Heat exchanger and manufacturing method therefor

A heat exchanger includes a plurality of heat transfer tubes housed in a predetermined case, a connecting tube body for connecting the plurality of heat transfer tubes, a predetermined tube expansion portion provided on each heat transfer tube, a first peripheral wall portion provided on the tube expansion portion, and a second peripheral wall portion that is positioned on an end portion of the connecting tube body and fitted to the tube expansion portion, wherein the first and second peripheral wall portions have different sectional shapes and are fitted together in a partial contact state including predetermined contact and non-contact portions. According to this configuration, the heat transfer tubes can be fixed to a side wall portion of the case and the connecting tube body can be connected to the heat transfer tubes easily and appropriately.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a heat exchanger used in a water heating application or the like in a water heater, for example, and a manufacturing method therefor.

Description of the Related Art

The present applicant has previously proposed the invention described in Japanese Patent Application Publication No. 2020-51682 as an example of a heat exchanger.

The heat exchanger described in this document is incorporated into a water heater or the like and used to heat water, and a plurality of heat transfer tubes are housed in a case to which a heating medium is supplied. End portions of the plurality of heat transfer tubes are drawn out to an exterior of the case by being passed through hole portions provided in a side wall portion of the case, and respective end portions of substantially semicircular arc-shaped connecting tube bodies are fitted to these parts. Thus, the plurality of heat transfer tubes are connected in series via the connecting tube bodies such that water can flow appropriately from one end side to the other end side thereof and water can be heated during the flowing process.

Further, a tube expansion portion is provided on the heat transfer tube as fixing means for fixing the heat transfer tube to the side wall portion of the case, and the tube expansion portion is brazed to the side wall portion. The tube expansion portion is configured to include both a press-fitted portion in which the outer peripheral surface of the heat transfer tube is press-fitted to an inner peripheral surface of the hole portion in the side wall portion, and a flared portion that has a flared shape and is positioned further toward an end portion tip end side of the heat transfer tube than the press-fitted portion.

When, in contrast to this configuration, tube expansion processing is implemented only to provide the press-fitted portion on the heat transfer tube, the aperture of the heat transfer tube on the end portion tip end side tends to shrink, and as a result, there is a danger that it will be difficult to connect the connecting tube body. According to the above configuration, on the other hand, the end portion of the connecting tube body can easily be fitted to the flared portion, and therefore this danger can be eliminated.

As described below, however, there remains room for improvement in the prior art described above.

When the flared portion having a flared shape is formed on the end portion tip end side of the heat transfer tube, although it becomes easier to fit the end portion of the connecting tube body into this part, the heat transfer tube and the connecting tube body cannot be fitted together in a contacting state in the location where the flared portion is formed. It is therefore difficult to realize a provisionally held state in which the connecting tube body is held with stability simply by fitting the end portion of the connecting tube body into the end portion of the heat transfer tube. As a result, when the case of the heat exchanger, in a state where the connecting tube bodies are fitted to the heat transfer tubes, is transported to a brazing operation position for brazing the connecting tube bodies to the heat transfer tubes during a manufacturing process of the heat exchanger, there is a danger that the connecting tube bodies will fall off the heat transfer tubes or the like. In order to improve the efficiency and appropriateness of the operation for manufacturing the heat exchanger, it is desirable to eliminate this danger as appropriate.

As means for eliminating this danger, the flared portion may be omitted. However, when the flared portion is simply omitted, it becomes difficult to appropriately control the fitting state between the heat transfer tube and the connecting tube body. When a fitting tolerance between the heat transfer tube and the connecting tube body is inappropriate such that interference between the heat transfer tube and the connecting tube body is large, it becomes difficult to fit and connect the connecting tube body to the heat transfer tube. Conversely, when a gap between the heat transfer tube and the connecting tube body is large, it becomes difficult to provisionally hold the connecting tube body on the heat transfer tube with stability.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

An object of the present invention is to provide a heat exchanger and a manufacturing method therefor with which heat transfer tubes can be fixed to a side wall portion of a case and a connecting tube body can be connected to the heat transfer tubes easily and appropriately.

To solve the problems described above, the present invention teaches the following technical means.

