JPWO2018181325A1 - Air preheater - Google Patents

Abstract

The air preheater is an air preheater that recovers the heat of the exhaust gas from the boiler and preheats the air, and includes a first tube provided in an exhaust gas passage through which the exhaust gas passes, and an outer peripheral side of the first tube. A second tube surrounding the first tube and extending along the first tube, wherein air flows through the first tube, and air having a higher temperature than air flowing through the first tube flows through the second tube. Flows.

Description

  The present invention relates to an air preheater.

  As a conventional boiler system, a system that heats a heat transfer medium by burning fuel is known (for example, see Patent Document 1). The heated heat transfer medium circulates through a circulation system in the system. Further, the combustion gas generated by the combustion of the fuel is subjected to processing such as heat exchange and then discharged to the outside as exhaust gas.

JP 2001-41415 A

  In recent years, fuels with high moisture content and high sulfur content tend to be used as fuel for boilers. Exhaust gas generated by burning such fuel contains a large amount of moisture. As the moisture in the exhaust gas increases, the acid dew point (sulfuric acid dew point) increases. Here, the boiler system as described above may be provided with an air preheater that recovers heat of exhaust gas and preheats air. If the acid dew point of the exhaust gas is high, low-temperature end corrosion (acid dew point corrosion) may occur in which the tube disposed at the downstream end where the temperature of the exhaust gas becomes low is corroded.

  As a method for suppressing the occurrence of such low-temperature end corrosion, there is a method for keeping the temperature of exhaust gas high. However, increasing the temperature of the exhaust gas means that the sensible heat loss of the exhaust gas increases, and as a result, the boiler efficiency of the boiler decreases.

  As described above, an object of the present invention is to provide an air preheater that can suppress tube corrosion while suppressing a decrease in boiler efficiency.

  An air preheater according to one embodiment of the present invention is an air preheater that recovers heat of exhaust gas from a boiler and preheats air, and a first tube provided in an exhaust gas passage that allows exhaust gas to pass therethrough, A second tube surrounding the first tube from the outer peripheral side and extending along the first tube, wherein air flows through the first tube and flows through the first tube through the second tube. Air with a higher temperature than air flows.

  An air preheater according to one embodiment of the present invention includes a first tube provided in an exhaust gas channel. The first tube can recover the heat of the exhaust gas flowing through the exhaust gas channel by the air flowing through the first tube. Here, the air preheater includes a second tube that surrounds the first tube from the outer peripheral side and extends along the first tube. Thus, a double tube is constituted by the first tube and the second tube. Further, air having a higher temperature than the air flowing through the first tube flows through the second tube which is the outer tube of the double tube. Thereby, even if the exhaust gas becomes low temperature by removing heat, the first tube is surrounded by the second tube through which the high-temperature air flows, so that the first tube is cooled by the low-temperature exhaust gas. Corrosion can be suppressed. As described above, it is possible to suppress the corrosion of the tube while suppressing the decrease in the boiler efficiency.

  In the air preheater according to one aspect, air that has passed at least once through the first tube may flow through the second tube. The air that has passed through the first tube at least once has a higher temperature than the air that is passing through the first tube on the most downstream side because the heat of the exhaust gas is recovered. Therefore, by flowing the air through the second tube, a structure for securing high-temperature air to flow from the location other than the air preheater to the second tube becomes unnecessary.

  In the air preheater according to one aspect, a plurality of first tubes may be provided in the exhaust gas channel, and a plurality of second tubes may be provided for a part of the plurality of first tubes. Among the plurality of first tubes, the second tube is provided only for the exhaust gas having a low passing temperature and a high possibility of low-temperature end corrosion, and the exhaust gas temperature to pass is high and the possibility of low-temperature end corrosion is high. No second tube is provided for low ones. Thereby, the manufacturing cost of the air preheater can be suppressed.

  An air preheater according to an aspect includes an air temperature detection unit that detects a temperature of air flowing through a second tube, and detects a temperature of exhaust gas on a downstream side of an area of the exhaust gas flow path where the first tube is provided. And a control unit that controls the flow rate of air flowing to the second tube based on the detection results of the air temperature detection unit and the exhaust gas temperature detection unit. By performing such control, an appropriate flow rate of air can be caused to flow through the second tube.

  ADVANTAGE OF THE INVENTION According to this invention, the air preheater which can suppress the corrosion of a tube can be provided, suppressing the fall of boiler efficiency.

