Note: Descriptions are shown in the official language in which they were submitted.
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RADIANT TUBE
TECHNICAL FIELD
[0001] The present invention relates to a radiant tube formed of cast metal
tubes and including at least one bent tube and a pair of straight tubes
connected
to the opposed ends of the bent tube, with combustion gas from a burner being
fed
through one of the pair of straight tubes.
BACKGROUND ART
[0002] As prior-art document information relating to a radiant tube of the
above-noted type, Patent Document 1 identified below is known. This Patent
Document 1 discloses a radiant tube including neck portions provided at two
open
ends of the bent tube, the neck portions extending straight for a
predetermined
length. It is described that with the above configuration, the compressive
stress
on the side of the bent tube and the compressive stress on the side of he
straight
tube act uniformly to the welded portions between the bent tube and the
straight
tubes, so that there is realized uniform distribution of the stress due to
thermal
expansion occurring at the welded portions, thus providing high resistance
against formation of crack at the welded portions.
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PRIOR ART DOCUMENT
PATE NT DOCUMENT
[0003] Patent Document 1: Japanese Unexamined Patent Application
Publication No. 10-227420 (paragraph 0007, paragraphs 0015-16, Fig.1).
SUMMARY OF THE INVENTION
OBJECT TO BE ACHIEVED BY INVENTION
[0004] However, with the radiant tube disclosed in Patent Document 1, its bent
tube is divided into a large-diameter portion disposed on the outer
circumferential
side relative to an arcuate center axis of the bent tube and a small-diameter
portion disposed on the inner circumferential side relative to the center axis
and
these large-diameter portion and the small-diameter portion are welded
together
in opposition to each other. Therefore, aside from the problem at the welded
portions between the bent tube and the straight tube, there was possibility
that a
crack due to thermal expansion or the like can occur at the two welded
portions
extending along the axis of the bent tube.
[0005] In view of the problem provided by the conventional radiant tube
exemplified above, the object of the present invention is to provide a radiant
tube
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which has high resistance against severe heat condition imposed by combustion
gas fed from a burner, thus being usable for a longer period of time.
MEANS FOR ACHIEVING THE OBJECT
[0006] According to the present invention, a radiant tube formed of a heat
resistant metal and including at least one bent tube which connects a pair of
straight tubes to each other, with combustion gas from a burner being fed
through
one of the pair of straight tubes;
wherein at least as the bent tube located closest to the burner, there is
employed a cast body having an outer diameter ranging from 150 to 210 mm and a
wall thickness ranging from 3 to 8 mm.
[0007] With the radiant tube having the above-described characterizing
feature,
as the bent tube located closest to the burner, thus being subject to the
severest
heat condition, there is employed a cast body having a wall thickness ranging
from 3 to 8 mm. Therefore, in comparison with e.g. a bent tube obtained by
welding end-to-end tubular bodies formed by pressing of a plate material, the
wall
thickness of the tube is more uniform, and no stress concentration occurs
which
would otherwise occur at the welded portions extending along the longitudinal
direction of the bent tube. Therefore, there will hardly occur e.g. a heat-
crack
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due to sharp temperature rise or sharp temperature drop caused by the
combustion gas of the burner. Consequently, there has been obtained a radiant
tube which has high heat resistance and which can be used for a longer period
of
time.
[0008] Moreover, since the thickness of the cast body is reduced to the range
from 3 to 8 mm, there is realized enhanced density of the metallographic
structure
due to increase in the cooling rate at the time of casting. Accordingly, with
enhancement in the heat resistance and heat-shock resistance of the bent tube
subject to the severest heat condition as being located closest to the burner,
there
is realized a radiant tube that can be used for an even longer period of time.
[0009] Further, the reduction of wall thickness at the portion of the bent
tube
subject to the severest heat condition facilitates deformation in response to
stress
application. As a result, the heat stress can be absorbed more easily and heat
crack due to sharp temperature rise due to the combustion gas from the burner
will occur less likely.