A heat exchanger provided by a first aspect of the present invention includes a case having a side wall portion, a heating medium being supplied into an interior of the case, a plurality of heat transfer tubes that are drawn out to an outside from the interior of the case by inserting end portions thereof respectively through a plurality of hole portions provided in the side wall portion, at least one connecting tube body for connecting the plurality of heat transfer tubes to each other, a tube expansion portion provided on each of the heat transfer tubes so as to form a press-fitted portion in which an outer peripheral surface of each heat transfer tube is press-fitted to an inner peripheral surface of each of the hole portions, a first peripheral wall portion provided on the tube expansion portion in a position further toward an end portion tip end side of each of the heat transfer tubes than the press-fitted portion, and a second peripheral wall portion positioned on an end portion of the connecting tube body and fitted to the tube expansion portion, wherein the first and second peripheral wall portions have different sectional shapes and are fitted together in a partial contact state including a contact portion in which respective circumferential direction parts of the first and second peripheral wall portions contact each other and a non-contact portion in which other parts are separated from each other via a gap.

In the heat exchanger according to the present invention, preferably, a plurality of contact portions positioned at equal intervals in the circumferential direction of the first and second peripheral wall portions are provided as the contact portion, and a plurality of non-contact portions respectively positioned between the plurality of contact portions in the circumferential direction of the first and second peripheral wall portions are provided as the non-contact portion.

Preferably, the heat transfer tubes and the connecting tube bodies are both formed using round pipes, the hole portions in the side wall portion are circular, the press-fitted portion and the second peripheral wall portion have a hollow, circular sectional shape, and the first peripheral wall portion has a hollow, non-circular sectional shape.

Preferably, the second peripheral wall portion is fitted into the first peripheral wall portion, and an inner peripheral surface of the first peripheral wall portion includes a plurality of first curved surface portions that have a larger curvature radius than an outer peripheral surface of the second peripheral wall portion and are provided at intervals in the circumferential direction so as to partially contact the outer peripheral surface of the second peripheral wall portion, and a plurality of second curved surface portions that are provided so as to connect the plurality of first curved surface portions to each other without contacting the outer peripheral surface of the second peripheral wall portion.

Preferably, the tube expansion portion extends inside the case beyond the press-fitted portion, and the second peripheral wall portion is fitted into the tube expansion portion so as to advance to a position further inside the case than the press-fitted portion.

Preferably, the tube expansion portion includes first and second bulge portions in which the outer peripheral surface of each heat transfer tube partially bulges outward in a radial direction so as to sandwich the side wall portion in an axial length direction of the heat transfer tube, and which are connected to respective sides of the press-fitted portion, and the first peripheral wall portion is positioned further toward the end portion tip end side of the heat transfer tube than the press-fitted portion and the second bulge portion of the tube expansion portion.

Preferably, the second bulge portion has a hollow, circular sectional shape and the first peripheral wall portion has a hollow, non-circular sectional shape, and the tube expansion portion includes an auxiliary portion that is positioned between the second bulge portion and the first peripheral wall portion in order to create variation in the sectional shape from the second bulge portion to the first peripheral wall portion.

A manufacturing method for a heat exchanger provided by a second aspect of the present invention includes a tube expansion step in which, in a state where end portions of a plurality of heat transfer tubes are respectively inserted through a plurality of hole portions provided in a side wall portion of a case into which a heating medium is supplied, tube expansion processing is implemented on each of the heat transfer tubes, thereby forming a tube expansion portion including a press-fitted portion, in which an outer peripheral surface of each heat transfer tube is press-fitted to an inner peripheral surface of the corresponding hole portion, and a first peripheral wall portion positioned further toward an end portion tip end side of the heat transfer tube than the press-fitted portion, and a tube body connection step performed after the tube expansion step to fit respective end portions of a connecting tube body for connecting the plurality of heat transfer tubes to each other to the first peripheral wall portion of each of the heat transfer tubes, wherein, in the tube expansion step, the first peripheral wall portion is formed in a different sectional shape to a second peripheral wall portion constituting the end portion of the connecting tube body, and in the tube body connection step, the first and second peripheral wall portions are fitted together such that parts thereof in a circumferential direction contact each other and other parts thereof are separated from each other via a gap.

Preferably, the tube expansion step is performed using a divided punch having an expandable and contractable portion that can be inserted into the heat transfer tube and caused to expand and contract in a radial direction, a site for expanding the press-fitted portion and the first peripheral wall portion being provided on an outer peripheral surface of the expandable and contractable portion.