It is a schematic structure figure of a boiler system provided with an air preheater concerning an embodiment of the present invention. FIG. 2 is a diagram illustrating a schematic configuration of an air preheater illustrated in FIG. 1. It is a figure which shows the schematic structure of the corrosion suppression structure of the air preheater shown in FIG.

  An embodiment of the present invention will be described with reference to the drawings. However, the following embodiment is an exemplification for describing the present invention, and is not intended to limit the present invention to the following contents. In the description, the same reference numerals will be used for the same elements or elements having the same functions, and redundant description will be omitted.

  A configuration of a boiler system 100 including an air preheater 50 according to the present embodiment will be described with reference to FIG. The boiler system 100 is a circulating fluidized bed boiler of an external circulating type (Circuling Fluidized Bed type). The boiler system 100 includes a fluidized bed type furnace 3 having a vertically long cylindrical shape. A fuel supply port 3a for supplying fuel is provided at an intermediate portion of the furnace 3, and a gas outlet 3b for discharging combustion gas is provided at an upper portion. The fuel supplied from the fuel supply device 5 to the furnace 3 is supplied into the furnace 3 through the fuel supply port 3a.

  A cyclone 7 functioning as a solid-gas separation device is connected to the gas outlet 3 b of the furnace 3. The outlet 7a of the cyclone 7 is connected to a subsequent gas processing system via a gas line. A return line 9 called a downcomer extends downward from a bottom outlet of the cyclone 7, and a lower end of the return line 9 is connected to a side surface of an intermediate portion of the furnace 3.

  In the furnace 3, the solid containing fuel supplied from the fuel supply port 3 a flows by the air for combustion and flow introduced from the lower air supply line 3 c, and the fuel flows, for example, about 800 to 900. Burns at ℃. The combustion gas generated in the furnace 3 is introduced into the cyclone 7 while accompanied by solid particles. The cyclone 7 separates the solid particles and the gas by the centrifugal action, returns the solid particles separated via the return line 9 to the furnace 3, and sends the combustion gas from which the solid particles have been removed to the gas line 7a through the outlet 7a. To the gas processing system at the subsequent stage.

  In the furnace 3, a solid material called “furnace bed material” is generated and accumulates at the bottom. Sintering and melting and solidification of the bed material caused by concentration of impurities (such as low-melting substances) in the furnace bed material, or It is necessary to suppress malfunction due to incombustible impurities. For this reason, in the furnace 3, the in-furnace bed material is periodically or continuously discharged to the outside from the discharge port 3d at the bottom. The discharged bed material is supplied to the furnace 3 again after removing unsuitable substances such as metal and coarse particles on a circulation line (not shown).

  The gas processing system includes a gas heat exchanger 13 connected to the outlet 7a of the cyclone 7 via a gas line, and a dust collector 15 connected to the outlet 13a of the gas heat exchanger 13 via a gas line. And The gas heat exchange device 13 is provided with a boiler tube 13b that superheats steam so as to cross the flow path of the exhaust gas. When the high-temperature exhaust gas sent from the cyclone 7 comes into contact with the boiler tube 13b, the heat of the exhaust gas is recovered by the steam in the tube, and the generated high-temperature steam is sent to the turbine for power generation through the boiler tube 13b. An air preheater 50 that recovers the heat of the exhaust gas and preheats the air is provided at the outlet 13 a in the gas heat exchange device 13. The configuration of the air preheater 50 will be described later. The dust collector 15 removes fine particles such as fly ash still entrained in the combustible gas. As the dust collector 15, for example, a bag filter, an electric dust collector, or the like is employed. The clean gas discharged from the discharge port 15a of the dust collector 15 is discharged to the outside from the chimney 19 via the gas line and the pump 17.

  The solid particles generated in the furnace 3 circulate in a circulation system 21 including the furnace 3, the cyclone 7, and the return line 9. In the following description, a fluid of solid particles is referred to as a heat transfer medium. In the circulation system 21, a heat exchange chamber 20 is formed between the return line 9 and the bottom of the furnace 3. A heat transfer medium is stored in the heat exchange chamber 20. In the heat exchange chamber 20, a heat exchanger 22 is provided.