[0010] Furthermore, the wall thickness reduction of the bent tube located
closest
to the burner provides increase in the rate of temperature rise due to the
combustion gas from the burner as well as decrease in the temperature drop
along
the direction of wall thickness. Therefore, the fuel consumption amount too
can
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be reduced in comparison with the conventional configuration.
Also, as the wall thickness reduction of the bent tube located closest to the
burner provides weight reduction of the radiant tube as a whole, the labor
required for its replacement has been reduced as well.
[0011] According to a further characterizing feature of the present invention,
the
bent tube has a smaller wall thickness at its portion near the connection to
the
straight tube than the remaining portion thereof.
[0012] The portion of the bent tube connected to the straight tube is
especially
vulnerable to insufficient strength when it is used due to e.g. the structural
weakness on account of being located near the tube end and embrittlement of
its
material under the influence of the heat received at the time of welding. With
the inventive arrangement described above, however, since the wall thickness
of
the portion near the connection is reduced relative to the remaining portion
of the
bent tube, the density of the metallographic structure is particularly
enhanced
due to increase in the cooling rate at the time of casting. As a result, there
is
ensured durability as good as that of the general portion of the bent tube
other
than its connection-vicinity portion, for the severe heat condition imposed by
the
combustion gas.
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[00131 According to a still further characterizing feature of the present
invention,
the radiant tube comprises a plurality of said bent tubes, all of which
comprise the
cast bodies having the wall thickness ranging from 3 to 8 mm.
[0014] Conceivably, only the bent tube that is located closest to the burner
may
comprise a cast body having the wall thickness ranging from 3 to 8 mm. With
the above inventive arrangement, however, all of a plurality of bent tubes
comprise cast bodies having the wall thickness ranging from 3 to 8 mm. With
this, there is obtained a radiant tube which has even higher reliability in
its heat
resistance and which can be used for an even longer period of time.
Moreover, as the arrangement allows even further weight reduction of the
radiant tube as a whole, the labor required for its replacement operation can
be
even further reduced.
[0015] According to a still further characterizing feature of the present
invention,
the straight tube has a wall thickness of 7 mm or less.
[0016] With the reduction of wall thickness of the straight tube, in addition
to
the wall thickness reduction of the bent tube, as proposed in the above
arrangement, in comparison with an arrangement of the straight tube alone
having a relatively large wall thickness, there can be ensured higher strength
at
the connection portion between the bent tube and the straight tube.
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[0017] According to a still further characterizing feature of the present
invention,
the straight tube has a smaller wall thickness at its portion near the
connection to
the bent tube than the remaining portion thereof.
[0018] The portion of the straight tube connected to the bent tube is
especially
vulnerable to insufficient strength when it is used due to e.g. the structural
weakness on account of being located near the tube end or embrittlement of its
material under the influence of the heat received at the time of welding. With
the inventive arrangement described above, however, since the wall thickness
of
the portion near the connection is reduced relative to the remaining portion
of the
straight tube, the density of the metallographic structure is particularly
enhanced
due to increase in the cooling rate at the time of casting. As a result, there
is
ensured durability as good as that of the general portion of the straight tube
other
than its connection-vicinity portion, for the severe heat condition imposed by
the
combustion gas.
[0019] According to a still further characterizing feature of the present
invention,
the straight tube comprises a cast bodies having a greater wall thickness than
that of the bent tube.
[0020] With the above-described arrangement, in comparison with an
arrangement using a straight tube having substantially same wall thickness as
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the bent tube, it becomes easier to obtain a radiant tube which has an even
higher
heat resistance and which can be used for an even longer period of time.
BRIEF DESCRIPTION OF THE DRAWING
[0021] [Fig. 1] is a partially cutaway side view schematically showing a
radiant
tube relating to the present invention.
MODE OF EMBODYING THE INVENTION
[0022] Next, one embodiment of the present invention will be described with
reference to the accompanying drawing. It is understood, however, that the
scope of the present invention is not to be limited by the following
description or
the illustration, but that the invention may be embodied in any modified
manner
as long as such modification does not deviate from the essential concept
thereof.
