Boiling-type heat transfer tube and method for manufacturing boiling-type heat transfer tube

Abstract

Provided are a boiling-type heat transfer tube capable of maintaining a high heat transfer coefficient from the boiling type heat transfer tube to a liquid refrigerant and obtaining high heat transfer efficiency, and a method for manufacturing the boiling-type heat transfer tube. This boiling-type heat transfer tube comprises a plurality of rows of fins (11) that protrude radially outward from the outer peripheral surface of a tube body and are formed in a spiral shape along the central axis of the tube body. The fin (11) has a leg part (11a) and a pair of protruding parts (11b, 11c). Cavities (13) continuous in the circumferential direction are defined between the fins (11) adjacent to each other in the tube axis direction. A recessed part (23) recessed in the tube axis direction is formed on a side wall surface (13b) of the cavity (13), and a plurality of flat ridge parts (17) in which an extension piece extending from the recessed part (23) is crimped to a bottom surface (13a) is formed on the bottom surface (13a) of the cavity (13).

Description

Boiling type heat transfer tube and method for manufacturing a boiling type heat transfer tube

[0001] The present invention relates to a boiling-type heat transfer tube used in a heat exchanger that boils a refrigerant outside the tube, and to a method for manufacturing a boiling-type heat transfer tube.

[0002] Boiling-type heat transfer tubes are incorporated into the evaporators of vapor compression type refrigerators, such as turbo chillers and screw chillers, and are immersed in a liquid refrigerant (e.g., Freon, liquid nitrogen, etc.) to heat and boil the liquid refrigerant. Various heat transfer surface shapes have been proposed for this type of boiling-type heat transfer tube.

[0003] For example, Patent Document 1 describes a technique for improving heat transfer performance by promoting the boiling of liquid refrigerant within a cavity and promoting turbulence of the liquid refrigerant and vaporized medium on the outer surface of the tube. Patent Document 2 describes a configuration in which material protrusions extending in the circumferential and axial directions are formed on the sides of fins formed on the outer surface of a heat transfer tube (pipe) so as to cover the bottom of the grooves between the fins. This configuration increases the proportion of the groove bottom covered by the material protrusions, thereby improving the evaporation efficiency of the liquid refrigerant.

[0004] Japanese Patent Publication No. 4-236097 Publication of Japanese Special Publication No. 2015-500456

[0005] By the way, in recent years, CO2 has been used in refrigeration systems. ２ In response to environmental concerns such as reducing emissions, inverter control, which allows for precise control of the evaporator that heats and boils the liquid refrigerant, and thereby improves energy efficiency, has been widely adopted. However, even with inverter control, the heat transfer rate from the heat transfer tubes to the liquid refrigerant is not always sufficient depending on the operating conditions, and further improvements in heat transfer efficiency are required.

[0006] According to the technologies described in Patent Documents 1 and 2 above, a certain degree of improvement in heat transfer efficiency can be expected. However, the small protrusions in the configuration of Patent Document 1 are formed at the bottom of the groove, and the angle of the corner at the base of the protrusions is greater than 90 degrees. In order to promote nucleation boiling in the cavity, many acute-angled foaming points are required within the cavity, but with the shape of the small protrusions, which can only produce obtuse-angled protrusions, it is difficult to secure many foaming points, and a significant nucleation boiling promotion effect cannot be expected.

[0007] On the other hand, the material protrusions in the configuration of Patent Document 2 are processed on the side surface of the fins, making it easier to create sharp corner angles. However, since the protrusions are not formed at the bottom of the groove where the degree of superheating is highest, the effect of promoting nucleation boiling is small. In addition, the material protrusions are formed to protrude significantly within the cavity, obstructing the flow of refrigerant vapor within the cavity. Refrigerant vapor and refrigerant liquid are present within the cavity, and the heat exchange due to refrigerant evaporation at the gas-liquid interface has a greater impact than the nucleation boiling described above. Therefore, the material protrusions in Patent Document 2, which obstruct the flow of vapor, cannot be said to have an appropriate shape.

[0008] Therefore, the present invention aims to provide a boiling-type heat transfer tube and a method for manufacturing a boiling-type heat transfer tube that can maintain a high heat transfer coefficient from the boiling-type heat transfer tube to the liquid refrigerant and obtain high heat transfer efficiency.

[0009] According to one embodiment of the present invention, the following configuration is provided: (1) A boiling heat transfer tube comprising a plurality of rows of fins that project radially outward from the outer circumferential surface of a tube and are formed spirally along the central axis of the tube, wherein each fin has a leg portion erected on the outer circumferential surface of the tube and a pair of overhanging portions whose radially outward tips extend in opposite directions with respect to the tube axis of the tube, a cavity is defined between adjacent fins in the tube axis direction, surrounded by the overhanging portion of one fin that protrudes toward each other, the overhanging portion of the other fin, and the pair of legs that face each other, a cavity that is continuous in the circumferential direction, a recess formed on the side wall surface of the cavity that is recessed in the tube axis direction, and a plurality of flat tiers formed on the bottom surface of the cavity, with extension pieces extending from the recess pressed against the bottom surface. (2) A method for manufacturing a boiling-type heat transfer tube as described in (1), comprising: pressing the outer circumferences of a plurality of rolling discs against the outer circumference of the tube body to plastically deform the outer circumference of the tube body to form a plurality of rows of fins projecting radially outward from the outer circumference of the tube body in a spiral shape along the central axis of the tube body; inserting a cutting disc having a cutting blade formed on its outer circumference between adjacent fins in the axial direction of the tube body to cut away the sides of the fins with the cutting blade to form the recess and the extension piece; inserting a pressing disc between the fins after the cutting disc has been inserted to press the extension piece into the bottom of the groove between the fins to form the step at the bottom of the groove; and pushing the tips of the fins radially inward after the pressing disc has been inserted to form a pair of the protruding portions.

[0010] According to the present invention, the heat transfer coefficient from the boiling-type heat transfer tube to the liquid refrigerant can be maintained at a high level, and high heat transfer efficiency can be obtained.

[0011] Figure 1 is a partial cross-sectional perspective view showing an axial cross-section of a boiling-type heat transfer tube. Figure 2 is a schematic partial enlarged cross-sectional view showing a portion of the outer surface of a boiling-type heat transfer tube cut out and enlarged. Figure 3 is a schematic partial cross-sectional perspective view showing the state after the legs of the fins shown in Figure 2 have been cut along line III-III and the protruding portion has been removed. Figure 4 is a cross-sectional view along line IV-IV shown in Figure 2. Figure 5 is a cross-sectional view showing a boiling-type heat transfer tube placed in a liquid refrigerant, taken in a cross-section perpendicular to the tube axis. Figure 6 is an explanatory diagram showing how the liquid refrigerant is heated in the cavity and evaporation bubbles are generated. Figure 7 is a schematic front view showing the main part of a processing device that forms fins etc. on the outer surface of a tube. Figure 8 is a side view of the processing device shown in Figure 7. Figure 9 is a schematic cross-sectional view showing how fins are processed by a gear disc and a pressing disc. Figure 10 is an explanatory diagram schematically showing the state of the fins cut by the gear disc. Figure 11 is an explanatory diagram schematically showing the state of the protrusions and cut pieces after the pressing disc is inserted between the fins. Figure 12 is an explanatory diagram showing the stepwise procedure for finishing the fins in the region. Figure 13 is a schematic process diagram showing how the fin shape changes stepwise as the tube is processed by a group of discs. Figure 14 is a schematic diagram of the test apparatus used to evaluate the heat transfer performance of the boiling type heat transfer tube. Figure 15 is an explanatory diagram showing the dimensions of the fins and ribs shown in Tables 2 and 3. Figure 16 is a graph showing the relationship between heat flux and overall heat transfer coefficient in Test Examples 1 to 3. Figure 17 is a photograph showing how a step is formed at the bottom of the groove by inserting a gear disc and a pressing disc between the formed fins. Figure 18 is a cross-sectional photograph showing the side view of the fin shown in Figure 17. Figure 19 is a photograph showing how an emboss is formed by crushing a pair of protruding parts with a gear disc.

