JP7734960B2 - Double-pipe heat exchanger manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a double-pipe heat exchanger having an outer pipe and an inner pipe.

Car air conditioners sometimes use an internal heat exchanger that improves the cooling efficiency of the evaporator by exchanging heat between the high-temperature liquid refrigerant from the condenser, which is part of the refrigeration cycle, and the low-temperature gaseous refrigerant from the evaporator, which is also part of the refrigeration cycle. This internal heat exchanger is a double-pipe heat exchanger consisting of an outer tube and an inner tube placed inside the outer tube, with a flow path for the high-temperature liquid refrigerant from the condenser formed between the outer and inner tubes, and a flow path for the low-temperature gaseous refrigerant from the evaporator formed inside the inner tube.

In double-pipe heat exchangers, if there is a gap between the outer and inner pipes, the inner pipe will not be able to follow the deformation of the outer pipe when an external force is applied, which could result in abnormal deformation or cracking of the inner pipe. Therefore, the outer and inner pipes are usually fixed in place so that they are in contact with each other.

For example, Patent Document 1 describes a double pipe comprising an outer pipe and an inner pipe, with a groove formed in the inner pipe, in which, in the straight pipe section, the inner pipe is not in contact with the outer pipe or is in contact only in one radial direction, and in the bent section, the inner pipe is in contact with the outer pipe in multiple radial directions.

In Patent Document 1, the outer and inner pipes are bent while the inner pipe remains positioned inside the outer pipe, thereby bringing the two into contact. This raises the risk of the inner pipe cracking when bending the outer and inner pipes, and also creates areas where the outer and inner pipes do not come into contact. Furthermore, if the outer and inner pipes do not come into contact, the cross-sectional area of the flow path between the outer and inner pipes becomes large, increasing the amount of refrigerant.

Patent Document 2 describes a double-pipe heat exchanger in which concave grooves are formed in the inner pipe to increase the heat transfer area and improve heat exchange efficiency, and a manufacturing method in which the inner pipe is moved while a groove-forming tool is pressed against the inner pipe to form the concave grooves in the inner pipe.

In Patent Document 2, the concave grooves in the inner tube are formed by rolling using a groove-forming tool, which poses problems such as expensive manufacturing equipment and a long processing time.

In Patent Document 3, the applicant proposes a method for manufacturing a double-pipe heat exchanger that allows for the formation of a corrugated portion on the inner pipe in a short period of time.

Patent No. 4350079 specification Patent No. 4628858 specification Japanese Patent Application Laid-Open No. 2020-82192

The present invention has been made in consideration of the problems of the conventional art, and aims to provide a method for manufacturing a double-pipe heat exchanger in which the inner pipe is not deformed or cracked when bending the double pipe, the cross-sectional area of the flow path between the outer pipe and the inner pipe can be reduced to reduce the amount of refrigerant, and the outer pipe can be easily reduced in diameter.

As a means for solving the above problem,
(1) The double-pipe heat exchanger according to the present invention is
A double-pipe heat exchanger including an outer pipe and an inner pipe inserted inside the outer pipe,
a plurality of bottoms and tops are formed on the outer surface of the inner tube;
The outer tube is characterized in that its diameter is reduced so that a plurality of points on the inner surface of the outer tube are in contact with the top of the inner tube.
Here, "diameter reduction" includes deforming a part of the outer surface of the outer tube inward in the radial direction, and deforming the entire periphery of the outer surface of the outer tube inward in the radial direction.

(2) It is preferable that the outer tube has a reduced diameter by forming a recess extending in the axial direction on the outer surface of the outer tube.

(3) It is preferable that the outer tube has a reduced diameter by forming a recessed groove extending in the axial direction on the outer surface of the outer tube.

(4) It is preferable that the outer tube has a reduced diameter by forming a spiral groove extending in the axial direction on the outer surface of the outer tube.

(5) A method for manufacturing a double-pipe heat exchanger according to the present invention includes the steps of:
A method for manufacturing a double-pipe heat exchanger including an outer pipe and an inner pipe inserted into the outer pipe,
The method is characterized in that the outer tube is pressed radially inward against an inner tube having a plurality of bottoms and tops formed on its outer surface, thereby reducing the diameter within a predetermined range in the axial direction and bringing multiple points on the inner surface of the outer tube into contact with the tops of the inner tube.