Preferably, the expandable and contractable portion of the divided punch is formed by combining a plurality of segments formed as separate members, and sites on the plurality of segments that correspond to the press-fitted portion are constituted by sites that include a plurality of first outer surface portions, the plurality of first outer surface portions having arc-shaped cross-sections with identical curvature radii and equal distances from a center of the expandable and contractable portion at the time of tube expansion, while sites on the plurality of segments that correspond to the first peripheral wall portion include a plurality of second outer surface portions, the plurality of second outer surface portions having arc-shaped cross-sections with non-identical curvature radii and unequal distances from the center of the expandable and contractable portion at the time of tube expansion.

Other features and advantages of the present invention will become more apparent from the embodiments of the invention, to be described below with reference to the attached figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described specifically below with reference to the figures.

A heat exchanger HE shown inFIG.1is incorporated into a water heater, for example, and used to heat water for use in a hot water supply.

The basic configuration of the heat exchanger HE is similar to that of the heat exchanger described in Japanese Patent Application Publication No. 2020-51682, and includes a substantially rectangular frame-shaped case1that is open at the top and bottom, a plurality of trunk pipes39, a plurality of fins9, a plurality of heat transfer tubes2housed therein, and a plurality of connecting tube bodies6for connecting the heat transfer tubes2to each other.

The heat exchanger HE is used in a reverse combustion type water heater, and a burner (not shown) is disposed in an upper portion of the case1so that combustion gas (an example of the heating medium) generated by the burner is supplied into the case1. Water passing through the trunk pipes39and the plurality of heat transfer tubes2is heated by the combustion gas, whereby hot water is generated.

The plurality of trunk pipes39serve to absorb heat used to heat water and cool a plurality of side wall portions10bto10dof the case1, and are provided to extend around respective inner surfaces of the plurality of side wall portions10bto10d. The plurality of trunk pipes39are connected via header portions35a,35bprovided on an outer surface portion of a side wall portion10aof the case1. As shown by dotted line arrows inFIG.1, water supplied to a water inlet38of the trunk pipes39passes through the trunk pipes39and the plurality of header portions35a,35b, then flows into the plurality of heat transfer tubes2, and after passing through the plurality of heat transfer tubes2reaches a hot water outlet37.

The plurality of heat transfer tubes2and the plurality of connecting tube bodies6are both formed using round metal (stainless steel, for example) pipes. As shown inFIGS.2and3, the plurality of heat transfer tubes2are fin tubes that are inserted through and joined to the plurality of fins9, and are laid horizontally inside the case1so as to be arranged in vertical and horizontal directions. Respective end portions of each heat transfer tube2are drawn out to the outside of the case1by being inserted through hole portions11provided in the side wall portions10a,10cof the case1.

The plurality of connecting tube bodies6are bend tubes having, for example, a substantially semicircular arc-shaped overall shape when seen from the side, and respective end portions60thereof are joined and connected to the end portions of the plurality of heat transfer tubes2. As a result, the plurality of heat transfer tubes2are connected in series via the plurality of connecting tube bodies6.

As shown inFIGS.4A and4B, each heat transfer tube2is provided with a tube expansion portion20in which the outer diameter and inner diameter are larger than in the other parts of the heat transfer tube2. The tube expansion portion20includes a press-fitted portion23, first and second bulge portions20a,20b, an auxiliary portion22, and a first peripheral wall portion21.

The end portion60of the connecting tube body6is fitted into the tube expansion portion20, and the end portion60has a hollow, circular sectional shape. A part62of the end portion60of the connecting tube body6that is joined to the tube expansion portion20so as to be positioned inside the tube expansion portion20corresponds to an example of a “second peripheral wall portion” of the connecting tube body according to the present invention (and will be referred to hereafter as the second peripheral wall portion62). Further, in this embodiment, a bulge portion63is formed on the connecting tube body6. The bulge portion63is set to contact an end portion tip end25of the heat transfer tube2.

The press-fitted portion23of the tube expansion portion20is a site that is positioned in the hole portion11of the side wall portion10aand press-fitted to an inner peripheral surface of the hole portion11, and by providing the press-fitted portion23, the side wall portion10aand the heat transfer tube2are fixed (provisionally fixed) to each other. The hole portion11is a circular hole portion (also seeFIG.5A), and the press-fitted portion23has a hollow, circular sectional shape.