[0012] Embodiments of the present invention will be described in detail below with reference to the drawings. <Configuration of Boiling Type Heat Transfer Tube> Figure 1 is a partial cross-sectional perspective view showing an axial cross-section of a boiling type heat transfer tube according to the present invention. Figure 2 is a schematic partial enlarged cross-sectional view showing a portion of the outer surface of the boiling type heat transfer tube cut out and enlarged. The boiling type heat transfer tube 100 shown in Figure 1 consists of a metal tube body 10 extending in one direction. Multiple rows of fins 11 (Figure 2) are formed on the outer surface of the tube body 10 along the circumferential direction TD, and continuous voids 13 are formed between the fins 11 in the circumferential direction. Multiple rows of ribs 15 are formed in a spiral shape on the inner surface of the tube body 10. The ribs 15 may be omitted, and the inner surface of the tube body 10 may be a smooth cylindrical inner surface.

[0013] As shown in Figure 2, the fins 11 protrude radially outward from the outer surface of the tube 10 and are formed in a spiral shape along the central axis Lc of the tube 10 shown in Figure 1. The number of fins 11 may be one or more. In this specification, the axial direction of the boiling heat transfer tube 100 is denoted as Ax, the circumferential direction as TD, and the radial direction as RD.

[0014] Each fin 11 has a leg portion 11a erected on the outer surface of the tube and a pair of protruding portions 11b and 11c. The pair of protruding portions 11b and 11c are formed by the radially outer tip of the leg portion 11a splitting into two, and extending in opposite directions with respect to the tube axis Ax. Of the pair of adjacent fins 11, the protruding portion 11b of one fin 11 and the protruding portion 11c of the other fin 11 are positioned to face each other with a gap between their ends, extending from the leg portion 11a. This defines a circumferentially continuous cavity 13.

[0015] Multiple raised sections 17 are formed on the bottom surface 13a of the cavity 13. As will be described in detail later, the raised sections 17 are formed by pressing the material and shavings obtained by machining the legs 11a of the fin 11 against the bottom surface 13a, and are formed in close contact with the bottom surface 13a to create a flat surface.

[0016] Figure 3 is a schematic partial cross-sectional perspective view showing the state after cutting the leg portion 11a of the fin 11 shown in Figure 2 along line III-III and removing the protruding portions 11b and 11c. Figure 3 is a model showing the shape of the fin 11 and the ridge portion 17, and does not necessarily correspond to the actual shape. As shown in Figure 3, each ridge portion 17 is arranged at equal intervals along the circumferential direction TD on the bottom surface 13a of the cavity portion 13. In addition, the side wall surface 13b of the cavity portion 13 has a plurality of uneven portions 19 formed thereon, each having a plurality of projections 21 that protrude in the pipe axis direction Ax and extend along the radial direction RD, and recesses 23 formed between adjacent projections 21. The ridge portion 17 is formed extending from the recesses 23 of the plurality of uneven portions 19. In other words, a recess 23 is formed on the side wall surface 13b of the cavity 13, recessed in the direction Ax of the pipe axis, and a plurality of flat ridges 17 are formed on the bottom surface 13a of the cavity 13, where the extension pieces extending from the recess 23 (described later) are pressed against the bottom surface 13a. The projection 21 protrudes from the circumferential TD edge of the recess 23 in the direction Ax of the pipe axis and extends radially. The uneven surface 19 will be described in detail later, but the recess 23 is the mark left when the leg portion 11a of the fin 11 is scraped off, and the extension pieces formed by this scraping (hereinafter also called scraped pieces) become the main material for forming the ridges 17. The projection 21 also includes burrs formed when the side wall surface 13b is plastically deformed when the leg portion 11a is scraped off.

[0017] Figure 4 is a cross-sectional view along the line IV-IV shown in Figure 2. The cavity 13 is defined by being surrounded by the protruding portion 11b of one fin 11 that protrudes toward each other, the protruding portion 11c of the other fin 11, and a pair of opposing leg portions 11a, 11a.

[0018] The stepped portion 17 and the uneven portion 19 form numerous steps on the bottom surface 13a and side wall surface 13b of the nearly smooth cavity portion 13. As will be described in detail later, by providing numerous steps within the cavity portion 13, the corners of each step act as foaming points, which promotes nucleation boiling.

[0019] Furthermore, as shown in Figure 2, embossing 25 is formed on the outer surface of the boiling-type heat transfer tube 100 at predetermined intervals along the circumferential direction. The embossing 25 connects adjacent protruding portions 11b and 11c in the axial direction Ax of the tube by recessing them inward in the radial direction RD. Multiple connection points for the protruding portions 11b and 11c on which the embossing 25 is formed are provided along the circumferential direction TD of the tube body 10. Here, the connection points are provided across three rows of fins 11, but the connection points may also be provided across one, two, or four or more rows of fins 11.

[0020] The aforementioned connection points result in narrow sections where the cross-sectional area of ​​the cavity 13 in the cross-section of the pipe body 10 in the axial direction Ax is smaller than the cross-sectional area of ​​the areas other than the connection points. In other words, the cavity 13 continuous in the circumferential direction TD has a shape that is locally narrowed at the circumferential position of the emboss 25. Due to these narrow sections, the cavity 13 is divided into multiple small sections SG along the circumferential direction TD. Since the emboss 25 is provided at equal intervals along the circumferential direction, the small sections SG are formed at regular intervals in the circumferential direction. It is preferable that the emboss 25 be arranged at regular intervals in the circumferential direction, but it is not limited to this, and may be arranged with a specific periodicity or randomly.

[0021] The boiling-type heat transfer tube 100 described above is manufactured from a thermally conductive metal material such as copper, copper alloy, aluminum, aluminum alloy, iron, stainless steel, titanium, or titanium alloy, and is particularly preferable if it is made of a material with high thermal conductivity, such as copper or a copper alloy.