(6) It is preferable that the diameter of the outer tube is reduced by pressing multiple movable pieces arranged at equal intervals around the outer tube radially inward against the outer tube, and then moving the outer tube and the inner tube axially. This is repeated multiple times to form recesses extending axially on the outer surface of the outer tube, thereby reducing the diameter.

(7) It is preferable to reduce the diameter by moving the outer tube and the inner tube in the axial direction while pressing a plurality of rollers having rotation axes on a plane perpendicular to the central axis of the outer tube against the outer tube, and forming recesses extending in the axial direction on the outer surface of the outer tube with the rollers.

(8) It is preferable to reduce the diameter by moving the outer tube and the inner tube in the axial direction while pressing multiple rollers having rotation axes on a plane perpendicular to the central axis of the outer tube against the outer tube, and forming axially extending grooves on the outer surface of the outer tube with the rollers.

(9) It is preferable to reduce the diameter by rotating a plurality of rollers, each having a rotation axis parallel to the central axis of the outer tube, against the outer tube while rotating the rollers around the outer tube and moving the outer tube and inner tube in the axial direction, and forming a spiral groove extending in the axial direction on the outer surface of the outer tube with the rollers.

According to the present invention , the inner surface of the outer pipe is in contact with the top of the inner pipe at multiple points, and the inner pipe is supported by the outer pipe, so there is no risk of the inner pipe being deformed or cracked when the double pipe is bent. Furthermore, the outer pipe is reduced in diameter, reducing the cross-sectional area of the flow path between the outer pipe and the inner pipe, allowing for a reduction in the amount of refrigerant.

According to the present invention , the outer tube can be easily reduced in diameter by simply pressing the outer tube radially inward using a plurality of radially movable drawing tools.

1 is a longitudinal cross-sectional view of a double-pipe heat exchanger according to the present invention. 2 is an enlarged cross-sectional view of the double-pipe heat exchanger of FIG. 1 taken along line II-II. FIG. 10 is a perspective view showing the double-pipe heat exchanger in a bent state. 4A is a side view showing a first embodiment of a method for reducing the diameter of an outer tube to bring the inner surface of the outer tube into contact with the top of the wave-shaped portion of an inner tube, and FIG. 4B is a cross-sectional view taken along line IVb-IVb thereof. 5A to 5C are cross-sectional views sequentially illustrating a method for reducing the diameter of the outer tube by the movable piece of FIG. 4 . 5 is an enlarged cross-sectional view of the outer tube and the inner tube after the diameter is reduced by the movable piece of FIG. 4 . 5 is a front view showing a recess formed in the outer tube after the diameter is reduced by the movable piece of FIG. 4 . 10 is a side view showing a second embodiment of a method for reducing the diameter of the outer tube to bring the inner surface of the outer tube into contact with the top of the corrugated portion of the inner tube. FIG. 9A to 9C are cross-sectional views sequentially illustrating a method for reducing the diameter of the outer tube by the roller of FIG. 8 . 9 is an enlarged cross-sectional view of the outer tube and the inner tube after the diameter is reduced by the roller of FIG. 8 . 9 is a front view showing the recesses formed in the outer tube after the diameter is reduced by the roller of FIG. 8 . 10 is a side view showing a third embodiment of a method for reducing the diameter of the outer tube to bring the inner surface of the outer tube into contact with the top of the corrugated portion of the inner tube. FIG. 13A to 13C are cross-sectional views sequentially illustrating a method for reducing the diameter of the outer tube by the roller of FIG. 12 . 13 is an enlarged cross-sectional view of the outer tube and the inner tube after the diameter is reduced by the rollers of FIG. 12 . 13 is a front view showing the grooves formed in the outer tube after the diameter is reduced by the rollers of FIG. 12 . FIG. 11 is a front view showing a modification of the third embodiment. 17 is an enlarged cross-sectional view of the outer tube and the inner tube after the diameter is reduced by the roller of FIG. 16 . 10 is a side view showing a fourth embodiment of a method for reducing the diameter of the outer tube to bring the inner surface of the outer tube into contact with the top of the corrugated portion of the inner tube. FIG. 19A to 19C are cross-sectional views sequentially showing a method of reducing the diameter of the outer tube by the roller of FIG. 18 . 19 is an enlarged cross-sectional view of the outer tube and the inner tube after the diameter is reduced by the roller of FIG. 18 . 19 is a front view showing the grooves formed in the outer tube after the diameter is reduced by the roller of FIG. 18 .

Embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 shows a double-pipe heat exchanger 1 according to a first embodiment of the present invention. This double-pipe heat exchanger 1 comprises an outer pipe 2 and an inner pipe 3 inserted inside the outer pipe 2. In this embodiment, the double-pipe heat exchanger 1 is an internal heat exchanger for a car air conditioner. A high-temperature, high-pressure liquid refrigerant flows through a first flow path 4 between the outer pipe 2 and the inner pipe 3, and a low-temperature, low-pressure gas refrigerant flows through a second flow path 5 within the inner pipe 3. The two refrigerants are described as flowing in countercurrent fashion. However, the opposite is also true: a low-temperature, low-pressure gas refrigerant may flow through the first flow path 4 and a high-temperature, high-pressure refrigerant may flow through the second flow path 5, or the two may flow in parallel.

As shown in Figure 2, the outer pipe 2 is made of an aluminum extrusion with a circular cross section and has a reduced diameter section 6 within a predetermined axial range, which has been reduced in diameter by a method described below. The inner surface 7 of the reduced diameter section 6 of the outer pipe 2 contacts the crests 10b of the corrugated sections 10 of the inner pipe 3, which will be described later, at multiple locations P. As shown in Figure 1, both ends of the outer pipe 2 are joined to the outer surface of the inner pipe 3 and closed. The outer pipe 2 is formed with an outer pipe inlet 8 through which high-temperature, high-pressure liquid refrigerant flows in, and an outer pipe outlet 9 through which the liquid refrigerant flows out after heat exchange with the gas refrigerant in the inner pipe 3.

As shown in FIG. 2, the inner pipe 3 is made of an aluminum extrusion with a circular cross section and has an outer diameter smaller than the inner surface of the outer pipe 2. When viewed from the outside, the inner pipe 3 has eight corrugated portions 10, each of which has a plurality of axially extending bottom portions 10a and top portions 10b that alternate in the circumferential direction. The corrugated portions 10 are formed by known methods such as pressing or rolling. The corrugated portions 10 may also extend spirally in the axial direction. As shown in FIG. 1, the inner pipe 3 has an inner pipe inlet 11 at one end through which a low-temperature, low-pressure gas refrigerant flows in, and an inner pipe outlet 12 at the other end through which the gas refrigerant flows out after heat exchange with the liquid refrigerant between the outer pipe 2 and inner pipe 3.

Next, we will explain the operation of the double-pipe heat exchanger 1.

In Figure 1, the heat held by the high-temperature, high-pressure liquid refrigerant flowing in the first flow path 4 between the outer tube 2 and the inner tube 3 is transferred to the low-temperature, low-pressure gas refrigerant flowing in the second flow path 5 of the inner tube 3 through heat transfer to the outer surface of the wall of the inner tube 3, heat conduction within the wall of the inner tube 3, and heat transfer from the inner surface of the wall of the inner tube 3.

This allows heat exchange to occur between the high-temperature, high-pressure liquid refrigerant flowing through the first flow path 4 between the outer tube 2 and the inner tube 3 and the low-temperature, low-pressure gas refrigerant flowing through the second flow path 5 within the inner tube 3. The high-temperature, high-pressure liquid refrigerant in the first flow path 4 between the outer tube 2 and the inner tube 3 is cooled, and the low-temperature, low-pressure gas refrigerant in the second flow path 5 within the inner tube 3 is heated. In particular, the inner tube 3 has a larger heat transfer area in the central portion having the corrugated portion 10 than when it does not have the corrugated portion 10, resulting in good heat exchange efficiency.

In the double-pipe heat exchanger 1 of the above embodiment, as shown in FIG. 2, multiple points on the inner surface of the outer pipe 2 contact the apex 10b of the corrugated portion 10 of the inner pipe 3, and the inner pipe 3 is supported by the outer pipe 2. Therefore, as shown in FIG. 3, there is no risk of the inner pipe 3 deforming or cracking when the double pipe is bent. Furthermore, because the outer pipe 2 is reduced in diameter, the cross-sectional area of the flow path for high-pressure liquid refrigerant between the outer pipe 2 and the inner pipe 3 is reduced, so the amount of refrigerant injected into the refrigeration cycle in which the double-pipe heat exchanger 1 is installed can be reduced.

Next, we will explain an embodiment of a method for manufacturing the double-pipe heat exchanger 1, particularly a method for contacting the outer pipe 2 with the inner pipe 3.

