Heat transfer tubes, heat exchangers, and flue gas treatment equipment

Abstract

The present invention aims to improve the durability of heat transfer tubes in heat exchangers and flue gas treatment devices. [Solution] The device comprises a heat transfer pipe body arranged in an exhaust gas passage, a connecting pipe arranged in the exhaust gas passage and formed of a material different from the heat transfer pipe body, one longitudinal end of which is connected to the longitudinal end of the heat transfer pipe body by welding, a first covering part made of metal that covers the outside of the connection part between the heat transfer pipe body and the connecting pipe, and a second covering part that covers the outside of the first covering part.

Description

[Technical Field]

[0001] The present disclosure relates to a heat transfer tube, a heat exchanger, and an exhaust gas treatment device. [Background technology]

[0002] Flue gas treatment equipment installed in thermal power plants and the like is composed of a heat recovery unit, an electrostatic precipitator, a desulfurization unit, a reheating unit, and other components. The soot and dust contained in the flue gas discharged from the boiler is removed by the electrostatic precipitator, and the sulfur dioxide gas contained in the flue gas is removed by the desulfurization unit. The heat recovery unit recovers heat from the flue gas. The reheating unit reheats the desulfurized flue gas using the heat recovered by the heat recovery unit, thereby suppressing the emission of white smoke.

[0003] The heat recovery device and the reheating device have a plurality of heat transfer tubes arranged in the exhaust gas passage. For example, the reheating device heats the exhaust gas by flowing a high-temperature heat medium through the plurality of heat transfer tubes and exchanging heat between the heat medium and the exhaust gas flowing through the exhaust gas passage. For example, a conventional reheating device is described in Patent Document 1 below. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7221437 Summary of the Invention [Problem to be solved by the invention]

[0005] The exhaust gas flowing into the reheating device contains mist containing corrosive impurities. Therefore, the heat transfer tubes of the reheating device are at risk of corrosion due to repeated deposition and evaporation of mist containing corrosive impurities. Therefore, stainless steel could be used as the material for the heat transfer tubes. In this case, the header made of carbon steel and the heat transfer tube made of stainless steel must be connected by welding. However, because the carbon steel that makes up the header and the stainless steel that makes up the heat transfer tube are different materials, corrosion occurs due to the potential difference.

[0006] To solve these problems, conventional heat transfer tubes have welds made of different materials covered from the outside with a coating (resin lining). However, deterioration of the resin lining is expected in the harsh environment where high-temperature exhaust gas flows, and there is a need to improve the durability of the coating.

[0007] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a heat transfer tube, a heat exchanger, and an exhaust gas treatment device that improve the durability of the heat transfer tube. [Means for solving the problem]

[0008] To achieve the above object, the heat transfer tube of the present disclosure comprises a heat transfer tube main body arranged in an exhaust gas passage; a connecting tube arranged in the exhaust gas passage and formed of a material different from the heat transfer tube main body, the connecting tube having one longitudinal end connected to the longitudinal end of the heat transfer tube main body by welding; a first covering portion made of metal that covers the outside of the connection portion between the heat transfer tube main body and the connecting tube; and a second covering portion that covers the outside of the first covering portion.

[0009] The heat exchanger of the present disclosure also includes a duct casing that forms an exhaust gas passage, an inlet header in which an inlet portion of a heat medium is provided, an outlet header in which an outlet portion of the heat medium is provided, and the heat transfer tube that is arranged in the exhaust gas passage and connects the inlet header and the outlet header.

[0010] The flue gas treatment device of the present disclosure also includes a heat recovery device that recovers a portion of the heat of the exhaust gas, a dust collection device that removes soot and dust contained in the exhaust gas after heat recovery, a desulfurization device that removes sulfur oxides contained in the exhaust gas after dust collection, and a reheating device to which the heat exchanger is applied that reheats the exhaust gas after desulfurization. [Effects of the Invention]

[0011] According to the heat transfer tube, heat exchanger, and flue gas treatment device of the present disclosure, the durability of the heat transfer tube can be improved. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of the flue gas treatment device of this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the heat recovery device and the reheating device of this embodiment. [Figure 3] FIG. 3 is a schematic plan view showing the heat exchanger of this embodiment. [Figure 4] FIG. 4 is a schematic side view showing the heat exchanger of this embodiment. [Figure 5] FIG. 5 is a cross-sectional view showing a connection portion between a header and a heat transfer tube. [Figure 6] FIG. 6 is an enlarged cross-sectional view showing the connection portion between the header and the heat transfer tube. [Figure 7] FIG. 7 is a perspective view showing a connection portion of the heat transfer tube to the header. [Figure 8] FIG. 8 is a cross-sectional view showing a connection portion of the heat transfer tubes. DETAILED DESCRIPTION OF THE INVENTION

[0013] Preferred embodiments of the present disclosure will be described in detail below with reference to the drawings. Note that the present disclosure is not limited to these embodiments, and when there are multiple embodiments, the present disclosure also includes configurations that combine the embodiments. Furthermore, the components in the embodiments include those that can be easily imagined by a person skilled in the art, those that are substantially identical, and those that are within the so-called equivalent range.

[0014] [Smoke exhaust treatment device] FIG. 1 is a schematic diagram showing the configuration of the flue gas treatment device of this embodiment.

[0015] As shown in FIG. 1, the flue gas treatment device 100 removes harmful substances such as soot and sulfur oxides (SOx) contained in flue gas (flue gas) G discharged from a boiler 111 in various power plants, factories, etc., as the flue gas G is released from a chimney 112.

[0016] The flue gas treatment device 100 includes a heat recovery device 101, an electrostatic precipitator 102, a blower (induced draft fan) 103, a desulfurization device 104, a reheating device 105, and a blower (desulfurization fan) 106. The flue gas treatment device 100 includes a first treatment system that sends flue gas G discharged from a boiler 111 to a chimney 112 through the heat recovery device 101, the electrostatic precipitator 102, and the blower 103, and a second treatment system that sends flue gas G discharged from the boiler 111 to the chimney 112 through the heat recovery device 101, the electrostatic precipitator 102, the blower 103, the desulfurization device 104, the reheating device 105, and the blower (desulfurization fan) 106.

[0017] In the case of the first treatment system, the blower 103 is driven, causing the exhaust gas G discharged from the boiler 111 to pass through the heat recovery device 101 and the electrostatic precipitator 102 and be sent to the chimney 112. In the case of the second treatment system, the blowers 103 and 106 are driven, causing the exhaust gas G discharged from the boiler 111 to pass through the heat recovery device 101, the electrostatic precipitator 102, the desulfurization device 104, and the reheating device 105 and be sent to the chimney 112. The arrangement of the devices is not limited to the arrangement described above.

