CA2164101A1 - Metal fiber membrane for gas burners - Google Patents

Abstract

The invention relates to a metal fiber membrane (1) for gas burners comprising a sintered metal fiber web with a pattern of a number of consecutive quadrangular porous zones (2) with each a width D, a length L and a surface of at least 100 cm2 and with intermediate sealed areas in the form of a grid (3) in which any distance D or L between each two adjacent rows of the grid is at least 40 mm, while the width B of each sealed area is between 5 mm and 20 mm. The porous zones are preferably rectangular. The membrane can be mounted in a flat or tubular arrangement in the burner.

Description

METAL FIBER MEMBRANE FOR GAS BURNERS

The invention relates to a porous metal fiber membrane for gas burners.

It is known from European patent 0157 432, to use sintered metal fiber membranes as burner membranes in premix gas surface radiation burners, provided the steel fibers used contain Cr and Al to make them resistant to high tempera-tures.

Further, from WO 93/18342 a porous metal fiber plate is known which contains a regular pattern of holes, all together com-prising bel.:_en 5 X and 35 X of the total surface area of the plate. The advantages of this embodiment include a more uniform flow of gas transversely through the plate over its entire surface.

In operation, these plates must be able to expand in all directions and then again contract in accordance with the heating and cooling cycles of the burner plates. When the plates have a considerable surface area, however, and are held in a fixed frame, this cyclical changing of dimensions cannot always proceed unhindered. Indeed, it must be kept in mind that the expansion coefficient of these plates between 20C and 1000C averages 15 x 1061C. In practice, this means, for example, that upon heating to 1000C, a 1 m long plate will expand by 15 mm in this longitudinal direction. In an open environment the fixed frame, however, will heat up to a maximum of 300C, since this frame, unlike the membrane, does not heat to incandescence during the gas burning. Hence there is nearly 10 mm of plate expansion which will have to be realized via the bulging of the plate instead of via the lateral expansion of the plate in its plane. This involves the risk that this bulging will develop as uncontrollable local deformations (creases or folds), thus causing the plate WO 95127871 ~ '~ 6 4 1 ~ - 2 - PCTIBE95/00031 to lose its original flat form. At the same time, there is the risk that the fiber layers in the plate will detach transversely from one another in some places, thus causing the porosity to u"dergo unacceptable changes.

It is an object of the invention is to prevent this u,lconlrollable deforming telldency.

This object is achieved by providing a metal fiber burner membrane for gas burners comprising a sintered metal fiber web with a pattern of a number of consecutive quadrangular porous zones with each a length L and a width D and with intermediate densified, solidified or sealed boundary areas or rows in the form of a grid. The surface of each porous zone should be at least 100 cm2 and any distance D or L
between each two adjacent rows of the grid is at least 40 mm, while the width B of each sealed area is between 5 mm and 20 mm. The grid should preferably be regular. This means that the porous zones should preferably all have the same shape and surface.

The width B must be a minimum of 5 mm in order to obtain sufficient rigidity in the intermediate sealed or solidified areas. If the width B is greater than 20 mm, however, then too much of the effective surface area of the membrane may be lost. Likewise, when the surface area of each porous zone or of a majority of them is smaller than 100 cm2, then again the effective burning surface decreases too much.

The porous zones can have the shape of a rectangle or of a square. The boundary grid lines of solidified or densified areas between the consecutive porous zones can be produced by locally compacting the metal fiber skeleton along these lines. Otherwise they can be produced by filling up the pores of the metal fiber skeleton of the membrane within these lines (or strips) at least in part - for example over at WO9S/27871 2 1 6 4 1 0 1 PCT~BE9S/00031 least half the thickness of the membrane - with heat resistant (e.g. ceramic) material, such that the membrane in these zones is sealed. This means that in the areas covered by the grid lines the membrane must become impermeable to the stream of gas sent transversely through the membrane. Hence the terms "densified", "solidified" or "sealed" are always to be understood as meaning that the material has been treated in order to obtain substantial impermeability to a stream of gas.
The solidified zones can also be obtained by keeping the metal fiber skeleton of the membrane fixed to a metal strip or bar on one side or pressed locally bel~_en parallel metal strips. These strips can be attached to the membrane either with a ceramic glue that is to be hardened, or by welding, by binding or with nuts and bolts or otherwise.