First Embodiment
4 shows the multiple movable pieces 21 of the first embodiment that bring the outer tube 2 into contact with the inner tube 3. Eight movable pieces 21 are arranged at equal intervals around the outer tube 2. Each movable piece 21 has a sector-shaped cross section in the radial direction, and the inner surface on the radially inner side is a curved surface with a radius slightly smaller than the outer peripheral surface of the outer tube 2, and has a predetermined length in the axial direction. Each movable piece 21 is movable in the radial direction of the outer tube 2 and can simultaneously press against the outer tube 2.

The inner tube 3 is inserted inside the outer tube 2, with both ends of the inner tube 3 protruding from both ends of the outer tube 2. Multiple movable pieces 21 are positioned at one end of the predetermined range L for reducing the diameter of the outer tube 2. Preferably, the multiple movable pieces 21 face the crests 10b of the corrugated portion 10 of the inner tube 3. When the multiple movable pieces 21 are moved radially inward and the outer tube 2 is pressed, as shown in Figure 6, the portions of the outer tube 2 pressed by the movable pieces 21 move radially inward toward the inner tube 3 and plastically deform while in contact with the crests 10b of the corrugated portion 10 of the inner tube 3, forming recesses 6a on the outer surface of the outer tube 2. Multiple recesses 6a are formed circumferentially, forming the reduced diameter portion 6.

As shown in Figure 5(a), the first recess 6a is formed using multiple movable pieces 21, and then, as shown in Figure 5(b), the outer tube 2 is moved axially to form a second recess 6a, and this process is repeated multiple times. The axial length s of the recess 6a previously formed by the multiple movable pieces 21 and the axial length s of the recess 6a subsequently formed are arranged to overlap by a predetermined dimension ΔL. By repeatedly forming recesses 6a of length s multiple times, recesses 6a are formed within a predetermined range L, as shown in Figure 5(c). Note that the recesses 6a of length s may be spaced apart in the axial direction so as not to overlap, or may be offset circumferentially.

As shown in Figure 7, multiple pressing operations by the multiple movable pieces 21 form multiple axially extending recesses 6a on the outer surface of the outer tube 2, and multiple locations P on the inner surface of the outer tube 2 come into linear contact with the peaks 10b of the corrugated portion 10 of the inner tube 3. Between circumferentially adjacent recesses 6a, which are not pressed by the multiple movable pieces 21, axially extending protrusions 13 are formed.

As shown in Figure 1, both ends of the outer pipe 2 are radially fastened to the outer periphery of the inner pipe 3 in the area where the corrugated portions 10 are not formed, and then secured by brazing or welding. Next, an outer pipe inlet 8 is attached to one end of the outer pipe 2, and an outer pipe outlet 9 is attached to the other end.

According to the manufacturing method of the double-pipe heat exchanger 1 of the first embodiment, the outer pipe 2 can be easily reduced in diameter simply by pressing the outer pipe 2 radially inward with multiple movable pieces 21 that can move radially.

Second Embodiment
In the second embodiment and the following embodiments, the process of inserting and positioning the outer tube 2 into the inner tube 3 and the process of fixing the outer tube 2 to the inner tube 3 are the same as those in the first embodiment, so the explanation will be omitted and only the process of bringing the outer tube 2 into contact with the inner tube 3 will be explained.

Figure 8 shows a second embodiment of rollers 22 that bring the outer tube 2 into contact with the inner tube 3. Four rollers 22 are arranged at equal intervals around the outer tube 2. Each roller 22 has a rotation axis 22a that is perpendicular to the central axis of the outer tube 2 and perpendicular to a line passing through the center of the outer tube 2, and is rotatable around this rotation axis 22a. The rollers 22 have a small diameter in the center and a larger diameter toward both ends, creating a so-called hourglass-shaped outer surface. The cross section of the roller 22, including its axis, is slightly smaller than the radius of curvature of the outer tube 2 and has a length approximately one-quarter of the outer circumference of the outer tube 2. Each roller 22 is movable radially around the outer tube 2 and can simultaneously press the outer tube 2. Preferably, the rollers 22 face the crests 10b of the corrugated portions 10 of the inner tube 3.