[0022] <Operation of Boiling Type Heat Transfer Tubes> Next, the heat transfer operation by the boiling type heat transfer tubes 100 described above will be explained. Figure 5 is a cross-sectional view of the boiling type heat transfer tubes 100 placed in the liquid refrigerant, taken in a cross-section perpendicular to the tube axis direction. The boiling type heat transfer tubes 100 described above are placed in the liquid refrigerant 31 in the evaporator with the tube axis direction horizontal, and heated water 33 is supplied into the boiling type heat transfer tubes 100. As a result, the liquid refrigerant 31 is heated, and evaporation bubbles 37 are generated on the bottom surface 13a and side wall surface 13b of the cavity 13. The generated evaporation bubbles 37 move along the circumferential surface of the tube from the bottom to the top of the cavity 13, as indicated by the arrow FL, and head toward the liquid surface (not shown) of the liquid refrigerant 31 from the top of the boiling type heat transfer tubes 100. In addition, some of the evaporation bubbles 37 escape to the outside of the cavity 13 through the gap between the protruding parts 11b and 11c of the fins 11 shown in Figure 4, and head toward the liquid surface of the liquid refrigerant 31.

[0023] Figure 6 is an explanatory diagram showing how the liquid refrigerant 31 is heated in the cavity 13 and evaporation bubbles 37 are generated. Figure 6 shows a partial cross-section of one of the small compartments SG of the cavity 13 described above. The inside of the cavity 13 is filled with the liquid refrigerant 31 shown in Figure 5. When heated water 33 for heating is supplied into the tube of the boiling heat transfer tube 100, as shown in Figure 4, the liquid refrigerant 31 inside the cavity 13 is heated by heat transfer Q1 from the bottom surface 13a of the cavity 13, which is the outer surface of the tube body 10, and heat transfer Q2 from the fins 11. At this time, heat transfer from the bottom surface 13a of the cavity 13, which is close to the heated water 33, to the liquid refrigerant 31 becomes particularly active.

[0024] Numerous ridges 17 are formed on the bottom surface 13a of the boiling-type heat transfer tube 100 in this configuration. The stepped corners of these ridges 17 act as foaming points, enhancing the nucleation boiling promotion effect. In addition, numerous irregularities 19 are formed on the side wall surface 13b of the cavity 13, increasing the surface area of ​​the side wall surface 13b. This promotes heat transfer from the side wall surface 13b to the liquid refrigerant 31. Moreover, the corners of the irregularities 19 act as foaming points, further enhancing the nucleation boiling promotion effect. Since the protrusions 21 of the irregularities 19 and the ridges 17 are formed by scraping the side wall surface 13b, the fins 11 themselves are locally thinned. As a result, the heat input from the heat transfer tube is more easily transferred to the fins 11 in accordance with the decrease in the heat capacity of the fins 11, improving the heat transfer efficiency to the liquid refrigerant 31.

[0025] As described above, when the nucleation boiling promotion effect is enhanced, the numerous evaporation bubbles 37 that are generated rise towards the liquid surface of the liquid refrigerant 31, so that inside the cavity 13, the liquid refrigerant 31 moves along the circumferential direction in accordance with the flow of the evaporation bubbles 37.

[0026] When the boiling-type heat transfer tube 100 is heated, a heating layer 30 of liquid refrigerant 31 (a dispersed region of minute dots schematically shown in Figure 4) is formed in the cavity 13 along the bottom surface 13a and the side wall surface 13b. When the evaporation bubbles 37 of the liquid refrigerant 31 generated by heating pass through the cavity 13, if the inner surface of the cavity 13 is covered by the evaporation bubbles 37 and does not come into contact with the liquid refrigerant 31, heat transfer to the liquid refrigerant 31 is hindered. Therefore, ideally, the heating layer 30 should be generated so that the inner surface of the cavity 13 is always covered with liquid refrigerant 31, and the evaporation bubbles 37 should flow along the circumferential direction TD further inside the heating layer 30.

[0027] In this configuration, inside the heating layer 30 shown in Figure 4, the liquid refrigerant 31 shown in Figure 5 gradually moves in the circumferential direction TD to follow the flow of the evaporative bubbles 37, and new liquid (new liquid refrigerant 31) is continuously supplied to the inner surface of the cavity 13. As a result, the heat exchange efficiency within the cavity 13 is improved. In this configuration, the step portion 17 is formed flat in contact with the bottom surface 13a, and the uneven portion 19 of the side wall surface 13b is removed to reduce the amount of protrusion from the side wall surface 13b, making it less likely to obstruct the flow of liquid refrigerant 31 and evaporative bubbles 37 within the cavity 13. This makes the flow of liquid refrigerant 31 and evaporative bubbles 37 within the small compartment SG smoother.

[0028] Here, the portion of the cavity 13 described above in which the embossing 25 is formed is distinguished from the portion in which the embossing 25 is not formed and is also called the "narrow portion." The positions in the protruding portions 11b and 11c shown in Figure 2 where the embossing 25 is formed are the narrow portions, and the other positions are not narrow portions.

[0029] In the narrow section of the cavity 13 where the embossing 25 is formed, the protruding portions 11b and 11c are recessed radially inward, causing the liquid refrigerant 31 flowing through the cavity 13 to be agitated in the narrow section, resulting in turbulent flow of the liquid refrigerant 31. This also promotes the exchange of fresh liquid, further improving heat exchange efficiency.

[0030] Furthermore, the narrow section is formed when the radially outer surface of the cavity 13 is recessed radially inward, and the surface on the bottom surface 13a of the cavity 13 protrudes only slightly in the radial direction. Therefore, the flow of evaporation bubbles 37 moving along the bottom surface 13a of the cavity 13 is not obstructed by the narrow section, and the flow resistance of the liquid refrigerant 31 in the narrow section is small. Thus, the liquid refrigerant 31 can be smoothly supplied to the entire circumferential direction of the cavity 13 without hindering the gas-liquid two-phase flow of evaporation bubbles 37 and liquid refrigerant 31 flowing from below to above the boiling heat transfer tube 100. In addition, in the cavity 13 above the boiling heat transfer tube 100, gas-liquid separation occurs due to the buoyancy of the evaporation bubbles 37, which promotes the supply of liquid refrigerant 31.

[0031] Furthermore, the cross-sectional shape of the cavity 13 in the axial direction Ax gradually changes along the circumferential direction, with the smallest cross-sectional area occurring in the narrow section where the embossing 25 is formed. Therefore, the flow FL of the liquid refrigerant 31 flowing through the cavity 13 narrows in the narrow section, increasing the flow resistance and decreasing the flow velocity. As a result, the liquid refrigerant 31 in the heating layer 30 continues to be heated in the same position within the cavity 13 on the near side of the flow direction in the narrow section. Consequently, heat input from the inner surface of the cavity 13 to the liquid refrigerant 31 accumulates, promoting the generation of evaporation bubbles 37.

[0032] In other words, by arranging multiple narrow sections along the circumferential direction of the cavity 13, multiple small compartments SG are formed in the cavity 13, each separated by a narrow section. In each of these small compartments SG, the circumferential movement of the liquid refrigerant 31 is suppressed, and the heating of the liquid refrigerant 31 within each small compartment SG is promoted. In this way, the localized heating of the liquid refrigerant 31 at multiple locations along the circumferential direction of the cavity 13 improves the efficiency of generating evaporation bubbles 37.

