U.S. patent application number 10/546325 was filed with the patent office on 2007-03-08 for alternate combustion type regenerative radiant tube burner apparatus.
Invention is credited to Shinichiro Fukushima, Toshiaki Hasegawa, Munehiro Ishioka, Shinya Kitahara, Susumu Mochida, Isao Mori, Hitoshi Oishi, Masaharu Suga, Yutaka Suzukawa, Takeshi Tada.
Application Number | 20070054227 10/546325 |
Document ID | / |
Family ID | 32923251 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070054227 |
Kind Code |
A1 |
Tada; Takeshi ; et
al. |
March 8, 2007 |
Alternate combustion type regenerative radiant tube burner
apparatus
Abstract
The present invention relates to an alternate combustion type
regenerative radiant tube burner apparatus which comprises a
circular plate positioned between a burner unit and a main
combustion chamber, so that preheated secondary combustion air is
fed through an opening of the circular plate to the main combustion
chamber. The burner apparatus has the circular plate (10), an
annular air throat (2), a cylindrical peripheral cover (30) and a
heat-transfer tube (11,16). A front end portion of the peripheral
cover is integrally connected with a peripheral edge portion of the
circular plate so as to prevent the secondary combustion air of the
air throat from leaking on a periphery of the circular plate to the
air diluting chamber (3). According to the present invention, the
concentration of nitrogen oxides in the combustion exhaust gas can
be definitely prevented from increasing, without being affected by
the installation condition or thermal deformation of the radiant
tube burner apparatus.
Inventors: |
Tada; Takeshi; (Kanagawa,
JP) ; Mochida; Susumu; (Kanagawa, JP) ;
Kitahara; Shinya; (Kanagawa, JP) ; Fukushima;
Shinichiro; (Tokyo, JP) ; Suzukawa; Yutaka;
(Tokyo, JP) ; Suga; Masaharu; (Okayama, JP)
; Mori; Isao; (Hiroshima, JP) ; Oishi;
Hitoshi; (Hiroshima, JP) ; Ishioka; Munehiro;
(Hiroshima, JP) ; Hasegawa; Toshiaki; (Saitama,
JP) |
Correspondence
Address: |
JACOBSON HOLMAN PLLC
400 SEVENTH STREET N.W.
SUITE 600
WASHINGTON
DC
20004
US
|
Family ID: |
32923251 |
Appl. No.: |
10/546325 |
Filed: |
February 25, 2004 |
PCT Filed: |
February 25, 2004 |
PCT NO: |
PCT/JP04/02203 |
371 Date: |
September 12, 2006 |
Current U.S.
Class: |
431/181 |
Current CPC
Class: |
F23L 15/02 20130101;
Y02E 20/348 20130101; F23D 14/22 20130101; F23C 3/002 20130101;
Y02E 20/34 20130101 |
Class at
Publication: |
431/181 |
International
Class: |
F23C 5/08 20060101
F23C005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2003 |
JP |
2003-46978 |
Claims
1. An alternate combustion type regenerative radiant tube burner
apparatus, which has a radiant tube (14) defining a main combustion
chamber (4) and burner units (15) provided on both end portions of
the radiant tube, the burner unit being provided with a regenerator
(13) and fluid passages (17, 18) for primary combustion air and
fuel, the regenerator being alternately in heat-transfer contact
with secondary combustion air or combustion exhaust gas, and the
fluid passages extending through the regenerator, comprising: a
circular plate (10) positioned between said burner unit (15) and
said main combustion chamber (4); an annular air throat (2)
provided between the circular plate and the regenerator, into which
the secondary combustion air flows after passing through said
regenerator; a cylindrical peripheral cover (30) extending from a
peripheral edge portion of said circular plate to a peripheral edge
portion of said regenerator; and a heat-transfer tube (16)
protruding forward of said circular plate and opening to said main
combustion chamber; wherein a fuel injection port (17a) and a
primary combustion air delivery port (18a, 18b) open in said
heat-transfer tube, an inner area of the heat-transfer tube
constitutes a primary combustion chamber (1) for mixing the primary
combustion air and the fuel, an air diluting chamber (3) in an
annular form opening to said main combustion chamber is formed
around a distal end portion of the heat-transfer tube, and said
circular plate has an opening (12) for communication between said
air throat and said air diluting chamber; and wherein a front end
portion of said peripheral cover is integrally connected with the
peripheral edge portion of said circular plate so as to prevent the
secondary combustion air of said air throat from leaking at a
periphery of said circular plate to said air diluting chamber.
2. The apparatus as defined in claim 1, wherein said peripheral
cover is contained in the radiant tube in such a condition that
thermal deformation of the cover in its axial direction is not
restrained by the radiant tube and the regenerator.
3. The apparatus as defined in claim 1, wherein a center (PC) of
the opening (12) is positioned in a zone radially outside of a
circle (R) centered on a center line (C) of said circular plate,
the circle (R) having a predetermined radius S and the radius S
equals (an inner diameter Dt of the radiant tube+ an outer diameter
Dr of the heat-transfer tube)/4].
4. The apparatus as defined in claim 1, wherein said opening (12)
of the circular plate is positioned in a range of a central angle
of the circular plate (.gamma.) equal to or less than 250
(.gamma..ltoreq.250.degree.).
5. The apparatus as defined in claim 1, wherein a velocity of
airflow at the opening (12) of said circular plate is set to be
equal to or lower than 30 m/s.
6. The apparatus as defined in claim 1, wherein said fuel injection
port (17a) and said primary combustion air delivery port (18a, 18b)
are positioned on a plane of said circular plate or in front
thereof.
7. The apparatus as defined in claim 1, wherein a flame stabilizing
tube (41) is located in said heat-transfer tube so as to surround
said fuel injection port (17a) and said primary combustion air
delivery port (18a, 18b), and a pilot fuel injection port (40)
opens in the flame stabilizing tube.
8. An alternate combustion type regenerative radiant tube burner
apparatus, which has a radiant tube (14) defining a main combustion
chamber (4) and burner units (15) provided on both end portions of
the radiant tube, the burner unit being provided with a regenerator
(13) and fluid passages (17, 18) for primary combustion air and
fuel, the regenerator being alternately in heat-transfer contact
with secondary combustion air or combustion exhaust gas, and the
fluid passages extending through the regenerator, comprising: a
circular plate (10) positioned between said burner unit (15) and
said main combustion chamber (4); an annular air throat (2) formed
between the circular plate and the regenerator, into which the
secondary combustion air flows after passing through said
regenerator; a cylindrical peripheral cover (30) extending from a
peripheral edge portion of said circular plate to a peripheral edge
portion of said regenerator; and a heat-transfer tube (16)
protruding forward of said circular plate and opening to said main
combustion chamber; wherein a fuel injection port (17a) and a
primary combustion air delivery port (18a, 18b) open in said
heat-transfer tube, an inner area of the heat-transfer tube
constitutes a primary combustion chamber (1) for mixing the primary
combustion air and the fuel, an air diluting chamber (3) in an
annular form opening to said main combustion chamber is formed
around a distal end portion of the heat-transfer tube, and said
circular plate has an opening (12) for communication between said
air throat and said air diluting chamber; and wherein a distance
(L4) between said regenerator and said circular plate is set in
such a manner that a ratio (L4/Dt) of the distance (L4) to an inner
diameter (Dt) of the radiant tube is equal to or less than 1.0.
9. The apparatus as defined in claim 8, wherein said peripheral
cover and said regenerator are connected with each other
displaceably in an axial direction.
10. The apparatus as defined in claim 8, wherein a velocity of
airflow at said opening (12) of the circular plate is set to be
equal to or less than 30 m/s, and a velocity of the exhaust gas at
the opening is set to be equal to or less than 35 m/s.
11. The apparatus as defined in claim 8, wherein a thickness (T) of
said peripheral cover is set to be equal to or less than 8 mm.
12. The apparatus as defined in claim 8, wherein said peripheral
cover is constituted from a heat-resistant metal alloy plate shaped
by bending.