Apart from that, the porous zones can be provided with non-cylindrically curved subzones with concave and convex sur-faces lying opposite one another as described in Belgian patent application No. 09301056 filed by the applicants.
Alternatively the porous zones can be provided with cylindrically curved subzones with concave and convex surfaces lying opposite one another. A regular pattern of holes can also be provided in the porous zones, such that all together these holes comprise between 5 X and 35 % of the porous membrane surface area, with each hole having a surface area of between 0.03 mm2 and 10 mm2, as is known from WO
93/18342 of present applicants. The burner according to the invention can thus be used as a radiant surface combustion burner, but also as a blue flame burner.

Finally, the invention also relates to a gas burner apparatus comprising a housing with inlet means for the gas mixture to be burned, possibly a distribution device and a membrane as described above.

Details will now be explained on the basis of a number of embodiments with reference to the accompanying drawings.

Figure 1 shows a perspective view of a flat metal fiber membrane according to the invention.
Figure 2 shows a cross-section of a gas burner apparatus with a cross-section of the membrane along the line I I in Figure 1.

Figure 3 is a top plan view of a second embodiment of the invention.

Figure 4 is a cross section along line IV-IV of theburner according to figure 3 showing other fixingmeans of the membrane in its sealed areas.

The plate-shaped metal fiber burner membrane 1 shown in Figure 1 comprises in essence a porous sintered metal fiber web with a thickness of between 0.8 mm and 4 mm. Plate thicknesses of 1 mm. 2 mm or 3 mm are very suitable. The consecutive porous zones or areas 2 have a porosity of between 60 X and 95 %, and preferably between 78 X and 88 %.
The metal fibers are of course resistant to high temperatures (over 1000C) and to thermal shocks. For this purpose they contain. for example. the known minimum amounts of aluminum and chrome. In particular. the FeCrAlY fibers described in European patent 0157 432 are very suitable. The equivalent fiber diameter lies between 8 ~m and 150- ~m, by preference between 15~m and 50 ~m. The fibers can be produced using a bundled drawing method. such as described in U:S. patent 3,379,000, or using a machining method as is known from U.S.
patent 4,930,199. The fibers can be processed into a non woven web and further into a sintered web membrane as described in U.S. patents 3,469,297 and 3.505,038, respectively.

When the grid 3 between the porous zones 2 is produced with a ceramic material 4 filling up the metal fiber skeleton of the membrane, the amount of this material (thickness and width B of the strip 4) and the distances D and L together determine the reinforcement and resistance to bending.

If so desired, the ceramic material 4, can be covered, for example, with strips of adhesive ceramic paper 15, either on the gas inlet side (as shown in Figure 2) or on both sides of the membrane 1.

For the solidified lined zones 3, parallel metal supporting elements such as strips 5 and 6 can also be used, with the membrane held between them. The strips can be attached to one anu~her with spot welds through the metal fiber skeleton. The attachment can also be achieved by introducing a ceramic glue 4 between the strips. The strips 5 on the gas inlet side can be equipped with upright edges 14, which then function as cooling fins; they also increase the bending strength of the strip. The metal strip 6 can be composed of AISI 430 steel, and the U-profile 5 of 18/8 chrome-nickel steel. If so desired, the grid can be discontinuous or interrupted at the crossing points 7 of the strips.

In order to s~rel,y~hen relatively thin fiber membranes 1, a supporting layer of expanded metal 17 can be attached to the gas inlet side of the membrane 1. This layer has a thickness, for example, of at most 1 mm and has consecutive diamond-shaped mesh openings with axial dimensions of 2 mm and 4 mm and with 10 meshes or openings per 6 cm in the axial direc-tion of the longer (= 4 mm) axes. This layer 17 is then preferably also sealed along the lines of the grid 3. Other already known gas permeable reinforcement layers 17 can of course also be utilized.

WO95127871 ~ l 6 4 ~ ~ PCT/BE95/00031 In order not to inhibit the lateral expansion of the porous zones 2 and to prevent them from bulging up irregularly, a preformed deformation can be pressed in the form of a spheri-cal subzone or shell 8 with shell diameter S and a radius of curvature R, in which S ~ R/2, in accordance with the teachings in the Belgian patent application 09301056 of applicants. The height of the spherical shell will be at most 15 mm. If a reinrorcement layer 17 is attached, it will by prefere,)ce u"de.=go the spherical shell deformation beforehand along with the membrane.

In accordance with patent application W0 93/18342, the mem-brane in the porous zones 2, either in the flat or the sphe-rical shell form, can be provided with a regular pattern of holes 9 to produce homogenous radiant combustion or possibly to effect blue flame combustion. The holes 9 can, for example, be circular or rectangular. For circular openings, the surface area of the holes per opening will usually be selected to be lower than 3 mm2 and, by preference, be~ En 0.4 and 1.5 nn~ ; and most preferably between 0.5 and 0.8 mm2. The consecutive circular holes are situated by prefe-rence at the corner points of adjoining equilateral triangles. The length of the triangle sides is then selected such that the total free passageway in the porous zones amounts to between 5 X and 25 X of their surface area, and by preference between 8 X and 16 X, as for example 10 X, 12 X or 15 %.