As shown in Figure 9(a), rollers 22 are positioned at one end of the predetermined range L within which the outer tube 2 is reduced in diameter. The rollers 22 are then moved radially inward to press against the outer tube 2. The outer tube 2 undergoes plastic deformation radially inward toward the crests 10b of the corrugated portion 10 of the inner tube 3, and as shown in Figure 10, multiple locations P on the inner surface of the outer tube 2 come into contact with the crests 10b of the inner tube 3. As shown in Figure 9(b), when the outer tube 2 is slid axially while the rollers 22 are pressed against the inner tube 3, the rollers 22 form axially extending recesses 6b on the outer surface of the outer tube 2, as shown in Figure 11. Multiple locations on the inner surface of the tube 2 come into linear contact with the crests 10b of the corrugated portion 10 of the inner tube 3. Axial protrusions 14 are formed between circumferentially adjacent recesses 6b because they are not pressed by the rollers 22.

According to the manufacturing method of the double-pipe heat exchanger 1 of the second embodiment, the outer pipe 2 can be easily reduced in diameter simply by pressing the outer pipe 2 radially inward with a plurality of radially movable rollers 22 and moving the outer pipe 2 in the axial direction.

Third Embodiment
12 shows a third embodiment of rollers 23 for bringing the outer tube 2 into contact with the inner tube 3. Four rollers 23 are arranged at equal intervals around the outer tube 2. Each roller 23 has a rotation axis 23a that is on a plane perpendicular to the central axis of the outer tube 2 and perpendicular to a line passing through the center of the outer tube 2, and is rotatable around this rotation axis 23a. The rollers 23 are narrower than the rollers 22 of the second embodiment and have an arcuate or flat outer peripheral surface. Each roller 23 is movable in the radial direction of the outer tube 2 and can simultaneously press the outer tube 2. Preferably, the rollers 23 face the crests 10b of the corrugated portion 10 of the inner tube 3.

As shown in Figure 13(a), when multiple rollers 23 are positioned at one end of a predetermined range L within which the outer tube 2 is reduced in diameter, and the multiple rollers 23 are moved radially inward to press against the outer tube 2, the outer tube 2 undergoes plastic deformation radially inward toward the crests 10b of the corrugated portions 10 of the inner tube 3. As shown in Figure 14, multiple locations P on the inner surface of the outer tube 2 come into contact with the crests 10b of the corrugated portions 10 of the inner tube 3. As shown in Figure 13(b), when the outer tube 2 is slid axially while the rollers 23 are pressed against the inner tube 3, the rollers 23 form axially extending grooves 6c on the outer surface of the outer tube 2, as shown in Figure 15. Multiple locations on the inner surface of the outer tube 2 come into linear contact with the crests 10b of the corrugated portions 10 of the inner tube 3.

According to the manufacturing method of the double-pipe heat exchanger 1 of the third embodiment, the outer pipe 2 can be easily reduced in diameter simply by pressing the outer pipe 2 radially inward with a plurality of radially movable rollers 23 and then moving the outer pipe 2 axially. Furthermore, because multiple points on the inner surface of the outer pipe 2 come into contact with the crests 10b of the corrugated portions 10 of the inner pipe 3, the first flow path 4 between the outer pipe 2 and inner pipe 3 does not become narrow.

In the third embodiment, as shown in FIG. 12, rollers 23 are arranged circumferentially to face the crests 10b of the corrugated portions 10, and the outer tube 2 is pressed radially inward by the multiple rollers 23, plastically deforming the outer tube 2 and bringing multiple locations on the inner surface of the outer tube 2 into contact with the crests 10b of the corrugated portions 10 of the inner tube 3. However, as shown in FIG. 16, rollers 23 may be arranged circumferentially to face the bottoms 10a of the corrugated portions 10, and the outer tube 2 is pressed radially inward by the multiple rollers 23, plastically deforming the outer tube 2, bringing multiple locations P on the inner surface of the outer tube 2 on both sides of the rollers 23 into contact with the crests 10b of the corrugated portions 10 of the inner tube 3, as shown in FIG. 17.

Fourth Embodiment
FIG. 18 shows a fourth embodiment of rollers 24 that bring the outer tube 2 into contact with the inner tube 3. Four rollers 24 are arranged at equal intervals around the outer tube 2. Each roller 24 has a rotation axis 24a parallel to the central axis of the outer tube 2 and is rotatable around this rotation axis 24a. The rollers 24 are narrower than the rollers 22 of the second embodiment and have an arcuate or flat outer peripheral surface. Each roller 24 is movable in the radial direction of the outer tube 2, allowing the rollers 24 to simultaneously press against the outer tube 2, and is also capable of rotating around the outer tube 2. In FIG. 18, the number of rollers 24 is not limited to four and can be any number, but three is preferred.