[0023] A radiant tube 1 shown in Fig. 1 includes four laterally oriented
straight
tubes 2A, 2B, 2C, 2D juxtaposed with an equal vertical spacing therebetween,
with the respective vertically adjacent straight tubes 2 pairs being connected
via
total three bent tubes 3A, 3B, 3C, so that the assembly as a whole forms a
laterally oriented W-shape.
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[0024] The radiant tube 1 is supported to a furnace wall 10 of a heating
furnace
such as a drying furnace, a sintering furnace, etc. via the uppermost straight
tube
2A and the lowermost straight tube 2D. To the terminal free ends of these
straight tubes 2A, 2D, there are connected burners 5 via heat reservoirs 4
formed
of ceramic honeycomb bodies having high heat recovery efficiency.
[0025] These burners 5 are composed of regenerative type burners in which the
fuel consumption required for burner combustion can be reduced, in such manner
that e.g. when the burner 5 connected to the uppermost straight tube 2A is
operated for combustion, exhaust gas is discharged through the lowermost
straight tube 2D while exhaust heat is collected by the lower heat reservoir
4, and
when the combustion is switched to the burner 5 connected to the lowermost
straight tube 2D, the combustive air is preheated using the exhaust heat
collected
by the lowermost heat reservoir 4.
[0026] By operating the switch valve 6 provided between the combustive air fun
7 for supplying the combustive air and each burners 5, it is possible to
switch over
between a state (indicated by the solid line) where the combustive air is
burned by
the burner 5 connected to the uppermost straight tube 2A, and exhaust gas is
discharged through the lowermost straight tube 2D while exhaust heat is
collected by the heat reservoir 4 connected to the lowermost straight tube 2D
and
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a further state (indicated by the broken line) where the combustive air is
burned
by the burner 5 connected to the lowermost straight tube 2D, and its exhaust
heat
is collected by the heat reservoir 4 connected to the uppermost straight tube
2A.
The exhaust gas past each heat reservoir 4 can be discharged into the
atmosphere via the switch valve 6 and an exhaust gas treating device (not
shown),
etc.
[0027] Each and every one of the four straight tubes 2A, 2B, 2C, 2D and the
three bent tubes 3A, 3B, 3C has an outer diameter of 180 mm and is formed of
cast
steel (an example of cast body formed of a heat resistant metal) containing 20-
35
wt.% of chrome and 30 to 50 wt.% of nickel.
The connection between the straight tube 2 and the bent tube 3 is realized
by means of welding these from the outer circumferential faces thereof, with
placing the respective end faces thereof in abutment with each other.
[0028] Among the three bent tubes 3A, 3B, 3C, the first bent tube 3A and the
third bent tube 3C located closest to the burners 5 each comprises a thin-
walled
cast body having a wall thickness ranging from 3 to 8 mm.
The second bent tube 3B located relatively distant from the burner 5 and
the four straight tubes 2A, 2B, 2C, 2D each comprises a cast body having a
wall
thickness of 5 mm or 10 mm.
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In the above, the languages "distant" and "closest" refer to the amounts of
distance from the burner 5 in the passageway of flame or combustion gas
generated from the burner 5 and moving inside the radiant tube 1.
[0029] In this way, as a thin-walled cast body having a wall thickness ranging
from 3 to 8 mm is employed as the bent tube 3 located closest to the burner 5,
there can be obtained a radiant burner 1 having high heat resistance and
usable
for an extended period of time.
A possible reason for the above is as follows. With a bent tube formed
integrally by casting, in comparison with e.g. a bent tube obtained by welding
end
faces of the right and left tubular bodies along the axial direction of the
tube, each
tubular body being obtained by pressing of a plate material, the former bent
tube
has a more uniform wall thickness and there occurs no local stress
concentration
that would otherwise occur at the welded portions extending along the
longitudinal direction of the bent tube, so that heat crack or the like due to
sharp
temperature rise or sharp temperature drop caused by combustion gas from the
burner will occur less likely.
