US20180304339A1

US20180304339A1 - Method for manufacturing fin-tube heat exchanger and combustion apparatus including fin-tube heat exchanger

Info

Publication number: US20180304339A1
Application number: US15/947,951
Authority: US
Inventors: Naohiro Ito; Seiichi Muto
Original assignee: Rinnai Corp
Current assignee: Rinnai Corp
Priority date: 2017-04-21
Filing date: 2018-04-09
Publication date: 2018-10-25
Also published as: CN108981168A; JP2018176262A; KR20180118534A

Abstract

The method for manufacturing a fin-tube heat exchanger includes the step of expanding each heat transfer tube extending through a corresponding tube insertion hole of a heat transfer fin with a tube expander inserted in heat transfer tubes. The expanding step includes a first sub-step of expanding the tube expander in a radial direction of the heat transfer tube to a radially expanded state while the tube expander is at rest in a first predetermined region to be expanded inside the heat transfer tube to bring the first predetermined region to be expanded into close contact with the tube insertion holes; and a second sub-step of releasing the tube expander from the radially expanded state to a radially contracted state and moving the tube expander to a second predetermined region to be expanded inside the heat transfer tube. The first and second sub-steps are alternately repeated.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method for manufacturing a fin-tube heat exchanger and to a combustion apparatus including a fin-tube heat exchanger.

Description of the Related Art

A fin-tube heat exchanger is known as a heat exchanger to be incorporated in a water heater. As illustrated in FIG. 7, the fin-tube heat exchanger includes a large number of thin plate-like heat transfer fins (3) that are disposed in parallel to each other at predetermined intervals. The heat transfer fins (3) have tube insertion holes (30) through which heat transfer tubes (31) extend. It is desirable that the inner periphery of each tube insertion hole (30) of the heat transfer fins (3) be in close contact with the outer circumferential surface of a corresponding heat transfer tube (31) to improve thermal efficiency. To fill the gap therebetween, the heat transfer tubes (31) are expanded.
For example, Japanese Patent Application Laid-open No. 2008-93713 discloses a method for expanding metal tubes. With reference to FIG. 8, the method for expanding tubes involves forcibly inserting and pushing a spherical body (40) of a tube expanding jig (4) into a metal tube (31 a) to expand the outer diameter of the metal tube (31 a). The body (40) is provided at a tip of a shaft (41) of the tube expanding jig (4) and has a larger diameter than that of the inner diameter of the metal tube (31 a). In this method, the body (40), which is coated with diamond-like carbon, of the expanding jig (4) is forcibly inserted into the metal tube (31 a) with application of a lubricant. The method is intended for a metal tube (31 a) made of copper or aluminum.
Unfortunately, in a case where the heat transfer tube (metal tube) (31) is made of stainless steel, forcibly inserting the body (40) of the expanding jig (4) into the heat transfer tube (31) with application of a lubricant removes the diamond-like carbon coating at an early point in time to generate high frictional heat, resulting in adhesion of the stainless steel on the surface of the body (40). Continuous use of the expanding jig (4) in such a condition may disadvantageously damage the inner surface of the heat transfer tubes (31), which in turn may require higher stress to push the body (40), resulting in a breakage of the shaft (41). This requires frequent replacement of the expanding jig (4) and a process of drying to remove the lubricant inside the heat transfer tubes (31). Such a high-cost method for manufacturing a heat exchanger with significantly low productivity is far from an ideal method for mass production.