13. A combustion method in an alternate combustion type
regenerative radiant tube burner apparatus; wherein a circular
plate (10) is positioned between a main combustion chamber (4) in a
radiant tube (14) and a regenerator (13) alternately in
heat-transfer contact with secondary combustion air or combustion
exhaust gas, primary combustion air and fuel are fed to the main
combustion chamber through a heat-transfer tube (11, 16) extending
through the circular plate; and wherein an air diluting chamber (3)
in an annular form opening to the main combustion chamber is formed
around a distal end portion of the heat-transfer tube so that the
secondary combustion air (A) heated up to a high temperature by
heat-transfer contact with the regenerator enters the air diluting
chamber, and said fuel and said primary combustion air are mixed
with said secondary combustion air in the main combustion chamber
while an ejected flow of the secondary combustion air induces the
combustion gas of the main combustion chamber, comprising: blocking
fluid communication between areas on both sides of said circular
plate, at a periphery of the circular plate, so that the fluid
communication between these areas are limited to an opening (12)
disposed in a range of central angle of said circular plate
(.gamma.) equal to or less than 250 (.gamma..ltoreq.250), a center
(PC) of the opening being positioned in a zone radially outside of
a circle (R) with a predetermined radius S about a center line (C)
of the circular plate, and the radius S being equal to (an inner
diameter Dt of the radiant tube+an outer diameter Dr of the
heat-transfer tube)/4; and injecting said secondary combustion air
to said air diluting chamber at a velocity equal to or less than 30
n/s by said opening (12), and directing an ejected flow of the
combustion exhaust gas of said main combustion chamber toward said
regenerator at a velocity equal to or less than 35 m/s by the
opening (12).
14. The method as defined in claim 13, wherein a quantity of supply
of said primary combustion air is set to be 10.about.20% of a
theoretical amount of air required for complete combustion of the
fuel.
15. The method as defined in claim 13, wherein said primary
combustion air and said fuel are fed to said primary combustion
chamber through a flame stabilizing section (41) and a pilot fuel
of 2.about.8% of supply of said fuel is fed to the flame
stabilizing section.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to an alternate combustion
type of regenerative radiant tube burner apparatus, and more
particularly, to such an apparatus which has a circular plate
positioned between a burner unit and a combustion chamber so that
preheated secondary combustion air is fed through an opening of the
plate to the combustion chamber.
BACKGROUND OF THE INVENTION
[0002] A radiant tube burner apparatus indirectly heating an inside
atmosphere of a furnace is practically in use as heating means of a
heating furnace for steel, furnace for heat treatment, and so
forth. As a kind of the radiant tube burner apparatus, the
alternate combustion type regenerative radiant tube burner
apparatus is known in the art, in which regenerators of honey-comb
structures are provided on both end portions of a radiant tube
(Japanese patent laid-open publications Nos. 8-247421, 11-108315,
11-108316, and U.S. patent publication No. 6,027,333).
[0003] Conventionally in the alternate combustion type regenerative
radiant tube burner apparatus, a velocity of injection of
combustion air tends to be set at a relatively high value in order
to reduce the concentration of nitrogen oxides in the combustion
exhaust gas. Therefore, loss of the total pressure in the burner
apparatus increases, so that a large capacity of intake blower and
a large capacity of exhaust blower have to be installed in many
cases. In view of such drawbacks, the present inventors have
proposed an alternate combustion type regenerative radiant tube
burner apparatus in Japanese patent application No. 2000-237677
(Japanese patent laid-open publication No. 2002-48334), wherein a
circular plate is positioned between a burner unit and a main
combustion chamber inside of a radiant tube so that fuel and
primary combustion air are fed to the main combustion chamber by
means of a heat-transfer tube extending through the circular plate.
According to this arrangement of the burner apparatus, a distal end
portion of the heat-transfer tube protrudes from the circular plate
into the main combustion chamber and opens thereto. An inside space
of the heat-transfer tube constitutes a primary combustion chamber
for mixing the primary combustion air and the fuel. An air diluting
chamber in an annular form, which opens to the main combustion
chamber, is defined around the distal end portion of the
heat-transfer tube. Secondary combustion air is heated up to a high
temperature by heat-transfer contact with a regenerator and then,
enters the air diluting chamber through an opening of the circular
plate. The secondary combustion air ejected to the main combustion
chamber is diluted with combustion gas of the main combustion
chamber. According to the radiant tube burner apparatus thus
arranged, it is possible to reduce the velocity of secondary
combustion air down to 20 m/s.about.50 m/s, thereby reducing the
loss of total pressure in the radiant tube burner apparatus without
increasing the concentration of nitrogen oxides in the combustion
exhaust gas.
[0004] However, in experiments for practical use of the radiant
tube burner apparatus with this arrangement, a phenomenon has taken
place in which the concentration of nitrogen oxides in the
combustion exhaust gas increases, depending on setting of an
attitude or orientation of the radiant tube burner apparatus. It
has been found that such increase in the concentration of nitrogen
oxides is also caused by thermal deformation of a tube portion.
[0005] The attitude or orientation of the radiant tube burner
apparatus is of the nature to be appropriately changed in its
design, depending on the purpose or structure of the furnace, i.e.,
the condition of its installation. Further, the thermal deformation
of the tube portion is apt to be remarkably caused in a
middle-sized or large-sized radiant tube burner apparatus with an
increased diameter. Therefore, it is necessary to definitely
prevent increase in the concentration of nitrogen oxides in the
combustion exhaust gas due to such causes.
[0006] It is an object of the present invention to provide an
alternate combustion type regenerative radiant tube burner
apparatus, which has the circular plate positioned between the
burner unit and the main combustion chamber so that the preheated
secondary combustion air is fed through the opening of the plate to
the main combustion chamber, wherein the concentration of nitrogen
oxides in the combustion exhaust gas can be definitely prevented
from increasing, without being influenced by the installation
condition or thermal deformation of the apparatus.
DISCLOSURE OF THE INVENTION
[0007] The present inventors have found that the concentration of
nitrogen oxides in the combustion exhaust gas can be definitely
prevented from increasing, by fully blocking the secondary
combustion air flowing through a peripheral edge portion of the
circular plate to the main combustion chamber. The present
inventors have attained the object of the invention as set forth
above, based on such findings.
[0008] The present invention provides an alternate combustion type
regenerative radiant tube burner apparatus having a radiant tube
(14) defining a main combustion chamber (4) and burner units (15)
provided on both end portions of the radiant tube, the burner unit
being provided with a regenerator (13) and fluid passages (17, 18)
for primary combustion air and fuel, the regenerator being
alternately in heat-transfer contact with secondary combustion air
or combustion exhaust gas, and the fluid passages extending through
the regenerator, comprising:
[0009] a circular plate (10) positioned between said burner unit
(15) and said main combustion chamber (4); an annular air throat
(2) provided between the circular plate and the regenerator, into
which the secondary combustion air flows after passing through said
regenerator; a cylindrical peripheral cover (30) extending from a
peripheral edge portion of said circular plate to a peripheral edge
portion of said regenerator; and a heat-transfer tube (16)
protruding forward of said circular plate and opening to said main
combustion chamber;
[0010] wherein a fuel injection port (17a) and a primary combustion
air delivery port (18a, 18b) open in said heat-transfer tube, an
inner area of the heat-transfer tube constitutes a primary
combustion chamber (1) for mixing the primary combustion air and
the fuel, an annular air diluting chamber (3) opening to said main
combustion chamber is formed around a distal end portion of the
heat-transfer tube, and said circular plate has an opening (12) for
communication between said air throat and said air diluting
chamber; and
[0011] wherein a front end portion of said peripheral cover is
integrally connected with the peripheral edge portion of said
circular plate so as to prevent the secondary combustion air of
said air throat from leaking at a periphery of said circular plate
to said air diluting chamber.