[0033] Here, the above-mentioned heat transfer efficiency will be described in further detail. Regarding the flow pattern of the gas-liquid two-phase flow in the heat transfer tube, for example, in "Refrigerant Condensation - From Basics to Applications", the Japan Society of Refrigeration and Air Conditioning Engineers Special Book Series (ISBN 978-4889671315), P277, etc., the case of a flow path whose cross-section perpendicular to the tube axis direction is rectangular is shown. In this flow path, the liquid flowing inside the tube is attracted to the rectangular corners (four corners) by surface tension, and a thin liquid film is formed on the straight parts (side parts). The thin liquid film can reduce the thermal resistance from the heat transfer tube to the vapor. Therefore, heat transfer from the heat transfer surface becomes easy, and the heat transfer performance can be improved. It has been demonstrated that the heat transfer performance in such a rectangular tube is higher than that of a circular tube with the same flow path cross-sectional area., Although the above literature describes the condensation phenomenon, the same applies to the evaporation and boiling phenomena.

[0034] On the other hand, in the boiling-type heat transfer tube 100 of this configuration, in addition to having corners (region Ac) in the cavity 13 shown in FIG. 4, a raised portion​​17 is formed on the bottom surface 13a, and a protrusion 21 is formed on the side wall surface​13b. There are many corner parts in the flow path. As described above, the corners themselves are effective in promoting nucleate boiling. In addition, since the liquid film is attracted to the corners by surface tension, thin liquid film parts are formed on the side parts and the surface parts connected to the corners. Specifically, the portion of region Ap shown in FIGS. 4 and 6 can be exemplified as the generation position of the thin liquid film part.

[0035] By forming a large number of thin liquid film parts shown in region Ap, heat transfer from the heat transfer tube to the liquid refrigerant 31 is promoted, and evaporation from the gas-liquid interface is promoted, so that the generation of evaporation bubbles 37 becomes active, and high heat transfer performance can be obtained. Thus, according to the boiling-type heat transfer tube 100 of this configuration, unlike the conventional configuration in which small protrusions are provided at the groove bottom of the cavity 13, the number of corners is significantly increased. Therefore, the area of the side parts and the surface parts connected to the corners becomes wide, a thin liquid film part is formed over a wide range, and heat exchange becomes active. Furthermore, since each of the many corners contributes to nucleate boiling, the heat transfer performance can be significantly improved by the synergistic effect of the thin liquid film part and the corners.

[0036] As described above, according to the boiling heat transfer tube 100 of this configuration, the nucleate boiling promotion effect by the above-described bottom portion 17 and the concavo-convex portion 19, the heat transfer amount increase effect by the increase in the surface area, the stirring effect of the liquid refrigerant 31, and the local heating effect by the small compartment SG synergistically promote the generation of evaporation bubbles 37. Thereby, evaporation bubbles 37 can be generated with high efficiency under any heat flux condition, and the heat transfer efficiency can be improved.

[0037] <Manufacturing Method of Boiling Heat Transfer Tube> Next, the manufacturing method of the above-described boiling heat transfer tube 100 will be described. FIG. 7 is a front view schematically showing a main part of a processing apparatus for forming fins or the like on the outer surface of the tube body 10. FIG. 8 is a side view of the processing apparatus shown in FIG. 7. The support shafts 41 shown in FIG. 7 are arranged at equal intervals in the circumferential direction at positions of a central angle of 120° centered on the mandrel 43 shown in FIG. 8. A disk group 47, which is a plurality of disk-shaped disks that are tools for fin forming and cavity / groove forming, is coaxially fixed to the three support shafts 41. Each support shaft 41 is supported so as to be rotatable about its axis. By rotating these support shafts 41, the tube body 10 is pressed against the circumferential end portion of the disk while rotating in the opposite direction to the rotation of the support shafts 41, so that the fins 11 and the cavity portions 13 are formed by plastic deformation. Further, the support shafts 4l are arranged at positions twisted with respect to each other with respect to the tube axis, whereby the tube body 10 is sent in the tube axis direction Ax while being processed in a spiral shape.

[0038] By using the disk group 47 configured as described above, spiral fins 11 are formed on the surface of the tube body 10 described above, and the tip portions on the outer side in the radial direction of the fins 11 are crushed and protruded portions 11b, 11c, cavity portions 13, bottom portions 仃, concavo-convex portions Each part shape such as 19 is formed. Here, an example of forming three spiral fins 11 by three support shafts 41 is described, but the number of fins 11 is not limited to this and can be adjusted as appropriate.

[0039] The group of discs 47 shown in Figure 7 includes rolling discs RD1 to RD13, a cutting disc (gear disc GD1), an indentation disc PD, a connecting disc (gear disc GD2), and finishing discs FD1 to FD5. The rolling discs RD1 to RD13 are discs for forming (rolling) the fins 11 in stages. The cutting disc is a disc that forms the aforementioned uneven portion 19 and the aforementioned extended piece, the cutting piece 51, and the gear disc GD1 is used. The indentation disc PD is a disc that presses the cutting piece 51 between the fins 11 to form the step portion 17. The connecting disc is a disc that forms the emboss 25 on the protruding portions 11b and 11c, and the gear disc GD2 is used. The finishing discs FD1 to FD5 are discs that crush the tips of the fins to finish them. The disk group 47 is formed similarly on the three support shafts 41, but the gear disk GD2 is placed on only one of the three support shafts 41, and spacers are placed on the other two support shafts 41 instead of the gear disk GD2.

[0040] The rolling discs RD1 to RD13 shown here are just examples, and their size and number can be changed as appropriate depending on the object to be molded. Also, although gear disc GD1 is used as the cutting disc here, it is not limited to this. The cutting disc can be any disc with cutting blades formed on its outer circumference, and may be a gear other than a spur gear, or a disc equipped with a dedicated cutting blade. In addition, the connecting disc may be a disc having protrusions at an appropriate circumferential pitch instead of gear disc GD2.

[0041] The pipe body 10 is sent from the upstream side to the downstream side in the direction of processing (from the left side to the right side in Figure 7), and fins 11 of a predetermined radial height are sequentially formed by the rolling discs RD1 to RD13 in region SC1. Region SC1 is arranged with rolling discs RD1 to RD9, whose outer diameter gradually increases in the direction of processing along the pipe axis Ax, and rolling discs RD10 to RD13 that shape the fins further ahead.

[0042] In the next region SC2, a gear disc GD1 and a press disc PD are arranged. The gear disc GD1 cuts the sides of the formed fins 11. The press disc PD forms a step 17.

[0043] Figure 9 is a schematic cross-sectional view showing the machining process of the fins 11 by the gear disc GD1 and the press disc PD. The gear disc GD1 is a disc with a spur gear formed on its outer circumference, and its diameter is slightly smaller than that of the previously used rolling disc RD13. When the gear disc GD1 is inserted between the pair of fins 11 after machining by the rolling disc RD13, the cutting edge and end face of the spur gear scrape away the sides of the fins 11, forming a cutting piece.

[0044] Here, each of the disks in the aforementioned disk group 47 is a rigid body, but the fins 11 are flexible protrusions that are easily elastically deformed. Therefore, when the gear disk GD1 is inserted between the fins 11, the fins 11 elastically deform along the side surface of the gear disk GD1, contacting the teeth of the gear disk GD1 and being cut.

