Recuperator

Abstract

To provide a recuperator having an improved heat exchange function between combustion exhaust gas and combustion air.SOLUTION: This recuperator 3 exchanges heat between combustion exhaust gas flowing in a radiant tube 1 and combustion air used for burning a burner used for the radiant tube 1, and a plurality of groove parts 7 extending in a zigzag shape in the longitudinal direction of the recuperator 3 are formed on an outer surface of the recuperator 3.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a recuperator that performs heat exchange between combustion exhaust gas flowing in a radiant tube and combustion air used to burn a burner used for the radiant tube in a radiant tube type heating device.

Background Art

[0002] As a recuperator for a radiant tube type heating device, conventionally, as shown in Patent Document 1, a configuration in which a double tube is provided on the exhaust side of the radiant tube has been disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the configuration shown above, the location for preheating the combustion air using the heat of the exhaust is limited to the outer peripheral surface of the outer tube of the double tube. Therefore, in order to increase the heat transfer area, as shown in Patent Document 2, a configuration in which fins having a spiral shape, a linear shape, a wave shape, etc. are provided on the outer peripheral surface of the outer tube has been disclosed. However, the main location for conducting the heat received by the fins to the inside of the outer tube is the line contact portion where the fins are joined to the outer tube.

[0005] In order to increase the contact area, a configuration has been disclosed, as shown in Patent Documents 3 to 5, in which a helical channel is formed on the outer surface of a double-walled tube, and heat is transferred from both the front and back surfaces. However, even if such a channel is formed on the outer surface of the outer tube, there is a problem in that the fluid in the channel flows too smoothly, and sufficient heat exchange does not occur.

[0006] Therefore, the present invention aims to provide a recuperator that further improves the heat exchange function between the combustion exhaust gas flowing inside the radiant tube and the combustion air used to burn the burner used in the radiant tube. [Means for solving the problem]

[0007] The present invention relates to a recuperator that performs heat exchange between combustion exhaust gas flowing through a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves are formed on the outer surface of the recuperator, extending in a zigzag shape in the longitudinal direction of the recuperator.

[0008] According to the above configuration, since multiple grooves extending in a zigzag shape in the longitudinal direction are formed on the outer surface of the recuperator, the combustion exhaust gas, which is the fluid flowing in the grooves, flows while colliding with the heat transfer surface due to the zigzag shape of the grooves, so that sufficient heat exchange occurs between the combustion exhaust gas and the combustion air flowing inside the recuperator. As a result, the heat exchange function of the recuperator can be improved. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a recuperator that further improves the heat exchange function between combustion exhaust gas and combustion air. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram of a radiant tube heating device equipped with a recuperator according to an embodiment of the present invention. [Figure 2] This is a cross-sectional view AA in Figure 1. [Figure 3] This is a cross-sectional view of BB as seen through the arrow in Figure 2. [Figure 4] This is a side view of the recuperator. [Figure 5] Figure 3 is a cross-sectional view of CC as seen through the arrow. [Figure 6] This is a perspective view of a part of the main body of the recuperator. [Figure 7] This is a schematic diagram of a protruding portion, which is another embodiment of the present invention. [Figure 8] This is a schematic diagram of a protruding portion, which is another embodiment of the present invention. [Modes for carrying out the invention]

[0011] Figure 1 is a schematic diagram of a radiant tube type heating device 10 equipped with a recuperator according to an embodiment of the present invention. As shown in Figure 1, the radiant tube type heating device 10 is equipped with a cylindrical radiant tube 1, which is heated from the inside, and the material to be processed placed in the furnace is heated by radiant heat from the outer surface of the radiant tube 1.

[0012] In the radiant tube heating device 10 of this embodiment, a U-shaped radiant tube 1 is used, and the main body 11 of the U-shaped radiant tube is placed inside the furnace. Both ends of the radiant tube 1 extend outside the furnace through the furnace wall. Note that the radiant tube 1 is not limited to a U shape, and may have other shapes such as a W shape or an I shape.

[0013] At one end of the radiant tube 1, a burner 2 for burning fuel gas such as hydrocarbon gas with combustion air is inserted. The burner 2 burns the fuel gas using the combustion air to generate high-temperature combustion exhaust gas, and heats the radiant tube 1 from the inside by using the combustion exhaust gas flowing through the radiant tube 1. The temperature of this combustion exhaust gas is preferably 800°C or higher and 1400°C or lower, particularly preferably 900°C or higher and 1300°C or lower.