[0018] The boiler 111 is provided with two exhaust gas passages 121a and 121b. The exhaust gas passage 121a is provided with a heat recovery device 101a, an electrostatic precipitator 102a, and a blower 103a, while the exhaust gas passage 121b is provided with a heat recovery device 101b, an electrostatic precipitator 102b, and a blower 103b. The two exhaust gas passages 121a and 121b merge into an exhaust gas passage 121c on the downstream side. The exhaust gas passage 121c branches into two exhaust gas passages 121d and 121e. The exhaust gas passage 121d constitutes part of the first treatment system, has an on-off valve 122 provided midway, and is connected to the chimney 112 on the downstream side.

[0019] The exhaust gas passage 121e constitutes a second treatment system, and is provided with a desulfurization device 104 and a reheating device 105. The exhaust gas passage 121e branches into two exhaust gas passages 121f and 121g on the downstream side. The exhaust gas passage 121f is provided with a blower 106a, and the exhaust gas passage 121g is provided with a blower 106b. The two exhaust gas passages 121f and 121g merge into an exhaust gas passage 121h on the downstream side. The exhaust gas passage 121d, which constitutes a part of the first treatment system, and the exhaust gas passage 121h, which constitutes a part of the second treatment system, merge into an exhaust gas passage 121i on the downstream side. The exhaust gas passage 121i is connected to the chimney 112.

[0020] The heat recovery device 101 (101a, 101b) recovers heat from the exhaust gas G (approximately 140°C) discharged from the boiler 111 by exchanging heat between the exhaust gas G and a heat medium (such as water). The exhaust gas G (approximately 90°C) whose heat has been recovered by the heat recovery device 101 is introduced into the electrostatic precipitator 102 (102a, 102b). The electrostatic precipitator 102 removes soot and dust from the exhaust gas G.

[0021] The flue gas G from which soot and dust have been removed by the electrostatic precipitator 102 is introduced into the desulfurization device 104. The desulfurization device 104 absorbs and removes sulfur oxides in the flue gas G using limestone (CaCO3) and produces gypsum (CaSO4.2H2O) as a by-product. The desulfurization device 104 has a mist eliminator 123. The mist eliminator 123 removes mist from the flue gas G after desulfurization.

[0022] The flue gas G (approximately 50°C) desulfurized by the desulfurization device 104 is introduced into the reheating device 105, which is a gas-gas heater. The reheating device 105 reheats the flue gas G using heat recovered by the heat recovery device 101 during the process of circulating the heat medium between the reheating device 105 and the heat recovery device 101. The heat recovery device 101 and the reheating device 105 are connected by a first heat medium circulation line L11 and a second heat medium circulation line L12. The first heat medium circulation line L11 is provided with a circulation pump 131. When the circulation pump 131 is driven, the heat medium from the reheating device 105 is returned to the heat recovery device 101 via the first heat medium circulation line L11. The second heat medium circulation line L12 is provided with a heater 132. The circulation pump 131 supplies the heat medium from the heat recovery device 101 to the reheating device 105 via the second heat medium circulation line L12. During this process, the heater 132 is operated as needed to heat the heat medium.

[0023] The temperature of the exhaust gas G drops as a result of being desulfurized in the desulfurization device 104, and if the temperature remains low, it will be difficult to diffuse and may turn into white smoke. The reheating device 105 reheats the exhaust gas G to raise its temperature (to about 90°C) for the purpose of diffusion and reducing white smoke, and then releases the exhaust gas G into the atmosphere from the chimney 112. Note that the temperature of the exhaust gas G described above is an example and is not limited thereto.

[0024] [Configuration of heat recovery equipment and reheating equipment] FIG. 2 is a schematic diagram showing the heat recovery device and the reheating device of this embodiment.

[0025] 2, the heat recovery unit 101 and the reheating unit 105 are complete countercurrent heat exchangers. However, the heat recovery unit 101 and the reheating unit 105 may be heat exchangers of other types, such as high-temperature preheating countercurrent type, high-temperature preheating parallel type, or medium-temperature preheating type.

[0026] The heat recovery device 101 includes a high-temperature heat recovery section 141, a medium-temperature heat recovery section 142, and a low-temperature heat recovery section 143. The high-temperature heat recovery section 141, the medium-temperature heat recovery section 142, and the low-temperature heat recovery section 143 are arranged in the exhaust gas passage 121a (121b). However, the heat recovery device 101 is not limited to this configuration. The heat recovery device 101 may be configured with one heat recovery section or multiple heat recovery sections.

[0027] The high-temperature heat recovery section 141 has a plurality of first heat transfer pipes 151. The first heat transfer pipes 151 are arranged in the exhaust gas passage 121a (121b) along the flow direction of the exhaust gas G. An end 151a of the first heat transfer pipe 151 located on the upstream side in the flow direction of the exhaust gas G is connected to a first header 152, and an end 151b located on the downstream side in the flow direction of the exhaust gas G is connected to a second header 153.

[0028] The medium-temperature heat recovery section 142 is arranged downstream of the high-temperature heat recovery section 141 in the flow direction of the exhaust gas G. The medium-temperature heat recovery section 142 has a plurality of second heat transfer pipes 154. The second heat transfer pipes 154 are arranged in the exhaust gas passage 121a (121b) along the flow direction of the exhaust gas G. An end 154a of the second heat transfer pipe 154 located on the upstream side in the flow direction of the exhaust gas G is connected to the first header 155, and an end 154b located on the downstream side in the flow direction of the exhaust gas G is connected to the second header 156.

[0029] The low-temperature heat recovery section 143 is arranged downstream of the medium-temperature heat recovery section 142 in the flow direction of the exhaust gas G. The low-temperature heat recovery section 143 has a plurality of third heat transfer pipes 157. The third heat transfer pipes 157 are arranged in the exhaust gas passage 121a (121b) along the flow direction of the exhaust gas G. An end 157a of the third heat transfer pipe 157 located on the upstream side in the flow direction of the exhaust gas G is connected to the first header 158, and an end 157b located on the downstream side in the flow direction of the exhaust gas G is connected to the second header 159.

[0030] The reheating device 105 includes a high-temperature heating section 161, a medium-temperature heating section 162, and a low-temperature heating section 163. The high-temperature heating section 161, the medium-temperature heating section 162, and the low-temperature heating section 163 are arranged in the exhaust gas passage 121e. The reheating device 105 is not limited to this configuration. The reheating device 105 may be configured with one heating section or multiple heating sections.