[0012] In the radiant tube burner apparatus according to the
present invention, the secondary combustion air, which has been
heated up to a high temperature by heat-transfer contact with the
regenerator, flows into the air throat and ejects through the
opening of the circular plate to the air diluting chamber. The fuel
takes a combustion reaction with the primary combustion air in the
primary combustion chamber, so that the fuel is diluted with
combustion gas produced by the combustion reaction. The flow of
secondary combustion air, which ejects through the opening of the
circular plate into the air diluting chamber, induces combustion
gas of the combustion chamber, so that a combustion gas
recirculation flow reverses into the air diluting chamber. This
recirculation flow moves along an outer circumferential surface of
the heat-transfer tube to the proximity of the opening and mixes
with the flow of secondary combustion air, so that the secondary
combustion air is diluted with the combustion gas. The flow of
secondary combustion air having an oxygen density or air ratio
reduced by mixing with the combustion gas is mixed with the mixed
gas flow (F) ejecting from the primary combustion chamber (the fuel
diluted with the combustion gas of the primary combustion chamber),
whereby a combustion reaction between the mixed gas and the
secondary combustion air generates and progresses in the main
combustion chamber.
[0013] In this kind of radiant tube burner apparatus, a
predetermined clearance between the outer edge portion of the
circular plate and a wall of the radiant tube is settled for
relieving thermal stress of the circular plate and the tube wall of
the radiant tube upon the thermal deformation of the tube wall.
This clearance is important for incorporating the circular plate
into the tube. However, when the radiant tube burner apparatus is
actually equipped on a heating furnace or the like, the tube is
installed in normal practice in such a position that a center axis
of the tube is directed horizontally, and the circular plate is
oriented vertically. Since the circular plate rests in a downward
eccentric position under the influence of gravity, the clearance
between the top edge of the circular plate and an inner wall
surface of the tube is enlarged. In a case where the opening of the
circular plate is located on a lower side, a relatively large gap
or space is created between the top edge of the circular plate and
the inner wall surface of the tube. Further, thermal expansion or
contraction of the radiant tube and the circular plate are caused
remarkably, owing to relatively significant temperature differences
between a combustion operation and a pause in the combustion
operation. Particularly, the thermal deformation of the radiant
tube and the circular plate due to such thermal expansion or
contraction appears remarkably in the middle-sized or large-sized
radiant tube burner apparatus with a large diameter. This kind of
thermal deformation also causes the relatively large gap or space
at the peripheral edge portion of the circular plate. As the
results of the aforementioned experiments conducted by the present
inventors, it has been found that a relatively large quantity of
secondary combustion air flow leaks through such a gap or space to
the main combustion chamber, so that this flow prevents the
combustion gas recirculation flow from being generated.
[0014] According to the aforementioned arrangement of this
invention, the air throat is defined in the peripheral cover
extending from the peripheral edge portion of the circular plate to
the peripheral edge portion of the regenerator, and the front edge
portion of the cover is integrally connected with the peripheral
edge portion of the plate to prevent the secondary combustion air
from leaking to the air diluting chamber through the periphery of
the plate. The combustion gas recirculation flow is not prevented
from occurring, and a sufficient effect of reduction in the
nitrogen oxides concentration can be obtained. Therefore, according
to the present invention, the combustion gas recirculation flow can
be surely generated without being affected by the installation
condition or thermal deformation of the radiant tube burner
apparatus, and a desired effect of reduction in the nitrogen oxides
concentration can be always obtained.
[0015] Further, the radiant tube burner apparatus as disclosed in
Japanese patent application No. 2000-237677 has an arrangement in
that the circular plate and the heat-transfer tube are supported
merely in a cantilever condition by the burner unit, and the
positions and attitudes of the plate and tube are apt to behave in
response to the thermal deformation. However, according to the
present invention, since the peripheral cover supports the circular
plate in a predetermined position, the positions and attitudes of
the plate and tube are stable.
[0016] Furthermore, provision of the peripheral cover permits the
clearance to be unintentionally enlarged, and enables intentional
enlargement of the clearance or change in setting of the clearance.
That is, flexibility of design in a combination of the tube and
burner unit or a relative position between the tube and the burner
unit can be obtained in accordance with the aforementioned
arrangement of the present invention, and this enables arbitrary
setting of clearance between the peripheral edge portion of the
circular plate and the tube wall, setting of clearance enough to
facilitate assembling or replacing works for the burner unit,
standardization of design of the burner unit adaptable to
differences of practical size or standard of the tube diameter and
the like, or design of the burner unit applicable to different
radiant tubes, and so forth. Nevertheless, the peripheral cover
surely prevents the secondary combustion air from leaking through
the periphery of the circular plate to the air diluting chamber,
and therefore, desired effect of reduction in the nitrogen oxides
concentration can be obtained by the present radiant tube burner
apparatus.
[0017] Preferably, the peripheral cover is contained in the radiant
tube in such a condition that the thermal deformation of the cover
in its axial direction is not restrained by the radiant tube and
the regenerator. The relief of thermal deformation in the axial
direction can prevent concentration of the thermal stress from
occurring at a joint portion of the plate and the cover. That is,
breakage or damage due to concentration of the thermal stress is
not caused at the joint portion of the plate and the cover.
[0018] The present invention also provides a radiant tube burner
apparatus with the aforementioned arrangement, wherein a center
(PC) of the opening (12) is positioned in a zone redially outside
of a circle (R) centered on a center line (C) of the circular
plate, the circle (R) having a predetermined radius S, and the
radius S equals [(an inner diameter Dt of the radiant tube+ an
outer diameter Dr of the heat-transfer tube)/4]. Preferably, each
of the centers (PC) of the openings (12) is positioned on a center
circle (PCD). The circle (PCD) has a center on a center line (C) of
the radiant tube and a diameter (PCR) larger than the diameter S.
More preferably, an area for positioning the opening of the
circular plate is limited to a central angle range (.gamma.) equal
to or less than 2500 about the center of the circular plate
(.gamma..ltoreq.250.degree.).
[0019] According to such an arrangement, the center (PC) of the
opening (12) is eccentrically positioned on the radially outer zone
of the circular plate, whereby the area for movement of the
combustion gas recirculation flow is enlarged and a combustion gas
inducing action of ejected flow of the secondary combustion air is
enhanced. As set forth above, the secondary combustion air is
prevented from leaking through the periphery of the circular plate
to the air diluting chamber. In connection with this, the
combustion gas of the main combustion chamber is effectively
induced by the secondary combustion air flow ejected from the
opening of the circular plate so that the secondary combustion air
is efficiently diluted with the combustion gas in the air diluting
chamber.
[0020] The velocity of the airflow at the opening (12) may be set
to be equal to or lower than 30 m/s, preferably equal to or lower
than 25 m/s, more preferably equal to or lower than 20 m/s. Such
reduction in velocity allows loss of the total pressure in the
radiant tube burner apparatus to be reduced. In spite of such
setting of a reduced airflow velocity, the nitrogen oxides
concentration of the combustion exhaust gas can be restricted by
fuel reforming (dilution) and the combustion gas recirculation,
according to the radiant tube burner apparatus having the
aforementioned arrangement.
[0021] Preferably, a fuel injection port (17a) and a primary
combustion air delivery port (18a, 18b) are positioned on a plane
of the circular plate or in front thereof, and the heat-transfer
tube protrudes forward from the circular plate to a relatively
large extent. According to such an arrangement, a relatively large
recirculation area of the combustion gas can be formed rearward of
a mixture gas outlet port (a distal end portion of the
heat-transfer tube).
[0022] More preferably, a flame stabilizing tube (41) is located to
surround the fuel injection port and the primary combustion air
delivery port, and a pilot fuel injection port (40) opens in the
flame stabilizing tube. This tube defines a flame stabilizing
section (42) therein for stabilizing the flame in the primary and
main combustion chambers.
