U.S. patent application number 15/558718 was filed with the patent office on 2018-04-26 for inwardly burning surface stabilized gas premix burner.
This patent application is currently assigned to Bekaert Combusting Technology B.V.. The applicant listed for this patent is BEKAERT COMBUSTION TECHNOLOGY B.V.. Invention is credited to Geert FOLKERS, Camillo Marino Josef HOGENBIRK, Rene HUITSING, Wilhelm Salvatore VAN DEN BERG, Rene VAN ZUTPHENT.
Application Number | 20180112867 15/558718 |
Document ID | / |
Family ID | 52991535 |
Filed Date | 2018-04-26 |
United States Patent
Application |
20180112867 |
Kind Code |
A1 |
HUITSING; Rene ; et
al. |
April 26, 2018 |
INWARDLY BURNING SURFACE STABILIZED GAS PREMIX BURNER
Abstract
The burner (100) comprises a cylindrical porous substrate (110);
and an end cap (130) at a first end of the cylindrical porous
substrate (110). The cylindrical porous substrate (110) is provided
for flow of a premix of combustible gas and air from the outside of
the cylindrical porous substrate (110) through the pores of the
cylindrical porous substrate (110) to an interior cavity (140), for
the combustible gas to be combusted on the inner surface of the
cylindrical porous substrate (110) thereby generating hot gas. The
burner has an opening (182) at the second end of the cylindrical
porous substrate (110) to exit the hot flue gas out of the interior
cavity (140). The cylindrical porous substrate (110) has a higher
permeability section (170), located at the opening (182) at the
second end. The higher permeability section (170) has a lower
resistance to gas flow than other sections of the cylindrical
porous substrate (110).
Inventors: |
HUITSING; Rene; (Middelstum,
NL) ; VAN DEN BERG; Wilhelm Salvatore;
(Zuidlaarderveen, NL) ; FOLKERS; Geert;
(Bruchterveld, NL) ; HOGENBIRK; Camillo Marino Josef;
(ASSEN, NL) ; VAN ZUTPHENT; Rene; (De Wijk,
NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BEKAERT COMBUSTION TECHNOLOGY B.V. |
Assen |
|
NL |
|
|
Assignee: |
Bekaert Combusting Technology
B.V.
Assen
NL
|
Family ID: |
52991535 |
Appl. No.: |
15/558718 |
Filed: |
April 6, 2016 |
PCT Filed: |
April 6, 2016 |
PCT NO: |
PCT/EP2016/057520 |
371 Date: |
September 15, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F23D 14/62 20130101;
F23D 2203/1023 20130101; F23D 2203/1055 20130101; F23C 6/04
20130101; F23D 2900/14122 20130101; F23C 2900/03005 20130101; F23D
2203/103 20130101; F23D 2203/106 20130101; F23C 3/002 20130101;
F23D 2203/1012 20130101; F23D 14/02 20130101 |
International
Class: |
F23C 6/04 20060101
F23C006/04; F23D 14/02 20060101 F23D014/02; F23C 3/00 20060101
F23C003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2015 |
EP |
15164068.7 |
Claims
1-15. (canceled)
16. A burner comprising, a cylindrical porous substrate; an end cap
at a first end of the cylindrical substrate; wherein the
cylindrical porous substrate and the end cap enclose an interior
cavity; wherein the cylindrical porous substrate is provided for
flow of a premix of combustible gas and air from the outside of the
cylindrical porous substrate through the pores of the cylindrical
porous substrate to the interior cavity, for the combustible gas to
be combusted on the inner surface of the cylindrical porous
substrate thereby generating hot gas; wherein the burner has an
opening at the second end of the cylindrical porous substrate, for
allowing the hot gas to exit the interior cavity; wherein the
cylindrical porous substrate has a higher permeability section,
wherein the higher permeability section is located at the opening
at the second end; and wherein the higher permeability section has
a lower resistance to gas flow than other sections of the
cylindrical porous substrate.