The gas burner apparatus according to the invention comprises a housing 11 with an inlet duct 12 for the gas mixture to be burned. A gas distribution means 13, such as for example a per-forated plate, can be provided upstream from the membrane 1. If so desired, a compression spring 10 can be installed on the gas inlet side of the porous zones 2. This spring 10 can thereby force a controlled expansion movement perpendicular to the membrane surface. The springs 10 can be placed between the distribution device 18 and the membrane 1. The height of the open spring is then approximately equal to the height of the bulge in the membrane.

Example 1 A rectangular metal fiber membrane of 90 cm by 30 cm was assembled as follows. The porous membrane 1 was constructed from a sintered web of FeCr-alloy fibers (containing at least 0.1% Y and produced by bundled drawing). The equivalent fiber diameter was 22 ~m. The porosity of the membrane was 80 % and its thickness was 2 mm. It had a pattern of holes 9 as described in W0 93/18342. The round holes had a diameter of 0.8 mm. The pitch, or distance between the holes was 2 mm, which resulted in a free passageway surface area of nearly 15 %.

The membrane was divided up into 12 square porous zones 2 (D
= L) of 14 x 14 cm. A preformed deformation 8 in the shape of a spherical shell with a height of 5 mm and a shell diameter S of 12 cm was pressed into each zone. A steel strip 5 (12 mm x 3 mm) was placed over the entire length along the center line of the gas inlet side of the membrane 1 and fastened to this membrane 1 by means of a ceramic glue (Aremco) 4. Strip segments (12 mm x 3 mm) were placed parallel to one another across this longitudinal strip every 14 cm and fastened with Aremco glue to form the edges or boundaries of the square porous zones.

This membrane, thus reinforced with solidified zones 3, was installed in a burner with the flame pointing downwards (gas flowing from top to bottom). The membrane was suitably clamped onto the bottom of a horizontal metal frame 11, which functioned as the housing, and the gas inlet 12 was connected at the top side of the frame 11. The frame itself was composed of tubular profiled sections through which a coolant was able to circulate. This construction prevents the membrane from deforming in an irregular manner, even after long-term use and with a cyclical burning schedule at varying intensities.

In general, an arrangement should be avoided in which a series of solidified strip areas 3 running parallel with one another also run parallel with the direction of movement of the material to be heated passing under the burner. Indeed, if these strip areas 3 and the direction of movement are kept mutually parallel, then there is a risk that at regular intervals, onto narrow longitudinal strips of the material to be heated (oriented along the direction of movement), less heat will be transmitted, namely, onto these narrow longitudinal strips which would then run directly opposite the solidified strip zones 3 of the membrane. This non parallel direction of movement of the material to be heated is suggested by the arrow 19 in figure 1.
-Example 2 A rectangular metal fiber membrane of 200 cm by 30 cm is assembled as follows. The porous membrane 1 is produced from a sintered web, provided with holes 9, as described in example 1.
The membrane is divided up into a grid of 4 adjacent parallel rectangular zones 2 of (D=) 7.5 cm by (L=) 200 cm. In practice the width D will preferably be choosen below 20 cm when L~20 cm. When L~50 cm, and certainly when L~lOOcm, then D will preferably be choosen smaller than 15 cm, and often even smaller than 10 cm. At the grid lines 3 be~ween the consecutive zones 2, the membrane is sealed by applying a ceramic strip coating 4 (Aremco glue) to the back side of the membrane. This strip coating has a width B = 10 mm. The membrane is mounted at the upper peripheral edge of a housing 11 on top of a metallic rectangular grate 20 of 200 cm by 30 9 .

cm. This grate 20 is built up of an outer frame 24 to which tubular elements 21 are fixed and which support the membrane.
The elements 21 can also consist of bars or rods in stead of tubes and with other cross sectional shapes than round. Each element 21 faces thereby a sealed area 4. It may however suffice to have an element 21 facing only each second sealed area 4. The elements 21 are fixed to the-outer frame 24 of the grate 20 at one end 25. At the other end 26 of said frame 24, pins 27 can be fixed onto which the tube ends 28 can slide in an axial direction. This sliding arrangement is useful in view of allowing thermal expansion of the elements 21 during the heating up before a burning cycle, respectively their retraction during the cooling down after a burning cycle. It may even be useful to subdivide the elements 21 in consecutive longitudinal sections linked to each other by means of a sliding pin arrangement 29. The membrane is then fixed at regular distances A eg. with binding wires 23 to the elements 21. The fixing or binding spots (23) are preferably distributed in an even pattern (at regular distances) over the surface of the membrane as shown in figure 3. In the binding areas the wires 23 may be covered on the burning side of the membrane with a local sealing spot 30 of ceramic glue.
Suitable distances A are situated between 2 cm and 20 cm.
During operation the porous zones may bulge outwardly to form cylindrical shells 8 whereby the shell dimension S should remain below half its radius of curvature R.