[0030] Further, as the wall thickness of the cast body is reduced to the range
from 3 to 8 mm, there is realized enhanced density of the metallographic
structure
due to increase in the cooling rate at the time of casting, whereby the heat
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resistance and heat-shock resistance are enhanced.
Moreover, the reduction of wall thickness facilitates deformation in
response to stress application. As a result, the heat stress can be absorbed
more
easily and heat crack due to sharp temperature rise due to the combustion gas
from the burner too will occur less likely.
[0031] Furthermore, the wall thickness reduction of the bent tube 3 located
closest to the burner 3 provides increase in the rate of temperature rise due
to the
combustion gas from the burner as well as decrease in the temperature drop
along
the direction of wall thickness. Therefore, the fuel consumption amount too
can
be reduced in comparison with the conventional configuration.
Also, since the weight reduction of the radiant tube as a whole, the labor
required for its replacement has been reduced as well.
[0032] The four straight tubes 2A, 2B, 2C, 2D constituting the radiant tube 1
are
manufactured with using the centrifugal casting technique.
On the other hand, all of the three bent tubes 3A, 3B, 3C are
manufactured with using the suction casting technique in which a negative
pressure is formed by means of e.g. a vacuum pump inside the cavity after
introduction of molten metal therein. Therefore, even with the realization of
wall thickness reduction, there occurs no shrinkage cavities or shrinkage
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looseness which generally tends to occur at the time of solidification of
molten
metal, so that there are obtained bent tubes having favorable surface
conditions.
Incidentally, for the purpose of further wall thickness reduction for
instance, wall thickness reduction may be implemented with the straight tubes
too with using the suction casting technique.
[0033] Incidentally, the outer diameter of the four laterally oriented
straight
tubes 2A, 2B, 2C, 2D and the three bent tubes 3A, 3B, 3C together constituting
the radiant tube 1 is not limited to 180 mm, but can range from 150 to 210 mm.
If the wall thickness is confined within this range, there can be readily
obtained
the advantageous effect due to the setting of wall thickness to 3 to 8 mm for
the
bent tubes 3A, 3B, 3C.
Example 1
[0034] Table 1 below shows results of analysis via simulation of various
properties respecting heat stress imposed on the third bent tube 3C when the
radiant tube 1 shown in Fig. 1 is actually used.
In this simulation, in simulating its use as the regenerative type
arrangement, combustion gas was fed alternatively from the respective burners
5
for a predetermined period and combustions were effected thereby.
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[00351 As shown in Table 1, with varying in many ways the wall thicknesses of
the respective bent tubes 3 and the respective straight tubes 2, the
relationships
between these thicknesses and the various properties about the heat stress
imposed on the third bent tube 3C after combustion gas was fed alternatively
from
the respective burners 5 for the predetermined period, were obtained.
The numerical values given to the bent tube wall thickness shown in the
table were applied to all of the three bent tubes 3A, 3B, 3C and similarly,
the
numerical values of the wall thickness of straight tube were applied to all of
the
four straight tubes 2A, 2B, 2C, 2D.
[0036] As the material for casting, KHR-48N was employed. KHR-48N is defined
as an austenitric super-heat-resistant alloy having acid resistance up to 1200
C
and good creep rupture strength and contains 27 wt.% of chrome, 47 wt.% of
nickel and 5 wt.% of tungsten.
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[0037]
[Table 1]
No. bent tube straight maximum 0.2% proof evaluation
wall tube wall stress stress of bent
thickness thickness (MPa) tube material
(mm) (mm) (MPa/1000 C)
1 3 5 48.6 135 0
2 5 5 44.9 128 0
3 7 5 47.2 115 0
4 10 5 50.1 87 x
5 10 53.3 120 0
6 6 10 50.3 128 0
7 7 10 51.9 115 0
8 8 10 53.5 102 0
9 10 10 56.1 87 x
13 10 57.4 78 X
5 [0038] From the determination results of 0.2% proof stress (MPa) of the
bent
tube material at 1000 C shown in Table 1 above, the following observations can
be
made.