17. A burner as in claim 16, wherein the higher permeability
section comprises or consists out of an annular section of the
cylindrical porous substrate.
18. A burner as in claim 16, wherein the cylindrical porous
substrate comprises a first porous substrate present over the full
height of the cylindrical porous substrate; wherein outside the
higher permeability section of the cylindrical porous substrate the
inner side of the first porous substrate is covered with a second
porous substrate; and wherein in the higher permeability section
when the burner is in use combustion occurs on the inner surface of
the first porous substrate.
19. Burner as in claim 16, wherein the cylindrical porous substrate
comprises a first porous substrate; wherein the inner side of the
first porous substrate is covered with a second porous substrate;
wherein the second porous substrate is a woven, knitted or braided
fabric comprising stainless steel fibers; and wherein the higher
permeability section is provided by differences in the structure of
the fabric compared to outside the higher permeability section.
20. Burner as in claim 18, wherein the second porous substrate is a
textile fabric comprising yarns, and wherein the yarns comprise a
plurality of metal fibers in the cross section of the yarns.
21. Burner as in claim 18, wherein the second porous substrate is a
weft knitted fabric; and wherein the weft direction of the weft
knitted fabric is provided in the circumferential direction of the
cylindrical porous substrate.
22. Burner as in claim 18, wherein the second porous substrate is a
weft knitted fabric; and wherein the weft direction of the weft
knitted fabric is provided in the axial direction of the
cylindrical porous substrate.
23. Burner as in claim 16; wherein the cylindrical porous substrate
has a higher porosity in the higher permeability section than in
other sections of the cylindrical porous substrate.
24. Burner as in claim 23, wherein the cylindrical porous substrate
comprises or consists out of a perforated plate; and wherein in the
higher permeability section, the higher porosity is provided by
means of a higher number of perforations per unit of surface area
and/or by larger perforations of a larger cross sectional area.
25. Burner as claim 16, wherein the burner comprises a flange at
the second end of the cylindrical porous substrate, and wherein in
use, hot flue gas exits the interior cavity via the central opening
in the flange.
26. Burner as in claim 25, wherein the flange is attached to the
cylindrical porous substrate at the inner side of the cylindrical
porous substrate.
27. Burner as in claim 16, wherein the end cap comprises
perforations for the passage of premix combustible gas and air
through the end cap to the inside of the interior cavity, for the
premix combustible gas to be combusted inside the interior
cavity.
28. Burner as in claim 27, wherein the end cap comprises
perforations in a first area around the centre point of the end
cap; wherein the first area has a diameter of less than 60% of the
diameter of the end cap; wherein the end cap comprises perforations
in a second area, wherein the second area is located outside the
area around the centre point of the end cap; with diameter more
than 75% of the diameter of the end cap; and wherein the end cap is
not perforated in the area between the first area and the second
area.
29. Burner as in claim 16, wherein the end cap is connected onto
the cylindrical porous substrate at the outer side of the
cylindrical porous substrate.
30. Heating device for heating a fluid, wherein the heating device
comprises at least two burners as in claim 16.
Description
TECHNICAL FIELD
[0001] The invention relates to the field of inwardly burning
surface stabilized gas premix burners. A surface stabilized gas
premix burner is a burner in which combustion occurs on a porous
surface, after a premix of combustible gas and air has flown
through pores of the porous surface.
BACKGROUND ART
[0002] US2012/247444A1, WO2009/151420A1 and U.S. Pat. No.
8,616,194B2 disclose examples of inwardly burning surface
stabilized gas premix burners. WO2009/151420A1 describes a burner
comprising a body defining an interior cavity and a burner surface
located in the body and defining an interior cavity. A diffusing
surface is located on an exterior portion of the body. Ports are
provided on the body extending through the diffusing and burner
surfaces, configured to provide fluid communication between the
interior cavity and ambient air outside the body. An opening is
provided larger than at least one of the ports in order to provide
fluid communication between the interior cavity and a space outside
of the body.
