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

Abstract

The present invention provides 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 achieve high heat transfer efficiency. [Solution] The boiling heat transfer tube is provided with multiple rows of fins 11 that protrude radially outward from the outer surface of the tube and are formed spirally along the central axis of the tube. Each fin 11 has a leg portion 11a and a pair of protruding portions 11b, 11c. A circumferentially continuous cavity 13 is defined between adjacent fins 11 in the direction of the tube axis. A recess 23 that is recessed in the direction of the tube axis is formed on the side wall surface 13b of the cavity 13, and a plurality of flat steps 17 are formed on the bottom surface 13a of the cavity 13, with extension pieces extending from the recess 23 being pressed against the bottom surface 13a.

Description

[Technical Field]

[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. [Background technology]

[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. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 4-236097 [Patent Document 2] Special Publication No. 2015-500456 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, in recent years, inverter control, which allows for precise control of the evaporator that heats and boils the liquid refrigerant, has been widely introduced in refrigeration systems in response to environmental issues such as reducing CO2 emissions, thereby improving energy efficiency. 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 corner angle 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 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. [Means for solving the problem]

[0009] According to one embodiment of the present invention, the following configuration is provided. (1) The tube has multiple rows of fins that protrude radially outward from the outer surface of the tube and are formed spirally along the central axis of the tube, Each fin has a leg portion erected on the outer surface of the tube and a pair of protruding portions whose radially outer tips extend in opposite directions with respect to the axial direction of the tube. Between the fins adjacent to each other in the axial direction of the tube, a cavity is defined that is continuous in the circumferential direction, surrounded by the protruding portion of one fin that extends toward each other, the protruding portion of the other fin, and a pair of opposing leg portions. A recess is formed on the side wall surface of the cavity portion, which is recessed in the direction of the pipe axis. Multiple flat, stepped portions are formed on the bottom surface of the cavity, with extensions extending from the recesses being pressed against the bottom surface. Boiling type heat transfer tube. (2) A method for manufacturing a boiling type heat transfer tube as described in (1), By pressing the outer circumferences of multiple rolling discs against the outer surface of the tube, the outer surface of the tube is plastically deformed, thereby forming multiple rows of fins that protrude radially outward from the outer surface of the tube in a spiral shape along the central axis of the tube. A cutting disc with cutting blades formed on its outer circumference is inserted between adjacent fins in the axial direction of the tube body, and the sides of the fins are cut away with the cutting blades to form the recess and the extension piece. After inserting the cutting disc, a pressing disc is inserted between the fins to push the extension into the bottom of the groove between the fins, thereby forming the step at the bottom of the groove. After inserting the aforementioned pressing disc, the tips of the fins are pushed radially inward to form a pair of the aforementioned protruding portions. A method for manufacturing boiling-type heat transfer tubes. [Effects of the Invention]