The first and second bulge portions20a,20bof the tube expansion portion20are annular bulge portions that are positioned respectively on an inside and an outside of the side wall portion10aof the case1so as to sandwich the side wall portion10ain an axial length direction of the heat transfer tube2, and have outer peripheral surfaces that partially bulge outward in a radial direction of the heat transfer tube2. The first and second bulge portions20a,20bare preferably disposed in contact with the side wall portion10a. By providing the first and second bulge portions20a,20b, the heat transfer tube2can be fixed to the side wall portion10amore reliably and firmly. A region between the first and second bulge portions20a,20bserves as the press-fitted portion23described above.

The auxiliary portion22is a site positioned between the second bulge portion20band the first peripheral wall portion21. The second bulge portion20b, similarly to the press-fitted portion23, has a hollow, circular sectional shape, whereas the first peripheral wall portion21, as will be described below, has a hollow, non-circular sectional shape. The auxiliary portion22is a site in which the sectional shape described above varies over a range extending from the second bulge portion20bto the first peripheral wall portion21, and is useful in facilitating processing for forming the first peripheral wall portion21.

The first peripheral wall portion21is a site that is further toward the end portion tip end25side of the heat transfer tube2than the second bulge portion20band the auxiliary portion22, and has a hollow, non-circular sectional shape. The end portion60(including the second peripheral wall portion62) of the connecting tube body6, meanwhile, has a hollow, circular sectional shape.

More specifically, as shown inFIG.5B, the first peripheral wall portion21includes three first and three second curved surface portions21a,21b, for example, as the inner peripheral surface thereof. The first curved surface portions21ahave a curvature radius R1that is larger than a curvature radius R0of an outer peripheral surface of the second peripheral wall portion62of the connecting tube body6and partially contact the outer peripheral surface of the second peripheral wall portion62so as to form contact portions Pa. The plurality of first curved surface portions21aand contact portions Pa are provided at equal angular intervals in a circumferential direction of the first and second peripheral wall portions21,62.

The second curved surface portions21bare provided to connect the plurality of first curved surface portions21ato each other without contacting the outer peripheral surface of the second peripheral wall portion62. A gap C is formed between the second curved surface portion21band the second peripheral wall portion62. The parts of the first and second peripheral wall portions21,62that are separated from each other via the gaps C constitute non-contact portions Pb. A curvature radius R2of the second curved surface portion21bhas a relationship of R2<R0<R1, for example.

The connecting tube body6is fitted into the heat transfer tube2so that the tip end of the end portion60thereof is positioned further inside the case1than the side wall portion10a. In so doing, a similar effect to that obtained by adding the end portion60of the connecting tube body6to a joint location between the heat transfer tube2and the side wall portion10aas a reinforcing member can be achieved, and as a result, the strength of the joint location between the heat transfer tube2and the side wall portion10ais improved. This is also effective in improving the strength of a joint location between the connecting tube body6and the heat transfer tube2.

In this embodiment, as shown inFIG.4B, brazed portions Ba, Bb are provided. The brazed portion Ba is a part where the vicinity of the second bulge portion20bis brazed to the side wall portion10a. The brazed portion Bb is a part where the end portion tip end25of the heat transfer tube2is brazed to the outer peripheral surface of the connecting tube body6, and the brazed portion Bb also advances into the aforementioned gaps C.

Next, an example of a manufacturing method for the heat exchanger HE will be described.

A divided punch5such as that shown inFIGS.6A to6D and7A to7Dis used when manufacturing the heat exchanger HE. To facilitate comprehension, the divided punch5will be described first.

The divided punch5is a substantially tubular member into which a mandrel4is inserted. The divided punch5is formed by combining a plurality of segments50ainto a bundle and fitting a plurality of elastic O-rings55to the exterior thereof so as to restrain the plurality of segments50aand prevent the divided punch5from breaking apart. The plurality of segments50acorrespond to a configuration in which a substantially cylindrical member is cut along an axial length direction thereof so as to be divided into six members, for example. An inclined surface56is provided on an inner peripheral surface of the divided punch5near a tip end portion thereof. Accordingly, as shown inFIGS.7A to7D, when the mandrel4is caused to advance so as to press against the inclined surface56, substantially the entire divided punch5expands in a radial direction against the elastic force of the O-rings55. When the mandrel4is withdrawn, the divided punch5is restored to its original unexpanded state, shown inFIGS.6A to6D, by the elastic force of the O-rings55.