[0014] At the other end of the radiant tube 1, a recuperator 3 is inserted. The recuperator 3 heats the combustion air supplied from the outside by performing heat exchange between the combustion exhaust gas of the burner 2 flowing inside the radiant tube 1 and the combustion air introduced into the recuperator 3 from the combustion air introduction pipe 4. The combustion air heated by the recuperator 3 is supplied to one end of the radiant tube 1 provided with the burner 2 through the combustion air supply pipe 5. The combustion air supplied to one end of the radiant tube 1 is used for the combustion of the burner 2, becomes high-temperature combustion exhaust gas, and flows through the main body 11 of the radiant tube 1 and heads towards the recuperator 3.

[0015] The combustion exhaust gas that has undergone heat exchange with the combustion air in the recuperator 3 is discharged from the exhaust port 6.

[0016] Figure 2 is a cross-sectional view taken along the line A-A of Figure 1, and Figure 3 is a view of the arrow in the cross-section B-B of Figure 2. Further, Figure 4 is a side view of the recuperator 3, and Figure 5 is a view of the arrow in the cross-section C-C of Figure 3. Note that the direction of the exhaust port 6 in Figure 3 is shown to be located opposite left and right to Figure 1 for convenience of explanation. As shown in Figures 2 to 5, the recuperator 3 includes a main body 31 and a tip 32 connected to the main body 31. The main body 31 has a double-tube structure including an outer tube 311 and an inner tube 312. Note that the wall thicknesses of the outer tube 311 and the inner tube 312 are preferably thinner in order to improve the efficiency of heat exchange, but as the structural strength of the main body 31 of the recuperator 3, the wall thickness of the outer tube 311 is preferably 1 mm or more, and the wall thickness of the inner tube 312 is preferably 3 mm or more.

[0017] The tip portion 32 includes a hemispherical portion 33 located at the tip 38 of the recuperator 3 and a cylindrical portion 34 connected to the hemispherical portion 33. The cylindrical portion 34 has a double-pipe structure including an outer pipe 341 and an inner pipe 342.

[0018] The outer pipe 311 of the main body portion 31 is connected to the outer pipe 341 of the cylindrical portion 34, and the inner pipe 312 of the main body portion 31 is configured to be connected to the inner pipe 342 of the cylindrical portion 34. As a method for performing the connection and joining, any method such as welding, adhesion, insertion, screwing, etc. may be used.

[0019] FIG. 6 is a perspective view of a part of the main body portion 31 of the recuperator. As shown in FIGS. 4 and 6, a plurality of groove portions 7 extending in a zigzag shape in the longitudinal direction of the recuperator 3 are formed on the outer surface of the outer pipe 311 of the main body portion 31. Each groove portion 7 extends in a zigzag shape in the longitudinal direction of the recuperator, but none of the groove portions 7 extends around the circumference of the recuperator 3. Specifically, each groove portion 7 extends in a zigzag shape within less than half a circumference in the circumferential direction of the recuperator 3, and the entire groove portion 7 can be visually recognized when viewed from the side surface centered on the center line extending in the axial direction of the groove portion 7.

[0020] The groove portion 7 has a plurality of bent portions 71, and the bent portions 71 have a two-stage bent shape. That is, the tip of the bent portion 71 is chamfered. The two-stage bent shape is more specifically a trapezoidal shape, and between the first-stage bend 71a and the second-stage bend 71b, the groove at that portion extends linearly parallel to the axial direction X of the recuperator 3. Also, the groove portion 7 has an expanded groove width at the tip 72 and is configured to communicate with the grooves of the adjacent groove portions 7. It is preferable that the amplitude of the bent portion 71 is larger than the groove width of the groove portion 7.

[0021] The protrusions 8 that form the side surface of the groove 7 are formed to protrude from the outer surface of the inner tube 312, and the tip 8a of the protrusion 8 is located near the inner surface of the radiant tube 1. An opening 81 is formed inside the protrusion 8 that penetrates the recuperator 3 in the axial direction, and the combustion air passing through the opening 81 and the combustion exhaust gas passing through the groove 7 exchange heat, heating the combustion air with the heat of the combustion exhaust gas. The groove 7, which is the flow path for the combustion exhaust gas, and the opening 81, which is the flow path for the combustion air, are arranged alternately, and each extends in a zigzag shape in the axial direction of the recuperator 3. Also, as shown in Figure 5, the rear end 39 of the recuperator 3 is closed by a flange 9, and the flange 9 has a hole 36a leading to the central flow path 36 and an inlet 81a leading to the opening 81.