SUMMARY OF THE INVENTION

An object of the present invention, which has been made in view of these circumstances, is to provide a method for manufacturing a fin-tube heat exchanger allowing for longer service life of a tube expander and exhibiting higher productivity, and a combustion apparatus including a fin-tube heat exchanger.
An embodiment of the present invention is a method for manufacturing a fin-tube heat exchanger including heat transfer fins having tube insertion holes and heat transfer tubes made of aluminum or stainless steel and respectively extending through the tube insertion holes of the heat transfer fins. The method includes the step of expanding each of the heat transfer tubes extending through the corresponding tube insertion hole with a tube expander inserted in the heat transfer tube. The expanding step includes a first sub-step of expanding the tube expander in a radial direction of the heat transfer tube to a radially expanded state while the tube expander is at rest in a first predetermined region to be expanded inside the heat transfer tube to bring the first predetermined region to be expanded into close contact with the tube insertion hole, and a second sub-step of releasing the tube expander from the radially expanded state to a radially contracted state and moving the tube expander to a second predetermined region to be expanded inside the heat transfer tube. The first and second sub-steps are alternately repeated.
In the manufacturing method, the tube expander is not moved in the heat transfer tubes before being released from the radially expanded state. In other words, the outer circumferential surface of the tube expander does not slide on the inner circumferential surface of the heat transfer tube. This prevents generation of frictional heat, eliminating the risk of adhesion of the stainless steel to the surface of the tube expander due to frictional heat, even in a case where the heat transfer tube is made of stainless steel. Accordingly, the manufacturing method does not require frequent replacement of the tube expander and application of a lubricant and thus eliminates the need for a process and cost of drying to remove the lubricant.
Furthermore, the alternate repeating of the first and second sub-steps can expand the heat transfer tube across substantially the entire length such that the outer circumferential surface of the heat transfer tube is brought into close contact with the tube insertion hole, improving the efficiency in brazing the heat transfer tube to the heat transfer fin. This can manufacture a heat exchanger exhibiting high thermal efficiency.
In the method for manufacturing the fin-tube heat exchanger described above, the tube expander may preferably include a cylinder including a plurality of head segments divided in a circumferential direction, a core rod to be respectively pushed from end of the heat transfer tube into opening of the cylinder in the heat transfer tube, and a conversion mechanism to convert axial pushing force of the core rod into a movement of the head segments in an expanding radial direction. The core rod is pushed into the cylinder to move the head segments in the expanding radial direction to expand the cylinder to the radially expanded state, and the core rod is pulled from the cylinder to move the head segments in a contracting radial direction to contract the cylinder to the radially contracted state.
The cylinder, which is composed of the head segments, of the tube expander has an adjustable outer diameter. The distance of the movement in the expanding radial direction of the head segments of the cylinder can be adjusted by the outer diameters of the core rods to be pushed into the openings of the cylinder or the amount of the pushing of the core rods into the openings of the cylinder. This can expand the heat transfer tube to a desired diameter. Furthermore, heat transfer tubes having various outer shapes can be expanded by changing the shapes of the head segments of the tube expander.
Another embodiment of the present invention is a combustion apparatus including a fin-tube exchanger including heat transfer fins having tube insertion holes, and heat transfer tubes made of aluminum or stainless steel and respectively extending through the tube insertion holes of the heat transfer tubes. A first predetermined region to be expanded inside each of the heat transfer tubes extending through the corresponding tube insertion hole is brought into close contact with the corresponding tube insertion hole with a tube expander that is expanded in a radial direction of the heat transfer tube to a radially expanded state while being at rest in the first predetermined region to be expanded. The tube expander released from the radially expanded state to the radially contracted state is movable to a second predetermined region to be expanded inside the heat transfer tube. The tube expander at rest in the second predetermined region to be expanded is configured to be radially expanded again to the radially expanded state to bring the second predetermined region to be expanded into close contact with the tube insertion hole.
In the combustion apparatus including the fin-tube heat exchanger described above, the tube expander preferably includes a cylinder including a plurality of head segments divided in a circumferential direction, a core rod to be respectively pushed from end of the heat transfer tubes into opening of the cylinder in each of the heat transfer tubes, and a conversion mechanism to convert axial pushing force of the core rod into a movement of the head segments in an expanding radial direction. The core rod is pushed into the cylinder to move the head segments in the expanding radial direction to expand the cylinder to the radially expanded state, while the core rod is pulled from the cylinder to move the head segments in a contracting radial direction to contract the cylinder to the radially contracted state.
The heat transfer tube in the combustion apparatus is expanded by the tube expander across substantially the entire length to enhance the contact between the heat transfer fins and the heat transfer tubes. The heat transfer tubes are thereby firmly brazed to the tube insertion holes of the heat transfer fins. This can improve the brazing efficiency and thus can provide a combustion apparatus with high thermal efficiency.
According to the method for manufacturing the fin-tube heat exchanger of the present invention, the tube expander can move to the second predetermined region to be expanded inside the heat transfer tube without sliding on the inner surface of the heat transfer tube. This prevents generation of frictional heat between the heat transfer tube and the tube expander, eliminating the risk of adhesion of the stainless steel to the surface of the tube expander, even in a case where the heat transfer tube is made of stainless steel. Accordingly, the manufacturing method does not require frequent replacement of the tube expander and allows for a longer service life of the tube expander. Furthermore, the tube expander can move in the heat transfer tube without application of a lubricant. This can reduce costs and eliminates the need for a process of drying to remove the lubricant, resulting in higher productivity.
According to the combustion apparatus including the fin-tube heat exchanger of the present invention, the contact between the heat transfer fins and the heat transfer tubes can be enhanced to improve the brazing efficiency. Therefore, the combustion apparatus including the heat exchanger can exhibit high thermal efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger;

