Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W o 93/18342 C A 2 1 1 7 6 0 5 PCT/BE93/00010
POROUS METAL FIBER PLATE
The invention relates to a porous metal fiber plate. Such plates,
in which the fibers are sintered to one another, are used, among
S other things, as filter media.
It is further known from the European patent 0 157 432 to use
these fiber webs as a membrane for radiant surface combustion
burners for gas mixtures, in as far as steel fibers containing Cr
and Al are used to make them resistant against high temperatures.
Since the porosity of these non-woven steels fiber webs, fiber
mats or sintered fiber plates is not always perfectly h
a uniform transverse gas flow over the entire surface of the plate
lS cannot always be guaranteed. For a number of applications, this
has turned out to be a drawback, e.g. for burner membranes and for
the gas-permeable support plates for fluid bed treatments in which
a controlled uniform flow is desired, linked with a small pressure
drop across the thickness of the plate.
It is an object of the invention to avoid this disadvantage of the
known gas-permeable metal fiber plates and thus to provide plates
with a controlled uniform gas flow. According to the invention,
this goal is achieved by providing a porous metal fiber plate in
which a regular pattern of transverse holes or passages has been
made which, all together, occupy an overall free passage area of
5 YO to 35 YO of the total surface area of the plate, while each
hole has a surface area of between 0.03 and 10 mmZ. Thus the gas
flow is forced primarily through these holes. This feature is
favourable i.a. in view of achieving a small pressure drop across
the plate.
W O 93/18342 PC~r/BE93/00010
CA2i 1 7605
-- 2 --
Insofar as said plates need to be utilized at very high tempera-
tures, the metal fibers that are used must be resistant to these
t~ u~es~ The equivalent fiber diameters may range between
about 8 ~m and 150 ~m. 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-
lable 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.
Z5 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
W O 93/18342 C ~ ~ P(~r/BE93/00010
enormously broad power range and which is therefor suitable for
both surface radiant and blue flame modes.
A further object of the invention deals with the design of burner
membrane plates which offer remarkably low CO and ~O~-emissions and
high yields.
Yet another obiect of the invention concerns the design of a gas
burner with less constraints as to prefiltration of the inflowing
gas stream.
The provision of a gas burning device with less danger for reso-
nances occuring in the gas stream and hence for avoiding whistling
effects emerging during operation is to be considered a further
object of the invention.
On the basis of several embodiments, further details will here-
after be explained. Additional solutions according to the inven-
tion for specific or partial problems or objectives and the
characteristics of these solutions, as well as the advantages they
entail, will also be made clear.
Figure 1 is a sketch of a porous plate with circular holes
according to the invention.
Figure 2 shows one possible way of assembling this plate in a
housing with supply means for the gas and transporta-
tion and flow means for it through the plate.
Figure 3 represents schematically a pipe-shaped device for
passing the gas flow through.
Figures 4 to 7 relate to top views of several alternative
patterns of transverse passages to be arranged in the
~ porous plates.
CA 2 i i 7605
W O 93/18342 PCr/BE93/00010
Figure 8 shows a cross-section of a gas burner device according
to the invention in which an acoustic muffling layer is
clamped between the burner membrane and the distribution
element.
Figure 9 presents a cross-section of a gas burner device in which
a number of muffling layers are included, possibly along
with empty interspaces.
The porous metal fiber plate 1 according to figure I comprises
holes 2 spaced at regular distances p (pitch) from one another.
These holes are by y,~re,~ace cylindrical in shape and, in parti-
cular, circular-cylindrical. By preference, the area of each hole
2 is the same and lies between 0.03 and 3 mm2, though more prefe-
rably between 0.4 and 1.5 mm2, respectively between 0.5 and
0.8 mmZ. As will be seen below, these dimensions are to be chosen
i.a. depending on the thickness of the plate 1, its porosity and
the intended application. When the hole 2 thus has a circular
cross-section, the diameter of each circle will be 0.8 mm for a
surface area of approximately 0.5 mm2. The holes 2 are by prefe-
rence made with a punching operation since this assures a smooth
cylinder wall. If so desired, holes can also be punched with
triangular, square, rectangular or other shapes. The holes may
also be made with laser beams. Thus, in principle, very small
holes with a diameter of at least 0.2 mm are possible for thin
plates.
Figures 4 to 7 illustrate other preferred shapes of passages :
slots of different shapes and their regular distribution over the
plate surface. Two examples of a suitable regular pattern of
adjacent rectangular slots 9 are shown in figure 4 (right side,
resp. left side). Circular passages 2 and rectangular slots 9 can
alternate over the surface as shown in figure 5. Similarly, oval
or elliptic slots 1I can alternate with circular holes 2 as
W O 93/18342 CA 2 i i ~ 6 05 P(~r/B E93/00010
represented in figure 7. A pattern of cruciform slots 10 is
possible also as illustrated in figure 6. A great number of
regular distributions of passages with different shapes is
conceivable in view i.a. of minimizing or avoiding any whistling
effect in the gas flow as will be explained further.
