular distribution over th plate surface. Two examples of a suitable regular pattern o adjacent rectangular slots 9 are shown in figure 4 (right side resp. left side). Circular passages .2 and rectangular slots 9 ca alternate over the surface as shown in figure 5. Similarly, ova or elliptic slots 11 can alternate with circular holes 2 a

represented in figure 7. A pattern of cruciform slots 10 possible also as illustrated in figure 6. A great number regular distributions of passages with different shapes conceivable in view i.a. of minimizing or avoiding any whistli effect in the gas flow as will be explained further.

Each of the slots 9, 10, 11 should preferably have a surface ar of between 1 and 10 mm ² . Rectangular, or substantially rectangul slots will have a slot width "w" of between 0,3 mm and 2 mm and length "1" of between 3 mm and 20 mm. Preferably the relatio 0,5 mm < w < 1 mm and 5 mm < 1 < 10 mm will apply. Anyway, in plate with rectangular slots 9 according to e.g. figure 4 or the overall free passage area occupies 20 % to 30 % of the tot surface area of the plate.

The pitch p between adjacent holes 2 is chosen such that the total surface area comprises 5 % to 25 % of the total surface ar of the plate, and preferably 8 % to 16 %. Values of 10 %, 12 % a 15 % are adequate. In order to assure a uniform flow over the su face, the successive holes are by preference ordered in a patte of adjacent, equilateral triangles in which each hole 2 occupi a corner of the triangle.

The porosity of the plate (between the holes 2) is always betwe 60 % and 95 %, but preferably between 78 % and 88 %. The pla surfaces can be flat, can have a relief (be embossed), or else c be curved or corrugated, for example.

The metal fibers that can be used for producing the porous plat and the production of the plates themselves, and in particul those that are resistant against very high temperatures, described in the same European patent application 390.255. general, stainless steel fibers are suitable. For the high tem

rature applications, such as in gas burners, steel fibers contai ning Cr and Al are to be used, preferably those containing also small amount of yttrium.

As represented in figure 2, the porous plate 1 according to th invention can be assembled in a standard manner in a housing with supply means 4 for the gas. When this device is intended t function as a gas burner, a flammable gas mixture (e.g. natural gas/air) can be supplied. The device thus formed can, moreover, comprise a distribution element 5 for the incoming gas flow. Normally speaking, this will be a plate with suitable holes o passages arranged in it such that a uniform flow of gas with suitable pressure reaches the inlet side of the porous plate 1. The surface area of the free passages in the distribution plate can amount to between 2 % and 10 %. In the case of a cylindrica burner (figure 3), the distribution plate 5 also serves as support element for the end plate 8. The distribution element can possibly be corrugated and can also function to neutraliz possible sound resonances in the gas flow or as a flame arreste or barrier should they backfire into the gas inlet side of th plate 1, e.g. as a result of damage (cracks) in the burner plate If so desired, the holes 2 can have a conical entrance 6 and cylindrical exit 7 or vice versa (plate upside down) : a cylin drical entrance and a conical exit.

A distribution element 5 is by preference also provided for th gas supply, along with an end plate 8 in the cylindrical devic according to figure 3. Due to the flexibility of the membran plate 1 with hole pattern 2, cylinders of relatively small dia meters can be bent from flat plates.

It has been found that with certain forms of burner housings an built-in constructions in the spaces to be heated up (e.g

boilers), resonance can occur at relatively high powers: e.g. ov 1000 kW/m ² . It also appears that excess air in the gas mixtu supply can have an influence on the tendency to resonate, alo with the fact as to whether the gas is either drawn (suctioned) 5 blown through the membrane. Finally, the pattern of holes utiliz in the membrane itself can also play a role in the resonan phenomenon.

The resonance phenomenon is presumably related to the high pre

10 sure gradient of the gas mixture between the relatively cold und side (inlet side) of the burner membrane and the very hot upp side (exit side : burning surface). By changing the flow ra variables, such as excess air and gas mixture flow rate, oscillation phenomenon presumably occurs between the flame fro

15 (i.e. the level of the flame bases) and the gas mixture enteri the holes. The tongues of flame therefore can dance up and do above the burner surface or even oscillate with their flame bas between a position in (or even under) the holes and a positi above the holes (above the burning surface). This can be acc

20 panied by annoying whistling sounds ranging from 1000 to 1500

This drawback can also be encountered when changing a burner f a blown gas to a drawn gas system.

As mentioned before it is an object of the invention to elimin 25 this disadvantage and to make the occurrence of whistling sou less critical. The measure taken, however, should not reduce of the other advantages of the concept with perforated bur membrane. In particular, the measure should not result in drastic increase in the total pressure drop over the burner o 30 (local) destabilizing of the flame front.