[0003] As is illustrated in U.S. Pat. No. 8,616,194B2, inwardly
burning surface stabilized gas premix burner can advantageously be
used in furnace air heaters.
DISCLOSURE OF INVENTION
[0004] It is an objective of the invention to provide an inwardly
burning surface stabilized gas premix burner with improved
lifetime.
[0005] A first aspect of the invention is a burner. The burner
comprises a cylindrical porous substrate; and an end cap at a first
end of the cylindrical substrate. The cylindrical porous substrate
and the end cap enclose an interior cavity. The cylindrical porous
substrate is provided for flow of a premix of combustible gas and
air from the outside of the cylindrical porous substrate through
the pores of the cylindrical porous substrate to the interior
cavity, for the combustible gas to be combusted on the inner
surface of the cylindrical porous substrate thereby generating hot
gas. The burner has an opening at the second end of the cylindrical
porous substrate, for allowing the hot gas to exit the interior
cavity. Preferably, the opening has a circular shape, preferably
with diameter at least 50% of the internal diameter of the interior
cavity. The cylindrical porous substrate has a higher permeability
section. The higher permeability section is located at the opening
at the second end of the cylindrical porous substrate. Preferably,
the higher permeability section forms the second end of the
cylindrical porous substrate. The higher permeability section has a
lower resistance to gas flow than other sections of the cylindrical
porous substrate; preferably than all the other regions of the
cylindrical porous substrate.
[0006] Inwardly burning surface stabilized gas premix burners have
all the heat generated by the combustion concentrated in the inner
cavity, before the heat is evacuated via the hot gas through the
opening at the second end. The amount of heat present in the inner
cavity poses lifetime issues to such burners. The burner of the
invention showed the surprising result that it has a longer
lifetime. The long lifetime seems to be achieved by avoiding that
parts of the burner get excessively hot when the burner is in use.
This result is the more surprising as locally near the opening at
the second end more combustible gas will be provided for
combustion.
[0007] In a preferred embodiment, the higher permeability section
comprises or consists out of an annular section of the cylindrical
porous substrate. Preferably the annular section has a height of at
least 3% of the diameter of the cylindrical porous substrate. More
preferably at least 10%, even more preferably at least 15%. And
preferably less than 30%, more preferably less than 20%.
[0008] Preferably, the annular section is provided at the second
end of the cylindrical porous substrate; more preferably the
annular section provides the second end of the cylindrical porous
substrate.
[0009] In a preferred embodiment, the cylindrical porous substrate
has a constant resistance to gas flow outside the higher
permeability section.
[0010] In a preferred embodiment, the cylindrical porous substrate
has a constant resistance to gas flow in the higher permeability
section.
[0011] In a preferred burner, the cylindrical porous substrate
comprises a first porous substrate present over the full height of
the cylindrical porous substrate. Outside the higher permeability
section of the cylindrical porous substrate the inner side of the
first porous substrate is covered with a second porous substrate.
In the higher permeability section when the burner is in use
combustion occurs on the inner surface of the first porous
substrate. The first porous substrate can e.g. be a woven wire mesh
or a perforated plate or an expanded metal sheet. The second porous
substrate can e.g. be a second woven wire mesh, a second perforated
plate, a second expanded metal sheet, a textile fabric (e.g. a
woven fabric, a knitted fabric, a braided fabric) comprising yarns
comprising a plurality of metal fibers in the cross section of the
yarns, a sintered powder object, or a sintered fiber object (e.g.
sintered metal fibers). In such embodiments, when the higher
permeability section comprises or consists out of an annular
section of the cylindrical porous substrate, the annular section
preferably has a height of at least 3% of the diameter of the
cylindrical porous substrate, and more preferably at least 10%; and
preferably less than 15%.