[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. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a partial cross-sectional perspective view showing the axial cross-section of a boiling-type heat transfer tube. [Figure 2] FIG. 2 is a schematic partial enlarged cross-sectional view showing a part of the outer peripheral surface of the boiling heat transfer tube cut out and enlarged. [Figure 3] FIG. 3 is a partial cross-sectional perspective view schematically showing a state in which the leg portions of the fins shown in FIG. 2 are cut along line III-III and the overhanging portions are removed. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV shown in FIG. 2. [Figure 5] FIG. 5 is a cross-sectional view showing the boiling heat transfer tube disposed in the liquid refrigerant in a cross-section perpendicular to the tube axis direction. [Figure 6] FIG. 6 is an explanatory view showing a state in which the liquid refrigerant is heated in the cavity portion to generate evaporation bubbles. [Figure 7] FIG. 16 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. [Figure 8] FIG. 19 is a side view of the processing apparatus shown in FIG. 7. [Figure 9] FIG. 22 is a cross-sectional view schematically showing a state of processing of the fins by the gear disk and the pushing disk. [Figure 10] FIG. 25 is an explanatory view schematically showing a state of the fins cut by the gear disk. [Figure 11] FIG. 28 is an explanatory view schematically showing a state of the protrusions and the cutting pieces after the pushing disk is inserted between the fins. [Figure 12] FIG. 31 is an explanatory view showing step by step the procedure of the finishing process of the fins in the region. [Figure 13] FIG. 34 is a process explanatory view schematically showing a state in which the fin shape changes step by step by processing the tube body with the disk group. [Figure 14] FIG. 37 is a schematic view of a test apparatus used for evaluating the heat transfer performance of the boiling heat transfer tube. [Figure 15] FIG. 40 is an explanatory view showing the respective dimensions of the fins and the ribs shown in Tables 2 and 3. [Figure 16] FIG. 43 is a graph showing the relationship between the heat flux and the overall heat transfer coefficient in Test Examples 1 to 3. [Figure 17] Figure 17 is a photograph showing how a gear disc and a pressing disc are inserted between the formed fins, creating a raised section at the bottom of the groove. [Figure 18] Figure 18 is a cross-sectional photograph showing the side view of the fin shown in Figure 17. [Figure 19] Figure 19 is a photograph showing how an embossed pattern is formed by crushing a pair of protruding parts with a gear disc. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described in detail below with reference to the drawings. <Configuration of boiling heat transfer tubes> 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. In addition, multiple rows of ribs 15 are formed in a spiral shape on the inner surface of the tube body 10. Note that 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 fin 11 shown in Figure 2 after cutting the leg portion 11a 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 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, recesses 23 are formed on the side wall surface 13b of the cavity 13, recessed in the direction Ax of the pipe axis, and multiple flat ridges 17 are formed on the bottom surface 13a of the cavity 13, where the extension pieces extending from the recesses 23 (described later) are pressed against the bottom surface 13a. The projections 21 protrude from the circumferential TD edge of the recesses 23 in the direction Ax of the pipe axis and extend radially. The uneven parts 19 will be described in detail later, but the recesses 23 are the marks left when the legs 11a of the fins 11 are scraped off, and the extension pieces formed by this scraping (hereinafter also called scraped pieces) are the main material that forms the ridges 17. In addition, the projections 21 include burrs formed when the side wall surface 13b is plastically deformed when the legs 11a are 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 compartments SG along the circumferential direction TD. Since the emboss 25 is provided at equal intervals along the circumferential direction, the small compartments 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 from 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 tube 100 described above will be explained. Figure 5 is a cross-sectional view of a boiling-type heat transfer tube 100 placed in a liquid refrigerant, taken perpendicular to the tube axis. The boiling-type heat transfer tube 100 is placed horizontally in the tube axis direction within the liquid refrigerant 31 in the evaporator, and heated water 33 is supplied into the boiling-type heat transfer tube 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 tube circumferential surface from the bottom to the top of the cavity 13, as indicated by the arrow FL, and move towards the liquid surface (not shown) of the liquid refrigerant 31 from the top of the boiling-type heat transfer tube 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 move towards 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 mentioned 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 evaporated 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 evaporated 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 inside 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 evaporative 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 evaporative 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 evaporative 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 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 within 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, we will elaborate further on the heat transfer efficiency mentioned above. Regarding the flow characteristics of gas-liquid two-phase flow in a heat transfer tube, for example, "Condensation of Refrigerants - From Fundamentals to Applications," Japan Society of Refrigerating and Air Conditioning Engineers Specialist Book Series (ISBN 978-4889671315), p. 277, shows the case of a flow channel with a rectangular cross-section perpendicular to the tube axis. In this flow channel, 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 sections (sides). This thin liquid film can reduce the thermal resistance from the heat transfer tube to the steam. Therefore, heat transfer from the heat transfer surface becomes easier, 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 round tube with the same flow channel cross-sectional area. Although the above literature describes the condensation phenomenon, the same applies to 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 Figure 4, a step 17 is formed on the bottom surface 13a and a projection 21 is formed on the side wall surface 13b, so there are many corners within the flow path. As mentioned above, the corners themselves are effective in promoting nucleated boiling, and in addition, because the liquid film is attracted to the corners by surface tension, a thin liquid film is formed on the edges and surfaces connected to the corners. Specifically, the region Ap shown in Figures 4 and 6 can be exemplified as a location where a thin liquid film is generated.

[0035] As numerous thin liquid film areas are formed 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 also promoted, activating the generation of evaporation bubbles 37, thus achieving high heat transfer performance. Thus, with this boiling-type heat transfer tube 100 configuration, unlike the conventional configuration in which small protrusions are provided at the bottom of the grooves in the cavity 13, the number of corners is significantly increased. Therefore, the area of ​​the edges and surfaces connected to the corners becomes larger, thin liquid film areas are formed over a wide area, and heat exchange becomes more active. Furthermore, since each of the numerous corner areas contributes to nucleated boiling, the synergistic effect between the thin liquid film areas and the corners can dramatically improve heat transfer performance.

[0036] Based on the above, the boiling-type heat transfer tube 100 of this configuration synergistically promotes the generation of evaporative bubbles 37 through the nucleation boiling promotion effect of the aforementioned ridge portion 17 and uneven portion 19, the increased heat transfer amount due to the increased surface area, the stirring effect of the liquid refrigerant 31, and the localized heating effect of the small compartment SG. As a result, evaporative bubbles 37 can be generated with high efficiency under all heat flux conditions, thereby improving heat transfer efficiency.

