CA2831302A1

CA2831302A1 - Radiant tube

Info

Publication number: CA2831302A1
Application number: CA2831302A
Authority: CA
Inventors: Makoto Hineno; Nobuyuki Sakamoto; Hiroaki Okano; Shigeki Nakamura
Original assignee: Kubota Corp
Current assignee: Kubota Corp
Priority date: 2011-03-31
Filing date: 2012-03-28
Publication date: 2012-10-04
Also published as: CN103429958A; JPWO2012133539A1; US20140053826A1; WO2012133539A1; KR20140045350A

Abstract

A radiant tube made of refractory metal is provided with at least one bent tube (3A (3C)) which connect straight tubes (2A, 2B (2C, 2D)). Combustion air from a burner (5) is fed through one of the straight tubes (2A, 2B (2C, 2D)), and the radiant tube is characterized by using a cast product with an external diameter of 150 - 210 mm and a thickness of 3 - 8 mm at least as the bent tube (3A (3C)) closest to the burner (5).

Description

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.

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 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 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 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.

[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.

F

[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 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.

[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 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-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.

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 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 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 for a predetermined period and combustions were effected thereby.

[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.

[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 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.

[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

[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.

[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.

[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 to 2 mm thinner than the remaining portions.

[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 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 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 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

Claims

1. 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.

2. The radiant tube according to claim 1, wherein the bent tube has a smaller wall thickness at its portion near the connection to the straight tube than the remaining portion thereof.

3. The radiant tube according to claim 1 or 2, wherein 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.

4. The radiant tube according to any one of claims 1-3, wherein the straight tube has a wall thickness of 7 mm or less.

5. The radiant tube according to any one of claims 1-4, wherein the straight tube has a smaller wall thickness at its portion near the connection to the bent tube than the remaining portion thereof.

6. The radiant tube according to any one of claims 1-5, wherein the straight tube comprises a cast body having a greater wall thickness than that of the bent tube.