[0012] Preferably, the second porous substrate is bonded onto the
first porous substrate by means of welding. Preferably the welding
is soft welding. Soft welding is performed such that when pulling
the second porous substrate from the first porous substrate, the
soft welded bonds between both substrates give way rather than that
breakages occur inside one the porous substrates itself. The test
method to determine that the burner deck is soft welded, is pulling
in peel-off mode: an edge portion of the second porous substrate is
removed from the first porous substrate, and folded over
180.degree.. Pulling the second porous substrate is then done by
hand or using pliers, wherein the pulling force is exerted parallel
with the first porous substrate, in a direction of 180.degree. to
the first porous substrate. In pulling, the force builds up until
the second porous substrate is progressively peeled off from the
first porous substrate. Within the limits of the described "pulling
in peel-off mode" the conclusion whether or not the second porous
substrate is soft-welded to the first porous substrate is
independent of further parameters. Soft welding has the benefit
that long lifetime of the cylindrical porous substrate is obtained.
This is a surprising result, as the bonding in soft welding is
certainly less strong than when standard welding is used in which
stronger bonds between both substrates is obtained. It is believed
that the soft welding has a positive effect on the combustion
properties, resulting in the longer lifetime of the cylindrical
porous substrate, and hence in a longer lifetime of the burner. In
a more preferred embodiment, the second porous substrate covers the
inside of the first porous substrate from the first end of the
cylindrical porous substrate up to a certain distance from the end
of the first porous substrate at the second end of the cylindrical
porous substrate.
[0013] In a preferred embodiment, the cylindrical porous substrate
comprises a first porous substrate. The inner side of the first
porous substrate is covered with a second porous substrate. The
second porous substrate is a woven, knitted or braided fabric
comprising stainless steel fibers. The higher permeability section
is provided by differences in the structure of the fabric compared
to outside the higher permeability section.
[0014] The first porous substrate can e.g. be a woven wire mesh or
a perforated plate or an expanded metal sheet. The second porous
substrate can e.g. be a second woven wire mesh, a second perforated
plate, a second expanded metal sheet, a textile fabric (e.g. a
woven fabric, a knitted fabric, a braided fabric) comprising yarns
comprising a plurality of metal fibers in the cross section of the
yarns, a sintered powder object, or a sintered fiber object (e.g.
sintered metal fibers). In such embodiments, when the higher
permeability section comprises or consists out of an annular
section of the cylindrical porous substrate, the annular section
preferably has a height of at least 3% of the diameter of the
cylindrical porous substrate, and more preferably at least 10%; and
preferably less than 15%.
[0015] Preferably, the second porous substrate is bonded onto the
first porous substrate by means of welding. Preferably the welding
is soft welding. As an example of embodiments wherein the higher
permeability section is provided by differences in the structure of
the fabric compared to outside the higher permeability section, the
fabric can be compressed to a lower thickness outside the higher
permeability section compared to the thickness in the higher
permeability section. A lower thickness of the fabric results in
lower permeability. Preferably, the thickness of fabric is in the
higher permeability section at least 40%, more preferably at least
50%, even more preferably at least 60% higher, even more preferably
at least 100% higher and still even more preferred 150% higher than
the average thickness outside the higher permeability section.
[0016] In another example of embodiments wherein the higher
permeability section is provided by differences in the structure of
the fabric compared to outside the higher permeability section, the
fabric differs in yarn density in order to create the section with
different permeability in the fabric, e.g. the fabric could be a
woven fabric with lower weft density in the higher permeability
section than outside the higher permeability section.
[0017] In a further preferred embodiment, the second porous
substrate is a textile fabric comprising yarns. The yarns comprise
a plurality of metal fibers in the cross section of the yarns. The
metal fibers in the yarns can be continuous filaments or can be
discrete length fibers. Further preferred are stainless steel
fibers as metal fibers, e.g. fibers out of a Fe, Cr and Al
comprising alloy such as FeCrAlloy. Preferred fabrics are woven
fabrics, knitted fabrics or braided fabrics. Further preferred are
weft knitted fabrics.