  Next, a schematic configuration of the air preheater 50 will be described with reference to FIG. As shown in FIG. 2, the gas heat exchange device 13 includes an exhaust gas flow path 101 through which the exhaust gas EG passes. The exhaust gas passage 101 includes side walls 101a and 101b facing each other, and side walls (not shown) facing each other in the front-back direction of the drawing. In the following description, the direction in which the exhaust gas EG flows in the exhaust gas channel 101 is referred to as a Z-axis direction. The upstream side with respect to the flow of the exhaust gas EG is defined as the negative side in the Z-axis direction, and the downstream side is defined as the positive side in the Z-axis direction. The direction in which the side wall portions 101a and 101b face each other (the left-right direction on the paper in the figure) is defined as the X-axis direction. The direction in which the side wall portion 101a is arranged (left side in the drawing) is defined as the negative side in the X-axis direction, and the side in which the side wall portion 101b is arranged (right side in the drawing) is the positive side in the X-axis direction. The front-back direction of the paper is the Y-axis direction. The back side of the paper is the negative side in the Y-axis direction, and the front side of the paper is the positive side in the Y-axis direction.

  The air preheater 50 recovers heat from the exhaust gas EG in the exhaust gas channel 101 and preheats the air. The air preheater 50 includes a plurality (three in this case) of heat exchange units 51A, 51B, and 51C provided in the exhaust gas passage 101. The heat exchange units 51A, 51B, and 51C are provided in the order from the upstream side to the downstream side of the flow of the exhaust gas EG, that is, from the negative side to the positive side in the Z-axis direction.

  The heat exchange units 51A, 51B, and 51C include a plurality of first tubes 52 provided in the exhaust gas channel 101, an inlet 53 communicating with each of the inlets of the plurality of first tubes 52, and a plurality of first tubes 52. And an outlet 54 communicating with each outlet of the tube 52. The first tube 52 extends in the X-axis direction between the side wall 101a and the side wall 101b. Air for performing heat exchange with the exhaust gas EG flowing through the exhaust gas channel 101 flows through the first tube 52. The first tube 52 extends at least over the entire length between the side wall portions 101a and 101b. The first tube 52 penetrates the side wall portion 101a and opens outside the exhaust gas passage 101, and penetrates the side wall portion 101b and opens outside the exhaust gas passage 101 (for example, see FIG. 3). The plurality of first tubes 52 are arranged so as to be parallel to each other so as to be arranged in the Z-axis direction and the Y-axis direction. A gap is provided between the adjacent first tubes 52 such that the exhaust gas EG can pass therethrough.

  The inlet 53 is a box-shaped member having an internal space, is provided outside the exhaust gas flow path 101, and is provided so that the respective inlets of the plurality of first tubes 52 communicate with the internal space. The inlet portion 53 expands the flow of air in the internal space so as to distribute the supplied air to each of the plurality of first tubes 52. The outlet portion 54 is a box-shaped member having an internal space, is provided outside the exhaust gas flow path 101, and is provided so that each of the outlets of the plurality of first tubes 52 communicates with the internal space. The outlet 54 collects the air flowing out of the plurality of first tubes 52.

  In the present embodiment, the inlet 53 of the heat exchange unit 51A is provided on the positive side wall 101b in the X-axis direction, and the outlet 54 is provided on the negative side wall 101a in the X-axis direction. The inlet 53 of the heat exchange unit 51B is provided on the negative side wall 101a in the X-axis direction, and the outlet 54 is provided on the positive side wall 101b in the X-axis direction. The inlet 53 of the heat exchange unit 51C is provided on the positive side wall 101b in the X-axis direction, and the outlet 54 is provided on the negative side wall 101a in the X-axis direction.

  In this embodiment, the air for heat exchange is supplied in the order of the heat exchange units 51C, 51B, and 51A. Specifically, a line L1 for supplying air from outside the air preheater 50 is connected to the inlet 53 of the heat exchange unit 51C. The outlet 54 of the heat exchange unit 51C and the inlet 53 of the heat exchange unit 51B are connected via a line L2. The outlet 54 of the heat exchange unit 51B and the inlet 53 of the heat exchange unit 51A are connected via a line L3. The outlet L of the heat exchange unit 51A is connected to a line L4 for allowing preheated air to flow out of the air preheater 50 to the outside. The air preheated by the air preheater 50 is supplied to various places in the boiler where air is used. For example, the air preheated by the air preheater 50 may be used as air for secondary combustion of the furnace 3 or may be used as fluidizing air or combustion air at the bottom of the furnace 3.