[0031] The high-temperature heating unit 161 has a plurality of first heat transfer pipes 171. The first heat transfer pipes 171 are arranged in the exhaust gas passage 121e along the flow direction of the exhaust gas G. An end 171a of the first heat transfer pipe 171 located downstream in the flow direction of the exhaust gas G is connected to a first header 172, and an end 171b located upstream in the flow direction of the exhaust gas G is connected to a second header 173.

[0032] The medium-temperature heating section 162 is arranged upstream of the high-temperature heating section 161 in the flow direction of the exhaust gas G. The medium-temperature heating section 162 has a plurality of second heat transfer pipes 174. The second heat transfer pipes 174 are arranged in the exhaust gas passage 121e along the flow direction of the exhaust gas G. An end 174a of the second heat transfer pipe 174 located downstream in the flow direction of the exhaust gas G is connected to the first header 175, and an end 174b located upstream in the flow direction of the exhaust gas G is connected to the second header 176.

[0033] The low-temperature heating section 163 is arranged upstream of the medium-temperature heating section 162 in the flow direction of the exhaust gas G. The low-temperature heating section 163 has a plurality of third heat transfer pipes 177. The third heat transfer pipes 177 are arranged in the exhaust gas passage 121e along the flow direction of the exhaust gas G. An end 177a of the third heat transfer pipe 177 located downstream in the flow direction of the exhaust gas G is connected to the first header 178, and an end 177b located upstream in the flow direction of the exhaust gas G is connected to the second header 179.

[0034] The first heat medium circulation line L11 has an upstream end connected to the second header 179 of the low-temperature heating section 163 in the reheating device 105, and a downstream end connected to the second header 159 of the low-temperature heat recovery section 143 in the heat recovery device 101. In addition, the second heat medium circulation line L12 has an upstream end connected to the first header 152 of the high-temperature heat recovery section 141 in the heat recovery device 101, and a downstream end connected to the first header 172 of the high-temperature heating section 161.

[0035] The second header 153 of the high-temperature heat recovery section 141 and the first header 155 of the medium-temperature heat recovery section 142 are connected via a first connection line L21. The second header 156 of the medium-temperature heat recovery section 142 and the first header 158 of the low-temperature heat recovery section 143 are connected via a second connection line L22. Furthermore, the second header 173 of the high-temperature heating section 161 and the first header 65 of the medium-temperature heating section 162 are connected via a first connection line L23. The second header 156 of the medium-temperature heating section 162 and the first header 178 of the low-temperature heating section 163 are connected via a second connection line L24.

[0036] The first heat medium circulation line L11 is provided with a circulation pump 131 and a drain tank 133, and the second heat medium circulation line L12 is provided with a heater 132. A steam line L13 is provided from a steam supply source (not shown) to the heater 132 and the drain tank 133, and a steam drain line L14 is provided to the drain tank 133. An on-off valve 134 is provided on the steam line L13.

[0037] Therefore, the reheated low-temperature heat medium is supplied to the heat recovery unit 101 from the first heat medium circulation line L11. In the heat recovery unit 101, the heat medium flows through the low-temperature heat recovery unit 143, the medium-temperature heat recovery unit 142, and the high-temperature heat recovery unit 141 in this order, and recovers heat from the exhaust gas G as it flows through the heat transfer tubes 151, 154, and 157. The heat medium from which the heat of the exhaust gas G has been recovered is discharged to the second heat medium circulation line L12. The high-temperature heat medium from which the heat has been recovered is supplied to the reheating unit 105 from the second heat medium circulation line L12. In the reheating unit 105, the heat medium flows through the high-temperature heating unit 161, the medium-temperature heating unit 162, and the low-temperature heating unit 163 in this order, and reheats the exhaust gas G as it flows through the heat transfer tubes 171, 174, and 177. The heat medium that has reheated the exhaust gas G is discharged to the first heat medium circulation line L11.

[0038] [Heat exchanger configuration] In this embodiment, the heat exchanger is described as being applied to the reheating device 105 (high-temperature heating section 161, medium-temperature heating section 162, low-temperature heating section 163) in the above-mentioned flue gas treatment device 100. However, the heat exchanger is not limited to the reheating device 105 in the flue gas treatment device 100. The heat exchanger may also be applied to the heat recovery device 101 in the flue gas treatment device 100 or a heat exchanger other than that in the flue gas treatment device 100.

[0039] Fig. 3 is a schematic plan view showing the heat exchanger of this embodiment, and Fig. 4 is a schematic side view showing the heat exchanger of this embodiment. The heat exchanger of this embodiment is of a horizontal type in which exhaust gas G flows horizontally and heat transfer tubes are arranged along a horizontal direction that intersects with the flow direction of the exhaust gas G. However, the heat exchanger is not limited to this type, and may be, for example, of a vertical type in which exhaust gas G flows horizontally and heat transfer tubes are arranged along a vertical direction that intersects with the flow direction of the exhaust gas G. Alternatively, the heat exchanger may be configured so that exhaust gas G flows along the vertical direction.

[0040] 3 and 4, the heat exchanger 11 (reheating device 105) includes one bundle 12. However, the heat exchanger 11 may include multiple bundles 12. That is, the bundle 12 corresponds to, for example, the high-temperature heating section 161, the medium-temperature heating section 162, and the low-temperature heating section 163 (all of which are shown in FIG. 1) that constitute the reheating device 105.

[0041] The bundle 12 is disposed inside a duct casing 13. The duct casing 13 has a rectangular cylindrical shape that is aligned in the horizontal direction, and an exhaust gas passage (gas path) 14 is defined inside the duct casing 13. The exhaust gas passage 14 is provided in the horizontal direction, and the exhaust gas G flows along the horizontal direction. The bundle 12 has a casing 21, an inlet header 22, an outlet header 23, and a plurality of heat transfer tubes 24. The bundle 12 is disposed inside the duct casing 13 and is supported by the inner wall portion of the duct casing 13.

[0042] The inlet header 22 and the outlet header 23 are fixed to and supported by the side wall portion 13a of the duct casing 13. The inlet header 22 and the outlet header 23 have a cylindrical tubular shape and are closed at each longitudinal end. The inlet header 22 and the outlet header 23 are arranged vertically and spaced apart horizontally. The inlet header 22 and the outlet header 23 are arranged in openings formed in the side wall portion 13a of the duct casing 13 and are fixed to and supported. One horizontal side of the inlet header 22 and the outlet header 23 faces the exhaust gas passage 14 defined in the duct casing 13, and the other horizontal side faces the outside of the duct casing 13 (exhaust gas passage 14).