[0045] Figure 10 is a schematic diagram illustrating the state of the fin 11 after it has been cut by the gear disc GD1. The side wall surface 13b of the fin 11 is cut by the gear disc GD1 up to a point partway through the radial height of the fin 11. As a result, a recess 23 is formed on the side wall surface of the fin 11, which has been thinned, and protrusions 21 are formed on both sides of the recess 23 in the circumferential direction TD by plastic deformation due to cutting, projecting in the direction Ax of the pipe axis, and cutting pieces 51 are formed extending from the side wall surface 13b as the fin 11 is removed. The protrusions 21 may contain burrs. The cutting pieces 51 are formed to protrude in the direction Ax of the pipe axis, similar to the protrusions 21. The diameter of the gear disc GD1 is set such that the outer edge of the disc does not reach the outer surface of the pipe body 10 (the bottom surface between the fins 11). Therefore, the protrusions 21 are formed between the top of the radially outer tip of the fin 11 and a point in the radial height of the side wall surface 13b. Furthermore, the cutting piece 51 is formed at the lowest height position where the projection 21 is formed.

[0046] Next, the pressing disc PD shown in Figure 9 is inserted between the fins 11 on which the projections 21 and cutting pieces 51 are formed. Figure 11 is a schematic diagram illustrating the appearance of the projections 21 and cutting pieces 51 after the pressing disc PD has been inserted between the fins 11. The pressing disc PD shears or stretches a portion of the projection 21 that protrudes from the side wall surface 13b of the fin 11 in the direction of the pipe axis Ax. The cutting pieces 51 are pressed against the bottom surface of the pipe body 10 to form a ridge 17. A portion of the cutting debris from the sheared or stretched projections 21 is pressed against the bottom surface of the pipe body 10 by the pressing disc PD, forming a ridge 17 together with the cutting pieces 51. The pressing disc PD has a smaller diameter than the rolling disc RD13, and is pressed in to the extent that the cutting pieces 51 do not become embedded in the bottom surface of the pipe body 10.

[0047] Numerous uneven scratches are formed on the surface of the projection 21 by shearing with the press disc PD. In addition, the stepped portion 17 formed by the cutting piece 51 adheres closely to the bottom surface 13a of the cavity 13 by being pressed against by the press disc PD and has numerous steps that protrude radially outward from the bottom surface 13a.

[0048] As a result, as shown in Figure 9, numerous protrusions 21 and ridges 17 with uneven surfaces are formed between the fins 11 after the push-in disc PD is inserted.

[0049] Next, in region SC3, the tops of the erected fins 11 are crushed using finishing discs FD1 to FD5 and gear disc GD2 to sequentially form the shape of the cavity 13. Figure 12 is an explanatory diagram showing the steps of the finishing process of the fins 11 in region SC3. In step 1, finishing discs FD1 and FD2 are sequentially pressed against the tops of the fins 11 to crush them. This extends the tops of the fins 11 in the direction of the pipe axis Ax. In step 2, the finishing disc FD3 is pushed between the fins 11 to form a thin-walled portion 53 at the tip of each fin 11. This finishing disc FD3 is provided on the three support shafts 41 shown in Figure 8, and the finishing disc FD on each support shaft 41 is sequentially pushed onto the fins 11. As a result, as shown in step 3, thin-walled portions 53 are formed on both sides of the fins 11 in the direction of the pipe axis Ax. Alternatively, instead of the finishing disc FD3, a disc with a cutting edge whose outer circumference is wavy in the circumferential direction may be used. In that case, the tip of the fin 11 will split into a Y shape in the cross section in the direction of the pipe axis Ax, thereby forming a protruding portion.

[0050] Next, in step 4, the gear disc GD2 is pressed against multiple rows of fins 11 to form embossing 25 as shown in Figure 2 at predetermined intervals in the circumferential direction TD. In other words, embossing 25 is formed at the points where the tooth tips of the gear disc GD2 and the tips of the fins 11 intersect, extending the thin-walled portion 53 in the direction of the pipe axis. The gear disc GD2 extends the thin-walled portion 53 to form adjacent protruding portions 11b and 11c in the pipe axis direction Ax, and these protruding portions 11b and 11c are connected to each other by recessing them radially inward at multiple locations along the circumferential direction TD.

[0051] Then, in step 5, the finishing discs FD4 and FD5 are pressed against the fins 11 to extend the thin-walled portion 53 other than the embossed portion 25 in the axial direction Ax of the tube to form protruding portions 11b and 11c, and adjust the shape of the cavity portion 13.

[0052] Figure 13 is a schematic diagram illustrating the process by which the shape of the fins 11 changes in stages as the tubular body 10 is processed by the disc group 47. In Figure 13, the processing proceeds sequentially from left to right. First, the fins 11 formed by the rolling discs RD1 to RD13 are cut into pieces 51 by the gear disc GD1, and then the cut pieces 51 are pressed into the groove bottom surface by the pressing disc PD to form a stepped portion 17. In addition, although not shown in the figure, uneven portions 19 are formed on the side wall surface 13b of the fins 11 shown in Figure 11.

[0053] Next, the tops of the fins 11 are crushed using finishing discs FD1 and FD2, and the thin-walled portion 53 is formed using finishing disc FD3. Then, the emboss 25 is formed using gear disc GD2, and the shape of the cavity portion 13 is refined using finishing discs FD4 and FD5. The ribs 15 on the inner surface of the pipe body 10 are formed sequentially using tools not shown, in the same manner as the formation of the fins 11.

[0054] In the procedure described above, the formation of the fins 11 and the formation of the cavity 13 are performed consecutively, but the formation of the fins 11 and the formation of the cavity 13 may be performed in separate steps. Furthermore, the method for forming the fins 11 is not limited to rolling, but may also be other processing methods such as cutting.

[0055] Next, the results of evaluating the heat transfer performance using the boiling-type heat transfer tube 100 with the above configuration will be described. Figure 14 is a schematic diagram of the test apparatus used to evaluate the heat transfer performance of the boiling-type heat transfer tube. The test apparatus consists of a stainless steel shell-and-tube heat exchanger with a condenser 61 and an evaporator 63 connected by steam piping and liquid piping. Multiple test heat transfer tubes 65 with an effective heat transfer length of 974 mm are horizontally installed in the center of the evaporator 63. The evaporator 63 is filled with liquid refrigerant 31, and the test heat transfer tubes 65 are immersed in the liquid refrigerant inside the evaporator 63. Heated water stored in a tank 67 is supplied to the test heat transfer tubes 65 of the evaporator 63. The temperature of the heated water supplied to the test heat transfer tubes 65 of the evaporator 63 is adjusted to a constant temperature by a cooling brine liquid heat exchanger including a cooling coil 69 located in the tank 67 or in the liquid piping, and an electric heater 71. The flow rate of the heated water is adjusted to a constant rate using an automatic control valve and a controller.

[0056] This heated water is supplied into the test heat transfer tube 65 from one end, the inlet side. The heated water discharged from the other end, the outlet side, of the test heat transfer tube 65 is returned to the tank 67. The liquid refrigerant 31 heated by the heated water inside the test heat transfer tube 65 then evaporates into refrigerant vapor, which is supplied to the condenser 61 through the vapor piping.