Each of the slots 9, 10, 11 should preferably have a surface area
of between 1 and 10 mm2. Rectangular, or substantially rectangular
slots will have a slot width "w" of between 0,3 mm and 2 mm and a
length "1" of between 3 mm and 20 mm. Preferably the relations
0,5 mm < w < 1 mm and 5 mm < 1 < 10 mm will apply. Anyway, in a
plate with rectangular slots 9 according to e.g. figure 4 or 5,
the overall free passage area occupies 20 ~/O to 30 % of the total
surface area of the plate.
The pitch p between adjacent holes 2 is chosen such that their
total surface area comprises 5 ~/0 to 25 ~/0 of the total surface area
of the plate, and preferably 8 ~~ to 16 ~/0. Values of 10 ~/0, 12 Y0 and
15 % are adequate. In order to assure a uniform flow over the sur-
face, the successive holes are by preference ordered in a pattern
of adjacent, equilateral triangles in which each hole 2 occupies
a corner of the triangle.
The porosity of the plate (between the holes 2) is always between
60 % and 95 ~/0, but preferably between 78 ~/0 and 88 %. The plate
surfaces can be flat, can have a relief (be embossed), or else can
be curved or corrugated, for example.
The metal fibers that can be used for producing the porous plates
and the production of the plates themselves, and in particular
those that are resistant against very high temperatures, are
described in the same Furopean patent application 390.255. In
general, stainless steel fibers are suitable. For the high tempe-
W o 93/18342 C A 2 i 1 7 6 o ~ CT/BE93/OO010
rature applications, such as in gas burners, steel fibers contai-
ning Cr and Al are to be used, preferably those containing also a
small amount of yttrium.
S As represented in figure 2, the porous plate I according to the
invention can be assembled in a standard manner in a housing 3
with supply means 4 for the gas. When this device is intended to
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.
~ormally speaking, this will be a plate with suitable holes or
passages arranged in it such that a uniform flow of gas with a
suitable pressure reaches the inlet side of the porous plate I.
The surface area of the free passages in the distribution plate S
IS can amount to between 2 ~/0 and I0 ~/0. In the case of a cylindrical
burner (figure 3), the distribution plate 5 also serves as a
support element for the end plate 8. The distribution element S
can possibly be corrugated and can also function to neutralize
possible sound resonances in the gas flow or as a flame arrester
or barrier should they backfire into the gas inlet side of the
plate I, 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 a
cylindrical exit 7 or vice versa (plate upside down) : a cylin-
drical entrance and a conical exit.
A distribution element S is by preference also provided for the
gas supply, along with an end plate 8 in the cylindrical device
according to figure 3. Due to the flexibility of the membrane
plate I 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 and
built-in constructions in the spaces to be heated up (e.g.
W O 93/18342 CA2 i 1 ~605 PCT/BE93/OO010
boilers), resonance can occur at relatively high powers: e.g. over
1000 kW/m2. It also appears that excess air in the gas mixture
supply can have an influence on the tendency to resonate, along
with the fact as to whether the gas is either drawn (suctioned) or
S blown through the membrane. Finally, the pattern of holes utilized
in the membrane itself can also play a role in the resonance
~h " .or
The resonance Fh ~ . ., is presumably related to the high pres-
sure gradient of the gas mixture between the relatively cold under
side (inlet side) of the burner membrane and the very hot upper
side (exit side : burning surface). By changing the flow rate
variables, such as excess air and gas mixture flow rate, an
oscillation j' presumably occurs between the flame front
(i.e. the level of the flame bases) and the gas mixture entering
the holes. The tongues of flame therefore can dance up and down
above the burner surface or even oscillate with their flame bases
between a position in (or even under) the holes and a position
above the holes (above the burning surface). This can be accom-
panied by annoying whistling sounds ranging from 1000 to 1500 Hz.
This drawback can also be encountered when changing a burner from
a blown gas to a drawn gas system.
As mentioned before it is an object of the invention to eliminate
this disadvantage and to make the occurrence of whistling sounds
less critical. The measure taken, however, should not reduce any
of the other advantages of the concept with perforated burner
membrane. In particular, the measure should not result in a
drastic increase in the total pressure drop over the burner or a
(local) destabilizing of the flame front.
The solution according to the invention consists of providing a
gas burner device which includes a housing comprising the
W O 93/18342 P(~r/BE93/00010
CA2 i i~6~5
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 mm2 and 10 mm2.
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 S of approximately 10 o~. Depending on the
circumstances, this free surface area could just as well lie
between S % and 20 %. Below S %, the pressure drop becomes too
high at high gas flow rates; above 20 % the distribution effect
for the gas mixture becomes insufficient at low flow rates.