[0043] An inspection hole 13b is provided in the side wall portion 13a of the duct casing 13, located between the inlet header 22 and the outlet header 23. A detachable cover 13c is provided for the inspection hole 13b. By opening the inspection hole 13b with the cover 13c, an operator can inspect the heat transfer tubes 24 and other components inside the duct casing 13.

[0044] The heat transfer tubes 24 are arranged partially horizontally. The heat transfer tubes 24 are arranged horizontally intersecting the longitudinal directions of the inlet header 22 and the outlet header 23. One longitudinal end of each of the heat transfer tubes 24 is connected to the inlet header 22, and the other longitudinal end of each of the heat transfer tubes 24 is connected to the outlet header 23. The heat transfer tubes 24 are located in the exhaust gas passage 14. That is, one end of each of the heat transfer tubes 24 is connected to the side of the inlet header 22 that faces the exhaust gas passage 14, and the interior of each of the heat transfer tubes 24 communicates with the interior of the inlet header 22. The other end of each of the heat transfer tubes 24 is connected to the side of the outlet header 23 that faces the exhaust gas passage 14, and the interior of each of the heat transfer tubes 24 communicates with the interior of the outlet header 23.

[0045] A casing 21 is disposed on the outside of the bundle 12. The casing 21 is disposed so as to surround the plurality of heat transfer tubes 24, and its longitudinal ends are connected to the inlet header 22 and the outlet header 23, and is supported on the inner wall of the duct casing 13. The casing 21 has openings on the upstream and downstream sides in the flow direction (horizontal direction) of the exhaust gas G.

[0046] A flange joint 32 having a connecting flange 31 on its side is fixed to the inlet header 22. The flange joint 32 is cylindrical and is connected to the side of the inlet header 22 that does not face the exhaust gas passage 14, with its interior communicating with the interior of the inlet header 22. The flange joint 32 is arranged at one end of the inlet header 22 in the longitudinal direction. A flange joint 34 having a connecting flange 33 on its side is fixed to the outlet header 23. The flange joint 34 is cylindrical and is connected to the side of the outlet header 23 that does not face the exhaust gas passage 14, with its interior communicating with the interior of the outlet header 23. The flange joint 34 is arranged at the other end of the outlet header 23 in the longitudinal direction.

[0047] The heat transfer tubes 24 have a plurality of straight portions 24a, a plurality of first curved portions 24b, and a plurality of second curved portions 24c. The plurality of straight portions 24a are formed by a base tube with spiral fins fixed around the periphery thereof. The plurality of first curved portions 24b and the plurality of second curved portions 24c are formed only by base tubes and are not provided with fins. Adjacent straight portions 24a are connected at one end by a first curved portion 24b, and adjacent straight portions 24a are connected at the other end by a second curved portion 24c. One end of the straight portion 24a of the heat transfer tube 24 located downstream in the flow direction of the exhaust gas G is connected to the inlet header 22, and one end of the straight portion 24a located upstream in the flow direction of the exhaust gas G is connected to the outlet header 23.

[0048] In one heat transfer tube 24, the straight portion 24a, the first curved portion 24b, and the second curved portion 24c are arranged at intervals in the horizontal direction, which is the flow direction of the exhaust gas G. A plurality of heat transfer tubes 24 form a heat transfer tube group by being arranged at intervals in the vertical direction perpendicular to the flow direction of the exhaust gas G. That is, one ends of the plurality of heat transfer tubes 24 are connected to the inlet header 22 at intervals in the longitudinal direction (vertical direction), and the other ends are connected to the outlet header 23 at intervals in the longitudinal direction (vertical direction).

[0049] The heat medium supply line L31 is made of piping and has a flange joint at its end. The flange joint of the piping of the heat medium supply line L31 is connected to a flange joint 32 of the inlet header 22 of the bundle 12. The heat medium discharge line L32 is made of piping and has a flange joint at its end. The flange joint of the piping of the heat medium discharge line L32 is connected to a flange joint 34 of the outlet header 23 of the bundle 12.

[0050] A plurality of support plates 41 are arranged inside the casing 21. The plurality of support plates 41 are arranged at intervals in the longitudinal direction (horizontal direction) of the casing 21 and fixed to the casing 21. A plurality of support holes are formed in the support plate 41, and the heat transfer tubes 24 are supported by inserting their straight portions 24a into the support holes. Furthermore, a resonance prevention plate 42 is arranged inside the casing 21. The resonance prevention plate 42 is arranged along the horizontal direction, is located between the plurality of heat transfer tubes 24, and is fixed around its periphery to the casing 21. The resonance prevention plate 42 divides the interior of the casing 21, in which the plurality of heat transfer tubes 24 are arranged, into two space sections. The resonance prevention plate 42 suppresses resonance of sound generated by the vibration of the plurality of heat transfer tubes 24.

[0051] [Heat exchanger operation] The heat exchanger 11 exchanges heat between the exhaust gas G and the heat medium, thereby heating the exhaust gas G with the heat of the heat medium.

[0052] That is, the heat medium is supplied to the bundle 12 from the heat medium supply line L31, and flows from the inlet header 22 to the multiple heat transfer tubes 24. When the heat medium flows through the multiple heat transfer tubes 24, the bundle 12 exchanges heat between the heat medium and the exhaust gas G flowing through the exhaust gas passage 14. That is, the heat medium flowing through the heat transfer tubes 24 heats the exhaust gas G flowing through the exhaust gas passage 14 outside the heat transfer tubes 24. The heat medium that has exchanged heat with the exhaust gas G flows from the multiple heat transfer tubes 24 to the outlet header 23 and is discharged to the heat medium discharge line L32.

[0053] As shown in FIGS. 3 and 4 , the heat transfer tube 24 has a straight portion 24a, a first curved portion 24b, and a second curved portion 24c, and is disposed in the exhaust gas passage 14. The exhaust gas G flowing through the exhaust gas passage 14 contains mist containing corrosive impurities. For this reason, the heat transfer tube 24 is formed of stainless steel. On the other hand, the inlet header 22, the outlet header 23, and the connecting pipe (stub 61), to which the respective ends of the heat transfer tube 24 are connected by welding, are formed of carbon steel. In this case, the carbon steel used for the inlet header 22 and the outlet header 23 and the stainless steel used for the heat transfer tube 24 are different materials. Therefore, if the heat transfer tube 24 is directly connected to the inlet header 22, the outlet header 23, and the connecting pipe (stub 61), as in the conventional case, corrosion is likely to occur due to the potential difference.