When the burner is to be used in an open environment, the housing 11 may be composed of steel. However, when the burner is to be used in a closed hot environment, its temperature may rise to a very high level during operation. Therefore its housing will then have to be composed preponderantly of ceramic parts. These parts of ceramic material have a much lower coefficient of thermal expansion than metallic parts.
So the difference of expansion between the membrane and the housing may become quite important when using a ceramic housing. The design of supporting the metallic porous membrane according to the invention is thus a breakthrough solution. in particular for making burners having large burning surfaces and which have to operate in closed environments.

It is also possible to design burners with cylindrically shaped membranes which are then suitably supported by means of a cylindrical grate 20 in the form of a cage mounted inside the membrane and with the tubular elements 21 running according to the generatrices of the cylinder. The densified grid lines 3 in the membrane can then consist of compressed narrow strips therein facing these generatrices. These lines 3 then can favour an easy bending of the membrane along these lines,similar to the teachings of the Belgian patent 890.312.
Similar to the embodiment shown in figure 3 of W0 93/18342 one end of the cylinder is then covered with a closing cap, whereas the premix gas is supplied to the other end. Having fixed the membrane in its grid lines 3 at regular distances A to the tubular elements. the adjacent porous sections between these grid lines can then bulge outwardly to allow for thermal expansion.

Claims (19)

1. A metal fiber membrane (1) for gas burners comprising a sintered metal fiber web with a pattern of a number of consecutive quadrangular porous zones (2) with each a width D, a length L and each a surface of at least 100 cm2 and with intermediate sealed boundary areas in the form of a grid (3) in which any distance D or L between each two adjacent rows of the grid is at least 40 mm, while the width B of each sealed area is between 5 mm and 20 mm.

2. A membrane according to claim 1, in which each of the consecutive porous zones (2) has the shape of a rectangle.

3. A membrane according to claim 1, in which the sealed areas (3) comprise a metal fiber skeleton densified by compaction.

4. A membrane according to claim 1, in which the sealed areas (3) comprise a metal fiber skeleton, the pores of which are sealed with ceramic material (4).

5. A membrane according to claim 1, in which the sealed areas comprise a metal fiber skeleton which is fixed to at least one supporting element (5,6,21).

6. A membrane according to claim 1, in which the solidified areas are discontinuous near their mutual crossing points (7) in the grid (3).

7. A membrane according to claim 1, in which each porous zone (2) is provided with one cylindrically curved subzone (8) with concave and convex surfaces lying opposite one another.

8. A membrane according to claim 1, in which the sintered metal fiber web on the gas inlet side is supported by a layer of expanded metal (17).

9. A membrane according to claim 1, with a thickness of between 0.8 mm and 4 mm.

10. A membrane according to claim 1, with a porosity of between 60 % and 95 % in the porous zones (2).

11. A membrane according to claim 10, with a porosity of between 78 % and 88 %.

12. A membrane according to claim 1, in which the metal fibers are resistant to high temperatures.

13. A membrane according to claim 13, in which the metal fibers are steel fibers containing aluminum and chrome.

14. A membrane according to claim 1, in which a regular pattern of holes (9) is present in the porous zones (2), such that all together these holes comprise between 5 % and 35 %
of the porous membrane surface area. with each hole having a surface area of between 0.03 mm2 and 10 mm2.

15. A membrane according to claim 14. in which the holes (9) are circular and have a surface area of between 0.5 mm2 and 0.8 mm2.

16. A membrane according to claim 14, in which the total free passageway surface area of the holes (9) is between 5 % and 25 % of the membrane surface area.

17. A membrane according to claim 16. in which this free passageway surface area is between 8 % and 16 %.

18. A membrane according to claim 15, in which the consecutive holes (9) are arranged such as to be situated on the corner points of adjoining equilateral triangles.

19. A gas burner apparatus comprising a housing (11) with an inlet means (12) for the gas to be burned. a gas distribution means (13) and a membrane (1) according to claim 1.