  The air preheater 50 configured as described above has a corrosion suppression structure 70 for suppressing low-temperature end corrosion (acid dew point corrosion) of the first tube 52 at a position on the downstream end side in the flow of the exhaust gas EG. Is provided. Such low-temperature corrosion is likely to occur when the exhaust gas contains a large amount of moisture, when the sulfur content is large, or when the moisture and the sulfur content are large. Examples of fuels that easily generate such exhaust gas include coal having a high sulfur content, petroleum coke, coal having a high moisture content, and woody fuel such as wood residue. In the present embodiment, the corrosion suppression structure 70 is provided in the heat exchange unit 51C disposed at the most downstream side with respect to the flow of the exhaust gas EG. Further, among the heat exchange units 51C, the corrosion suppression structure 70 is provided for the plurality of first tubes 52 arranged at the most downstream side with respect to the flow of the exhaust gas EG. That is, the corrosion suppression structure 70 is not provided at a location where the possibility of low-temperature end corrosion is low due to the high temperature of the flowing exhaust gas EG. Here, the corrosion suppression structure 70 is not provided for the first tubes 52 of the heat exchange units 51A and 51B and a part of the first tubes 52 on the upstream side among the heat exchange units 51C.

  The detailed configuration of the corrosion suppression structure 70 will be described with reference to FIG. The corrosion suppressing structure 70 includes a first tube 52, a second tube 71, an inlet 72 for the second tube 71, and an outlet 73 for the second tube 71. The outflow end 52a of the first tube 52 in the corrosion suppression structure 70 extends so as to protrude more to the negative side in the X-axis direction than the side wall 101a in order to provide the inlet 72. The inflow end 52b of the first tube 52 in the corrosion suppression structure 70 extends so as to protrude more toward the positive side in the X-axis direction than the side wall 101b in order to provide the outlet 73.

  The second tube 71 surrounds the first tube 52 from the outer peripheral side and extends along the first tube 52. It may extend so as to coincide with the central axis of the second tube 71 and the central axis of the first tube 52, but it is shifted from the central axis of the first tube 52 as long as it does not affect the flow of air. Is also good. The inflow side end 71a of the second tube 71 extends so as to protrude more toward the negative side in the X-axis direction than the side wall 101a. However, the end portion 71a has a smaller amount of protrusion than the end portion 52a of the first tube 52, and is arranged on the positive side in the X-axis direction with respect to the end portion 52a. The outflow end 71b of the second tube 71 extends so as to protrude more toward the positive side in the X-axis direction than the side wall 101b. However, the end portion 71b has a smaller protrusion amount than the end portion 52b of the first tube 52, and is disposed on the negative side in the X-axis direction with respect to the end portion 52b. Note that the ends 71a and 71b of the second tube 71 do not have to protrude from the side walls 101a and 101b, and may extend at least over the entire length between the side walls 101a and 101b. Although the second tubes 71 are provided for all of the first tubes 52 shown in FIG. 3, the second tubes 71 are provided for a part of the plurality of first tubes 52. It is provided. That is, the second tube 71 is not provided in the first tube 52 on the upstream side with respect to the flow of the exhaust gas EG, and is exposed to the exhaust gas EG.

  The material of the first tube 52 and the second tube 71 is not particularly limited, but a general material such as carbon steel may be applied in order to reduce material cost. That is, the adoption of the corrosion suppression structure 70 does not require the use of an expensive material that is resistant to corrosion for the tube material itself. However, the present invention does not exclude the use of expensive materials for the first tube 52 and the second tube 71.

  The inlet 72 is a box-shaped member having an internal space, is provided outside the exhaust gas flow path 101, and is provided so that the respective inlets of the plurality of second tubes 71 communicate with the internal space. The inlet 72 expands the flow of air in the internal space so as to distribute the supplied air to each of the plurality of second tubes 71. The inlet 72 for the second tube 71 is provided inside the outlet 54 of the heat exchange unit 51C, and airtightness is ensured so as not to communicate with the internal space of the outlet 54. The end 52a of the first tube 52 extends so as to penetrate the inlet 72 and is arranged to open in the internal space of the outlet 54.

  The outlet portion 73 is a box-shaped member having an internal space, is provided outside the exhaust gas channel 101, and is provided so that each of the outlets of the plurality of second tubes 71 communicates with the internal space. The outlet 73 collects the air flowing out of the plurality of second tubes 71, respectively. The outlet 73 for the second tube 71 is provided inside the inlet 53 of the heat exchange unit 51C, and airtightness is ensured so as not to communicate with the internal space of the inlet 53. Note that the end 52 b of the first tube 52 extends so as to penetrate the outlet 73, and is arranged to open in the internal space of the inlet 53.