The solution according to the invention consists of providin gas burner device which includes a housing comprising

following elements, positioned in succession downstream one after the other: means of supply for the gas which is to be burned, a distribution element, at least one acoustic muffling layer through which gas can pass, and a porous plate as burner membrane provided with a regular pattern of holes that, taken together, make up 5 % to 35 % of the surface area of the plate, with each hole having a surface area of between 0.03 mm ² and 10 mm ² .

Details will be explained on the basis of a number of embodiments, thereby referring to figures 8 and 9. The embodiments are to be understood only as examples.

The gas burner device according to figure 8 includes a housing 16 with the following elements positioned in succession downstream from one another : a supply duct 15 for the gas mixture and a distribution element 5 in the form of a perforated metal plate which lies against the bent edge 22 of said supply duct 15. The housing 16 is attached to the supply duct with a weld 17. The distribution plate 5 is, for example, 0.4 mm thick and provided with holes 18, each having a diameter of 0.4 mm. The holes or passages 18 can be placed in the corner points of a pattern of adjacent equilateral triangles with a triangle side (i.e. pitch between the holes) of 1.25 mm. This means a free passage surface area of the plate 5 of approximately 10 %. Depending on the circumstances, this free surface area could just as well lie between 5 % and 20 %. Below 5 %, the pressure drop becomes too high at high gas flow rates; above 20 % the distribution effec for the gas mixture becomes insufficient at low flow rates.

Against the outlet side of distribution element 5 lies a welde wire mesh 13 of stainless steel wire with a wire diameter of, fo example, 0.125 mm and a gas permeability of 48 mesh. Depending o the circumstances, a permeability can be chosen of between 30 mes

and 60 mesh. Two or more meshes 13 can also be stacked on top one another, preferably of different permeabilities.

Downstream from the welded wire mesh (or meshes) 13, whi operates as an acoustic muffling layer, is the porous membra plate 1, which is provided with a regular pattern of holes 1

This porous plate is again preferably a sintered metal fiber pla in which the fibers are heat-resistant, i.e. resistant against t high burner temperatures occurring during operation and resista against thermal shocks. The fibers, therefore, are preferab steel fibers with a suitable Cr and Al content: e.g. FeCrAll fibers as described hereinbefore.

Plate 1, for example, is 2 mm thick and has a porosity of 80.5 between the holes. The fiber diameter in the example 2 below w 22 μm and the diameter of the cylinder-shaped punched holes w 0.8 mm, while the spacing between the centers of the holes (i. the pitch) was 1.5 mm. Plate 1 is clamped against the housing 1 with a ceramic mat 14 inserted between the two.

In order to minimize resonance with specific gas flow profiles specially shaped burners for situations in which the gas mixtu is being drawn (sucked) and/or for specific construction par meters related to the combustion space to be heated, considerati can be given to providing an intermediate space 23 or 24 betwe the acoustically muffling layer 13 and the distribution element and/or the membrane 1, respectively, as shown, for example, figure 9. In this way various embodiments of the device are th created. The device can, for example, include one muffling lay 13 that is in surface contact with the distribution element 5. another embodiment the layer 13 can be in surface contact wi both element 5 and porous plate 1.

Another possibility is to build up the muffling layer 13 as a laminate made up of two wire meshes 25 and 26 with a porous mass interposed between them. If so desired, the porosity, and there¬ fore also the pressure drop over this laminate, can be changed under the influence of the gas pressure of the incoming mixture or via external operating means (not shown). The porous mass 27 can, for example, be a resilient mass of fibers, e.g. steel wool. Besides a more intense distributive effect on the mixture, this transverse compression respectively relaxation of the laminate can decrease the pressure drop over the membrane 1 at high flow rates so that again the danger of resonance becomes less critical.

According to another embodiment, the muffling layer 13 can consist wholly or partially of a porous mass of fibers 27. If so desired, this mass can fill up the whole interspace between plate 1 and element 5. By preference, mineral fibers are to be utilized (e.g. rockwool or steel wool) .

Finally, the porous plate 1 can also include a 'laminate of wire meshes sintered to one another. Woven or knitted wire meshes of heat-resistant wires can be used for this purpose. A suitable laminate structure is described in U.S. patent 3.780.872. On the whole _. these laminates will be more rigid than those made of sintered fiber webs. Therefore they are mounted by preference in flat burners. A pattern of holes is of course also punched through these laminates as described above.

When the gas burner devices are intended only for operation at relatively low powers, or when the tendency to resonate does not per se need to be avoided, then sintered porous plates 1 as such - made of shavings or cut fibers, or else of wire meshes such as ^• described above - can also be utilized. In this case a muffling layer 2 is not required and embodiments according to or analogous

to those described in the Belgian patent application 09200209 a then applicable. Instead of FeCrAlloy fibers, ceramic fibers wires can also be used.