[0057] Multiple heat transfer tubes 73 (effective length 974 mm), each with an O-ring at its end, are horizontally installed in the condenser 61. Brine liquid supplied from the tank 75 is supplied to each heat transfer tube 73. The vapor refrigerant sent from the evaporator 63 is cooled and pressurized by the heat transfer tubes 73, and condenses and liquefies on the outer surface of the heat transfer tubes. A baffle plate is installed at the vapor refrigerant inlet of the condenser 61 to prevent the vapor refrigerant supplied from the evaporator 63 from directly contacting the heat transfer tubes 73. The liquefied liquid refrigerant 31 is returned to the evaporator 63 by gravity through the liquid piping.

[0058] In the test apparatus with the above configuration, the saturated evaporation pressure in the evaporator 63 was measured using an absolute pressure transmitter from a pressure outlet located at the top of the evaporator 63. The inlet and outlet temperatures of the heated water were measured by measuring the mixed mean temperature at both ends of the test heat transfer tube 65 using a platinum resistance thermometer that had been pre-calibrated. Specifically, the inlet and outlet temperatures of the heated water were measured by installing platinum resistance thermometers (Pt100Ω, JIS-A class), which had been calibrated to ±0.05°C using a temperature calibration device (ISO-Tech millik 0.5mA model 923-0.5MA) and a secondary standard thermometer (resistance thermometer) in a loop calibration system, at both ends of the test heat transfer tube 65. At both ends of the test heat transfer tube 65, the tips of the platinum resistance thermometers were positioned so that they were in the center of the flow path. The heated water flow rate was measured using a positive displacement flow meter installed on the outlet side of the test heat transfer tube 65. The test conditions are shown in Table 1. Furthermore, prior verification confirmed that there was no change in the trend between using one and three heat transfer tubes in the evaporator.

[0059]

[0060] Overall heat transfer coefficient Ko [kW / (m²) based on pipe outer surface area Ao ２・K)], the heating water heat transfer amount Qs [kW], the outer surface area of the tube Ao [m ２ , and the logarithmic mean temperature difference ΔTm [K] between the refrigerant and the heating water are shown in the following formulas (1) to (4). The outer surface area of the tube Ao is based on the assumed smooth surface from the fin outer diameter of the test heat transfer tube.

[0061]

[0062] Here, Ws is the heating water flow rate [kg / s], cp ｓ is the specific heat of the heating water [kJ / (kg·K)], T ｓ，ｉｎ is the inlet temperature of the heating water [°C], T ｓ，ｏｕｔ is the outlet temperature of the heating water [°C], Tr is the refrigerant saturation temperature [°C], Do is the fin outer diameter of the test heat transfer tube [m], and L is the effective length of the heat transfer tube [m]. The refrigerant saturation temperature Tr was obtained using the refrigerant thermophysical property database (REFPROP Ver. 10) from the measured saturation vapor pressure. The heat flux qo [kW / m ２ based on the outer heat transfer area of the tube is defined by formula (5).

[0063]

[0064] Also, the heat transfer coefficient ho [kW / (m ２ ·K)] based on the outer heat transfer area of the tube is defined by formula (6).

[0065]

[0066] The inner heat transfer coefficient h ｉ [kW / (m ２ ·K)] is assumed to be in agreement with the Dittus - Boelter equation in terms of functional form and is defined by formula (7).

[0067]

[0068] Here, κ ｓ is the thermal conductivity of the heating water [kW / (m·K)], Pr ｓ is the Prandtl number of the heating water. Also, the representative length of the Reynolds number Re ｓ of the heating water is taken as the maximum inner diameter D ｉｍａｘ [m] of the processed part and is defined by formula (8).

[0069] s]] [[ID=5Note that the Dittus-Boelter coefficient DBC in the formula ｉ The values ​​used were those obtained from a preliminary test conducted using the Wilson-Plot method.

[0071] Tables 2 and 3 show the dimensions and number of ribs of the test heat transfer tubes for test examples 1 to 3 that were fabricated. Figure 15 is an explanatory diagram showing the dimensions of the fins 11 and ribs 15 shown in Tables 2 and 3.

[0072]

[0073]

[0074] Each test heat transfer tube has the aforementioned fins and cavities formed on its outer surface and ribs as shown in Table 3 formed on its inner surface. The test heat transfer tube in Test Example 1 is the boiling-type heat transfer tube 100 of the embodiment described above. The test heat transfer tubes in Test Examples 2 and 3 are for comparison with Test Example 1, and both have embossed protrusions 11b and 11c that form the cavities 13, which are flattened at equal intervals along the circumferential direction, but do not have the stepped portion 17 and the uneven portion 19. Test Example 2 was manufactured using the test conditions of Test Example 1 in Japanese Patent Publication No. 2021-134952, and Test Example 3 was manufactured using the test conditions described in Japanese Patent Publication No. 2017-20736.

[0075] For the test heat transfer tubes described in Test Examples 1 to 3 above, the overall heat transfer coefficient Ko, which indicates the heat transfer performance from the radially inner side of the inner circumference to the radially outer side of the outer circumference, was determined. The results are shown in Figure 16.

[0076] Figure 16 is a graph showing the relationship between the heat flux qo and the overall heat transfer coefficient Ko in Test Examples 1 to 3. As shown in Figure 16, across the entire range of heat flux, the overall heat transfer coefficient of Test Example 1 was larger than that of Test Examples 2 and 3. In the case of Test Examples 2 and 3, the overall heat transfer coefficient was at most 7.5 kW / (m²). ２ K) is approximately the same, but in the case of Test Example 1, the increase in the overall heat transfer coefficient with increasing heat flux is significant, and when the heat flux reaches 20 kW / m ２ 8 kW / (m ２ It has reached K). For example, the heat flux is 40 kW / m ２Before and after the test, the overall heat transfer coefficient of Test Example 1 becomes more than 1.3 times that of Test Example 3, and the difference between Test Example 1 and Test Examples 2 and 3 widens significantly. Therefore, the heat exchanger using the test heat transfer tube (boiling type heat transfer tube 100) of Test Example 1 can maintain a high heat transfer rate from the heat transfer tube to the liquid refrigerant, enabling highly efficient heat transfer across the entire operating range.

[0077] Next, we will explain the results of observing the shape of the heat transfer tube surface after cutting the test heat transfer tube of Test Example 1. Figure 17 is a photograph showing how a gear disc GD1 and a pressing disc PD are inserted between the formed fins 11, and how a stepped portion 17 is formed at the bottom of the groove. Figure 17 corresponds to the area P1 in Figure 13. As shown in Figure 17, numerous stepped portions 17 are formed in the groove between the fins 11, and a stepped portion is formed on the outer edge of the stepped portion 17, rising from the bottom of the groove. The corners formed by this stepped portion become foaming points, enhancing the nucleation boiling promotion effect.

[0078] Figure 18 is a cross-sectional photograph showing the side view of the fin 11 shown in Figure 17. The side view of the fin 11 has a surface area 19 including the aforementioned protrusions 21 and recesses 23, corresponding to the pitch of the teeth of the gear disc. The circumferential spacing between adjacent pairs of protrusions 21 widens as they move radially outward. Here, if the spreading angle of the pair of protrusions 21 is θp, the circumferential pitch of the pair of protrusions 21 is Pp, the depth from the outer circumference of the fin 11 to the root of the recess 23, which is the indentation depth of the gear disc teeth, is tp, and the circumferential length of the root is Wp, then the dimensions are as follows.