[0022] The main body 31 of the recuperator 3 is constructed by joining together multiple divided sections 31a that are separated in the longitudinal direction. The bent section 71 is located at the joint 31b of the divided sections 31a. In other words, the divided sections 31a are joined at the trapezoidal upper base portion of the bent section 71 (the portion that extends parallel to the axial direction of the recuperator 3).

[0023] The divided parts 31a and tip 32 of the main body 31 of the recuperator 3 are each manufactured using a 3D printer with ceramic powder as the material. Examples of ceramics include silicon carbide (SiC), alumina, mullite, silicon nitride, and aluminum nitride, but silicon carbide is particularly preferred when used in high-temperature furnaces, as it has excellent heat resistance, thermal shock resistance, and high thermal conductivity. The recuperator 3 is manufactured by joining the divided parts 31a, which are manufactured using a 3D printer, together, and then joining the tip 32 to the main body 31. Adhesives are used to join the divided parts 31a together and to join the main body 31 and the tip 32 together. Alternatively, the recuperator 3 may be manufactured as a single unit without joining the divided parts 31a together.

[0024] As shown in Figure 3, the inner tube of the recuperator 3 is provided with an eductor 35 to improve the suction of combustion air into the inner tube. The eductor 35 is formed in the inner tube 342 of the cylindrical portion 34 and includes an injector 343 that protrudes inward from the inner surface of the inner tube 342 so that the central flow path 36 narrows in diameter toward the main body portion 31, and a narrowed diameter portion 315 that curves inward along its entire circumferential direction near the cylindrical portion 34 within the inner tube 312 of the main body portion 31.

[0025] According to the above configuration, by providing an injector 343 on the inner surface of the inner tube 342, the flow velocity of the combustion air entering the inner tube 342 is increased, generating negative pressure. As a result, the suction of combustion exhaust gas into the inner tube 342 can be promoted. Furthermore, by providing a reduced diameter section 315 inside the inner tube 312 of the main body section 31, the suction of combustion air into the inner tube 312 is promoted, increasing the flow velocity from the inner tube 312 towards the combustion air supply pipe, and improving the efficiency of heat exchange. In other words, by providing an eductor 35 inside the inner tube 342 of the cylindrical section 34 and the inner tube 312 of the main body section 31, the suction of combustion air into the inner tube 342 is promoted, and the flow velocity of the combustion air flowing through the opening 81 is increased, thereby improving the efficiency of heat exchange (theoretically, the faster the fluid flow velocity, the better the heat conduction).

[0026] (Flow of combustion air and combustion exhaust gas in a radiant tube heating device) Combustion air introduced from the combustion air introduction pipe 4 is guided from the inlet 81a opening in the flange 9 to the opening 81 of the protruding portion 8 inside the recuperator 3. The combustion air flowing through the opening 81 of the recuperator 3 is heated by heat exchange with the combustion exhaust gas generated by the combustion of the burner 2 and flowing through the groove 7 between the outer tube 311 of the recuperator 3 and the radiant tube 1.

[0027] The flow direction of the combustion air flowing through the opening 81 and the flow direction of the combustion exhaust gas flowing through the groove 7 are in opposite directions (counter-flow), and they flow side by side in a zigzag pattern, colliding with the heat transfer surface as they move. As a result, heat exchange between the combustion air and the combustion exhaust gas is promoted (theoretically, counter-flow is better for heat conduction than parallel flow, and heat conduction is better when the fluid collides with the heat transfer plate than when it follows the heat transfer plate).

[0028] In the recuperator 3, the combustion exhaust gas, heated by the combustion air, flows through the groove 7 to the rear end 39 of the recuperator 3, where it hits the flange 9, changes direction, and is discharged from the exhaust port 6.

[0029] The combustion air, heated by the combustion exhaust gas, passes through the opening 81 at the tip 32 of the recuperator 3, through the space 37 between the outer tube 341 and the inner tube 342 of the cylindrical section 34, folds back within the hemispherical section 33, and enters the central flow path 36 inside the inner tube 342 of the cylindrical section 34. The combustion air that has entered the inner tube 342 of the cylindrical section 34 then enters the inner tube 312 of the main body section 31.

[0030] The combustion air flowing through the inner tube 312 of the main body 31 is supplied through the combustion air supply pipe 5 to one end of the radiant tube 1 on which the burner 2 is installed. The burner 2 burns the fuel gas with the combustion air supplied from the combustion air supply pipe 5, generating new combustion exhaust gas.