[0023] From another aspect, the present invention provides an
alternate combustion type regenerative radiant tube burner
apparatus having a radiant tube (14) defining a main combustion
chamber (4) and burner units (15) provided on both end portions of
the radiant tube, the burner unit being provided with a regenerator
(13) and fluid passages (17, 18) for primary combustion air and
fuel, the regenerator being alternately in heat-transfer contact
with secondary combustion air or combustion exhaust gas, and the
fluid passages extending through the regenerator, comprising:
[0024] a circular plate (10) positioned between said burner unit
(15) and said main combustion chamber (4); an annular air throat
(2) provided between the circular plate and the regenerator, into
which the secondary combustion air flows after passing through said
regenerator; a cylindrical peripheral cover (30) extending from a
peripheral edge portion of said circular plate to a peripheral edge
portion of said regenerator; and a heat-transfer tube (16)
protruding forward of said circular plate and opening to said main
combustion chamber;
[0025] wherein a fuel injection port (17a) and a primary combustion
air delivery port (18a, 18b) open in said heat-transfer tube, an
inner area of the heat-transfer tube constitutes a primary
combustion chamber (1) for mixing the primary combustion air and
the fuel, an air diluting chamber (3) in an annular form opening to
said main combustion chamber is formed around a distal end portion
of the heat-transfer tube, and said circular plate has an opening
(12) for communication between said air throat and said air
diluting chamber; and
[0026] wherein a distance (L4) between said regenerator and said
circular plate is set in such a manner that a ratio (L4/Dt) of the
distance (L4) to an inner diameter (Dt) of the radiant tube is
equal to or less than 1.0.
[0027] Preferably, the ratio (L4/Dt) is equal to or less than 0.8,
more preferably, equal to or less than 0.5.
[0028] Preferably, the peripheral cover and the regenerator are
connected displaceably in an axial direction, a velocity of airflow
at the opening (12) of the circular plate is set to be equal to or
less than 30 m/s, and a velocity of the exhaust gas at the opening
is set to be equal to or less than 35 m/s.
[0029] A thickness of the peripheral cover is set to be,
preferably, equal to or less than 8 mm, more preferably, equal to
or less than 6 mm, and the peripheral cover is constituted from a
part other than a metal casting part, e.g., a heat-resistant metal
alloy plate shaped by bending.
[0030] From yet another aspect, the present invention provides a
combustion method in an alternate combustion type regenerative
radiant tube burner apparatus; wherein a circular plate (10) is
positioned between a main combustion chamber (4) in a radiant tube
(14) and a regenerator (13) alternately in heat-transfer contact
with secondary combustion air or combustion exhaust gas, primary
combustion air and fuel are fed to the main combustion chamber
through a heat-transfer tube (11, 16) extending through the
circular plate; and wherein an air diluting chamber (3) in an
annular form opening to the main combustion chamber is formed
around a distal end portion of the heat-transfer tube so that the
secondary combustion air (A) heated up to a high temperature by
heat-transfer contact with the regenerator enters the air diluting
chamber, and said fuel and said primary combustion air are mixed
with said secondary combustion air in the main combustion chamber
while an ejected flow of the secondary combustion air induces the
combustion gas of the main combustion chamber, comprising:
[0031] blocking fluid communication between areas on both sides of
said circular plate, at a periphery of the circular plate, so that
the fluid communication between these areas are limited to an
opening (12) disposed in a range of central angle of said circular
plate (.gamma.) equal to or less than 250.degree.
(.gamma..ltoreq.250.degree.), a center (PC) of the opening being
positioned in a zone radially outside of a circle (R) with a
predetermined radius (S) about a center line (C) of the circular
plate, and the radius (S) being equal to [(an inner diameter Dt of
the radiant tube+ an outer diameter Dr of the heat-transfer
tube)/4]; and
[0032] injecting said secondary combustion air to said air diluting
chamber at a velocity equal to or less than 30 m/s by said opening
(12), and directing an ejected flow of the combustion exhaust gas
of the main combustion chamber toward said regenerator at a
velocity equal to or less than 35 m/s by the opening (12).
[0033] According to such a combustion method, the fluid
communication at the periphery of the circular plate is blocked,
and a fluid passage (area) for inducing the combustion gas of the
main combustion chamber into the air diluting chamber is always
ensured in the angular range (.theta.) in which any opening is not
provided. The opening is positioned radially outward of the
circular plate, and therefore, the area for inducing the combustion
gas recirculation flow toward the main combustion chamber is formed
between the flow of the secondary combustion air and the outer
surface of the heat-transfer tube. Thus, the combustion air diluted
with the combustion gas is surely mixed with injected fuel (fuel
diluted with the combustion gas by the primary combustion) from the
rear side of the injected flow, so that the nitrogen oxides
concentration of the combustion exhaust gas can be surely
reduced.
[0034] Preferably, the quantity of supply of the primary combustion
air is set to be 10.about.20% of the theoretical amount of air
required for complete combustion of the fuel. More preferably, the
primary combustion air and the fuel are fed to the primary
combustion chamber through the flame stabilizing section (41), and
a pilot fuel of 2.about.8% of the fuel supply is fed to the flame
stabilizing section.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIGS. 1 and 2 are a schematic cross-sectional view and a
vertical cross-sectional view generally showing a preferred
embodiment of an alternate combustion type regenerative radiant
tube burner apparatus according to the present invention;
[0036] FIG. 3(A) is a cross-sectional view taken along line I-I of
FIG. 4, and FIG. 3(B) is a cross-sectional view taken along line
II-II of FIG. 4;
[0037] FIG. 4 is a cross-sectional view taken along line III-III of
FIG. 3;
[0038] FIG. 5(A) is a cross-sectional view showing a relation
between a circular plate and a clearance, and FIG. 5(B) is a
cross-sectional view which conceptually illustrates an examined
model for studying a relation between a volume of air leaking
through a gap and a flow rate of air at an opening;
[0039] FIG. 6(A) is a diagram showing a relation between the size
of gap and the volume of air leaking through the gap, and FIG. 6(B)
is a diagram showing a relation between the size of gap and a
concentration of NOx in a combustion exhaust gas;
[0040] FIGS. 7(A) and 7(B) are cross-sectional views conceptually
illustrating the distribution of the gap around the circular plate,
and FIG. 7(C) is a table showing a relation between an angular
range of the blocked gap and the concentration of NOx in the
combustion exhaust gas;
[0041] FIG. 8(A) is a schematic perspective view conceptually
showing a relation between the circular plate and a regenerator on
an exhaust side, and FIG. 8(B) is a diagram showing a variation in
a cross-sectional area of an effluent flow of the combustion
exhaust gas;
[0042] FIGS. 9 and 10 are a perspective view and a vertical
cross-sectional view of the burner apparatus provided with a
peripheral cover, which schematically show streams of a secondary
combustion air effluent from the openings of the circular plate
into an air diluting chamber;
[0043] FIGS. 11 and 12 are a perspective view and a vertical
cross-sectional view of the burner apparatus without the peripheral
cover, which schematically show the streams of the secondary
combustion air effluent from the opening of the circular plate into
the air diluting chamber.
BEST MODE FOR CARRYING OUT THE INVENTION
[0044] With reference to the attached drawings, a preferred
embodiment of the present invention is described hereinafter.
Structure of Burner Apparatus
[0045] As shown in FIG. 1, an alternate combustion type
regenerative radiant tube burner apparatus (referred to as "burner
apparatus" hereinafter) has a radiant tube 14 forming a main
combustion chamber 4. Burner units 15 are provided on both end
portions of the tube 4. A four-way valve 19 is connected with the
respective burner units 15 so that each of the burner units 15 is
selectively connected to either an air supply system or an exhaust
system. Each of the burner units 15 is provided with a ceramic
regenerator 13 having a honey-comb structure. A fuel feed passage
17 and a primary air passage 18 of the burner unit 15 extend
through the regenerator 13. The burner units 15 in a pair are
alternately changed over to either an operation mode as shown in
FIG. 1(A) or an operation mode as shown in FIG. 1(B) at
predetermined time intervals, so that the burner units 15
alternately carry out a combustion operation. The four-way valve 19
is changed over simultaneously with change-over operation of the
burner units 15, so that the valve 19 alternately takes either of
positions as shown in FIGS. 1(A) and 1(B) respectively, similar to
the burner units 15.
[0046] In FIGS. 2 to 4, a construction of the burner unit as shown
in FIG. 1 is illustrated.
[0047] Each of the end portions of the burner apparatus extends
through a furnace wall W, and an air case 20 and a burner gun 21 of
the burner unit 15 are exposed to an outside of a furnace. The air
case 20 and the burner gun 21 are connected with a primary air
supply pipe 22, a secondary air supply pipe 23 and a fuel feed pipe
24, respectively. The fuel feed passage 17 and the primary air
passage 18 extend through a center part of the regenerator 13 and
protrude forward from a front face of the regenerator 13. A fuel
injection nozzle 17a and primary air delivery ports 18a, 18b are
open to the primary combustion chamber 1.