As shown in Figure 19(a), when the roller 24 is rotated around the outer tube 2 while pressed against the outer tube 2 and the outer tube 2 is slid in the axial direction, the roller 24 forms a spiral groove 6d extending in the axial direction on the outer surface of the outer tube 2, as shown in Figures 19(b) and 21, and multiple points P on the inner surface of the outer tube 2 come into contact with the top 10b of the inner tube 3, as shown in Figure 20.

According to the manufacturing method of the double-pipe heat exchanger 1 of the fourth embodiment, the outer pipe 2 can be easily reduced in diameter simply by moving the outer pipe 2 in the axial direction while pressing the outer pipe 2 radially inward with a plurality of radially movable rollers 24 and rotating them around the outer pipe 2.

The present invention is not limited to the above-described embodiment, and various modifications are possible.
For example, in the above embodiment, eight corrugated portions 10 are provided on the inner pipe 3, but the number is not limited to eight and any number may be provided. The height between the bottom 10a and the crest 10b of each corrugated portion 10 can also be changed as appropriate. Furthermore, the corrugated portion 10 may have a plurality of bottoms 10a and crests 10b extending spirally in the axial direction, repeated in the circumferential direction.

1...Double tube heat exchanger 2...Outer tube 3...Inner tube 4...First channel 5...Second channel 6...Reduced diameter section 6a, 6b...Concave section 6c, 6d...Concave groove 7...Inner surface 8...Outer tube inlet 9...Outer tube outlet 10...Wave shaped part 10a...Bottom part 10b...Top part 11...Inner tube inlet 12...Inner tube outlet 13, 14... Convex portion 21... Movable piece 22, 23, 24... Roller

Claims (4)

A method for manufacturing a double-pipe heat exchanger including an outer pipe and an inner pipe inserted into the outer pipe,
A method for manufacturing a double-pipe heat exchanger, characterized in that a plurality of movable pieces arranged at equal intervals around the outer pipe are pressed radially inward against the outer pipe, and the outer pipe and the inner pipe are moved axially, and this is repeated multiple times to form axially extending recesses on the outer surface of the outer pipe, thereby reducing the diameter of the outer pipe relative to the inner pipe, which has a plurality of bottoms and tops formed on its outer surface, and bringing multiple points on the inner surface of the outer pipe into contact with the tops of the inner pipe . A method for manufacturing a double-pipe heat exchanger including an outer pipe and an inner pipe inserted into the outer pipe,
A method for manufacturing a double-pipe heat exchanger, characterized in that: while pressing a plurality of rollers, each having a rotation axis on a plane perpendicular to the central axis of the outer tube, against the outer tube, the outer tube and the inner tube are moved axially, and the rollers form recesses extending in the axial direction on the outer surface of the outer tube, thereby reducing the diameter of the outer tube relative to the inner tube, which has a plurality of bottoms and tops formed on its outer surface, and bringing multiple points on the inner surface of the outer tube into contact with the tops of the inner tube . A method for manufacturing a double-pipe heat exchanger including an outer pipe and an inner pipe inserted into the outer pipe,
A method for manufacturing a double-pipe heat exchanger, characterized in that a plurality of rollers having rotation axes on a plane perpendicular to the central axis of the outer tube are pressed against the outer tube, and the outer tube and the inner tube are moved axially, so that the rollers form axially extending grooves on the outer surface of the outer tube, thereby reducing the diameter of the outer tube relative to the inner tube, which has a plurality of bottoms and tops formed on its outer surface, and bringing multiple points on the inner surface of the outer tube into contact with the tops of the inner tube . A method for manufacturing a double-pipe heat exchanger including an outer pipe and an inner pipe inserted into the outer pipe,
A method for manufacturing a double-pipe heat exchanger, comprising the steps of: pressing a plurality of rollers, each having a rotation axis parallel to the central axis of the outer tube, against the outer tube; rotating the rollers around the outer tube while moving the outer tube and the inner tube axially; forming a spiral groove extending in the axial direction on the outer surface of the outer tube with the rollers; thereby reducing the diameter of the outer tube relative to the inner tube, which has a plurality of bottoms and tops formed on its outer surface, and bringing multiple points on the inner surface of the outer tube into contact with the tops of the inner tube .