The divided punch5according to this embodiment is formed by combining the plurality of separate segments50a, and therefore the entire length region thereof serves as an expandable and contractable portion50. A tip end portion of the mandrel4is preferably formed in a tapered shape such as a truncated conical shape or a conical shape. In this embodiment, the tip end portion of the mandrel4is formed in a truncated conical shape and includes a plurality of planar portions40that are capable of contacting the inclined surface56of the plurality of segments50aby surface contact.

As is clearly illustrated in the enlarged main part view ofFIG.6A, substantially annular first and second projecting portions51,52, a first outer surface portion53positioned between the first and second projecting portions51,52, an auxiliary portion forming portion54, and a second outer surface portion57are provided on an outer peripheral surface of the divided punch5near the tip end portion thereof.

Here, the first and second projecting portions51,52are sites for forming the first and second bulge portions20a,20bof the heat transfer tube2.

The first outer surface portion53is a site for forming the press-fitted portion23of the heat transfer tube2. As shown inFIG.6C, the respective first outer surface portions53of the plurality of segments50aall have the same curvature radius R3, and when the heat transfer tube2is expanded, as shown inFIG.7C, the first outer surface portions53each have an arc-shaped cross-section on which a distance Lc from a center of the expandable and contractable portion50is equal in each location.

The second outer surface portion57is a site for forming the first peripheral wall portion21of the heat transfer tube2. As described above, however, the plurality of first and second curved surface portions21a,21bare provided on the inner peripheral surface of the first peripheral wall portion21. Therefore, to correspond to this, as shown inFIG.6D, two types of segments50a′,50a″ are provided as the plurality of segments50a, and two types of second outer surface portions57(57a,57b) having different curvature radii are formed thereon. The second outer surface portions57aof the segments50a′ are curved surfaces having an arc-shaped cross-section that corresponds to the first curved surface portion21ashown inFIG.5B, while the second outer surface portions57bof the segments50a″ are curved surfaces having an arc-shaped cross-section that corresponds to the second curved surface portion21b. When the heat transfer tube2is expanded, as shown inFIG.7D, distances La, Lb from the center of the expandable and contractable portion50to the respective second outer surface portions57a,57bare unequal.

The auxiliary portion forming portion54is a site for forming the auxiliary portion22of the heat transfer tube2, described above. The shapes and sizes of the second outer surface portions57a,57band the auxiliary portion forming portion54differ between the two types of segments50a′,50a″, but the shapes and sizes of the other sites are the same.

When manufacturing the heat exchanger HE, the divided punch5described above is used to implement a tube expansion operation on the heat transfer tube2by means of procedures shown inFIGS.8A to8C.

First, in a state where the end portion of the heat transfer tube2has been inserted through the hole portion11in the side wall portion10aof the case1, as shown inFIG.8A, the divided punch5is inserted into the end portion of the heat transfer tube2, as shown inFIG.8B. Next, as shown inFIG.8C, the divided punch5is expanded so as to expand the end portion of the heat transfer tube2. Thus, the tube expansion portion20described with reference toFIGS.4A,4B,5A, and5Bcan be provided on the heat transfer tube2, and the heat transfer tube2can also be fixed (provisionally fixed) to the side wall portion10a. Thereafter, the divided punch5is returned to its original size and then withdrawn from the heat transfer tube2, whereupon the end portion60of the connecting tube body6is fitted into the end portion of the heat transfer tube2. This operation is performed on each of the plurality of heat transfer tubes2, but by using a plurality of divided punches5, the operation can be performed simultaneously on the plurality of heat transfer tubes2. When the process described above is complete, a brazing operation is performed to provide the brazed portions Ba, Bb described above.

With the heat exchanger HE according to this embodiment, the following actions are obtained.

As shown inFIG.5B, the first peripheral wall portion21of the heat transfer tube2and the second peripheral wall portion62of the connecting tube body6have different sectional shapes, and the plurality of first curved surface portions21aof the first peripheral wall portion21are fitted to the outer peripheral surface of the second peripheral wall portion62in a state of partial contact therewith (the first and second peripheral wall portions21,62are fitted together in a fitting state including the plurality of contact portions Pa and non-contact portions Pb). Therefore, even if interference constituting the fitting tolerance between the first and second peripheral wall portions21,62is comparatively large, the first and second peripheral wall portions21,62can be fitted together smoothly and easily. As a result, the ease of an assembly operation can be improved.