Against the outlet side of distribution element S lies a welded
wire mesh 13 of stainless steel wire with a wire diameter of, for
example, 0.125 mm and a gas permeability of 48 mesh. Depending on
the circumstances, a permeability can be chosen of between 30 mesh
W O 93/18342 CA 2 i 1 7 6 05 PC~r/BE93/00010
_ g _
and 60 mesh. Two or more meshes 13 can also be stacked on top of
one another, preferably of different permeabilities.
Downstream from the welded wire mesh (or meshes) 13, which
S operates as an acoustic muffling layer, is the porous membrane
plate 1, which is provided with a regular pattern of holes 12.
This porous plate is again preferably a sintered metal fiber plate
in which the fibers are heat-resistant, i.e. resistant against the
high burner temperatures occurring during operation and resistant
against thermal shocks. The fibers, therefore, are preferably
steel fibers with a suitable Cr and Al content: e.g. FeCrAlloy
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 was
22 ~m and the diameter of the cylinder-shaped punched holes was
0.8 mm, while the spacing between the centers of the holes (i.e.
the pitch) was 1.5 mm. Plate I is clamped against the housing 16,
with a ceramic mat 14 inserted between the two.
In order to minimize resonance with specific gas flow profiles in
specially shaped burners for situations in which the gas mixture
is being drawn (sucked) and/or for specific construction para-
meters related to the combustion space to be heated, consideration
can be given to providing an intermediate space 23 or 24 between
the acoustically muffling layer 13 and the distribution element 5
and/or the membrane 1, respectively, as shown, for example, in
figure 9. In this way various embodiments of the device are thus
created. The device can, for example, include one muffling layer
13 that is in surface contact with the distribution element 5. In
another embodiment the layer 13 can be in surface contact with
both element S and porous plate 1.
W O 93/18342 P(~r/BE93/00010
CA21 1 7605
- 10 -
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 I and
element S. 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
W O 93/18342 C ~ 2 i 1 ~6 05 PCr/BE93/00010
to those described in the Belgian patent application 09200209 are
then applicable. Instead of FeCrAlloy fibers, ceramic fibers or
wires can also be used.
S EXAMPLE I
A flat sintered porous metal fiber plate I produced according to
the invention and possessing the characteristics given below can
be used as a membrane for a gas burner device. The characteristics
and advantages of this concept with respect to previously presen-
ted burner membranes are explained below.
The steel fibers to be used are resistant against high tempera-
tures and, for this purpose, contain by percent weight, for
example, 15 to 22 % Cr, 4 to 5.2 ~0 Al, 0.05 to 0.4 % Y, 0.2 to
0.4 % Si and at most 0.03 % C. They have a diameter of between 8
and 35 ~m - for example, approximately 22 ~m. The fibers can be
obtained by a technique of bundled drawing, as known, for example,
from U.S. patent 3.379.000 and as is mentioned in U.S. patent
4.094.673. They are processed into a non-woven fiber web according
to a method described in or similar to the method which is known
from U.S. patents 3.469.297 or 3.127.668. Afterwards, these webs
are consolidated by pressing and sintering into a porous plate I
with a porosity of between 78 % and 88 D/o, Porosities of 80.5 %,
83 ~~ and 85.5 ~/0 are very common.
It is also possible to use thicker metal fibers as heat-resistant
fibers in the porous plate, e.g. fibers with equivalent diameters
of between 35 and 150 ~m and consisting of wire shavings or
cuttings from a plate of the desired heat-resistant alloy (e.g.
FeCrAlloy). These fibers look rather like steel wool and can be
manufactured according to a shaving process as disclosed e.g. in
U.S. patent 4.930.199.
W O 93/18342 r A 2 1 1 7 6 o ~ CT/BE93/00010
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, û.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/m2 for a radiant surface combustion and
can be increased to that of a blue flame surface burner with an
output or power of up to 4000 kW/m~, depending on factors such as
W o 93/18342 CQ 2 i 1 7 6 o 5 PCT/BE93/000l0
- 13 -
the excess air in the gas mixture in relation to a stoichiometric
gas combustion mixture.
The porosity of the plate I results in the fact that a small
portion of the gas always penetrates through the pores between the
holes 2 to the hot exit surface. As explained below, this greatly
promotes a uniform and stable burning over a broad load or power
range. Especially at higher flow rates, the portion of gas that
passes between the holes through the plate increases proportional-
ly. It is now precisely at these higher flow rates (and conse-
quently higher powers if the percent of excess air remains the
same in the gas mixture) that the tendency to blow away the blue
flame at the level of the holes needs to be counteracted. The
burning of the gas at the surface of the plate between the holes
I5 2 maintains, as it were, a stable (blue) flame front over the
whole plate surface and prevents this front (or the blue flame
tongues within it) from being blown away from the plate surface.