[0018] In a more preferred embodiment, the second porous substrate
is a weft knitted fabric. The weft direction of the weft knitted
fabric is provided in the circumferential direction of the
cylindrical porous substrate.
[0019] In another more preferred embodiment, the second porous
substrate is a weft knitted fabric. The weft direction of the weft
knitted fabric is provided in the axial direction of the
cylindrical porous substrate.
[0020] In a preferred embodiment of the invention, the cylindrical
porous substrate has a higher porosity in the higher permeability
section than in other sections of the cylindrical porous substrate.
In such embodiments, when the higher permeability section comprises
or consists out of an annular section of the cylindrical porous
substrate, the annular section preferably has a height of at least
3% of the diameter of the cylindrical porous substrate, and more
preferably at least 15%; and preferably less than 30%.
[0021] In a more preferred embodiment, the cylindrical porous
substrate comprises or consists out of a perforated plate. The
porosity of the plate is the percentage of the surface area which
is open for gas flow passage. In the higher permeability section,
the higher porosity is provided by means of a higher number of
perforations per unit of surface area and/or by larger perforations
of a larger cross sectional area. Preferably, the porosity in the
higher permeability section is in relative terms at least 20%
higher than outside the higher permeability section, more
preferably at least 40% higher than outside the higher permeability
section, more preferably at least 60% higher than outside the
higher permeability section. And even more preferably the porosity
in the higher permeability section is less than double the
permeability outside the higher permeability section; even more
preferably less than 60% higher than outside the higher
permeability section.
[0022] The porosity in the higher permeability section is e.g.
between 15 and 33%. The porosity outside the higher permeability
section is e.g. between 14 and 17%. It is also possible to have a
section of the perforated plate at the end cap, wherein that
section has a porosity lower than the porosity outside the higher
permeability section, but preferably at least having 7%
porosity.
[0023] A preferred burner comprises a flange at the second end of
the cylindrical porous substrate. In use, hot flue gas exits the
interior cavity via the central opening in the flange. The flange
can be provided to allow mounting the burner into a supporting
structure, e.g. inside a heat exchanger. To this end, the flange
can be provided with holes for mounting the flange to a supporting
structure, e.g. by means of bolts.
[0024] It is also possible that the flange is formed by a raised
collar in the plate onto which the burner is mounted.
[0025] It is also possible that the flange is formed by mechanical
deformation of material of the cylindrical porous surface,
preferably by deformation of the first porous surface. An example
of such embodiment is where the first porous surface is formed by a
perforated metal plate formed into cylindrical shape, and wherein
the end of the metal plate is deformed to form a flange.
[0026] In a further preferred burner, the flange is attached to the
cylindrical porous substrate at the inner side of the cylindrical
porous substrate; preferably by means of welding. An Inwardly
burning surface stabilized gas premix burner according to the prior
art showed to have further reduced lifetime when the flange is
attached at the inner side to the cylindrical porous substrate, way
of attachment which facilitates manufacturing operations.
Surprisingly however, the inventive burner with the flange attached
to the cylindrical porous substrate at the inner side of the
cylindrical porous substrate showed excellent lifetime.
[0027] In a preferred burner, the end cap comprises perforations
for the passage of premix combustible gas and air through the end
cap to the inside of the interior cavity for being combusted inside
the interior cavity. Such perforations can e.g. be circular or slit
shaped, or a combination of circular perforations and slit shaped
perforations. In a preferred embodiment, the perforations include
circular perforations with a diameter less than 0.9 mm, e.g. a
diameter 0.8 mm. In embodiments where slit shaped perforations are
provided, slits preferably have a width of less than 0.6 mm; e.g.
0.5 mm.
[0028] More preferred is a burner wherein the end cap comprises
perforations in a first area around the centre point of the end
cap. The first area has a diameter of less than 60%, preferably of
less than 50% of the diameter of the end cap. The end cap further
comprises perforations in a second area, wherein the second area is
located outside the area around the centre point of the end cap;
wherein the area has a diameter of more than 75%, preferably of
more than 80% of the diameter of the end cap. The end cap is not
perforated in the area between the first area and the second area.