Further, since the first and second peripheral wall portions21,62are in partial contact with each other, an appropriate degree of frictional force is generated therebetween. Moreover, as shown inFIG.5B, the plurality of contact portions Pa form three point-contact portions positioned at equal intervals. Therefore, when the connecting tube body6is fitted to the heat transfer tube2, the connecting tube body6can be provisionally held with stability. As a result, the danger of the connecting tube body6inadvertently falling off the heat transfer tube2before the operation for brazing the connecting tube body6to the heat transfer tube2is performed can be eliminated.

In this embodiment, when the tube expansion portion20is formed by implementing tube expansion processing on the heat transfer tube2, the first peripheral wall portion21may be set so that a certain degree of interference occurs in relation to the second peripheral wall portion62of the connecting tube body6. When, in contrast to this embodiment, the first and second peripheral wall portions21,62have identical hollow, circular cross-sections and the interference is large, it becomes difficult to fit the first and second peripheral wall portions21,62together, and to avoid this, it is necessary to perform precision finishing so that the fitting tolerance therebetween is within a narrow predetermined dimension range. According to this embodiment, however, this need can be eliminated or mitigated, and therefore the sizes of the first and second peripheral wall portions21,62may be finished comparatively roughly so that a certain degree of interference occurs as the fitting tolerance therebetween. As a result, the manufacturing operation can be further facilitated, enabling an improvement in productivity. When the heat transfer tube2and the connecting tube body6are made of stainless steel, with which it is more difficult to improve the dimension precision of the respective parts than with copper or the like, for example, the above effects of this embodiment are even more welcome.

The press-fitted portion23of the tube expansion portion20is press-fitted to the inner peripheral surface of the hole portion11provided in the side wall portion10aof the case1, and the first and second bulge portions20a,20bsandwich the respective sides of the side wall portion10a. Hence, the heat transfer tube2can be fixed (provisionally fixed) to the side wall portion10aappropriately, favorable fitting precision can be achieved between the hole portion11and the heat transfer tube2, and the brazed portion Ba can be provided appropriately.

Furthermore, the part including the end portion tip end25of the heat transfer tube2and the vicinity thereof is the site that is subjected to tube expansion processing in order to form the first peripheral wall portion21, described above, and therefore the dimension precision of this part can also be improved. More specifically, when the first and second bulge portions20a,20bare formed near the end portion tip end25of the heat transfer tube2, there is a danger that the aperture of the part including the end portion tip end25and the vicinity thereof will shrink in reaction thereto, but according to this embodiment, this danger can be appropriately eliminated.

Meanwhile, according to the manufacturing method for the heat exchanger HE described above, the respective locations of the tube expansion portion20can be provided appropriately by a single tube expansion operation using the divided punch5. As a result, the productivity of the heat exchanger HE can be improved.

FIGS.9A and9B to12A and12Bshow other embodiments of the present invention. In these figures, identical or similar elements to those of the embodiment described above have been allocated identical reference symbols to the above embodiment, and duplicate description thereof has been omitted.

In an embodiment shown inFIG.9A, two second curved surface portions21bare provided on the inner peripheral surface of the first peripheral wall portion21of the heat transfer tube2such that the gap C is formed in two locations, and the remaining parts of the inner peripheral surface form the first curved surface portions21a. In this embodiment, two contact portions Pa and two non-contact portions Pb are provided.

As shown inFIG.9B, this configuration can be formed by dividing the six segments50aof a divided punch5A into two segments50a″ having an outer surface portion that corresponds to the second curved surface portion21band four segments50a′ having an outer surface portion that corresponds to the first curved surface portion21a. Note that a mandrel having a circular cross-section is used as the mandrel4(likewise in the other embodiments shown inFIGS.10A,10B,11A, and11B).

In this embodiment, two contact portions Pa and two non-contact portions Pb are provided, and the two contact portions Pa are arranged opposite each other with the center of the first and second peripheral wall portions21,62therebetween, which is favorable for stabilizing the fitting state between the first and second peripheral wall portions21,62.