[0031] The combustion exhaust gas generated by the combustion of fuel air by burner 2 heats radiant tube 1 from the inside, and the radiant heat from the outer surface of radiant tube 1 heats the processing material placed in the furnace where the radiant tube heating device 10 is installed.

[0032] The radiant tube type heating device 10 with the above configuration can provide the following effects.

[0033] (1) Since multiple grooves 7 extending in a zigzag shape in the longitudinal direction are formed on the outer surface of the recuperator 3, the combustion exhaust gas, which is the fluid flowing through the grooves 7, flows while colliding with the heat transfer surface due to the zigzag shape of the grooves 7, so that sufficient heat exchange occurs between the combustion exhaust gas and the combustion air flowing inside the recuperator. As a result, the heat exchange function of the recuperator 3 can be improved.

[0034] (2) Since the bent portion 71 has a two-stage bend shape, the number of collisions with the heat transfer surface increases, and the bending angle at each stage can be reduced. As a result, heat exchange is further promoted, and the flow of combustion exhaust gas in the groove portion 7 can be made smoother.

[0035] (3) The groove portion 7 has a wider width at its tip, which makes it easier for fuel exhaust gas to enter the groove portion 7 from its tip, thereby smoothing the flow of fuel exhaust gas in the groove portion 7.

[0036] (4) The inner tube is formed with an injector having an inner tube projection that protrudes inward from the inner surface of the inner tube in order to improve the suction force of fluid into the inner tube. As a result, the flow velocity of combustion air entering the inner tube can be increased, which in turn promotes the suction of combustion exhaust gas into the inner tube and increases the flow velocity of combustion air flowing through the opening 81, thereby improving the efficiency of heat exchange.

[0037] (5) The recuperator 3 has a double-tube structure consisting of an outer tube and an inner tube, and the depth of the groove formed in the outer tube extends to the vicinity of the inner tube, so that the flow rate of fuel exhaust gas flowing through the groove 7 increases. As a result, heat exchange between the combustion exhaust gas and the combustion air can be performed more effectively.

[0038] (6) The projection 8 that forms the groove 7 is formed to protrude from the outer surface of the inner tube 312, and the tip of the projection 8 is located near the inner surface of the radiant tube 1, so that the amount of combustion exhaust gas that does not flow through the groove 7 can be reduced, and heat exchange between the combustion exhaust gas flowing through the groove 7 and the combustion air flowing through the opening 81 of the projection 8 can be made more effective.

[0039] (7) Since the recuperator 3 is constructed by joining together multiple divided parts in the longitudinal direction, the manufacturability of the recuperator 3 can be improved. Furthermore, by placing a bent section with a two-stage folding shape at the joint of the divided parts, the alignment of the grooves becomes easier (because the divided parts are joined in the portion that extends parallel to the axial direction of the recuperator 3), and the joining of the divided parts can be made easier. In addition, since the length of the parts is shortened by using divided parts, handling becomes easier and the possibility of damage to the parts can be reduced.

[0040] (8) Since the recuperator 3 is manufactured using a 3D printer with ceramic powder as the material, the recuperator 3 having grooves 7 can be easily manufactured. In particular, by using SiC powder, which has high heat resistance, as the material, the recuperator 3 can be used even in high-temperature furnaces.

[0041] In the above embodiment, the inner and outer surfaces of the protrusion 8 are flat, but the shape of the inner and / or outer surfaces may be changed to increase the contact area between the combustion air and the combustion exhaust gas. Figure 7 is a schematic diagram of a protrusion in another embodiment in which the inner surface of the protrusion 8 is corrugated, and Figure 8 is a schematic diagram of a protrusion in yet another embodiment in which fins are provided on the inner and outer surfaces of the protrusion.

[0042] As shown in Figures 7 and 8, the inner surface 8b and / or outer surface 8c of the protrusion 8 may be corrugated in order to increase the contact area between the combustion air and the combustion exhaust gas, and fins 82 may be provided on the inner surface 8b and / or outer surface 8c of the protrusion 8.

[0043] In the above alternative embodiment, the following effects can be achieved.

[0044] Since the inner surface 8b and / or outer surface 8c of the protrusion 8 that forms the groove 7 are corrugated, the contact area between the combustion exhaust gas and the combustion air is increased. As a result, the heat exchange function of the recuperator 3 can be improved.

[0045] Since fins 82 are provided on the inner surface 8b and / or outer surface 8c of the protrusion 8 that forms the groove 7, the contact area between the combustion exhaust gas and the combustion air is increased. As a result, the heat exchange function of the recuperator 3 can be improved.