[0079] θp: 31.9° Pp: 0.89mm tp: 0.37mm Wp: 0.21mm

[0080] A spreading angle θp in the range of 20° to 50° is preferable because it facilitates the formation of the protrusion 21, provides a stirring effect on the liquid flow within the cavity 13, and improves heat exchange efficiency. Furthermore, a ratio of the height Hf to the depth tp of the fin 11, tp / Hf, in the range of 0.5 to 0.9 is preferable because it provides an appropriate volume of material for removing the fin 11 to form the step 17, allowing for the formation of a step 17 of the desired size.

[0081] Figure 19 is a photograph showing how an emboss 25 is formed by crushing a pair of protruding portions 11b and 11c with a gear disc GD2. Figure 19 corresponds to the area P2 in Figure 13. As shown in Figure 19, the surface of the tube is covered with protruding portions 11b and 11c formed by multiple rows of fins continuous in the circumferential direction TD, and a connected emboss 25 is formed when the protruding portions 11b and 11c, each consisting of three rows of fins, are crushed at once by the gear disc GD2.

[0082] Here, if we let Pe be the circumferential pitch of the emboss 25, We be the circumferential width of the emboss 25, Lh be the distance between the embossings, and Lg be the gap distance between the protruding portion 11b and the protruding portion 11c, then the dimensions are as follows.

[0083] Pe: 0.619mm We: 0.174mm Lh: 0.408mm Lg: 0.106mm

[0084] It is preferable that the circumferential pitch Pe of the emboss 25 and the distance Lh between the embossings are longer than the circumferential pitch Pp of the pair of protrusions 21 described above (Pe > Pp, Lh > Pp). This ensures that at least one uneven portion 19 is placed within one section of the cavity 13 separated by the emboss 25, thereby enhancing the heat transfer effect.

[0085] Thus, the present invention is not limited to the embodiments described above. It is also intended and within the scope of protection to be provided for the combination of each configuration of the embodiments, as well as for modifications and applications by those skilled in the art based on the description in the specification and well-known technology.

[0086] As described above, the following is disclosed in this specification: (1) A boiling heat transfer tube comprising a plurality of rows of fins projecting radially outward from the outer surface of a tube and formed spirally along the central axis of the tube, wherein each fin has a leg portion erected on the outer surface of the tube and a pair of overhangs whose radially outward tips extend in opposite directions with respect to the tube axis of the tube, a cavity is defined between adjacent fins in the tube axis direction, surrounded by the overhang of one fin projecting toward each other, the overhang of the other fin projecting toward each other, and the pair of legs facing each other, a recess formed in the tube axis direction on the side wall surface of the cavity, and a plurality of flat tiers formed on the bottom surface of the cavity, with extension pieces extending from the recess pressed against the bottom surface. In this boiling-type heat transfer tube, multiple ridges formed on the bottom surface of the cavity allow for highly efficient heat transfer to the outside of the heat transfer tube through the ridges closest to the center of the tube. Furthermore, the formation of the ridges increases the surface area of ​​the bottom surface inside the cavity, which also contributes to highly efficient heat transfer. In addition, steps are formed on the ridges by rising from the bottom surface of the cavity, and the numerous corners of these steps act as foaming points, promoting nucleated boiling. As a result, the liquid refrigerant moves in accordance with the flow of evaporative bubbles, ensuring a continuous supply of fresh liquid to the bottom surface of the cavity, enabling highly efficient heat transfer. Moreover, since the ridges are formed by extensions extending from the side walls of the cavity, the volume of the ridges can be secured without adding any new material. In addition, the fins themselves are made thinner, making it easier for the heat input of the heat transfer tube to be transferred to the entire fin, improving the heat transfer efficiency to the liquid refrigerant.

[0087] (2) The boiling-type heat transfer tube according to (1), wherein the side wall surface of the cavity has a plurality of irregularities, each having projections that protrude from the recess in the direction of the tube axis and extend radially. With this boiling-type heat transfer tube, the surface area is increased by the irregularities on the side wall surface, thereby promoting heat transfer from the side wall surface. In addition, the corners of the irregularities act as foaming points, enhancing the nucleation boiling promotion effect.

[0088] (3) A boiling-type heat transfer tube according to (1) or (2), wherein multiple connection points are provided along the circumferential direction, where adjacent protruding portions in the axial direction of the tube are recessed radially inward to connect them, and the connection points are narrow sections in which the cross-sectional area of ​​the cavity in the axial direction of the tube is smaller than the cross-sectional area other than the connection points, and the cavity is divided into multiple small sections along the circumferential direction by the narrow sections. With this boiling-type heat transfer tube, multiple connection points are provided along the circumferential direction to connect the protruding portions, thereby forming narrow sections in the cavity. As a result, the cross-sectional area in the axial direction of the tube becomes smaller at the connection points than at other circumferential positions, and the flow of liquid refrigerant is narrowed. As a result, the flow of liquid refrigerant is suppressed, making it easier to heat at the same location and promoting the generation of evaporation bubbles. In other words, the liquid refrigerant is heated locally at multiple locations along the circumferential direction within the cavity, and evaporation bubbles are generated efficiently.

[0089] (4) The boiling type heat transfer tube described in (3), wherein the connecting portion is provided across multiple rows of the cavities. With this boiling type heat transfer tube, each small section divided in the circumferential direction of the multiple rows of cavities is formed evenly. As a result, the liquid refrigerant is heated under the same conditions, and heat exchange with less unevenness can be performed.

[0090] (5) The platform portion is a boiling type heat transfer tube according to any one of (1) to (4), which is arranged at equal intervals along the circumferential direction on the bottom surface of the cavity. With this boiling type heat transfer tube, the arrangement of the platform portion within the cavity becomes uniform, and the occurrence of uneven heat transfer can be reduced.

[0091] (6) A method for manufacturing a boiling-type heat transfer tube according to any one of (1) to (5), comprising: pressing the outer circumferences of a plurality of rolling discs against the outer circumference of the tube body to plastically deform the outer circumference of the tube body to form a plurality of rows of fins projecting radially outward from the outer circumference of the tube body in a spiral shape along the central axis of the tube body; inserting a cutting disc with a cutting blade formed on its outer circumference between adjacent fins in the tube axis direction of the tube body to cut away the sides of the fins with the cutting blade to form the recess and the extension piece; inserting a pressing disc that pushes the extension piece into the groove bottom between the fins after the cutting disc has been inserted to form the step at the bottom of the groove; and pushing the tips of the fins radially inward after the pressing disc has been inserted to form a pair of the protruding parts. According to this method for manufacturing a boiling-type heat transfer tube, by cutting away the sides of the fins and pressing them into the groove bottom to form a plurality of step, heat can be transferred to the outside of the heat transfer tube with high efficiency through the step near the center of the heat transfer tube. Furthermore, the formation of the tiered section increases the surface area of ​​the bottom of the cavity, which also enables highly efficient heat transfer. In addition, the tiered section has steps formed by rising from the bottom of the cavity, and the numerous corners of these steps act as foaming points, promoting nucleated boiling. As a result, the liquid refrigerant moves in accordance with the flow of the evaporative bubbles, and a continuous supply of fresh liquid is constantly provided to the bottom of the cavity, enabling highly efficient heat transfer. Moreover, since the tiered section is formed by extensions extending from the side walls of the cavity, the volume of the tiered section can be secured without adding any new material. In addition, the fins themselves are made thinner, making it easier for the heat input from the heat transfer tubes to be transferred to the entire fin, thereby improving the heat transfer efficiency to the liquid refrigerant.