FIG. 2 is a schematic view of a water heater including the heat exchanger illustrated in FIG. 1;

FIG. 3 is a schematic view of a tube expander in a radially contracted state that is used in a step of expanding heat transfer tubes in a method for manufacturing a heat exchanger according to an embodiment;

FIG. 4 is a cross-sectional view of the tube expander in a radially contracted state that is used in a step of expanding heat transfer tubes in a method for manufacturing a heat exchanger according to an embodiment;

FIG. 5 is a schematic view of a tube expander in a radially expanded state that is used in a step of expanding heat transfer tubes in a method for manufacturing a heat exchanger according to an embodiment;

FIG. 6 is a cross-sectional view of the tube expander in a radially expanded state that is used in a step of expanding heat transfer tubes in a method for manufacturing a heat exchanger according to an embodiment;

FIG. 7 is a plan view of a heat transfer fin and heat transfer tubes of a conventional heat exchanger; and

FIG. 8 is a partial cross-sectional view of a metal tube for illustrating a conventional method for expanding the metal tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be described with reference to the attached drawings. A heat exchanger (51) illustrated in FIG. 1 includes large number of thin plate-like heat transfer fins (2) for absorbing heat. The heat transfer fins (2) are disposed in parallel to opposite side walls (a front side wall (501) and a rear side wall (502)) of a case (5) between the side walls (501, 502). Herein, the outer face of the front side wall (501) is a front face of the heat exchanger (51). When viewed from the front face of the case (5), a depth direction corresponds to a front-back direction, a width direction corresponds to a horizontal direction, and a height direction corresponds to a vertical direction.
For example, the heat exchanger (51) is used in a water heater illustrated in FIG. 2, which is an example of combustion apparatuses. The water heater includes a casing (55) accommodating a container (53) and a fan case (34). The container (53) includes a burner (33) having a downward combustion face (33 a) and disposed at an upper portion of the container (53). The fan case (34) is communicated with the container (53) and includes a fan (34 a) driven by a motor (M) to transfer an air-fuel mixture gas to the burner (33) in the container (53).
The heat exchanger (51) is of a sensible heat recovery type and disposed at a middle portion of the container (53). A heat exchanger (52) disposed below the heat exchanger (51) is of a latent heat recovery type. In this water heater, the water from a water supply pipe (38) disposed upstream of and in communication with the heat exchanger (52) is heated with latent heat of exhaust gas from the burner (33) at the lower heat exchanger (52), and is then heated with sensible heat of the exhaust gas at the upper heat exchanger (51), to discharge hot water heated to a predetermined temperature from a hot-water supply pipe (39) disposed downstream of and in communication with the heat exchanger (51). It is noted that the case (5) illustrated in FIG. 1 is a part of the container (53) and is heated by the combustion face (33 a) of the downward burner (33) disposed above the case (5), so that the exhaust gas flows from an upper portion to a lower portion of the case (5).
Heat transfer tubes (21) are made of stainless steel and each have a vertically oval cross-section. The heat transfer tubes (21) are staggered in two rows in a substantially lower half space of the case (5) such that the heat transfer tubes (21) in the upper row are eccentric from the heat transfer tubes (21) in the lower row by a half of a pitch in the horizontal direction. The heat transfer tubes (21) extend through the front and rear side walls (501, 502). Each two adjacent ends of the heat transfer tubes (21) extending outward from the case (5) are covered by a cover (5 a). The covers (5 a) are fixed to the front and rear side walls (501, 502) of the case (5). Such a structure allows a fluid to meander through the heat transfer tubes (21) via the covers (5 a).
The heat transfer fins (2) are thin metal plates made of stainless steel. The heat transfer fins (2) have tube insertion holes (20) or burring holes formed by burring. The heat transfer tubes (21) extend through the tube insertion holes (20). Like the heat transfer tubes (21), the tube insertion holes (20) are staggered in two rows such that the tube insertion holes (20) in the upper row are eccentric from the tube insertion holes (20) in the lower row by a half of a pitch in a horizontal direction.
To assemble the heat transfer tubes (21) in the tube insertion holes (20), each of the tube insertion holes (20) has a vertically oval shape which is large enough to receive the heat transfer tube (21) therethrough while being in substantially contact with the heat transfer tube (21). It is desirable, however, that the inner periphery of each tube insertion hole (20) be in close contact with the outer circumferential surface of the heat transfer tube (21) to improve brazing efficiency and thermal efficiency. Accordingly, the heat transfer tube (21) is expanded in diameter with a tube expander (100) inserted into the heat transfer tube (21).