[0037] <Manufacturing method for boiling type heat transfer tubes> Next, the manufacturing method of the boiling-type heat transfer tube 100 described above will be explained. Figure 7 is a schematic front view showing the main parts of a processing apparatus for forming fins and the like on the outer surface of a pipe body 10. Figure 8 is a side view of the processing apparatus shown in Figure 7. The support shafts 41 shown in Figure 7 are arranged at equal intervals in the circumferential direction at a central angle of 120° from the mandrel 43 shown in Figure 8. A group of discs 47, which are made up of multiple stacked disc-shaped discs that are tools for forming fins and cavities / grooves, are fixed coaxially 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 pipe body 10 is pressed against the circumferential ends of the discs while rotating in the opposite direction to the rotation of the support shafts 41, thereby forming the fins 11 and cavities 13 by plastic deformation. In addition, each support shaft 41 is positioned at a twisted position relative to the pipe axis, so that the pipe body 10 is processed in a spiral manner and fed in the pipe axis direction Ax.

[0038] By using the disc group 47 configured in this way, spiral fins 11 are formed on the surface of the aforementioned tube 10, and the radially outer tips of these fins 11 are crushed to form the shapes of various parts such as protruding parts 11b, 11c, hollow parts 13, stepped parts 17, and uneven parts 19. Here, an example is described in which three spiral fins 11 are formed by three support shafts 41, 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 the disc that forms the aforementioned uneven portion 19 and the cutting piece 51 which is the extension piece, and gear disc GD1 is used. The indentation disc PD is the disc that presses the cutting piece 51 between the fins 11 to form the step portion 17. The connecting disc is the disc that forms the emboss 25 on the protruding portions 11b and 11c, and gear disc GD2 is used. The finishing discs FD1 to FD5 are discs that crush the tips of the fins to finish them. The disc group 47 is formed similarly on the three support shafts 41, but the gear disc GD2 is placed on only one of the three support shafts 41, with spacers placed on the other two support shafts 41 instead of the gear disc 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, while 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 with 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 left to right 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 easily undergo elastic deformation. 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 its radial height. 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, formed by plastic deformation during cutting and projecting in the direction of the pipe axis Ax. Cut pieces 51 are formed extending from the side wall surface 13b, which are the material removed from the fin 11. The protrusions 21 may contain burrs. The cut pieces 51 are formed to project in the direction of the pipe axis Ax, 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 push-in disc PD shown in Figure 9 is inserted between the fins 11 on which the protrusions 21 and cutting pieces 51 are formed. Figure 11 is a schematic diagram illustrating the appearance of the projection 21 and cutting piece 51 after the press disc PD is inserted between the fins 11. The projection 21 is sheared or stretched by the press disc PD at a point where it protrudes from the side wall surface 13b of the fin 11 in the direction of the pipe axis Ax. The cutting piece 51 is pressed against the bottom surface of the pipe body 10 to form a ridge 17. A portion of the cutting shavings from the sheared or stretched projection 21 is pressed against the bottom surface of the pipe body 10 by the press disc PD, forming a ridge 17 together with the cutting piece 51. The press disc PD has a smaller diameter than the rolling disc RD13, and is pressed in to the extent that the cutting piece 51 does 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 caused by the press-in 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-in 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 illustrating the step-by-step procedure for finishing 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, finishing disc FD3 is pushed between the fins 11 to form thin-walled portions 53 at the tips of each fin 11. These finishing discs FD3 are provided on the three support shafts 41 shown in Figure 8, and the finishing discs FD on each support shaft 41 are sequentially pressed onto the fins 11. As a result, thin-walled portions 53 are formed on both sides of the fins 11 in the direction of the pipe axis Ax, as shown in step 3. Alternatively, a disc with a wavy cutting edge on its outer circumference may be used instead of the finishing disc FD3. In that case, the tip of the fin 11 will split in a Y-shape in the cross-section in the direction of the pipe axis Ax, 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 at predetermined intervals in the circumferential direction TD, as shown in Figure 2. 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, excluding 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 the ridge 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. [Examples]