Preferably the perforations in the second area are one or two rows
of perforations in a circular configuration around the centre point
of the end cap.
[0029] Although the end cap can be connect onto the cylindrical
porous substrate at the inner side of the cylindrical porous
substrate; preferably, the end cap is connected onto the
cylindrical porous substrate at the outer side of the cylindrical
porous substrate. Such embodiment--wherein the end cap is connected
onto the cylindrical porous substrate at the outer side of the
cylindrical porous substrate--synergistically adds to the longer
lifetime as it allows that the burner dimensions can be better
maintained during manufacturing of the burner. Burner dimensions
are important for the proper operation of the burner. Improper
operation of the burner could lead to local overheating of the
burner, and breakdown of the burner leading to shorter lifetime of
the burner. Preferably, the end cap is connected onto the
cylindrical porous substrate by means of welding.
[0030] A second aspect of the invention is a heating device for
heating a fluid. The heating device comprises at least two of
burners as in the first aspect of the invention. Preferably, the
burners are linearly aligned in the heating device. The fluid to be
heated can e.g. be air. The heating device can be a furnace air
heater. In a preferred furnace air heater, each of the burners is
provided to exit hot gas through the opening at the second end of
the burner into a specific tube of a tubular heat exchanger.
BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS
[0031] FIG. 1 shows an example of a burner according to the
invention.
[0032] FIGS. 2 and 3 show cross sections of burners according to
the invention.
[0033] FIG. 4 shows an example of an end cap that can be used in
burners according to the invention.
[0034] FIG. 5 shows a heating device according to the second aspect
of the invention.
MODE(S) FOR CARRYING OUT THE INVENTION
[0035] FIG. 1 shows an example of a burner 100 according to the
invention.
[0036] FIG. 2 shows a section of the burner taken according to the
plane 2-2 parallel with and through the central axis of the burner.
The burner 100 comprises a cylindrical porous substrate 110; and an
end cap 130 at a first end of the cylindrical substrate. The
cylindrical porous substrate 110 and the end cap 130 enclose an
interior cavity 140.
[0037] The end cap 130 of the exemplary burner 100 has perforations
132 for the passage of premix combustible gas and air through the
end cap 130 to the inside of the interior cavity 140 for being
combusted inside the interior cavity 140. The end cap 130 of the
exemplary burner 100 is connected onto the porous substrate at the
outer side of the porous substrate.
[0038] The cylindrical porous substrate 110 is provided for flow of
a premix of combustible gas and air from the outside of the
cylindrical porous substrate 110 through the pores of the
cylindrical porous substrate to the interior cavity 140, for the
combustible gas to be combusted on the inner surface of the
cylindrical porous substrate thereby generating hot gas. The burner
100 has an opening 182 at the second end of the cylindrical porous
substrate, for allowing the hot gas to exit the interior cavity. In
the example shown, the burner 100 comprises a flange 180 at the
second end. The opening 182 is provided in the flange 180. In the
exemplary burner 100, the flange 180 is attached to the cylindrical
porous substrate 110 at the inner side of the cylindrical porous
substrate 110, by means of welding. The flange 180 can be used to
mount a burner to a support structure.
[0039] The cylindrical porous substrate 110 of the exemplary burner
100 comprises a first porous substrate 112 present over the full
height of the cylindrical porous substrate 110. Outside the higher
permeability section 170 of the cylindrical porous substrate 110
the inner side of the first porous substrate 112 is covered with a
second porous substrate 114. Due to fact that outside the higher
permeability section 170 of the cylindrical porous substrate 110,
premix gas and air needs to flow through the first porous substrate
112 and through the second porous substrate 114, the resistance to
gas flow is higher and therefore, the gas permeability is lower. In
the higher permeability section 170, combustion will occur on the
inner surface of the first porous substrate 112. Outside the higher
permeability section 170, combustion will occur on the inner
surface of the second porous substrate 114. In the example, the
higher permeability section consists out of annular section of the
cylindrical porous substrate 110; however other shapes of the
higher permeability sections are possible.