[0048] A guide pipe 25 extends through a center part of each of the
regenerator 13 and a metal wire mesh 13a, and the pipe 25 is
connected with a proximal heat-transfer tube 11. A primary air
supply tube 26 forming the primary air passage 18 is located in a
guide pipe 25 and the heat-transfer tube 11. As shown in FIG. 3(A),
a fuel feed tube 27 forming the fuel feed passage 17 is positioned
at a center part of the primary air supply tube 26, and a pilot
fuel tube 28 for feeding a pilot fuel is positioned within the tube
26 in parallel with the tube 27.
[0049] As shown in FIG. 4, a distal end of the air supply tube 26
is closed by a distal end plate 29 perpendicular to an axis of the
tube 26. The fuel injection nozzle 17a of the fuel feed tube 27 is
positioned on the plate 29. The primary air delivery ports 18a, 18b
are formed on the plate 29 and a pilot fuel injector 40 extends
through the plate 29.
[0050] As shown in FIG. 3(B), the fuel injection nozzle 17a is
positioned at a center part of the distal end plate 29, and the
primary air delivery ports 18a, 18b are disposed at equal intervals
in a concentric formation around the fuel injection nozzle 17a.
Each of the ports 18a arrayed in a form of outside circle has a
diameter greater than the diameter of each of the ports 18b arrayed
in a form of inside circle, so that the port 18a injects a larger
quantity of primary air, compared to the quantity of primary air
injected from the port 18b. The pilot fuel injector 40 is
positioned in the circular array of the ports 18b, and the fuel
injected from the injector 40 takes a combustion reaction with,
mainly, the flow of air injected from the ports 18b.
[0051] As shown in FIG. 4, a circular plate 10 is located in front
of the regenerator 13. The proximal heat-transfer tube 11 extends
through a center part of the plate 10. The plate 10 and the tube 11
are integrally assembled by fixing means (not shown) such as
welding. The plate 10 and the tube 11 may be constituted by an
integrally formed single metal part.
[0052] An annular air throat 2 is formed to surround the proximal
heat-transfer tube 11. The throat 2 is in communication with the
respective cells (honey-comb fluid passages) of the regenerator 13.
The tube 11 slightly protrudes forward of the circular plate 10. A
flame stabilizing tube 41 protrudes forward from an outer periphery
of the primary air supply tube 26 so that a flame stabilizing
section 42 is formed in close proximity to an injection port of the
fuel injection nozzle 17a.
[0053] The distal heat-transfer tube 16 extends forward from a
periphery of the proximal heat-transfer tube 11. An inner diameter
of the tube 16 corresponds to an outer diameter of the tube 11. A
rear end portion of the tube 16 is integrally and airtightly
assembled to a periphery of a front end portion of the tube 11 by
fixing means (not shown) such as welding. The tubes 11, 16 may be
an integrally formed single metal part.
[0054] The distal heat-transfer tube 16 defines a primary
combustion chamber 1 therein. The fuel of the fuel injection nozzle
17a mixes with the primary air of the primary air delivery ports
18a, 18b in the chamber 1. A mixed gas flow F of the fuel and the
primary air effluent from a distal end opening of the tube 16
enters the main combustion chamber 4.
[0055] An annular air diluting chamber 3 is formed to surround the
distal heat-transfer tube 16. The circular plate 10 is provided
with a plurality of openings 12 (three openings in this
embodiment), which opens to the air diluting chamber 3. The
secondary combustion air supplied to the air throat 2 is ejected to
the air diluting chamber through the respective openings 12.
Preferably, a velocity of an airflow through the opening 12 is set
to be in a range from 20 cm/s to 30 cm/s.
[0056] As shown in FIG. 3(B), each of the openings 12 is configured
in a form of curved slot cricumferentially extending, and the
openings 12 are arranged symmetrically, spaced at an equal angle.
In this embodiment, outlines of the plate 10, the tube 16 and the
plate 29 are circles concentric with a center line C of the radiant
tube 14. An outside edge 12a of the opening 12 and an inside edge
12b thereof are curved with predetermined radiuses of curvature
about the center line C. A center PC of each of the openings 12 is
positioned on a center circle PCD. A radius PCR of the circle PCD
corresponds to an average value of the radiuses of the edges 12a,
12b.
[0057] The openings 12 are deviated to a range of angle .gamma.
about the center of plate 10. In the range of angle .theta. about
the center of the plate 10, the plate 10 functions as a blind plate
which does not have any opening. The centers PC of the openings 12
are positioned radially outside of a circle R with a radius S which
is centered on the center line C.
[0058] The radius S of the circle R and the radius PCR of the
circle PCD are set to be as follows: PCR>S=(Dr+Dt)/4
[0059] wherein Dt is an inner diameter of the radiant tube 14 and
Dr is an outer diameter of the heat-transfer tube 16.
[0060] The area for positioning the openings 12 is restricted to
the range of central angle (.gamma.) equal to or less than
250.degree. (.gamma..ltoreq.250.degree.).
[0061] If the opening 12 is designed to have another configuration
such as a circle, or if the opening 12 is designed to have an
uncertain or irregular configuration, the center PC of the opening
12 can be determined to be, e.g., a center of the mass (a
gravitational center) with respect to the configuration of the
opening. Further, the open area ratio of the circular plate 10 (the
total area of the openings 12/the cross-sectional area of the air
diluting chamber 3) is set to be in a range from 20% to 50%.
Preferably, the openings 12 are positioned as radially outward as
possible, as shown in FIG. 4, provided that an edge part dimension
E required for a structural strength is ensured (e.g., E.gtoreq.5
mm).
[0062] A diameter (outer diameter) of the circular plate 10 is set
to be 2 mm.about.3 mm smaller than the inner diameter Dt of the
tube 14, so that clearance of 1 mm.about.1.5 mm on the average can
be ensured around the plate 10 for inserting the plate 10 into the
tube 14 upon assembly of the burner apparatus.
[0063] FIG. 5(A) is a cross-sectional view showing the relation
between the circular plate 10 and the clearance.
[0064] A gap G having a gap size Gt is defined between an outside
surface of the circular plate 10 and an inside surface of the tube
14 by means of the clearance around the circular plate. Influence
of the gap G is described hereinafter.
Influence of Gap
[0065] FIG. 5(B) is a cross-sectional view showing an examined
model for studying the ratio of the flow rates at the gap G and the
opening 12.
[0066] In FIG. 5(B), a plate 100 is illustrated, which is located
in a fluid passage of air and which is perpendicular to a direction
of the flow of air A. The plate 100 is provided with a principal
opening 101 having a diameter Dm and a subsidiary opening 102
having a diameter Dg. The openings 101, 102 are round openings
extending through the plate 100. It is assumed that the principal
opening 101 corresponds to the aforementioned opening 12 and that
the subsidiary opening 102 corresponds to the aforementioned gap G.
It is also assumed that the fluid (air) is an incompressible fluid.
As regards a pressure loss of the fluid, only a friction resistance
is taken into consideration. It is supposed, on an analysis basis,
that the diameter of the opening 102 is 2 mm (Dg=2 mm) and the
thickness of the plate 100 is 10 mm (t=10 mm).
[0067] In order to reduce the velocity Um of the airflow A1 from 50
m/s to 20 m/s while the flow rate of the airflow A1 effluent from
the opening 101 is kept constant, the diameter Dm is changed from
23 mm(Um=50 m/s) to 36 mm(Um=20 m/s). This results in change of
velocity Ug of the airflow A2 through the opening 102, wherein the
velocity Ug varies from Ug=41.8 m/s (Um=50 m/s) to Ug=12.8 m/s
(Um=20 m/s), although detailed explanation of an analytical method
is omitted. That is, as the airflow velocity Um at the opening 101
is reduced, the airflow velocity Ug at the opening 102 is
extensively reduced, compared to a ratio of reduction in the flow
velocity at the opening 101.