With reference to FIG. 3, the tube expander (100) includes a cylinder (1) that can be accommodated in the heat transfer tube (21) and a pair of core rods (first and second core rods (12, 13)). The cylinder (1) includes an oval cylindrical portion (11) and small-diameter cylindrical portions (10) extending from the opposite ends of the oval cylindrical portion (11). The small-diameter cylindrical portions (10) respectively have first and second openings (10 a, 10 b). The first and second core rods (12, 13) are respectively inserted through the first and second openings (10 a, 10 b) into the cylinder (1). The oval cylindrical portion (11) has an outer shape substantially the same as the oval cylindrical shape of the heat transfer tube (21). It is noted that the heat transfer fins (2) illustrated in FIG. 3 and any other drawing are rectangular plates each having a single insertion hole (20) for the purpose of illustration.
The cylinder (1) is composed of four head segments (1 a, 1 b, 1 c, and 1 d) divided in a circumferential direction. These head segments (1 a to 1 d) are banded all together substantially without a gap therebetween. To keep the banding, O-rings (14) as banding members are respectively attached to the opposite base ends of the small-diameter cylindrical portions (10). The banding member may be any elastic member other than an O-ring, and may be a spring, for example. The cylinder (1) in such a state is referred to as being in a radially contracted state. As illustrate in FIG. 3, the cylinder (1) in the radially contracted state has an outer diameter slightly smaller than the inner diameter of the transfer tube (21) so as to be movable in the heat transfer tube (21) without sliding on the inner surface of the heat transfer tube (21).
With reference to FIG. 4, the cylinder (1) has flaring cavities (15 a, 15 b) respectively flaring toward the first and second openings (10 a, 10 b) at the opposite ends of the cylinder (1). The flaring cavities (15 a, 15 b) each have a circular cross-section. The flaring cavities (15 a, 15 b) are formed in a central portion of the cylinder (1) and in communication with each other via a space (16) having a larger diameter than minimum-diameter portions of the flaring cavities (15 a, 15 b).
The first and second core rods (12, 13) respectively have tapering shafts (12 a, 13 a) each having a circular cross-section at their tips. As illustrated in FIG. 4, a latching member (13 b) is provided at the tip of the tapering shaft (13 a) of the second core rods (13). The latching member (13 b) has a head (130) having a diameter larger than that of the minimum-diameter portion of the flaring cavity (15 b) and smaller than that of the space (16). The second core rod (13) and the head segments (la to 1 d) are assembled and banded with the O-ring (14) such that the head (130) resides in the space (16) in the cylinder (1) and the tapering shaft (13 a) resides in the flaring cavity (15 b) with a gap therebetween. The tapering shaft (12 a) of the first core rod (12) has no latching member (13 b) described above.
The flaring cavities (15 a, 15 b) and the tapering shafts (12 a, 13 a) constitute a conversion mechanism to expand the head segments (la to 1 d).
Next, a step of expanding the heat transfer tubes (21) is described. The step of expanding tubes involves first and second sub-steps.
In the first sub-step, the cylinder (1) in the radially contracted state with the second core rod (13) extending through the second opening (10 b) is inserted into the heat transfer tube (21) that is assembled in the tube insertion hole (20) of the heat transfer fin (2). The cylinder (1) is then rested in a first predetermined region to be expanded (22) of the heat transfer tube (21). The tapering shaft (12 a) at the tip of the first core rod (12) is pushed into the flaring cavity (15 a) of the cylinder (1) through the first opening (10 a). As the tapering shaft (12 a) is pushed into the flaring cavity (15 a) of the cylinder (1), the cylinder (1) is pushed, so that the tapering shaft (13 a) is also forcibly pushed into the flaring cavity (15 b). The flaring cavities (15 a, 15 b) of the cylinder (1) are thereby respectively pressed by the tapering shafts (12 a, 13 a) of the first and second core rods (12, 13) in a radial direction. The pressing force acts against elastic restoring force of the O-ring (14) and is converted into a force to move the head segments (la to 1 d) apart from each other in the radial direction. This operation forms gaps (S) between the head segments (la to 1 d), so that the outer diameter of the cylinder (1) is evenly expanded, as illustrated in FIG. 5. The cylinder (1) in such a state is referred to as being in a radially expanded state. The cylinder (1) in the radially expanded state can expand the first predetermined region to be expanded (22) of the heat transfer tube (21).
In the first sub-step, the first and second core rods (12, 13) are respectively pushed into the first and second openings (10 a, 10 b) at the opposite ends of the cylinder (1). This prevents the cylinder (1) from being displaced from the first predetermined region to be expanded (22) during the expanding step.