[0055] Next, we will explain the results of evaluating the heat transfer performance using the boiling-type heat transfer tube 100 with the above configuration. Figure 14 is a schematic diagram of the test apparatus used to evaluate the heat transfer performance of boiling-type heat transfer tubes. The test apparatus consists of a stainless steel shell-and-tube heat exchanger with a condenser 61 and an evaporator 63 connected by steam 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. The test heat transfer tubes 65 of the evaporator 63 are supplied with heated water stored in a tank 67. The temperature of the heated water supplied to the test heat transfer tubes 65 of the evaporator 63 is regulated to a constant temperature by a cooling brine liquid heat exchanger, which includes 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 regulated to a constant flow 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 at both ends of the test heat transfer tube 65 using a platinum resistance thermometer that had been pre-calibrated for temperature. Specifically, the inlet and outlet temperatures of the heated water were measured by placing 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 mass 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] [Table 1]

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

[0061]

number

[0062] Here, Ws is the heated water flow rate [kg / s], cp s The specific heat of heated water [kJ / (kg·K)], T s,in The temperature of the heated water inlet is [°C], T s,outis the hot water outlet temperature [°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 from the measured saturation vapor pressure using the refrigerant thermophysical property database (REFPROP Ver. 10). The heat flux qo [kW / m 2 based on the outer tube heat transfer area was defined by Equation (5).

[0063]

Number

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

[0065]

Number

[0066] The inner side heat transfer coefficient h i [kW / (m 2 ·K)] was defined by Equation (7) assuming that the functional form coincides with the Dittus - Boelter equation.

[0067]

Number

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

[0069]

Number

[0070] Note that the Dittus - Boelter coefficient DBC in the equationi 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 as shown in Tables 2 and 3.

[0072] [Table 2]

[0073] [Table 3]

[0074] Each test heat transfer tube has the aforementioned fins and cavities formed on its outer surface and the ribs 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 heat flux qo and the overall heat transfer coefficient Ko in Test Examples 1-3. As shown in Figure 16, across the entire heat flux range, 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). 2The temperature is approximately 20 kW / m², but in the case of Test Example 1, the increase in the overall heat transfer coefficient with increasing heat flux is significant, and the heat flux is 20 kW / m². 2 8kW / (m 2 It has reached K). For example, the heat flux is 40 kW / m 2 Before and after the test, the overall heat transfer coefficient in Test Example 1 becomes more than 1.3 times that in 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) in 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 from 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 region 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 step act as 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 protrusions 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 fins 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 fins 11 to form the stepped portion 17, allowing for the formation of a stepped portion 17 of the desired size.

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

[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 matters are disclosed in this specification: (1) The tube has multiple rows of fins that protrude radially outward from the outer surface of the tube and are formed spirally along the central axis of the tube, Each fin has a leg portion erected on the outer surface of the tube and a pair of protruding portions whose radially outer tips extend in opposite directions with respect to the axial direction of the tube. Between the fins adjacent to each other in the axial direction of the tube, a cavity is defined that is continuous in the circumferential direction, surrounded by the protruding portion of one fin that extends toward each other, the protruding portion of the other fin, and a pair of opposing leg portions. A recess is formed on the side wall surface of the cavity portion, which is recessed in the direction of the pipe axis. Multiple flat, stepped portions are formed on the bottom surface of the cavity, with extensions extending from the recesses being pressed against the bottom surface. Boiling type heat transfer tube. 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 uneven portions having projections that protrude from the recess in the direction of the tube axis and extend along the radial direction. In this boiling-type heat transfer tube, the surface area is increased by the irregularities on the side walls, thereby promoting heat transfer from the side walls. In addition, the corners of the irregularities act as foaming points, enhancing the nucleation boiling effect.

[0088] (3) Multiple connection points are provided along the circumferential direction, where adjacent protruding portions in the axial direction of the pipe are connected by recessing them radially inward. The boiling heat transfer tube according to (1) or (2), wherein the connection point is a narrow 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 section, and the cavity is divided into a plurality of small sections along the circumferential direction by the narrow section. In this boiling-type heat transfer tube, multiple connection points are provided along the circumferential direction to connect the protruding sections, creating narrow sections within 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 locations, constricting the flow of the liquid refrigerant. This suppresses the flow of the liquid refrigerant, making it easier to heat 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, leading to the efficient generation of evaporation bubbles.

[0089] (4) The boiling type heat transfer tube described in (3), wherein the connection points are provided across multiple rows of the cavities. In this boiling-type heat transfer tube, each small compartment, which is divided circumferentially into multiple rows of cavities, is formed evenly. As a result, the liquid refrigerant is heated under the same conditions, enabling uniform heat exchange.

[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 portion. This boiling-type heat transfer tube ensures even distribution within the cavity of the platen, reducing uneven heat transfer.