[0054] Therefore, the heat transfer tubes 24 of this embodiment are made of stainless steel, and are provided with coating portions 50 at the welded portions (connections) with the inlet header 22 and the outlet header 23, which are made of carbon steel. By covering the connecting pipes (stubs 61) and welded portions of the heat transfer tubes 24 with the inlet header 22 and the outlet header 23 from the outside with coating portions 50, contact between the welded portions and moisture is suppressed, and corrosion due to potential differences in the welded portions can be suppressed.

[0055] [Heat transfer tube] FIG. 5 is a cross-sectional view showing a connection portion between a header and a heat transfer tube.

[0056] 5, the covering portion 50 covering the welded portion (connection portion) of the heat transfer tube 24 has a first covering portion 51 and a second covering portion 52. That is, the heat transfer tube 24 of this embodiment includes a straight portion (heat transfer tube main body) 24a, a stub (connecting pipe) 61, the first covering portion 51, and the second covering portion 52. The first covering portion 51 has a protective function or a sacrificial function.

[0057] The straight portion 24a is made of stainless steel. The straight portion 24a is configured by fixing spiral-shaped fins 24a2 to the outer periphery of a mother tube 24a1. However, an end 24a3 of the straight portion 24a, which is connected to the inlet header 22 (outlet header 23), is not provided with fins 24a2 and has the same shape as the mother tube 24a1. The stub 61 is made of carbon steel, a different material from the straight portion 24a. The stub 61 has a cylindrical shape with the same outer and inner diameters as the end 24a3 of the straight portion 24a. One longitudinal end of the stub 61 is connected to the end 24a3 of the straight portion 24a by welding, and the other longitudinal end is connected to the inlet header 22.

[0058] The first covering portion 51 is a metal pipe. The first covering portion 51 has a cylindrical shape and covers the outside of the connection portion between the straight portion 24a and the stub 61. The second covering portion 52 covers the outside of the first covering portion 51.

[0059] The straight portion 24a of the heat transfer tube 24 is made of stainless steel, which is a material with a higher potential than the stub 61, which is made of carbon steel. The first covering portion 51 is made of carbon steel, which is the same material as the stub 61. The second covering portion 52 is made of a lining.

[0060] The following provides a detailed description of the end 24a3 of the straight portion 24a of the heat transfer tube 24 and the covering portion 50. Fig. 6 is an enlarged cross-sectional view showing the connection portion between the header and the heat transfer tube, and Fig. 7 is a perspective view showing the connection portion of the heat transfer tube to the header.

[0061] As shown in FIGS. 6 and 7 , the heat transfer tube 24 is connected by a weld 71, with one end of a stub 61 welded to the end 24a3 of the straight portion 24a. The weld 71 is ring-shaped. In this case, the end 24a3 of the straight portion 24a is made of stainless steel, and the stub 61 is made of carbon steel, resulting in a dissimilar material joint. The end 24a3 of the straight portion 24a of the heat transfer tube 24 is connected to the inlet header 22 via the stub 61. In this case, the other end of the stub 61 is connected to the inlet header 22 by pipe expansion. The inlet header 22 is disposed vertically and has a connecting hole 22a that penetrates horizontally at a predetermined position. The connecting hole 22a is cylindrical, with a groove 22b formed on its inner circumferential surface. The groove 22b has a ring shape that continues circumferentially around the connecting hole 22a. The groove 22b is recessed relative to the inner circumferential surface of the connecting hole 22a.

[0062] The outer diameter of the stub 61 is slightly smaller than the inner diameter of the connecting hole 22a of the inlet header 22. The other end of the stub 61 is inserted into the connecting hole 22a. At this time, it is preferable that the end face of the other end of the stub 61 is continuous with the inner peripheral surface of the inlet header 22 without any step. At this time, the groove 22b of the connecting hole 22a of the inlet header 22 is covered by the outer peripheral surface of the stub 61. In this state, first, a tube expansion tool (not shown) is inserted into the stub 61 through an operation hole provided on the opposite side of the inlet header 22 from the connecting hole 22a. Next, the pressure part of the tube expansion tool is positioned so as to face the groove 42b of the connecting hole 22a via the stub 61. Then, the pressure part is moved radially outward of the stub 61 to expand the diameter of the other end of the stub 61.

[0063] As a result, an expanded diameter portion 61a is formed at one end of the stub 61, and the expanded diameter portion 61a engages with the groove 22b of the connecting hole 22a in the inlet header 22. The stub 61 is connected to the inlet header 22 by having the expanded diameter portion 61a formed at the other end engage with the groove 22b of the connecting hole 22a. Thereafter, the other end of the stub 61 is seal-welded to the inner circumferential surface of the inlet header 22, and a ring-shaped seal weld portion 72 is formed.

[0064] The first covering portion 51 has a cylindrical shape and is made of carbon steel. The inner diameter of the first covering portion 51 is slightly larger than the outer diameter of the stub 61. However, the inner diameter of the first covering portion 51 is preferably such that, when the first covering portion 51 is attached to the outside of the stub 61, the inner circumferential surface of the first covering portion 51 fits tightly against the outer circumferential surface of the stub 61 without any gaps. The thickness of the first covering portion 51 is preferably greater than the thickness of the stub 61. Furthermore, the axial length of the first covering portion 51 is specified. When the first covering portion 51 is attached to the outside of the stub 61 and the end 24a3 of the straight portion 24a of the heat transfer tube 24, the first covering portion 51 preferably has a length such that it covers the welded portion 71 between the straight portion 24a and the stub 61 from the outside, one end of the first covering portion 51 contacts the end 24a3 of the straight portion 24a, and the other end of the first covering portion 51 contacts the inlet header 22.

[0065] The inner peripheral surface of the first covering portion 51 is in close contact with the outer peripheral surface of the end 24a3 of the straight portion 24a of the heat transfer tube 24 without any gaps. One longitudinal end of the first covering portion 51 is welded all around to the outer peripheral surface of the end 24a3 of the straight portion 24a, and the two are connected by a weld 73. The weld 73 is ring-shaped. The other longitudinal end of the first covering portion 51 is welded all around to the outer peripheral surface of the inlet header 22, and the two are connected by a weld 74. The weld 74 is ring-shaped.

