Note: Descriptions are shown in the official language in which they were submitted.
WO95/27871 2 1 6 4 1 0 1 PCT/BE9S/00031
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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
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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.
WO95/27871 21 6 4 1 0 I PCT/BE95/00031
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.
WO95127871 2 1 6 4 1 0 1 PCT/BE95/00031
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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
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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
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WO 95/27871 PCT/BE95/00031
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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
WO 95/27871 2 1 6 4 1 0 1 PCT/BE95/00031
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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
2 1 64 1 0 1
WO 9S/27871 PCT/BE95/00031
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
WO 95127871 2 1 6 4 1 0 1 PCT/BE95/00031
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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.