[0092] (7) The method for manufacturing a boiling-type heat transfer tube according to (6), wherein by inserting the cutting disc, a plurality of uneven surfaces are formed on the side wall surface of the cavity, each having a projection that protrudes from the recess in the direction of the tube axis and extends radially, and the pressing disc cuts off a portion of the protruding projections. According to this method for manufacturing a boiling-type heat transfer tube, the surface area is increased by the uneven surfaces of the side wall, thereby promoting heat transfer from the side wall surface. In addition, the corners of the uneven surfaces become foaming points, enhancing the nucleation boiling promotion effect. Furthermore, by inserting the pressing disc, a portion of the projections protruding from the side of the fins is cut off, maintaining the fluidity of the liquid refrigerant flowing inside the cavity.

[0093] (8) The method for manufacturing a boiling-type heat transfer tube according to (6) or (7), wherein the cutting disc is a gear disc with spur teeth formed on its outer circumference. According to this method for manufacturing a boiling-type heat transfer tube, equipment costs can be reduced by using a general-purpose spur gear as the cutting disc.

[0094] (9) A method for manufacturing a boiling-type heat transfer tube according to any one of (6) to (8), wherein adjacent protruding portions in the axial direction of the tube are connected by recessing them radially inward at multiple locations along the circumferential direction, thereby forming narrow sections in the cross-section in the axial direction of the tube where the cross-sectional area of ​​the cavity is smaller than the cross-sectional area of ​​the other locations, and the cavity is divided into multiple small compartments along the circumferential direction. According to this method for manufacturing a boiling-type heat transfer tube, by providing multiple connection points along the circumferential direction where protruding portions are connected by recessing them, narrow sections are formed in the cavity. The small compartments between these narrow sections are uniform in the circumferential direction, and the liquid refrigerant is heated under the same conditions. As a result, the liquid refrigerant is heated uniformly in the circumferential direction, enabling heat exchange with less unevenness. In addition, the formation of a closed space makes the cross-sectional area in the axial direction of the tube smaller at the connection points than at other circumferential locations, narrowing the flow of the liquid refrigerant. This suppresses the flow of the liquid refrigerant, making it easier to heat at the same location and promoting the generation of evaporation bubbles. In other words, the liquid refrigerant is heated locally at multiple points along the circumferential direction within the cavity, allowing for efficient generation of evaporation bubbles.

[0095] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to these examples. It is clear to those skilled in the art that various modifications or alterations can be conceived within the scope of the claims, and these will naturally also fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any way without departing from the spirit of the invention.

[0096] This application is based on a Japanese patent application (JP 2024-159285) filed on September 13, 2024, the contents of which are incorporated by reference within this application.

[0097] 10 Tube body 11 Fin 11a Leg portion 11b, 11c Protruding portion 13 Cavity portion 13a Bottom surface 13b Side wall surface 15 Rib 17 Step portion 19 Uneven portion 21 Projection portion 23 Recess 25 Embossing 30 Heating layer 31 Liquid refrigerant 33 Heating water 37 Evaporation bubbles 41 Support shaft 43 Mandrel 47 Disc group 51 Cut piece (extended piece) 53 Thin-walled portion 61 Condenser 63 Evaporator 65 Test heat transfer tube 67 Tank 69 Cooling coil 71 Electric heater 73 Heat transfer tube 75 Tank 100 Boiling type heat transfer tube RD1-RD13 Rolled disc GD1, GD2 Gear disc PD Pressed disc FD1-FD5 Finishing disc

Claims

1. A boiling heat transfer tube comprising a plurality of rows of fins that project radially outward from the outer surface of a tube and are formed spirally along the central axis of the tube, wherein each fin has a leg portion erected on the outer surface of the tube and a pair of overhangs at the radially outward tips of the leg portions that extend in opposite directions with respect to the tube axis of the tube, a cavity is defined between adjacent fins in the tube axis direction, surrounded by the overhang of one fin that protrudes toward each other, the overhang of the other fin, and the pair of legs that face each other, thereby creating a circumferentially continuous cavity, recesses formed in the tube axis direction on the side walls of the cavity, and a plurality of flat tiers formed on the bottom surface of the cavity, with extension pieces extending from the recesses pressed against the bottom surface.

2. The boiling heat transfer tube according to claim 1, wherein the side wall surface of the cavity portion has a plurality of uneven portions having projections that protrude from the recess in the direction of the tube axis and extend radially.

3. Multiple connection points are provided along the circumferential direction, where adjacent protruding portions in the axial direction of the pipe are recessed radially inward to connect them, and the connection points are narrow sections in which the cross-sectional area of ​​the cavity in the cross section in the axial direction of the pipe is smaller than the cross-sectional area of ​​the other sections, and the cavity is divided into multiple small sections along the circumferential direction by the narrow sections, as described in claim 1 or 2.

4. The boiling-type heat transfer tube according to claim 3, wherein the connecting portion is provided across multiple rows of the cavities.

5. The boiling heat transfer tube according to any one of claims 1 to 4, wherein the plate portion is arranged at equal intervals along the circumferential direction on the bottom surface of the cavity.

6. A method for manufacturing a boiling-type heat transfer tube according to any one of claims 1 to 5, comprising: pressing the outer circumferences of a plurality of rolling discs against the outer circumference of the tube body to plastically deform the outer circumference of the tube body to form a plurality of rows of fins projecting radially outward from the outer circumference of the tube body in a spiral shape along the central axis of the tube body; inserting a cutting disc having a cutting blade formed on its outer circumference between adjacent fins in the axial direction of the tube body to cut away the sides of the fins with the cutting blade to form the recess and the extension piece; inserting a pressing disc between the fins after the cutting disc has been inserted to push the extension piece into the bottom of the groove between the fins to form the step at the bottom of the groove; and pushing the tips of the fins radially inward after the pressing disc has been inserted to form a pair of the protruding portions.

7. The method for manufacturing a boiling-type heat transfer tube according to claim 6, wherein by inserting the cutting disc, a plurality of uneven portions are formed on the side wall surface of the cavity portion, each having a projection that protrudes from the recess in the direction of the tube axis and extends radially, and the pressing disc cuts off a portion of the protruding projections.

8. The method for manufacturing a boiling-type heat transfer tube according to claim 6 or 7, wherein the cutting disc is a gear disc with spur teeth formed on its outer circumference.

9. A method for manufacturing a boiling-type heat transfer tube according to any one of claims 6 to 8, wherein adjacent protruding portions in the axial direction of the tube are connected by recessing them radially inward at multiple locations along the circumferential direction, thereby forming a narrow section in which the cross-sectional area of ​​the cavity in the cross section in the axial direction of the tube is smaller than the cross-sectional area of ​​the other locations, and the cavity is divided into a plurality of small sections along the circumferential direction.

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