By setting the wall thickness of the bent tube 3 to 8 mm or less, it is
possible to ensure values greater than 100 MPa. Further, the values of 7 mm or
10 less are better than the values of 8 mm or less and the values of 6 mm
or less are
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even better. And, the smaller the wall thickness, the higher the values tend
to
be.
Further, respecting the determination results of the maximum stress too,
there is the tendency of being able to ensure numeric values of 55 MPa or less
by
setting the wall thickness of the bent tube 3 to 8 mm or less.
[0039] Incidentally, it is understood that the respective tendencies described
above can be seen basically throughout in both of the cases of the wall
thickness of
the straight tube 2 portion being 5 mm and 10 mm and the tendencies are not
much affected by the wall thickness of the straight tube 2.
However, in the case of setting the wall thickness of straight tube to 5 mm,
as far as the determination values of the 0.2% proof stress of the bent tube
are
concerned, radiant tubes whose straight tubes have greater wall thickness than
those of their bent tubes tend to show higher numeric values.
Example 2
[0040] In this Example 2, as materials other than KHR-48N, Alloy 230 and
KHR-35H were employed. And, like Example 1 above, various properties about
the heat stress imposed on the third bent tube 3C when the radiant tube 1
shown
in Fig. 1 is actually used were analyzed via simulation.
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[00411 In this example too, with varying in many ways the wall thicknesses of
the respective bent tubes 3 and the respective straight tubes 2, the
relationships
between these thicknesses and the various properties about the heat stress
imposed on the third bent tube 3C after combustion gas was fed alternatively
from
the respective burners 5 for the predetermined period, were obtained.
[0042] Table 2 shows the results of Alloy 230 (containing 22 wt.% chrome, 57
wt.% nickel, 2 wt.% molybdenium and 14 wt.% tungsten). Table 3 shows the
results of KHR-35H (containing 25 wt.% chrome and 35 wt.% nickel).
[0043] [Table 2]
No. bent tube straight maximum 0.2% proof evaluation
wall tube wall stress stress of bent
thickness thickness (MPa) tube material
(mm) (mm) (MPa/1000 C)
1 5 5 26.3 87 0
2 10 10 32.9 65 X
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[0044] [Table 3]
No. bent tube straight maximum 0.2% proof evaluation
wall tube wall stress stress of bent
thickness thickness (MPa) tube material
(mm) (mm) (MPa/1000 C)
1 5 5 33.9 100 0
2 10 10 42.4 86
[0045] From the determination results of 0.2% proof stress (MPa) of
the bent
tube materials at 1000 C shown in Table 2 and Table 3 above, with the
materials
other than KHR-48N too, higher values were obtained with smaller wall
thicknesses of the bent tube 3.
Further, respecting the determination results of the maximum stress too,
there is observed a similar tendency of being able to obtain smaller values
with
smaller wall thicknesses of the bent tube 3.
[0046] Incidentally, the mark " x " employed in the respective tables above
representing evaluation result indicates that there occurred crack or
deformation
especially around the bent tube to such a level to impair the function of the
radiant tube as a heating means.
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[0047] (About the Analysis Method)
In the analyses of the various properties relating to heat stress imposed on
the third bent tube 3c conducted in Example 1 and Example 2, a software:
"Solid
Works Simulation"produced by Solid Works Corp. was used and as its model type,
there was employed a linear isotropic elasticity model with two burner-heat
introducing side ends (the right ends of the straight tubes 2A, 2D in Fig. 1)
being
completely restricted to the wall face of the furnace.
[0048] Referring to the size conditions of the radiant tube 1 as the target of
analysis, there were set the width (the length from the base end of the
straight
tube 2A, 2D restricted to the wall face to the curved leading end of the bent
tube
3A, 3C) : 2276 mm x height (the length from the upper face of the uppermost
straight tube 2A to the lower face of the lowermost tube 2D): 1087 mm; and the
outer diameter of the tube was set as 187 mm for all of the straight tubes 2A,
2B,
2C, 2D and the three bent tubes 3A, 3B, 3C.