  Air A2, which has a higher temperature than air A1 flowing through the first tube 52, flows through the second tube 71. As the air A2 flowing through the second tube 71, air supplied from any location may be adopted as long as the temperature is higher than the air A1. In addition, the temperature of the air A1 is not particularly limited, but may be about 30 to 90 ° C. However, in the present embodiment, the air that has passed through the first tube 52 flows through the second tube 71 at least once. In the present embodiment, the air discharged from the heat exchange unit 51A is adopted as the air A2. The temperature of such air A2 may be approximately 200 ° C. or higher. Therefore, the inlet 72 is connected to the line L6 branched from the line L4 extending from the outlet 54 of the heat exchange unit 51A (see also FIG. 2). As a result, the air supplied from the line L6 flows to the second tube 71 via the inlet 72. In addition, a line L7 is connected to the outlet 73. The connection destination of the line L7 is not particularly limited, but may be connected to various places in the boiler where air is used, similarly to the line L4. However, since the air A2 passes through the second tube 71 outside the double tube, the air pressure decreases. Therefore, the line L7 can reuse the recovered heat by supplying air to a position that does not require a high pressure (for example, a furnace upper part, seal air, or the like).

  The flow rate of the air A2 to the second tube 71 may be controlled according to the situation. Specifically, the corrosion suppression structure 70 may include an air temperature detection unit 81, an exhaust gas temperature detection unit 82, a flow rate adjustment unit 83, and a control unit 84. The air temperature detector 81 detects the temperature of the air A2 flowing through the second tube 71. The air temperature detecting section 81 may directly measure the air A2 in the second tube 71, but may be located immediately before flowing into the second tube 71 (for example, the line L6 and the inlet 72) or immediately after flowing out. (For example, the line L7 and the outlet 73) may be measured. The exhaust gas temperature detector 82 detects the temperature of the exhaust gas EG on the downstream side of the exhaust gas flow path 101 from the region where the first tube 52 is provided. That is, the exhaust gas temperature detection unit 82 detects the temperature of the exhaust gas EG in a region of the exhaust gas passage 101 downstream of the heat exchange unit 51C of the air preheater 50. The flow rate adjusting unit 83 adjusts the flow rate of the air A2 to the second tube 71. The flow rate adjusting unit 83 may be configured by, for example, a valve provided in the line L6. The control unit 84 controls the flow rate of the air flowing to the second tube 71 based on the detection results of the air temperature detection unit 81 and the exhaust gas temperature detection unit 82. The control unit 84 adjusts the flow rate of the air A2 by sending a control signal corresponding to the flow rate to the flow rate adjustment unit 83.

  Next, the operation and effect of the air preheater 50 according to the present embodiment will be described.

  First, an air preheater according to a comparative example will be described. The air preheater according to the comparative example does not include the corrosion suppression structure 70 as in the present embodiment, and does not include the second tube 71 in the first tube 52 on the most downstream side, so that the exhaust gas EG It is exposed to. Here, when a fuel with a high moisture content and a high sulfur content is used as the fuel for the boiler, the exhaust gas EG generated by burning the fuel contains a large amount of moisture. As the moisture in the exhaust gas increases, the acid dew point (sulfuric acid dew point) increases (roughly 110 to 130 ° C., depending on the fuel used). When the acid dew point of the exhaust gas EG is high, in the air preheater according to the comparative example, the first tube 52 disposed at the downstream end where the temperature of the exhaust gas EG becomes low corrodes at low temperature end corrosion (acid dew point corrosion). ) May occur.

  As a method for suppressing the occurrence of such low-temperature end corrosion, there is a method for keeping the temperature of the exhaust gas EG high. However, raising the temperature of the exhaust gas EG means that the sensible heat loss of the exhaust gas EG increases, and as a result, the boiler efficiency of the boiler decreases. Further, when the first tube 52 is made of a material that is resistant to corrosion, a high-grade material must be adopted, and there is a problem that the manufacturing cost increases.