[0068] FIG. 6(A) is a diagram showing a relation between the gap
size Gt and the flow rate leaking through the gap G, and FIG. 6(B)
is a diagram showing a relation between a concentration of NOx in
the combustion exhaust gas and the gap size Gt. The leakage flow
rate of air and the concentration of NOx as shown in FIG. 6 are
those of the burner apparatus having a nominal diameter of 7B (the
diameter of the tube=7 inches), in which the velocity of flow at
the opening 12 is set to be 20 m/s.
[0069] In FIG. 6(A), the flow rate of air, which leaks through the
gap G having a gap size Gt of 1 mm (Gt=1 mm), is defined as a
reference flow rate (=1). Increase in the flow rate of the leakage
air involved in enlargement of the gap size Gt is indicated in FIG.
6(A) by the ratio of flow rate relative to the reference flow
rate.
[0070] As shown in FIG. 6(A), the flow rate of air leaking through
the gap G is increased substantially in proportion to enlargement
of the gap size Gt. Similarly, the NOx concentration of the
combustion exhaust gas gradually increases as the gap size Gt
enlarges in a range of the gap size Gt equal to or less than 1.5
mm, until the NOx concentration increases up to 115 ppm (corrected
to 11% O.sub.2), as shown in FIG. 6(B). However, the NOx
concentration of the combustion exhaust gas rapidly increases in a
range of the gap size Gt exceeding 1.5 mm, and it increases up to
approximately 190 ppm when the gap size is equal to 3.0 mm.
[0071] That is, enlargement of the gap size Gt results in
relatively rapid increase of the NOx concentration of the
combustion exhaust gas, in spite of relatively remarkable decrease
in the velocity (Ug) of airflow at the gap G involved in decrease
of the velocity (Um) of airflow at the opening 12. Such a variation
of the NOx concentration is a peculiar characteristic observed in
the burner apparatus of the present invention, in which effect of
reduction of the NOx concentration depends on distribution of
airflow in the air diluting chamber 3. This apparently differs from
a property of a conventional burner apparatus in which reduction of
the NOx concentration depends on the velocity (Um) of high speed
airflow.
[0072] In FIG. 7, there is shown a relation between the NOx
concentration of the combustion exhaust gas and a position or an
angular range of the gap G.
[0073] In a case where the gap G exists throughout the whole
circumference of the circular plate 10 in the burner apparatus
having the gap size Gt of 3 mm (Gt=3 mm) and a nominal diameter of
7B (the diameter of the tube=7 inches), the NOx concentration
increases up to approximately 150.about.200 ppm (corrected to 11%
O.sub.2). When the gap G is blocked in an angular range of
90.degree. (.alpha.=90.degree.), the NOx concentration decreases
down to 140.about.180 ppm. When the gap G is blocked in an angular
range of 180.degree. (.alpha.=180.degree.), the NOx concentration
decreases down to 130.about.150 ppm. In a case where the gap G is
blocked throughout the whole circumference (.alpha.=360.degree.),
the NOx concentration decreases down to 115.about.120 ppm, which is
the minimum value. Simultaneously, a deviation of the NOx
concentration also decreases as the angular range .alpha. is
enlarged. When the gap G is blocked throughout the whole
circumference (.alpha.=360.degree.), the NOx concentration is
stabilized in a range from 115 ppm to 120 ppm.
Positional Relation Between Circular Plate and Regenerator
[0074] As previously mentioned, the burner unit 15 is alternately
changed over to either of the operation modes as shown in FIGS.
1(A) and 1(B) at predetermined time intervals. Therefore, the
opening 12 is not only used to inject the secondary combustion air
of the air throat 2 into the air diluting chamber, but also used as
an exhaust port for exhausting the combustion exhaust gas of the
main combustion chamber 4. Therefore, the length of the air throat
2, i.e., a distance L4 (FIG. 4) between the circular plate 10 and
the regenerator 13 is settled so that the combustion exhaust gas G
flowing through the opening 12 into the air throat 2 can enter the
respective cells of the regenerator 13 substantially uniformly
while spreading over the whole area of an end face of the
regenerator 13.
[0075] FIG. 8(A) is a schematic perspective view conceptually
showing a relation between the circular plate 10 and the
regenerator 13, and FIG. 8(B) is a diagram showing a relation
between a distance L from the plate 10 and a cross-sectional area
Ga of an ejected flow of the combustion exhaust gas. For
simplification of description hereinafter, it is assumed that the
opening 12 is a center circular opening 112 of the plate 10.
[0076] In order to set the flow rate of air at the opening 112 to
be 26 m/s, a diameter Dm of the opening 112 is set to be 79 mm.
Further, in order to set the flow rate of air at the opening 112 to
be 70 m/s, the diameter Dm of the opening 112 is set to be 48 mm.
In either of these cases, the effluent flow of the combustion
exhaust gas G entering the air throat 2 from the opening 112
spreads toward the inlet face of the regenerator 13. In FIG. 8(B),
the cross-sectional area Ga of the effluent flow of the combustion
exhaust gas in relation with L/Dn is indicated in each of a case of
Dm=79 mm(26 m/s) and a case of Dm=48 mm(70 m/s), wherein L is a
distance between the circular plate 10 and the cross-section of the
effluent flow and wherein Dn is a diameter of the regenerator
13.
[0077] If the diameter Dn of the regenerator 13 is 144 mm (Dn=144
mm) and the diameter of the heat-transfer tube 16 is 90 mm, the
cross-sectional area of the whole cell part of the regenerator 13
is as follows:
(14.4/2).sup.2.times.3.14-(9/2).sup.2.times.3.14=approx.99.2
cm.sup.2
[0078] It can be considered that the effluent flow of the
combustion exhaust gas enters the cell part of the regenerator 13,
while inevitably avoiding the outline of the heat-transfer tube 16.
Therefore, it can be regarded that the effluent flow spreading to
this cross-sectional area (approximately 99.2 cm) uniformly enters
the whole cell part of the regenerator 13.
[0079] The value of L/Dn for the cross-sectional area Ga of the
effluent flow equal to 99.2 cm.sup.2 (Ga=99.2 cm.sup.2) is 0.42 in
a case of Dm being equal to 79 mm(26 m/s), whereas the value of
L/Dn for Ga equal to 99.2 cm.sup.2 (Ga=99.2 cm.sup.2) is 1.05 in a
case of Dm being equal to 48 mm(70 m/s). Therefore, if the velocity
of air is reduced from 79 m/s to 26 m/s, the distance Lm between
the circular plate 10 and the inlet face of the regenerator 13 can
be reduced to 0.42/1.05(=0.4).
[0080] In the burner apparatus as shown in FIGS. 1 to 4, the
opening 12 is eccentric and deformed, and it is somewhat different
from the examined model as shown in FIG. 8(A). However, the
tendency itself is substantially the same. Therefore, enlargement
of the open area of the opening 12 (i.e., reduction in the velocity
of flow through the opening 12) enables considerable reduction of
the distance L4 (FIG. 4) between the regenerator 13 and the
circular plate 10.
Cylindrical Peripheral Cover
[0081] As described in "Influence of Gap", if the velocity of air
at the opening 12 is reduced to 20.about.30 m/s, the velocity of
air at the gap G (FIG. 5(A)) is, relatively speaking, remarkably
reduced. However, since the size of gap Gt (FIG. 5(A))
significantly affects the NOx concentration of the combustion
exhaust gas, it is desired that the gap G is fully blocked.
Further, as described in "Positional Relation between Circular
Plate and Regenerator", the reduction of the velocity of air
through the opening 12 (enlargement of the open area of the opening
12) enables reduction of the distance L4 (FIG. 4) between the
regenerator 13 and the circular plate 10.
[0082] Therefore, the burner apparatus according to the present
invention is provided with a cylindrical peripheral cover 30 which
extends between the regenerator 13 and the circular plate 10. The
peripheral cover 30 blocks fluid communication between the air
throat 2 and the air diluting chamber 3 throughout the whole
circumference of the circular plate 10, so that the cover 30
prevents leakage of air through the gap G, which may, otherwise,
cause disturbance of gaseous flow distribution in the air diluting
chamber 3. Thus, an effect of reduction in the NOx concentration
can be surely obtained from this burner apparatus.