[0040] Preferred examples for the first porous substrate are a
woven metal wire mesh or a perforated metal plate. Preferred
example for the second porous substrate--that can be combined with
any of the first porous substrates--is a textile fabric comprising
yarns, wherein the yarns comprise a plurality of metal fibers in
the cross section of the yarns. It is preferred when the textile
fabric is soft welded onto the first porous substrate. A specific
example for the second porous substrate is a weft knitted fabric
out of yarns spun from FeCrAlloy fibers. As an example, the weft
direction of the weft knitted fabric can be provided in the
circumferential direction of the cylindrical porous substrate. As
an alternative example, the weft direction of the weft knitted
fabric can be provided in the axial direction of the cylindrical
porous substrate.
[0041] FIG. 3 shows the section of an alternative burner 300
according to the invention. The burner 300 is to a large extent
similar to the burner shown in FIGS. 1 and 2. Parts in FIG. 3 with
the same reference numeral as in FIGS. 1 and 2 are identical as in
the burner shown in FIGS. 1 and 2. The burner 300 in FIG. 3 differs
from the burner shown in FIG. 2 in that the cylindrical porous
substrate 316 is formed out of a perforated metal plate welded at
its contact edge to form the cylindrical porous substrate 316. The
perforated metal plate, and thus the cylindrical porous substrate,
has a higher porosity in the higher permeability section 370 than
in its other sections 374. In the example, the higher permeability
section 370 is an annular section of the cylindrical porous
substrate 316, at the second end of the cylindrical porous
substrate 316. The perforations in the perforated metal plate can
be circular perforations and/or slits. Preferably, circular
perforations have a diameter of less than 0.9 mm, e.g. 0.8 mm.
Slits preferably have a width of less than 0.6 mm; e.g. 0.5 mm. The
higher porosity in the higher permeability section 370 is e.g.
provided by means of a higher number of perforations (circular
perforations and/or slits) than in the other sections 374 of the
cylindrical porous substrate 316. The porosity in the higher
permeability section 370 is e.g. 26%. Outside the higher
permeability section, the porosity of the perforated metal plate is
e.g. 15%. The perforated metal plate can also have a small section
at the end cap, wherein the section has a porosity of e.g. 9%.
[0042] FIG. 4 shows an example of an end cap 430 that can be used
in the invention. The end cap 430 has a diameter D. the end cap 430
comprises perforations 434 in a first area around the centre point
of the end cap. The first area has a diameter D1 of less than 60%
of the diameter of the end cap. The end cap 430 comprises
perforations 436 in a second area. The second area is located
outside the area around the centre point of the end cap; wherein
the area has a diameter D2 of more than 75%. The end cap 430 is not
perforated in the area between the first area and the second area.
The perforations of in the first area and the perforations in the
second area e.g. have a diameter of 0.8 mm.
[0043] FIG. 5 shows a heating device 500 for heating a fluid
according to the second aspect of the invention. The heating device
500 comprises a number of burners 100 as in the first aspect of the
invention. The burners 100 are linearly aligned in the heating
device 500. The flanges 580 of the burners 500 are attached to a
mounting plate 592. The burners are provided in a premixing chamber
594, which is provided with walls 596. A premix of combustible gas
and air in fed (see arrow 598) into the premixing chamber 594
through one of the walls of the premixing chamber. Hot gas
generated by combustion of the premix gas on the inner surface of
the cylindrical porous substrate 510 flows from the interior cavity
140 of the burner 100 through opening 582 into a tube 590, via the
combustion chamber 597 provided between the burners 100 and the
tubes 590. Each of the burners is provided to exit hot gas through
the opening at the second end of the burner into a specific tube of
a tubular heat exchanger. The heating device can be a furnace air
heater.
* * * * *