[0066] Before the other end of the stub 61 is connected to the inlet header 22, the first covering portion 51 is positioned outside the stub 61 and the end 24a3 of the straight portion 24a of the heat transfer tube 24. That is, first, one end of the stub 61 is connected to the end 24a3 of the straight portion 24a of the heat transfer tube 24 by welding. Next, the first covering portion 51 is inserted outside the stub 61 from the other end side of the stub 61 and moved axially until it is positioned on the end 24a3 side of the straight portion 24a. In this state, the other end of the stub 61 is inserted into the connecting hole 22a of the inlet header 22 and expanded to connect. Note that when converting the heat transfer tube 24 from carbon steel to stainless steel, for example, the expanded carbon steel is cut and removed while leaving a portion of the expanded portion, and a new stainless steel heat transfer tube 24 is installed. At this time, the first covering part 51 is inserted on the stainless steel heat transfer tube 24 side and moved to the opposite side from the inlet header 22, and the carbon steel remaining in the inlet header 22 is welded to the stainless steel heat transfer tube 24. Thereafter, the first covering part 51 is moved to the inlet header 22 side and fixed.

[0067] When the other end of stub 61 is connected to connecting hole 22a of inlet header 22, first covering portion 51 on the end 24a3 side of straight portion 24a moves toward stub 61. First covering portion 51 covers welded portion 71 from the outside, and is positioned at a position where the other end abuts against inlet header 22. One end of first covering portion 51 is fixed to end 24a3 of straight portion 24a via welded portion 73, and the other end is fixed to the outer peripheral surface of inlet header 22 via welded portion 74. By covering welded portion 71 between end 24a3 of straight portion 24a and one end of stub 61 from the outside, first covering portion 51 can protect welded portion 71 from the surrounding corrosive environment.

[0068] The second coating portion 52 covers the entire first coating portion 51 from the outside. That is, the second coating portion 52 covers the welding portion (connection portion) 73 between one end portion of the first coating portion 51 and the end portion 24a3 of the straight portion 24a in the heat transfer tube 24. At this time, the welding portion 73 is covered without a gap by the same coating portion 75 as the second coating portion 52 or another coating portion. Further, the second coating portion 52 covers the welding portion (connection portion) 74 between the other end portion of the first coating portion 51 and the inlet header 22. The second coating portion 52 is a lining. In particular, in order to protect the outer surface of the welding portion 73 from galvanic corrosion, the outer surface of the welding portion 73 is covered with a predetermined thickness by a material different from the end portion 24a3 of the straight portion 24a, the stub 61, and the first coating portion 51. As a method of providing the lining as the second coating portion 52, for example, there is a coating method. As the material of the lining, for example, corrosion-resistant FRP (fiber reinforced plastic), resin (for example, HF281 of Oji Rubber Kasei Co., Ltd., etc.), rubber sheet (hard rubber, chloroprene, etc.), glass, fluororesin, etc. are applied.

[0069] The second coating portion 52 does not only cover the first coating portion 51 but also covers the welding portions 73 and 74. The coating film thickness of the second coating portion 52 is preferably 400 μm or more. In this case, it is preferable that the thickness of the first coating portion 51 is thicker than the thickness of the second coating portion 52. For example, when the thickness of the first coating portion 51 is t1 and the thickness of the second coating portion 52 is t2, it is preferable that t2 < t1. The second coating portion 52 can protect the welding portion 73 between the end portion 24a3 of the straight portion 24a and one end portion of the first coating portion 51 from the surrounding corrosive environment. Further, the second coating portion 52 can suppress the deterioration of the first coating portion 51 and the welding portion 74.

[0070] For the heat transfer tube 24, the first coating portion 51 covers the outside of the welding portion �1 between the end portion 24a3 of the straight portion 24a and one end portion of the stub 61, and the second coating portion 52 covers the outside of the first coating portion 51. Therefore, the welding portion 71 for heterogenous welding of stainless steel and carbon steel can be covered by the first coating portion 51 and protected from the surrounding corrosive environment. Further, the deterioration (thinning) of the first coating portion 51 can be suppressed by the second coating portion 52.

[0071] However, the second covering portion 52 is, for example, a resin lining, and may deteriorate with long-term use. However, even if the second covering portion 52 deteriorates, the welded portion 71 where the stainless steel and the carbon steel are welded together is covered by the first covering portion 51, which suppresses corrosion of the welded portion 71, improves the durability of the heat transfer tube 24, and extends its lifespan.

[0072] If the weld 71, where stainless steel and carbon steel are welded together, were covered only with the second coating 52, as in the prior art, the second coating 52 would deteriorate over time, potentially exposing the weld 71 to a corrosive environment. Therefore, the deterioration state of the second coating 52 on the heat transfer tube 24 must be identified during periodic inspection of the reheating device 105, and a resin lining must be reapplied as necessary. When inspecting the heat transfer tube 24, workers visually inspect the second coating 52 through the inspection hole 13b (see FIG. 3 ). However, the inspection hole 13b is small, making it difficult to visually inspect the second coating 52 on all of the heat transfer tubes 24, and the inspection process takes a long time. If the periodic inspection takes a long time, the reheating device 105 would be shut down for a long period of time, which would hinder the treatment of the exhaust gas G.

[0073] In this embodiment, the first covering portion 51 covers the welded portion 71 of the dissimilar material joint, and the second covering portion 52 covers the first covering portion 51. Therefore, even if the second covering portion 52 deteriorates, only the first covering portion 51 is exposed, not the welded portion 71 of the dissimilar material joint. Therefore, the welded portion 71 can be protected from the surrounding corrosive environment for a long period of time.

[0074] Furthermore, if the first covering portion 51 also deteriorates due to further long-term use, the first covering portion 51 can be replaced. Because the first covering portion 51 is fixed by the welds 73 and 74, the first covering portion 51 can be removed by removing the welds 73 and 74, and a new first covering portion 51 can be fixed. On the other hand, if the weld 71 is covered only by the second covering portion 52 as in the conventional technology, when the second covering portion 52 deteriorates, the weld 71 corrodes, and the stub 61 needs to be replaced. However, because the stub 61 is connected to the inlet header 22 and the outlet header 23 by a pipe expansion process, removal is difficult and the work takes a long time.

[0075] 3, when the covering portion 50 (first covering portion 51 and second covering portion 52) is disposed at the connection portion between the heat transfer tube 24 and the inlet header 22 and outlet header 23, the opening area of ​​this portion becomes smaller, reducing the flow rate of the exhaust gas G. In other words, the covering portion 50 acts as a resistance, increasing the flow rate of the exhaust gas G flowing through the straight portion 24a of the heat transfer tube 24 where the fins are provided, thereby improving the heat exchange efficiency.