[0046] Furthermore, unlike the above embodiment, an opening (not shown) may be provided at the tip 32 of the recuperator 3, allowing a portion of the fuel exhaust gas to be introduced into the recuperator 3 from the tip 32.

[0047] In the recuperator 3, when the combustion air is heated to a high temperature (preheated) by the heat of the combustion exhaust gas and then guided from the combustion air supply pipe 5 to the burner 2 to burn the fuel gas, the energy saving effect is improved, but the combustion temperature becomes too high during combustion, increasing the amount of NOx generated during combustion, and combustion exhaust gas containing a large amount of NOx is exhausted from the exhaust port 6 at the other end of the radiant tube 1.

[0048] In contrast, when combustion air and a portion of the combustion exhaust gas are mixed in the recuperator 3, and the combustion exhaust gas is combined with the heated combustion air and introduced to the burner 2 from the combustion air supply pipe 5 to burn the fuel gas, the oxygen concentration of the combustion air decreases, and the combustion rate in the burner 2 slows down. As a result, the combustion temperature during combustion is prevented from becoming too high, and the amount of NOx generated during combustion can be reduced. It is preferable to increase the amount of combustion exhaust gas mixed with the heated combustion air as the temperature of the combustion air heated by the combustion exhaust gas increases.

[0049] The present invention and its embodiments are summarized as follows.

[0050] (1) One embodiment of the present invention is a recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves are formed on the outer surface of the recuperator, extending in a zigzag shape in the longitudinal direction of the recuperator.

[0051] According to the above configuration (1), since multiple grooves extending in a zigzag shape in the longitudinal direction are formed on the outer surface of the recuperator, the combustion exhaust gas, which is the fluid flowing in the grooves, flows while colliding with the heat transfer surface due to the zigzag shape of the grooves, so that sufficient heat exchange occurs between the combustion exhaust gas and the combustion air flowing inside the recuperator. As a result, the heat exchange function of the recuperator can be improved.

[0052] (2) In the above configuration (1), the groove portion has a plurality of bent portions, The aforementioned bent portion has a two-stage folding shape.

[0053] According to the above configuration (2), by making the bent portion a two-stage folding shape, the number of collisions with the heat transfer surface can be increased and the folding angle at each stage can be reduced, and as a result, heat exchange is further promoted, so that the flow of combustion exhaust gas in the groove can be made smoother.

[0054] (3) In configuration (1) or (2) above, the groove is widened at the tip.

[0055] According to the above configuration (3), by making it easier for fuel exhaust gas to enter the groove from the tip of the groove, the flow of fuel exhaust gas in the groove can be made smoother.

[0056] (4) In any one of the above configurations (1) to (3), the inner and / or outer surfaces of the protrusions that form the groove are wavy in shape.

[0057] According to the above configuration (4), the contact area between the combustion exhaust gas and the combustion air is increased, thereby improving the heat exchange function of the recuperator.

[0058] (5) In any one of the above configurations (1) to (3), fins are provided on the inner surface and / or outer surface of the projection that forms the groove.

[0059] According to the above configuration (5), the contact area between the combustion exhaust gas and the combustion air is increased, thereby improving the heat exchange function of the recuperator.

[0060] (6) In any one of the above configurations (1) to (5), the recuperator has a double-tube structure consisting of an outer tube and an inner tube. The inner tube is formed with an injector having an inner tube projection that protrudes inward from the inner surface of the inner tube in order to improve the suction force of fluid into the inner tube.

[0061] According to the above configuration (6), by forming an injector in the inner tube, the flow velocity of combustion air entering the inner tube can be increased, and as a result, the suction of combustion exhaust gas into the inner tube is promoted, and the efficiency of heat exchange can be improved by increasing the flow velocity of combustion air flowing through the opening 81.

[0062] (7) In any one of the above configurations (1) to (6), the recuperator has a double-tube structure consisting of an outer tube and an inner tube. The depth of the groove formed in the outer tube extends to the vicinity of the inner tube.

[0063] According to the above configuration (7), the flow rate of fuel exhaust gas flowing through the groove increases, so that heat exchange between the combustion exhaust gas and the combustion air can be performed more effectively.

[0064] (8) In the configuration (7) above, the projection that forms the groove is formed to protrude from the outer surface of the inner tube, and the tip of the projection is located near the inner surface of the radiant tube.