The various properties of the respective steel materials employed in the
analyses are shown in Table 4 below.
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[00491 [Table 41
steel type KHR-48N Alloy 230 KHR-35H
failure criterion max von Mises max von max von Mises
stress Mises stress stress
elastic modulus 105,000 MPa 72,200 MPa 93,000 MPa
Poisson's ratio 0.3 0.3 0.3
mass density 8200 kg/m3 8970 kg/m3
8050 kg/m3
coefficient of 1.6e-005/ C 1.61e-005/t
1.8e-005fC
thermal
expansion
[0050] Incidentally, the mutually welded portions of the bent tube and the
straight tube (the area extending for 10 to 30 mm from respective end faces in
abutment at the time of welding) are portions where shortage of strength tends
to
occur more easily during use, due to structural strength shortage on account
of
being located near the tube end face and embrittlement of material due to heat
applied thereto during the welding operation. Therefore, for these welded
portions, in order to ensure sufficient resistance against the severe heat
condition
from combustion gas, these portions are formed even thinner, specifically from
1
to 2 mm thinner than the remaining portions.
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[0051] Further, when the radiant tube 1 is put to an actual use, as a means
for
receiving the mechanical load, in many cases, adjacent bent tubes or a portion
of a
bent tube and a portion of a straight tube will be supported to each other via
an
interconnecting piece provided separately. In such case, in the bent tube and
the
straight tube, supported portions thereof to be welded to the interconnecting
piece
are formed locally thick (e.g. about 10 mm). As specific examples of the
supported portions, they are the portions in the base ends of the bent tubes
3A, 3C
shown in Fig. 1 which portions are in vertical opposition to each other, the
lower
face of the base end portion on the lower side of the bent tube 3B, the upper
face
portion of the nearest straight tube 2D, etc.
[0052] It is understood that the values given to the wall thicknesses of the
bent
tubes and the straight tubes defined in the appended claims and recited in the
detailed disclosure are to be applied to the general portions thereof other
than
these welded portions and the supported portions.
interconnecting a plurality of tubular portions for such purposes as adjusting
the
extending direction of the pipe, branching from a single tube into a plurality
of
tubes or converging a plurality of pipes into a single pipe and has a bent
curved
portion or a bent portion to such ends. Thus, it is understood that the bent
tube
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as used in the present invention is not limited to the U-shaped pipe
illustrated in
Fig. 1, but is inclusive also of joint pipes having any desired shapes.
[0054] [Other Embodiments]
<1> All of the bent tubes 3A, 3B, 3C, including the second bent tube 3B
relatively distant from the burner 5, can be formed as thin-walled cast bodies
having a wall thickness ranging from 3 to 8 mm.
[0055] <2> When the invention is used not as the regenerative type burner 5,
but as a non-regenerative type in which combustion gas is fed invariably from
the
burner 5 connected to the uppermost straight tube 2, only the first bent tube
3A
located closest to this constantly used burner 5 may be formed as a thin-
walled
cast body having a wall thickness ranging from 3 to 8 mm. Alternatively,
however, all of the bent tubes 3A, 3B, 3C can be formed as thin-walled cast
bodies
having a wall thickness ranging from 3 to 8 mm.
[0056] <3> The shape of the radiant tube 1 is not limited to the W-shape
described above, but can be a trident shape.
[0057] <4> The numbers of the bent tubes and the straight tubes together
constituting the radiant tube 1 are not limited to those exemplified above. As
long as there is provided at least one bent tube as a part of its
configuration, the
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radiant tube can be configured as e.g. U-shaped radiant tube including a pair
of
straight tubes and only one bent tube interconnecting the pair of straight
tubes.
Industrial Applicability
[0058] The present invention may be used as a technique relating to a radiant
tube formed of a heat resistant metal and including at least one bent tube for
interconnecting a pair of straight tubes, and a combustion gas from a burner
is fed
through one of the pair of straight tubes.
Description of Reference Numerals
[0059] 2 straight tubes (2A, 28, 2C, 2D)
3 bent tubes (3A, 3C)
5 burners
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