[0083] As shown in FIG. 4, the periphery cover 30 extends backward
from a peripheral edge of the circular plate 10. An outer diameter
of the periphery cover 30 is substantially the same as that of the
plate 10 or slightly smaller than that of the plate 10. The plate
10 and the heat-transfer tube 16 require strength in high
temperature. Therefore, they are constituted from metal casting
parts with high strength, or the like. On the other hand, the
peripheral cover 30 can be a part other than the metal casting
part, which has a circular cross-section. For example, the
peripheral cover 30 may be a part constituted from two semicircular
plates welded to each other, each shaped by bending a plate of
SUS310S (JIS). A front end of the peripheral cover 30 is fixed in
an air-tight manner to the peripheral edge portion of the plate 10
by fixing means 31 such as welding.
[0084] The peripheral cover 30 fully blocks the fluid communication
between the air throat 2 and the air diluting chamber 3 at the
outer peripheral edge portion of the circular plate 10. Therefore,
the fluid communication between these areas 2, 3 is limited to the
opening 12. The peripheral cover 30 also carries the circular plate
10 in a predetermined position so that the position and attitude of
the plate 10 and those of the heat-transfer tube 11 are stable.
[0085] A rear end portion of the peripheral cover 30 is located on
an outer cylindrical casing 13b of the regenerator 13. An inner
surface of the rear end portion of the cover 30 is not integrally
connected with an outer surface of the front end portion of the
casing 13b. Rather, it is connected in a displaceable manner with
the casing 13b, which is in sliding contact. An annular spacer 13c
is integrally attached to the outer periphery of the casing 13b,
spaced at a distance rearward from the cover 30.
[0086] An overall length L3 of the peripheral cover 30 is
relatively short, involved in reduction of the distance L4.
Therefore, excessive thermal stress or thermal deformation is not
caused on the cover 30, and cracks by thermal stress or the like
due to heat load can be prevented from occurring on the cover 30.
Further, a thickness T of the cover 30 can be set to be equal to or
less than 8 mm, e.g., 5-6 mm, owing to reduction in the thermal
stress. Therefore, sufficient volume of the air throat 2 and
flexibility of design in regard to layout of the openings 12 can be
ensured, in spite of provision of the cover 30.
[0087] In addition, the thermal deformation of the peripheral cover
30 is not restrained in its axial direction by the radiant tube 14
or the casing 13b, and therefore, the thermal deformation of the
cover 30 is permissible in the axial direction. Thus, the thermal
stress does not concentrate on a joint portion (the fixing means
31) of the plate 10 and the cover 3, and breakage, damage or the
like due to the thermal stress concentration or thermal deformation
is not caused.
Dimensions of Heat-Transfer Tube and Opening
[0088] The dimensions of the heat-transfer tube 16 are set to be in
the following range: Dr/Dt=0.2.about.0.6 L1/Dr=1.28.about.5.2
L2/Dr=0.64.about.5.2
[0089] wherein symbols L1, L2 are as follows:
[0090] L1: Distance between the distal end of the fuel injection
nozzle 17a and the distal end of the heat-transfer tube 16; and
[0091] L2: Distance between the circular plate 10 and the distal
end of the heat-transfer tube 16.
[0092] For instance, the dimensions Dt, Dr, L1, L2 in the burner
apparatus having a nominal diameter of 7B (the diameter of the
tube=7 inches) are set to be as follows: Dt=approx.180 mm(7B)
Dr=approx.90 mm(3B) L1=approx.200 mm L2=approx.200 mm.
[0093] The openings 12 have the same size and configuration, and a
total amount L5 of circumferential edge lengths of the openings 12
is set to fall under L5/Dt=2.0.about.4.0, preferably,
L5/Dt=2.5.about.3.5. For example, the total amount L3 of the
circumferential edge lengths of the openings 12 is set to fall
under approximately 350.about.700 mm, preferably, 450.about.600 mm,
with respect to the burner apparatus having a nominal diameter of
7B (the diameter of the tube=7 inches).
[0094] Further, the heat-transfer tube 16 physically separates the
secondary combustion air and the fuel until the flow A of secondary
combustion air and the flow F of the mixed gas join together. The
dimension L2 of protrusion of the tube 16 is so set as to ensure a
term of time for diluting the secondary combustion air in the air
diluting chamber 2 and diluting the fuel in the primary combustion
chamber 1.
Operation of Burner Apparatus
[0095] The operation of the burner apparatus with the arrangement
as previously mentioned is described hereinafter.
[0096] Each of the burner units 15 of the burner apparatus
alternately performs a combustion operation (supply of the
combustion air and the fuel) and a pause in the combustion (exhaust
of the combustion exhaust is gas) as illustrated in FIG. 1(A) and
FIG. 1(B), in response to switching control of supply of the
primary air and the fuel to the burner gun 21 and switching control
of the four-way valve 19 synchronized therewith.
[0097] The primary combustion air and the fuel are fed to the
burner gun 21 (FIG. 2) in combustion operation by the primary air
supply pipe 22 and the fuel feed pipe 24, so that the air and the
fuel are introduced from the primary air delivery ports 18a, 18b
and the fuel injection nozzle 17a (FIGS. 3 and 4) into the primary
combustion chamber 1, and they are mixed with each other in the
chamber 1. The quantity of the supplied air is set to fall under
approximately 10.about.20% of the theoretical amount of air
required for complete combustion of the fuel.
[0098] The pilot fuel of the pilot fuel feed pipe 28 is normally
injected through the pilot fuel injector 40 into the flame
stabilizing tube 41. The quantity of the supplied pilot fuel is set
to fall under approximately 2.about.8% of the quantity of the fuel
fed by the nozzle 17a. The flame stabilizing section 42 in the tube
41 acts to prevent flame failure, whereby the combustion reactions
in the chambers 1 and 4 are stabilized.
[0099] A large quantity of the secondary combustion air, which is
much greater than a quantity of the primary combustion air, is fed
into the air case 20 through the secondary air supply pipe 23, as
shown in FIG. 2. The secondary combustion air flows into the narrow
passages (passages of cells) of the regenerator 13 at a high
temperature. The secondary combustion air is heated up to a
temperature equal to or higher than 700.degree. C., preferably,
equal to or higher than 900.degree. C. by heat-transfer contact
with the honey-comb walls (cell walls) of the regenerator 13 and
then, the heated air flows into the air throat 2.
[0100] In FIGS. 9 and 10, there are illustrated streams of the
secondary combustion air flowing from the openings 12 of the
circular plate 10 to the air diluting chamber 3. In FIGS. 11 and
12, there are illustrated streams of the secondary combustion air A
flowing into the air diluting chamber 3 in a condition that the
peripheral cover 30 is not provided. The burner apparatus as shown
in FIGS. 9 to 12 is positioned in a heating furnace or the like so
that the openings 12 are located on the underside, and the plate 10
is displaced downward under the gravity. Therefore, a relatively
large gap G is formed between the top portion of the periphery of
the plate 10 and the inner circumferential surface of the radiant
tube 14.
[0101] In the burner apparatus with the peripheral cover 30 (FIGS.
9 and 10), the streams of the secondary combustion air A entering
the air throat 2 are delivered into the air diluting chamber 3
through the openings 12 which is eccentrically disposed on one side
of the circular plate 10. The flow rate of the air A through the
opening 12 is set to be, for example, equal to or lower than 30
m/s, preferably equal to or lower than 25 m/s, more preferably
equal to or lower than 20 m/s. This is considerably a low velocity
range. The cover 30 blocks the fluid communication between the air
throat 2 and the chamber 3 on the outer periphery of the plate 10,
and therefore, the fluid communication between the areas 2, 3 is
limited to the fluid commination at the openings 12. The flows of
air A effluent from the openings 12, which have the aforementioned
peculiar positions, sizes and configurations, induce the combustion
gas of the main combustion chamber 4. As a result, recirculation
flows R of combustion gas reversed toward the chamber 3 are
created.