[0076] [Other embodiments] In the above-described embodiment, the heat transfer tube main body is the straight section 24a and the connecting tube is the stub 61, but the present invention is not limited to this configuration. Fig. 8 is a cross-sectional view showing the connecting section of the heat transfer tube.

[0077] As shown in Fig. 8, the straight portion 24a of the heat transfer tube 24 has a first straight portion 24aa and a second straight portion 24ab, and is disposed in the exhaust gas passage 14 (see Fig. 3). The first straight portion 24aa is made of stainless steel. On the other hand, the second straight portion 24ab is made of carbon steel. In this case, the carbon steel used for the second straight portion 24ab and the stainless steel used for the first straight portion 24aa are different materials, and therefore corrosion is likely to occur due to a potential difference.

[0078] Therefore, in the heat transfer tube 24 of this embodiment, a coating portion 80 is provided at the welded portion (connection portion) between the first straight portion 24aa made of stainless steel and the second straight portion 24ab made of carbon steel. By covering the welded portion between the first straight portion 24aa and the second straight portion 24ab from the outside with the coating portion 80, the heat transfer tube 24 is prevented from coming into contact with moisture, and corrosion due to a potential difference at the welded portion can be prevented.

[0079] The covering portion 80 has a first covering portion 81 and a second covering portion 82. That is, the heat transfer tube 24 of this embodiment includes a first straight portion (heat transfer tube main body) 24aa, a second straight portion (connecting tube) 24ab, the first covering portion 81, and the second covering portion 82.

[0080] The first straight portion 24aa is made of stainless steel. The second straight portion 24ab is made of carbon steel. The first straight portion 24aa and the second straight portion 24ab are unfinned pipes at least at their connected ends, and have the same shape. The ends of the first straight portion 24aa and the second straight portion 24ab are welded all around and connected by a welded portion 91. The welded portion 91 has a ring shape. In this case, the first straight portion 24aa is made of stainless steel and the second straight portion 24ab is made of carbon steel, resulting in a dissimilar material joint.

[0081] The first covering portion 81 has a cylindrical shape and is made of carbon steel. The inner peripheral surface of the first covering portion 81 is in close contact with the outer peripheral surface of the end 24a3 of the straight portion 24a of the heat transfer tube 24 without any gaps. One longitudinal end of the first covering portion 81 is welded all around to the outer peripheral surface of the first straight portion 24aa, and they are connected by a weld 92. The other longitudinal end of the first covering portion 81 is welded all around to the outer peripheral surface of the second straight portion 24ab, and they are connected by a weld 92.

[0082] The second covering portion 82 covers a welded portion 92 between one end of the first covering portion 81 and the first straight portion 24aa. The second covering portion 82 also covers a welded portion 92 between the other end of the first covering portion 81 and the second straight portion 24ab. At this time, the welded portion 92 is covered without any gaps by the same covering portion 93 as the second covering portion 82 or by a different covering portion. The second covering portion 82 is a lining, and in particular, in order to protect the outer surface of the welded portion 92 from potential corrosion, the outer surface of the welded portion 92 is covered to a predetermined thickness with a material different from the first straight portion 24aa, the second straight portion 24ab, and the first covering portion 81.

[0083] The second covering portion 82 not only covers the first covering portion 81 but also covers the welded portion 92. The second covering portion 82 can protect the welded portion 92 between the first straight portion 24aa and one end of the first covering portion 81 from the surrounding corrosive environment. In addition, the second covering portion 82 can suppress deterioration of the first covering portion 81 and the welded portion 92.

[0084] In the above-described embodiment, the first covering portion 51, 81 is cylindrical, but the first covering portion 51, 81 may be divided into multiple segments in the circumferential direction, and the multiple segments may be arranged outside the heat transfer tube 24, and then connected by welding to form a cylindrical shape.

[0085] [Effects of this embodiment] The heat transfer tube of the first embodiment includes a straight portion 24a or a first straight portion 24aa as a heat transfer tube main body arranged in an exhaust gas passage, a stub 61 or a second straight portion 24ab as a connecting tube arranged in the exhaust gas passage 14 and formed of a material different from that of the straight portion 24a or the first straight portion 24aa, and one longitudinal end of which is connected by welding to the longitudinal end of the straight portion 24a or the first straight portion 24aa, a first covering portion 51, 81 made of metal that covers the outside of a welded portion (connection portion) 71 between the straight portion 24a and the stub 61 or a welded portion (connection portion) 91 between the first straight portion 24aa and the second straight portion 24ab, and a second covering portion 52, 82 that covers the outside of the first covering portion 51, 81.

[0086] According to the heat transfer tube of the first aspect, the first covering portion 51 covers the outside of the welded portion 71 between the end portion 24a3 of the straight portion 24a and one end portion of the stub 61 in the heat transfer tube 24, so that the welded portion 71, where stainless steel and carbon steel are welded together, is not exposed to a corrosive environment. Furthermore, the second covering portion 52 covers the outside of the first covering portion 51 in the heat transfer tube 24, so that deterioration of the first covering portion 51 can be suppressed. As a result, the durability of the heat transfer tube 24 can be improved.

[0087] The heat transfer tube according to the second aspect is the heat transfer tube according to the first aspect, further comprising: the straight portion 24a and the first straight portion 24aa formed of a material having a higher potential than the stub 61 and the second straight portion 24ab; and the first covering portion 51, 81 formed of the same material as the stub 61 and the second straight portion 24ab. This makes it possible to suppress corrosion of the welded portions 73, 92 between the first covering portion 51, 81 and the stub 61 or the second straight portion 24ab.

[0088] The heat transfer tube according to the third embodiment is the heat transfer tube according to the second embodiment, except that the straight portion 24a and the first straight portion 24aa are made of stainless steel, and the stub 61, the second straight portion 24ab, and the first covering portion 51, 81 are made of carbon steel, thereby suppressing corrosion of the welded portions 73, 92 between the first covering portion 51, 81 and the stub 61 or the second straight portion 24ab.

[0089] A heat transfer tube according to a fourth aspect is the heat transfer tube according to any one of the first to third aspects, and further, the second covering portion 52, 82 is formed by a lining, thereby making it possible to prevent corrosion due to joining of dissimilar materials.

[0090] A heat transfer tube according to a fifth aspect is the heat transfer tube according to any one of the first to fourth aspects, further comprising a first covering portion 51, 81 having a thickness greater than a second covering portion 52, 82. This allows the welded joints 71, 91 of dissimilar materials to be protected for a long period of time.