In a second sub-step following the first sub-step, the first and second core rods (12, 13) are removed from the cylinder (1) in a pulling direction. The radial pressing force of the tapering shafts (12 a, 13 a) of the first and second core rods (12, 13) acting on the cylinder (1) is thereby released, and the elastic restoring force of the O-ring (14) returns the cylinder (1) to the radially contracted state, as illustrated in FIGS. 3 and 4. The first core rod (12) is then pulled out through the first opening (10 a), whereas the second core rod (13) is not pulled out through the second opening (10 b) because the head (130) of the latching member (13 b) at the tip of the tapering shaft (13 a) is caught at a boundary between the flaring cavity (15 b) and the space (16). The second core rod (13) in such a state is pulled further in the pulling direction (indicated by the arrow in FIG. 4), so that the cylinder (1) kept in the radially contracted state is pulled by the head (130) to a second predetermined region to be expanded (22) inside the heat transfer tube (21). The cylinder (1) is then rested in the second predetermined region to be expanded (22), and the first sub-step is performed again. The second predetermined region to be expanded (22) inside the heat transfer tube (21) is generally a non-expanded region adjacent to the first predetermined region expanded in the first sub-step.
The alternate repeating of the first and second sub-steps can sequentially expand each heat transfer tube (21) to bring the outer circumferential surface of the heat transfer tube (21) into close contact with the corresponding tube insertion hole (20) of the heat transfer fin (2).
According to the manufacturing method described above, the tube expander (100) with the cylinder (1) being in the radially contracted state can move in the heat transfer tube (21) without sliding on the inner surface of the heat transfer tube (21), causing no frictional heat. This prevents adhesion of the stainless steel of the heat transfer tube (21) from the inner circumferential surface of the heat transfer tube (21) to the outer surface of the cylinder (1) due to frictional heat. This does not require frequent replacement of the cylinder (1) of the tube expander (100) and allows for a long service life of the tube expander (100). Additionally, since the tube expander (100) can move in the heat transfer tube (21) without application of a lubricant, manufacturing costs can be reduced. Since application of a lubricant is not required, the need for a process of drying to remove lubricant adhering to the inner surface of the heat transfer tube (21) is also eliminated. As a result, higher productivity of the heat exchanger (51) can be achieved.
The tube expander (100) described above can expand each heat transfer tube (21) in shape and size to bring the heat transfer tube (21) into close contact with the corresponding tube insertion hole (20) of the transfer fin (2) without a gap therebetween. This can improve efficiency in brazing the heat transfer tubes (21) to the tube insertion holes (20). Furthermore, the heat exchanger (51) including the heat transfer tubes (21) in highly close contact with the heat transfer fins (2) excels in thermal efficiency. Accordingly, incorporating the heat exchanger (51) in a water heater can provide a combustion apparatus exhibiting high thermal efficiency.
An advantage of the cylinder (1), which is composed of the head segments (la to 1 d), of the tube expander (100) lies in its capability to evenly expand non-circular or oval heat transfer tubes (21).
Since the first and second core rods (12, 13) respectively have the tapering shafts (12 a, 13 a) at the tips thereof and the cylinder (1) has the flaring cavities (15 a, 15 b) at the respective openings (10 a, 10 b), the degree of expansion of the transfer tube (21) can be variously changed depending on the pushing amount of the first and second core rods (12, 13) into the respective flaring cavities (15 a, 15 b). This can enhance the contact between the heat transfer tubes (21) and the heat transfer fins (2).
Since the cylinder (1) in the radially contracted state can move in the heat transfer tube (21), regions to be expanded (22) inside the heat transfer tube (21) may be any desired regions. Furthermore, a positional displacement of the cylinder (1) in the heat transfer tube (21) can be readily corrected by axially moving the second core rod (13).
In the embodiment described above, the outer shape of the cylinder (1) of the tube expander (100) is vertically oval to conform to the cross-sectional shape of the heat transfer tube (21). In other words, the head segments (la to 1 d) can define various shapes conforming to not only a circular cross-sectional shape but also any other cross-sectional shape of the heat transfer tube.
Furthermore, although the first and second core rods (12, 13) are respectively inserted through the opposite ends of the cylinder (1) of the tube expander (100) in the embodiment described above, a core rod may be inserted only through one end to expand the cylinder (1) while the other end of the cylinder (1) is temporarily fixed with a stopper or any other member.
Moreover, although the tapering shafts (12 a, 13 a) and the flaring cavities (15 a, 15 b) each have a circular cross-section in the embodiment described above, these components may have any appropriate cross-sectional shape other than the circular one.
It is noted that the present invention may be also adapted to heat transfer tubes (21) made of aluminum.
Besides the water heater described above, the heat exchanger (51) may be incorporated in any other combustion apparatus, such as a condensing water heater, a heat source in a hot water supply system of a storage type, a water heater with a bath reheating function, a water heater dedicated for hot-water supply, a heat source for hot-water supply and heating, and a hot-water heater.