The tongue-shaped flames above each hole remain, as it were, with
their base - or root - anchored to the plate surface.
The largely horizontal orientation of the fibers within the porous
plate also promotes the isolating effect of the membrane. Indeed,
the heat conduction runs primarily in the outside surface (radiant
side) of the plate and much less in the depth (throughout the
thickness) of the plate. Moreover, there is the ongoing uniform
cooling effect of the cold gas supply in direct contact with the
layer of fibers on the gas inlet side. In turn, this uniform heat
distribution at the level of the plate surface promotes the uni-
form combustion of the gas layer and a stable burning state over
a broad load or power range at the exit side of the plate between
the consecutive holes 2. With a porous membrane layer I, that on
its gas inlet side is attached, for example, to a supporting steel
plate and in which the porous layer together with the support
W O 93/18342 P(~r/BE93/00010
~2 i i 7~o~
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 ~m. 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
W O 93/183~2 C A 2 i 1 7 6 o 5 PCT/BE93/OOO10
_ 15 _
p = 2 mm in a regular grid of adjacent equilateral triangles. The
distribution plate 5 (0.4 mm thick) was at a distance of 5 mm from
plate I and was provided with holes of 0.4 mm diameter and with a
pitch of 1.5 mm. This resulted in a free passage surface area of
6.5 %. There were no sound resonances or whistling sounds during
operation.
The pressure drop in the gas mixture over the plate (mbar)
increases somewhat more rapidly than linearly with the resulting
power (kW/m2). At a pressure drop of 0.05 mbar, a power of
150 kW/m2 was noted and at a pressure drop of 3 mbar, a power of
3500 kW/m2 was attained. The gas mixture was composed of 8.1 ~~
natural gas and 91.9 ~~ air. Natural gas with a relatively low
calorific value of 10 kWh/Nm3 was used and a 30 % excess of air was
applied.
A radiant surface burner state was noted up to something like
800 kW/m2. At higher powers, the burning changed into a blue flame
mode. The temperature of the membrane surface (gas outlet side)
increased to approximately 850 degrees C at around 700 kW/m2 and
gradually fell when going to higher powers (blue flame mode) to
approximately 600 degrees C. The membrane temperature on the gas
inlet side remained below 150 degrees C and even decreased to
below 100 degrees C in the blue flame mode. The measured N0~
emission (ppm) rose gradually over the whole power range up to
2000 kW/m2. However, it was only about 10 ppm at 700 kW/m2, and for
powers around 2000 kW/m2 and up, it stabilized at about 15 to
20 ppm. The measured NO~ values are in fact the data reduced to
their value at 0 ~/O 02 in the combustion gases. These very low N0
values are probably to be explained by the fact that the flame
tongues above the holes remain small so that the temperature in
their cores remains relatively low. The C0 content was nearly zero
over the entire power range.
W O 93/18342 C A 2 i i ~ PCT/BE93/00010
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 low
C0 and ~0~ emissions and it offers high yields.
ExamDle 2
In the embodiment of figure 8 the porous plate I is in surface
contact with the 48 mesh wire mesh 13. A gas mixture of natural
gas and air was passed through the compact combination in housing
16 of this wire mesh 13 clamped together between the 2 mm thick
porous plate 1 and the distribution element 5 with free passage
surface area of 10% (both described above). The square burner
surface measured 150 mm x 150 mm. Various proportions of excess
air were utilized (1.1 to 1.3) and the flow rates were increased
such that powers were developed ranging from 500 kW/m2 to 5000
kW/m2 .
In the table below the resonance results are given in column
[1 + 2 + 3]. By way of comparison, the burning tests are repeated
in the table for embodiments with a combination of only plate 1
and distribution element 5: column [1 + 3] and for the embodiment
without wire mesh 13 and without plate 5: column [1]. The minus
sign in the table refers to the desired absence of whistling
sounds during burning, while the plus sign indicates the presence
of an annoying whistling sound. Whistling sounds, moreover,
indicate an oscillation of the flame bases 20 in the holes 12 as
suggested with arrow 21.
W o 93/18342 C A 2 i 1 7 6 0 5 PCT/BE93/00010
- 17 -
TABLE 1
LOAD EXCESS OF AIR [1] [1+3] [1+2+3]
[KW/m2] [n]
500 1.2
1.3
+
1000 1.2
1.3
1,1 + +
2000 1.2 +
1.3
I . 1 + +
3000 1.2 + +
1.3 +
1 . 1 + +
4000 1.2 + +
1.3 + +
1 . 1 + +
5000 1.2 + +
1.3 + +
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 air
(1.2 or 1.3). Moreover, the favorable effect of wire mesh 13
appears to show up especially with the higher power loads (above
1000 kW/m2).