Likewise in an embodiment shown inFIG.10A, similarly toFIG.9A, two second curved surface portions21bare provided on the inner peripheral surface of the first peripheral wall portion21of the heat transfer tube2such that the gap C is formed in two locations, while the remaining sites of the inner peripheral surface form the first curved surface portions21a. In other words, two contact portions Pa and two non-contact portions Pb are provided.

Note, however, that a divided punch having a plurality of segments50cdivided into four parts, as shown inFIG.10B, is used as a divided punch5B for acquiring this configuration. Of the plurality of segments50c, two segments50c′ have an outer surface portion that corresponds to the first curved surface portion21a, and two segments50c″ have an outer surface portion that corresponds to the second curved surface portion21b.

Likewise in this embodiment, similarly to the embodiment ofFIG.9A, the two contact portions Pa are arranged opposite each other with the central portions of the first and second peripheral wall portions21,62therebetween, and as a result, the fitting state between the first and second peripheral wall portions21,62can be stabilized.

In an embodiment shown inFIG.11A, only one second curved surface portion21bis provided on the inner peripheral surface of the first peripheral wall portion21of the heat transfer tube2, and the remaining part of the inner peripheral surface forms the first curved surface portion21a. The location on the first and second peripheral wall portions21,62in which the second curved surface portion21bis provided serves as the non-contact portion Pb, and the other location serves as the contact portion Pa.

As shown inFIG.11B, this configuration can be obtained by using the four segments50cas a divided punch5C, forming one segment50c″, among the four segments50c, to have an outer surface portion that corresponds to the second curved surface portion21b, and forming the remaining segments50c′ to have an outer surface portion that corresponds to the first curved surface portion21a.

According to this embodiment, although the first and second peripheral wall portions21,62have only one contact portion Pa, in the contact portion Pa, the first curved surface portion21ais in surface contact with the outer peripheral surface of the second peripheral wall portion62over a range of at least half of the entire circumference thereof. As a result, the fitting state between the first and second peripheral wall portions21,62can be stabilized.

In an embodiment shown inFIGS.12A and12B, the end portion60of the connecting tube body6is externally fitted to the tube expansion portion20of the heat transfer tube2. According to this embodiment, although a disadvantage occurs in that the end portion60of the connecting tube body6cannot be inserted into the case1beyond the side wall portion10aof the case1, it is possible to employ such a configuration. In the case of this embodiment, as shown inFIG.12B, the contact portions Pa are constituted by sites in which parts of the outer peripheral surface of the first peripheral wall portion21of the heat transfer tube2partially contact the inner peripheral surface of the second peripheral wall portion62of the connecting tube body6.

The present invention is not limited to the content of the embodiments described above, and the specific configurations of the respective parts of the heat exchanger according to the present invention may be freely subjected to various design modifications within the intended scope of the present invention. The specific configurations of the respective processes of the manufacturing method for a heat exchanger according to the present invention may be modified freely within the intended scope of the present invention.

In the embodiments described above, the tube expansion operation is performed using a divided punch having six or four segments, but the number of segments is not limited thereto. Further, the sizes of the plurality of segments may be uniformly aligned so that the plurality of segments are arranged at equal angular intervals, or instead, the plurality of segments may be configured to have non-uniform sizes.

In the present invention, a flared portion having a flared shape may additionally be formed in a position at the furthest tip end (a position even further toward the end portion tip end side than the first peripheral wall portion) of the tube expansion portion of the heat transfer tube.

The heat transfer tube is not limited to an entirely straight tube shape and may have a meandering shape, a spiral shape, or the like. The trunk pipe39of the embodiment described above may also be included in the heat transfer tube according to the present invention. Not all of the plurality of heat transfer tubes provided in the heat exchanger need have the intended configuration of the present invention, and as long as some of the heat transfer tubes have an attachment structure configured as intended by the present invention, the resulting configurations belong to the technical scope of the present invention.

The heat exchanger according to the present invention is not limited to a reverse combustion system in which combustion gas advances downward, and may be applied to a normal combustion system in which combustion gas advances upward. Moreover, the heat exchanger according to the present invention may be configured so as not to include the trunk pipes. Furthermore, the heat exchanger is not limited to use in a water heater. The heating medium is not limited to combustion gas, and high-temperature exhaust gas generated from a power generation system, for example, may be used instead.