Claims

What is claimed is:

1. A method for manufacturing a fin-tube heat exchanger comprising heat transfer fins having tube insertion holes and heat transfer tubes made of aluminum or stainless steel and respectively extending through the tube insertion holes of the heat transfer fins, the method comprising the step of:

expanding each of the heat transfer tubes extending through the corresponding tube insertion hole with a tube expander inserted in the heat transfer tube, wherein

the expanding step comprises:

a first sub-step of expanding the tube expander in a radial direction of the heat transfer tube to a radially expanded state while the tube expander is at rest in a first predetermined region to be expanded inside the heat transfer tube to bring the first predetermined region to be expanded into close contact with the corresponding tube insertion hole; and

a second sub-step of releasing the tube expander from the radially expanded state to a radially contracted state and moving the tube expander to a second predetermined region to be expanded inside the heat transfer tube, wherein

the first sub-step and the second sub-step are alternately repeated.

2. The method for manufacturing the fin-tube heat exchanger according to claim 1, wherein

the tube expander comprises:

a cylinder comprising a plurality of head segments divided in a circumferential direction;

a core rod to be respectively pushed from end of the heat transfer tube into opening of the cylinder in the heat transfer tube; and

a conversion mechanism to convert axial pushing force of the core rod into a movement of the head segments in an expanding radial direction, wherein

the core rod is pushed into the cylinder to move the head segments in the expanding radial direction to expand the cylinder to the radially expanded state, and the core rod is pulled from the cylinder to move the head segments in a contracting radial direction to contract the cylinder to the radially contracted state.

3. A combustion apparatus comprising:

a fin-tube heat exchanger including:

heat transfer fins having tube insertion holes; and

heat transfer tubes made of aluminum or stainless steel and respectively extending through the tube insertion holes of the heat transfer tubes, wherein

a first predetermined region to be expanded inside each of the heat transfer tubes extending through the corresponding tube insertion hole is brought into close contact with the corresponding tube insertion hole with a tube expander that is expanded in a radial direction of the heat transfer tube to a radially expanded state while being at rest in the first predetermined region to be expanded,

the tube expander released from the radially expanded state to a radially contracted state is movable to a second predetermined region to be expanded inside the heat transfer tube, and

the tube expander at rest in the second predetermined region to be expanded is configured to be radially expanded again to the radially expanded state to bring the second predetermined region to be expanded into close contact with the corresponding tube insertion hole.

4. The combustion apparatus comprising the fin-tube heat exchanger according to claim 3, wherein

the tube expander comprises:

the core rod is pushed into the cylinder to move the head segments in the expanding radial direction to expand the cylinder to the radially expanded state, while the core rod is pulled from the cylinder to move the head segments in a contracting radial direction to contract the cylinder to the radially contracted state.