[0091] A heat transfer tube according to a sixth aspect is the heat transfer tube according to any one of the first to fifth aspects, further comprising: an inner circumferential surface of the first covering portion (51, 81) that tightly fits tightly against the outer circumferential surface of the straight portion (24a) of the heat transfer tube (24); one longitudinal end of the first covering portion (51, 81) is connected to the straight portion (24a) or the first straight portion (24aa) by welding; and a second covering portion (52, 82) that covers a welded portion (73, 92) between the one end of the first covering portion (51, 81) and the straight portion (24a) or the first straight portion (24aa). This allows the second covering portion (52, 82) to protect the welded portion (73, 92) of the first covering portion (51, 81), which is a dissimilar material joint, from the surrounding corrosive environment.

[0092] A heat transfer tube according to a seventh aspect is the heat transfer tube according to the sixth aspect, and further comprises a stub 61 as a connecting tube, the other longitudinal end of which is connected to the inlet header 22 or the outlet header 23, the other longitudinal end of the first covering portion 51 is connected to the inlet header 22 or the outlet header 23 by welding, and the second covering portion 52 covers a welded portion 74 between the other end of the first covering portion 51 and the inlet header 22 or the outlet header 23. In this way, the second covering portion 52 can suppress deterioration of the welded portion 74.

[0093] A heat transfer tube according to an eighth aspect is the heat transfer tube according to the sixth aspect, further comprising: a first covering portion 81 having a second longitudinal end connected by welding to the connecting pipe and the second straight portion 24ab serving as the second heat transfer tube main body; and a second covering portion 82 covering a welded portion 92 between the second straight portion 24ab and the second end of the first covering portion 81. This allows the second covering portion 82 to suppress deterioration of the welded portion 92.

[0094] The heat exchanger according to the ninth aspect includes a duct casing 13 that forms an exhaust gas passage 14, an inlet header 22 in which an inlet portion of the heat medium is provided, an outlet header 23 in which an outlet portion of the heat medium is provided, and heat transfer tubes 24 that are arranged in the exhaust gas passage 14 and connect the inlet header 22 and the outlet header 23. This can improve the durability of the heat transfer tubes 24.

[0095] The flue gas treatment device according to the tenth aspect includes a heat recovery device 101 that recovers a portion of the heat from the flue gas G, an electrostatic precipitator 102 that removes soot and dust contained in the flue gas G after the heat recovery, a desulfurization device 104 that removes sulfur oxides contained in the flue gas G after the dust collection, and a reheating device 105 that employs a heat exchanger 11 that reheats the desulfurized flue gas G. This can improve the durability of the heat transfer tubes 24. [Explanation of symbols]

[0096] 11 Heat exchanger 12 bundles 13 Duct casing 14 Exhaust gas passage 21 Casing 22 Inlet Header 23 Exit Header 24 Heat transfer tube 24a Straight section (heat transfer tube body) 24a1 bare pipe 24a2 Fins 24a3 end 24aa First straight section (heat transfer tube body) 24ab 2nd straight section (connecting pipe) 31,33 Connecting flange 32,34 Flange joint 41 Support plate 42 Resonance prevention plate 50,80 Covering part 51,81 First coating part 52,82 Second coating section 61 Stub (connecting pipe) 71, 73, 74, 91, 92 Welded parts 72 Seal Weld 100 Smoke treatment equipment 101 Heat recovery device 102 Electrostatic Precipitator 103 Blower 104 Desulfurization equipment 105 Reheating device 106 Blower 111 Boiler 112 Chimney 121a, 121b, 121c, 121d, 121e, 121f, 121g, 121h, 121i Exhaust gas passage 122 On-off valve 123 Mist Eliminator 131 Circulation Pump 132 Heater 133 Drain tank 134 On-off valve L11 First heat medium circulation line L12 Second heat medium circulation line L13 steam line L14 Steam drain line L21, L23 First connection line L22, L24 Second connection line L31 Heat medium supply line L32 Heat transfer medium discharge line G. Exhaust gas

Claims

1. a heat transfer tube body disposed in the exhaust gas passage; a connecting pipe disposed in the exhaust gas passage, the connecting pipe being made of a material different from that of the heat transfer pipe body, and having one longitudinal end connected to the longitudinal end of the heat transfer pipe body by welding; a first covering portion made of metal that covers the outside of a connection portion between the heat transfer tube main body and the connecting tube; a second covering portion that covers the outside of the first covering portion; Equipped with One longitudinal end of the first covering portion is connected to the heat transfer tube main body by welding, and the second covering portion covers a connection portion between the one end of the first covering portion and the heat transfer tube main body. Heat transfer tube.

2. the heat transfer tube main body is formed of a material having a higher potential than the connecting tube, and the first covering portion is formed of the same material as the connecting tube. The heat transfer tube according to claim 1 .

3. the heat transfer tube main body is made of stainless steel, and the connecting tube and the first covering portion are made of carbon steel. The heat transfer tube according to claim 2.

4. the second covering portion is formed by a lining. The heat transfer tube according to claim 1 .

5. The thickness of the first covering portion is greater than the thickness of the second covering portion. The heat transfer tube according to claim 1 .

6. an inner circumferential surface of the first covering portion closely contacts an outer circumferential surface of the heat transfer tube main body without any gap; The heat transfer tube according to claim 1 .

7. the connecting pipe has the other longitudinal end connected to a header, the first covering portion has the other longitudinal end connected to the header by welding, and the second covering portion covers a connection portion between the other end of the first covering portion and the header. The heat transfer tube according to claim 6.

8. the connecting pipe is a second heat transfer pipe body, the other end of the first covering portion in the longitudinal direction is connected to the second heat transfer pipe body by welding, and the second covering portion covers a connection portion between the other end of the first covering portion and the second heat transfer pipe body. The heat transfer tube according to claim 6.

9. a duct casing that forms an exhaust gas passage; an inlet header provided with an inlet portion for the heat medium; an outlet header provided with an outlet portion for the heat medium; the heat transfer tube according to claim 1 being disposed in the exhaust gas passage and connecting the inlet header and the outlet header; A heat exchanger comprising:

10. a heat recovery device that recovers a portion of the heat of the exhaust gas; a dust collector that removes soot and dust contained in the exhaust gas after heat recovery; a desulfurization device for removing sulfur oxides contained in the exhaust gas after dust collection; A reheating device to which the heat exchanger according to claim 9 is applied, which reheats the exhaust gas after desulfurization; A smoke treatment device comprising:

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