EXAMPLE 1

A flat sintered porous metal fiber plate 1 produced according the invention and possessing the characteristics given below c be used as a membrane for a gas burner device. The characteristi and advantages of this concept with respect to previously prese ted burner membranes are explained below.

The steel fibers to be used are resistant against high temper tures and, for this purpose, contain by percent weight, f example, 15 to 22 % Cr, 4 to 5.2 % Al , 0.05 to 0.4 % Y, 0.2 0.4 % Si and at most 0.03 % C. They have a diameter of between and 35 μm - for example, approximately 22 μm. The fibers can obtained by a technique of bundled drawing, as known, for exampl from U.S. patent 3.379.000 and as is mentioned in U.S. pate 4.094.673. They are processed into a non-woven fiber web accordi to a method described in or similar to the method which is kno from U.S. patents 3.469.297 or 3.127.668. Afterwards, these w are consolidated by pressing and sintering into a porous plat with a porosity of between 78 % and 88 %. Porosities of 80.5 83 % and 85.5 % are very common.

It is also possible to use thicker metal fibers as heat-resist fibers in the porous plate, e.g. fibers with equivalent diamet of between 35 and 150 μm and consisting of wire shavings cuttings from a plate of the desired heat-resistant alloy (e. FeCrAlloy). These fibers look rather like steel wool and can manufactured according to a shaving. process as disclosed e.g. U.S. patent 4.930.199.

This porous plate 1 is now placed in a mould and, with a suitable punching device (stamp with punching pins), it is provided with a regular pattern of perfectly delimited circular cylindrical passages or holes 2 having a diameter of, for example, 0.8 mm. With a pitch of 2 mm between every pair of adjacent holes, a free surface area of nearly 15 % is obtained. Compared to a plate without holes, this design increases the flexibility and thus at the same time it facilitates the process of shaping, for example, into cylinders. The holes also form barriers against the spreading or propagation of cracks that may form in the membrane plate 1 as a result of the fluctuating thermal stress during operation. If so desired, the pattern of holes can be supplemented with a waffle pattern such as is described in EP 390.255.

Whenever holes are to be punched in solid steel plate, the thick¬ ness of the plate must always be thinner than the diameter of the holes. Surprisingly however, it has been found that this is not required for the punching of holes in the porous plates according to the invention. Thus there is a broad range of choice for the ratio of plate thickness to diameter or size of the holes or passages.

The great advantages of the invention concept, however, appear when the gas mixture to be burned is passed through the porous membrane plate 1. Indeed, the gas mixture now flows mainly through the holes 2, because of which the pressure drop over the membrane 1 is noticeably lower (than for plates without holes) for a particular flow rate or by which higher flow rates - and conse¬ quently larger thermal outputs or powers - can be achieved for a particular pressure drop value. The power range can now be selec¬ ted between 150 and 900 kW/m ² for a radiant surface combustion an can be increased to that of a blue flame surface burner with a output or power of up to 4000 kW/m ² , depending on factors such a

the excess air in the gas mixture in relation to a stoichiometr gas combustion mixture.

The porosity of the plate 1 results in the fact that a sma portion of the gas always penetrates through the pores between t holes 2 to the hot exit surface. As explained below, this great promotes a uniform and stable burning over a broad load or pow range. Especially at higher flow rates, the portion of gas th passes between the holes through the plate increases proportiona ly. It is now precisely at these higher flow rates (and cons quently higher powers if the percent of excess air remains t same in the gas mixture) that the tendency to blow away the bl flame at the level of the holes needs to be counteracted. T burning of the gas at the surface of the plate between the hol 2 maintains, as it were, a stable (blue) flame front over t whole plate surface and prevents this front (or the blue fla tongues within it) from being blown away from the plate surfac The tongue-shaped flames above each hole remain, as it were, wi their base - or root - anchored to the plate surface.

The largely horizontal orientation of the fibers within the poro plate also promotes the isolating effect of the membrane. Indee the heat conduction runs primarily in the outside surface (radia side) of the plate and much less in the depth (throughout t thickness) of the plate. Moreover, there is the ongoing unifo cooling effect of the cold gas supply in direct contact with t layer of fibers on the gas inlet side. In turn, this uniform he distribution at the level of the plate surface promotes the un form combustion of the gas layer and a stable burning state ov a broad load or power range at the exit side of the plate betw the consecutive holes 2. With a porous membrane layer 1, that its gas inlet side is attached, for example, to a supporting st plate and in which the porous layer together with the supp

plate have the same pattern of holes, this isolating effect will on the whole be smaller and the powers that can be attained will be lower. On the other hand, with another variant embodiment : a porous membrane without holes that is attached to a gas distribu- tion plate support with a regular pattern of many small holes (e.g. hole diameters of 0.3 mm and a pitch or center-to-center distance of adjacent holes of 1.25 mm), the attainable gas flow rate for a given pressure drop will remain more limited than with the plate according to the invention. Further with this arrange- ment, the high powers per unit of burner surface area are not attainable.