[0065] According to the above configuration (8), the amount of combustion exhaust gas that does not flow through the groove can be reduced, and heat exchange between the combustion exhaust gas flowing through the groove and the combustion air flowing through the opening of the protrusion can be made more effective.

[0066] (9) In the above configuration (2), the recuperator is constructed by joining together a plurality of divided parts in the longitudinal direction, The bent portion is located at the joint of the divided body.

[0067] According to the above configuration (9), the recuperator is constructed by joining divided parts, thus improving the manufacturability of the recuperator. Furthermore, by placing a bent section having a two-stage folding shape at the joining part of the divided parts, the alignment of the grooves becomes easier, and the joining of the divided parts can be easily performed. In addition, since the length of the parts is shortened by using divided parts, handling becomes easier and the possibility of damage to the parts can also be reduced.

[0068] (10) In any one of the above configurations (1) to (9), the recuperator is manufactured using a 3D printer with ceramic powder as the material.

[0069] According to the above configuration (10), a recuperator having grooves can be easily manufactured.

[0070] (11) In the above configuration (10), the ceramic powder is SiC powder.

[0071] According to the above configuration (11), by using SiC powder, which has particularly high heat resistance, as the ceramic powder, the recuperator can be used even in a high-temperature furnace.

[0072] Various modifications and alterations can be made without departing from the spirit and scope of the invention as described in the claims. [Industrial applicability]

[0073] This invention provides a recuperator with improved heat exchange function between combustion exhaust gas and combustion air, thus having great industrial value. [Explanation of Symbols]

[0074] 1 Radiant Tube 11 Main body 2 burners 3 Recuperators 31 Main body 31a Split body 31b Joint part 311 Outer tube 312 Inner tube 313 Heat exchange member 314 Reduced diameter section 315 Reduced diameter section 32 Tip 33 Hemisphere 34 Cylindrical section 341 Outer tube 342 Inner tube 343 Injector 35 Eductors 36 Central channel 36a Hole 37 Space 38 Tip 39 Rear end 4. Combustion air inlet pipe 5. Combustion air supply pipe 6 Exhaust vents 7 Groove 71 Flexed section 71a Fold 71b Fold 72 Tip 8 Protrusion 8a Tip 8b Inside surface 8c Outside surface 81 Opening 81a Inlet 82 fins 9 Flange 10. Radiant tube heating device

Claims

1. A recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves extending in a zigzag shape in the longitudinal direction of the recuperator are formed on the outer surface of the recuperator. The groove portion has a plurality of bent portions, The aforementioned bent portion has a two-stage folding shape, which is the recuperator.

2. The recuperator according to claim 1, wherein the groove has a wider groove width at its tip.

3. A recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves extending in a zigzag shape in the longitudinal direction of the recuperator are formed on the outer surface of the recuperator. A recuperator in which the inner and / or outer surfaces of the protrusions forming the groove are wavy.

4. A recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves extending in a zigzag shape in the longitudinal direction of the recuperator are formed on the outer surface of the recuperator. A recuperator in which fins are provided on the inner and / or outer surfaces of the protrusions that form the grooves.

5. A recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves extending in a zigzag shape in the longitudinal direction of the recuperator are formed on the outer surface of the recuperator. The aforementioned recuperator has a double-tube structure consisting of an outer tube and an inner tube. A recuperator is provided in which an injector is formed in the inner tube, having an inner tube projection that protrudes inward from the inner surface of the inner tube in order to improve the suction force of fluid into the inner tube.

6. A recuperator that performs heat exchange between combustion exhaust gas flowing inside a radiant tube and combustion air used to burn a burner used in the radiant tube, Multiple grooves extending in a zigzag shape in the longitudinal direction of the recuperator are formed on the outer surface of the recuperator. The aforementioned recuperator has a double-tube structure consisting of an outer tube and an inner tube. A recuperator in which the depth of the groove formed in the outer tube extends to the vicinity of the inner tube.

7. The recuperator according to claim 6, wherein the projection forming the groove is formed to protrude from the outer surface of the inner tube, and the tip of the projection is located near the inner surface of the radiant tube.

8. The aforementioned recuperator is constructed by joining together multiple divided sections in the longitudinal direction. The recuperator according to claim 1, wherein the bent portion is located at the joint portion of the divided body.

9. The recuperator according to any one of claims 1 to 8, wherein the recuperator is manufactured using a 3D printer with ceramic powder as the material.

10. The recuperator according to claim 9, wherein the ceramic powder is SiC powder.

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