[0102] According to the burner apparatus with the peripheral cover
30, a flow passage (area) for inducing the recirculation flows R
into the air diluting chamber 3 is always ensured in the angular
range (.theta.) in which the opening 12 is not disposed. In
addition, since the openings 12 are disposed in a radially outer
zone of the circular plate 10, an area for inducing the
recirculation flows R and moving the flows R toward the main
combustion chamber 4 is created between the flows A of the
secondary combustion air and an outer surface of the heat-transfer
tube 11. Therefore, effect of combustion gas recirculation can be
enhanced in the limited area inside of the radiant tube as set
forth above, and the secondary combustion air can be diluted with a
relatively large quantity of combustion gas, even under
circumstances where the velocity of the air flow A is lowered.
[0103] On the other hand, under the condition where the peripheral
cover 30 is absent (FIGS. 11 and 12), the secondary combustion air
in the air throat 2 flows into the air diluting chamber 3 not only
through the openings 12, but also through the periphery of the
circular plate 10 (the gap G) as peripheral air flows J. The flows
J act to be in opposition against the recirculation flows R and
prevent the recirculation flows R from entering the chamber 3.
Therefore, the quantity of combustion gas mixed with the secondary
combustion air is reduced.
[0104] Thus, according to the burner apparatus provided with the
peripheral cover 30 (FIGS. 9 and 10), the recirculation flows R can
enter the air diluting chamber 3 without being obstructed by the
peripheral air flows J. The flows R move around the outer surface
of the heat-transfer tube 16 and approach the proximity of the
openings 12, so that the flows R effectively mix with the flows of
air A in order to dilute the secondary combustion air with a
relatively large quantity of combustion gas.
[0105] The fuel flowing into the primary combustion chamber 1 in
the heat-transfer tube 16 mixes with the primary combustion air to
take a partial combustion reaction under a condition of low air
ratio, so that the fuel is diluted with the combustion gas which is
produced in the combustion chamber 1. The atmosphere in the
combustion chamber 1 is normally kept in a high temperature state
by the surrounding high temperature atmosphere. The mixture of the
fuel, primary combustion air and combustion gas is activated by
reforming action in the chamber 1 under a condition of low oxygen
concentration or low air ratio, and then, the mixture flows from
the chamber 1 to the main combustion chamber 4 as a flow of gaseous
mixture F. The outer surface of the heat-transfer tube 11 is
normally kept in a high temperature state, because it is in contact
with the combustion exhaust gas at a high temperature while the
combustion pauses, as described later.
[0106] The flow of air A, the oxygen concentration of which has
been lowered by mixing with the recirculation flow R, mixes with
the flow of gaseous mixture F delivered from the distal end opening
of the heat-transfer tube 16, whereby a combustion reaction occurs
and proceeds between the flows A and F in the main combustion
chamber 4.
[0107] In such a burner apparatus, a large quantity of nitrogen
oxides are not produced by the combustion reaction in the main
combustion chamber 4, in spite of the secondary combustion air flow
A introduced at a relatively low speed into the chamber 4. For
instance, in measurements conducted by the present inventors, the
concentration of nitrogen oxides in the combustion exhaust gas were
reduced to a level equal to or less than 130 ppm, regardless of the
installation condition and thermal deformation of the burner
apparatus, even if the velocity of airflow A at the opening 12 was
equal to or less than 20 m/s. This is considered to mainly result
from the formation of the openings 12 having the aforementioned
particular sizes and configurations in the certain region of the
circular plate 10, and change of property of the fuel caused during
flowing inside of the heat-transfer tube 11 at a high temperature,
and further, combustion gas recirculation enhanced by the flow of
air A at a low speed, owing to installation of the peripheral cover
30, and so forth. In addition, the fuel diluted in the primary
combustion chamber 1 as set forth above takes a slow combustion
reaction with the flow of high temperature air A having a low speed
and a low oxygen density, so that a uniform temperature
distribution can be obtained over a relatively wide area of the
radiant tube 14.
[0108] Further, in the burner apparatus according to the present
invention, the fluid communication between the areas 2 and 3 on
either sides of the circular plate is restricted only to the
openings 12, owing to provision of the peripheral cover 30, and
therefore, the distance between the outer circumferential surface
of the circular plate 10 and the inner circumferential surface of
the radiant tube 14 can be arbitrarily designed as desired. Thus,
it is possible to ensure a clearance sufficient enough to
facilitate an assembly or replacement work of the burner unit 15.
It is also possible to adopt a practical design, such as a
standardized design of the burner unit applicable to different
dimensions or different standards of the tube diameter Dt, a design
of the burner unit 15 applicable to different dimensions of the
radiant tube 14, and so forth. Nevertheless, effective reduction in
the concentration of nitrogen oxides can be desirably obtained,
because the cover 30 prevents the secondary combustion air from
leaking out from the periphery of the plate 10 into the air
diluting chamber 3 as previously described.
[0109] As shown in FIG. 1, the combustion gas flows in the radiant
tube 1 toward the burner unit 21 pausing the combustion operation,
except the counterflow of the aforementioned recirculation gas. The
burner unit 21 pausing the combustion operation receives the
combustion exhaust gas from the openings 12 of the circular plate
10 into the air throat 2. The combustion gas in the air throat 2
enters the honey-comb passages (cells) of the regenerator 13 after
flowing through the areas surrounding the heat-transfer tubes 11
and 16, so that the combustion gas is in heat-transfer contact with
the honey-comb walls (cell walls) of the regenerator 13 at a
relatively low temperature to heat the regenerator up to a
temperature equivalent to the temperature of the combustion exhaust
gas. The combustion exhaust gas, after being cooled by
heat-exchange with the regenerator 13, is exhausted to the
atmosphere through the four-way valve 19.
[0110] Since the velocity of air at the openings 12 of the burner
unit 21 on the combustion side is set to be a relatively low, the
combustion exhaust gas flowing through the opening 12 on the
exhaust side also has a low velocity. For example, when the
velocity of air at the opening 12 is set to be equal to or less
than 20 m/s, the velocity of the combustion exhaust gas at the
opening 12 on the exhaust side is restricted to approximately 25
m/s or less, even if increase in the volume of combustion exhaust
gas is taken into consideration. This results in reduction of the
pressure loss in the burner units 21 on each of the combustion side
and the exhaust side. Further, the exhaust gas entering the air
throat 2 flows into the respective cells of the regenerator 13
relatively uniformly, in spite of reduction of the distance L4.
That is, the flow rates of exhaust gas through the respective
honey-comb passages are uniformed over the whole area of the
regenerator 13 so as to uniformly heat the regenerator, since the
exhaust gas at a relatively low speed is uniformly distributed over
the whole area of the end face of the regenerator 13 without
concentrically entering only a part of the regenerator 13 opposing
the opening 12. This prevents the regenerator 13 from exhibiting a
local temperature drop or a local overheating, and the regenerator
13 can desirably take efficient performances of heat accumulation
and heat exchange. Strictly speaking, the flow rate of the
combustion exhaust gas is increased in comparison with the flow
rate of the primary air, owing to supply of the primary combustion
air and the fuel, combustion reaction, thermal expansion and the
like. However, such increase of the flow rate of the combustion
exhaust gas is considered to be negligible, as set forth above.
[0111] Although the present invention has been described as to a
preferred embodiment, the present invention is not limited thereto,
but may be carried out in any of various modifications or
variations without departing from the scope of the invention as
defined in the accompanying claims.
[0112] For insurance, the circular plate 10 is formed with the
three openings 12 symmetrically arranged, in the aforementioned
embodiment, but more openings or less openings may be
unsymmetrically formed on the plate 10 in accordance with the
present invention.
[0113] Further, the size and configuration of the heat-transfer
tube 11 is not limited to those in the aforementioned embodiment,
but can be appropriately modified or designed in accordance with
the present invention.
INDUSTRIAL APPLICABILITY
[0114] The present invention is applied to the alternate combustion
type regenerative radiant tube burner apparatus, which has the
circular plate positioned between the burner unit and the main
combustion chamber so that the preheated secondary combustion air
is fed through the opening of the plate to the main combustion
chamber. According to the present invention, the concentration of
nitrogen oxides in the combustion exhaust gas can be definitly
prevented from increasing, without being influenced by the
installation condition or thermal deformation of the apparatus.
* * * * *