  On the other hand, the air preheater 50 according to the present embodiment includes a first tube 52 provided in the exhaust gas passage 101. The first tube 52 can recover the heat of the exhaust gas EG flowing through the exhaust gas channel 101 by the air flowing through the first tube 52. Here, the air preheater 50 includes a second tube 71 that surrounds the first tube 52 from the outer peripheral side and extends along the first tube 52. Thus, the first tube 52 and the second tube 71 form a double tube. Further, air having a higher temperature than the air flowing through the first tube 52 flows through the second tube 71 which is the outer tube of the double tube. Thereby, even if the exhaust gas EG becomes low temperature by removing heat, the first tube 52 is surrounded by the second tube 71 through which high-temperature air flows. Corrosion by the low-temperature exhaust gas EG can be suppressed. As described above, it is possible to suppress the corrosion of the tube while suppressing the decrease in the boiler efficiency.

  In the air preheater 50, since high-temperature air flows through the second tube 71 exposed to the exhaust gas EG, the temperature of the tube wall of the second tube 71 is maintained at a temperature at which low-temperature end corrosion does not occur. it can. Accordingly, as a material for the first tube 52 and the second tube 71, a high-grade material that is resistant to corrosion does not have to be used, so that manufacturing costs can be suppressed.

  In the air preheater 50, the air that has passed at least once through the first tube 52 flows through the second tube 71. Since the air that has passed through the first tube 52 at least once has recovered the heat of the exhaust gas EG, the air has passed through the first tube 52 of the corrosion suppression structure 70 on the most downstream side more than the air A1 that has passed through the first tube 52. The temperature is high. Therefore, by flowing the air through the second tube 71, a structure for securing high-temperature air to flow from the location other than the air preheater 50 to the second tube 71 becomes unnecessary.

  In the air preheater 50, a plurality of the first tubes 52 are provided in the exhaust gas channel 101, and the second tubes 71 are provided for a part of the plurality of the first tubes 52. Of the plurality of first tubes 52, the second tube 71 is provided only for those having a low passing temperature of exhaust gas and a high possibility of low-temperature end corrosion, and a high passing temperature of exhaust gas and low-temperature end corrosion are possible. The second tube 71 is not provided for those having low properties. Thereby, the manufacturing cost of the air preheater 50 can be suppressed.

  The air preheater 50 includes an air temperature detector 81 that detects the temperature of the air flowing through the second tube 71, and a temperature of the exhaust gas EG on the downstream side of a region of the exhaust gas passage 101 where the first tube 52 is provided. And a control unit 84 that controls the flow rate of air flowing to the second tube 71 based on the detection results of the air temperature detection unit 81 and the exhaust gas temperature detection unit 82. By performing such control, an appropriate flow rate of air can flow through the second tube 71.

  The present invention is not limited to the above embodiment.

  For example, in the above-described embodiment, air flows from the downstream heat exchange unit 51C to the heat exchange unit 51B and the heat exchange unit 51A in this order with respect to each heat exchange unit. Instead of this, for example, the order of introducing air to the heat exchange units may be changed, and air may be simultaneously supplied to the heat exchange units 51A, 51B, 51C in parallel.

  Further, in the above-described embodiment, the air A2 is extracted from the line L4, but the extraction location of the air A2 is not particularly limited. For example, the air A2 may be drawn from the lines L2, L3 and the like. If the temperature of the air drawn from the lines L2 and L3 is not sufficient, auxiliary heating may be performed.

  In the above-described embodiment, the air A2 and the air A1 flow so as to face each other, but may flow in the same direction.

  50: air preheater, 52: first tube, 71: second tube, 81: air temperature detection unit, 82: exhaust gas temperature detection unit, 84: control unit, 101: exhaust gas flow path.

Claims (4)

An air preheater that recovers the heat of the boiler exhaust gas and preheats the air,
A first tube provided in an exhaust gas passage through which the exhaust gas passes,
A second tube surrounding the first tube from the outer peripheral side and extending along the first tube;
Air flows through the first tube;
An air preheater, wherein air having a higher temperature than air flowing through the first tube flows through the second tube.   The air preheater according to claim 1, wherein the air after passing through the first tube flows at least once through the second tube. A plurality of the first tubes are provided in the exhaust gas channel,
The air preheater according to claim 1, wherein the second tube is provided for a part of the plurality of first tubes. An air temperature detector that detects the temperature of the air flowing through the second tube;
An exhaust gas temperature detection unit that detects the temperature of the exhaust gas on a downstream side of a region where the first tube is provided in the exhaust gas channel;
A control unit that controls a flow rate of air flowing to the second tube based on detection results of the air temperature detection unit and the exhaust gas temperature detection unit, The control unit according to any one of claims 1 to 3, Air preheater.

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