Another advantage with respect to the known plate membranes without holes relates to the fact that now it is much less neces- sary - if at all - to pre-filter the gas being supplied since it passes mainly through the larger passages (holes) 2 and only to a very limited extent through the small pores in the plate 1. The membrane plates according to the invention also need to be cleaned with a reverse flow much less frequently than was the case with porous plates lacking holes or passages.

The plate thickness, its porosity and the size of the passages or holes must of course all be coordinated with one another so that for any burner state no backfiring towards the gas inlet side will occur.

In a burning test the following observations were noted for a sintered fiber plate 1 made of the known FECRALLOY fibers with a diameter of 22 μm. The plate was 2 mm thick, had a porosity of 80.5 % and was built into a gas burning device of the type illus¬ trated in figure 2. A pattern of holes was punched into the mem¬ brane 1 as shown in figure 2: diameter of the cylinder holes was 0.8 mm and a regular geometric pattern of holes with pitch

p = 2 mm in a regular grid of adjacent equilateral triangles. T distribution plate 5 (0.4 mm thick) was at a distance of 5 mm f plate 1 and was provided with holes of 0.4 mm diameter and wit pitch of 1.5 mm. This resulted in a free passage surface area 5 6.5 %. There were no sound resonances or whistling sounds duri operation.

The pressure drop in the gas mixture over the plate (mb increases somewhat more rapidly than linearly with the resulti

10 power (kW/m ² ). At a pressure drop of 0.05 mbar, a power 150 kW/m ² was noted and at a pressure drop of 3 mbar, a power 3500 kW/m ² was attained. The gas mixture was composed of 8.1 natural gas and 91.9 % air. Natural gas with a relatively l calorific value of 10 kWh/N ³ was used and a 30 % excess of air

15 applied.

A radiant surface burner state was noted up to something li 800 kW/m ² . At higher powers, the burning changed into a blue fl mode. The temperature of the membrane surface (gas outlet si

20 increased to approximately 850 degrees C at around 700 kW/m ² gradually fell when going to higher powers (blue flame mode) approximately 600 degrees C. The membrane temperature on the inlet side remained below 150 degrees C and even decreased below 100 degrees C in the blue flame mode. The measured

25 emission (ppm) rose gradually over the whole power range up 2000 kW/m ² . However, it was only about 10 ppm at 700 kW/m ² , and powers around 2000 kW/m ² and up, it stabilized at about 15 20 ppm. The measured N0 _X values are in fact the data reduced their value at 0 % 02 in the combustion gases. These very low

30 values are probably to be explained by the fact that the fl tongues above the holes remain small so that the temperature their cores remains relatively low. The CO content was nearly z over the entire power range.

By way of conclusion, therefore, it has been found that for burner applications with the invention, a porous plate concept is avai¬ lable for the first time that can be used over an enormously broad power range and is therefore suitable both for surface radiant and blue flame modes. In addition, the concept offers remarkably lo CO and N0 _X emissions and it offers high yields.

Example 2

In the embodiment of figure 8 the porous plate 1 is in surfac contact with the 48 mesh wire mesh 13. A gas mixture of natural gas and air was passed through the compact combination in housin 16 of this wire mesh 13 clamped together between the 2 mm thic porous plate 1 and the distribution element 5 with free passag surface area of 10% (both described above). The square burne surface measured 150 mm x 150 mm. Various proportions of exces air were utilized (1.1 to 1.3) and the flow rates were increase such that powers were developed ranging from 500 kW/m ² to 500 kW/m ² .

In the table below the resonance results are given in column

[1 + 2 + 3]. By way of comparison, the burning tests are repeate in the table for embodiments with a combination of only plate and distribution element 5: column [1 + 3] and for the embodimen without wire mesh 13 and without plate 5: column [1]. The minu sign in the table refers to the desired absence of whistlin sounds during burning, while the plus sign indicates the presenc of an annoying whistling sound. Whistling sounds, moreover indicate an oscillation of the flame bases 20 in the holes 12 a suggested with arrow 21.

TABLE 1

We can infer from the table that a smaller excess of air (1.1 results more readily in resonance than does a larger excess of ai (1.2 or 1.3). Moreover, the favorable effect of wire mesh 1 appears to show up especially with the higher power loads (abo 1000 kW/m ² ).