[0091] (6) A method for manufacturing a boiling type heat transfer tube according to any one of (1) to (5), By pressing the outer circumferences of multiple rolling discs against the outer surface of the tube, the outer surface of the tube is plastically deformed, thereby forming multiple rows of fins that protrude radially outward from the outer surface of the tube in a spiral shape along the central axis of the tube. A cutting disc with cutting blades formed on its outer circumference is inserted between adjacent fins in the axial direction of the tube body, and the sides of the fins are cut away with the cutting blades to form the recess and the extension piece. After inserting the cutting disc, a pressing disc is inserted between the fins to push the extension into the bottom of the groove between the fins, thereby forming the step at the bottom of the groove. After inserting the aforementioned pressing disc, the tips of the fins are pushed radially inward to form a pair of the aforementioned protruding portions. A method for manufacturing boiling-type heat transfer tubes. According to this boiling-type heat transfer tube manufacturing method, multiple ridges are formed by carving out the sides of the fins and pressing them into the bottom of the grooves, enabling highly efficient heat transfer to the outside of the heat transfer tube through the ridges near the center of the tube. Furthermore, the formation of the ridges increases the surface area of ​​the bottom of the cavity, which also contributes to highly efficient heat transfer. In addition, steps are formed on the ridges 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 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 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 to the heat transfer tube to be transferred to the entire fin, improving the heat transfer efficiency to the liquid refrigerant.

[0092] (7) 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 pipe axis and extends radially. The method for manufacturing a boiling type heat transfer tube according to (6), wherein the pressing disc cuts off a portion of the protruding projection. In this boiling-type heat transfer tube manufacturing method, the surface area is increased by the irregularities on the side walls, thereby promoting heat transfer from the side walls. Furthermore, the corners of the irregularities act as foaming points, enhancing the nucleation boiling promotion effect. Additionally, by inserting the indentation disc, some of the protrusions extending from the sides of the fins are removed, maintaining the fluidity of the liquid refrigerant flowing within 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. This method for manufacturing boiling-type heat transfer tubes allows for the use of general-purpose spur gears as cutting discs, thereby reducing equipment costs.

[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 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 multiple small sections along the circumferential direction. According to this boiling-type heat transfer tube manufacturing method, multiple connection points are created along the circumferential direction by recessing and connecting the protruding portions, thereby forming narrow sections within the cavity. These small compartments between the 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 minimal unevenness. Furthermore, the formation of closed spaces reduces the cross-sectional area in the axial direction of the tube at the connection points compared to other circumferential positions, 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 locations along the circumferential direction within the cavity, leading to efficient generation of evaporation bubbles. [Explanation of Symbols]

[0095] 10. Body 11 fins 11a Legs 11b, 11c Protruding section 13 Cavity 13a Bottom 13b Side wall 15 Ribs 17 Danbe 19 Uneven part 21 Protrusion 23 Recess 25 Embossed 30 heating layer 31 Liquid Refrigerant 33 Heating water 37 Evaporation bubbles 41 Spindle 43 Mandrels 47 disks 51 Cutting piece (extending piece) 53 Thin-walled section 61 Condenser 63 Evaporator 65 Test heat transfer tubes 67 tanks 69 Cooling coil 71 Electric heater 73 Heat transfer tubes 75 tanks 100 boiling type heat transfer tubes RD1~RD13 Rolling Discs GD1, GD2 Gear Disc PD push-in disc FD1~FD5 Finishing Discs

Claims

1. The tubular body has multiple rows of fins that protrude radially outward from its outer surface and are formed spirally along the central axis of the tubular body. Each fin has a leg portion erected on the outer surface of the tube and a pair of protruding portions whose radially outer tips extend in opposite directions with respect to the axial direction of the tube of the tube. Between adjacent fins in the axial direction of the tube, a cavity is defined that is continuous in the circumferential direction, surrounded by the protruding portion of one fin that extends toward each other, the protruding portion of the other fin, and a pair of opposing leg portions. A recess is formed on the side wall surface of the cavity portion, which is recessed in the direction of the pipe axis. Multiple flat, stepped portions are formed on the bottom surface of the cavity, with extensions extending from the recesses being pressed against the bottom surface. Boiling type heat transfer tube.

2. The side wall surface of the cavity has a plurality of irregularities formed thereon, each having a projection that extends radially from the recess in the direction of the pipe axis. A boiling-type heat transfer tube according to claim 1.

3. Multiple connection points are provided along the circumferential direction, where adjacent protruding portions in the axial direction of the pipe are connected by recessing them radially inward. The aforementioned connection point is a narrow section in the axial direction of the pipe where the cross-sectional area of ​​the cavity is smaller than the cross-sectional area of ​​the area outside the connection point, and the cavity is divided into a plurality of small sections along the circumferential direction by the narrow section. A boiling-type heat transfer tube according to claim 1.

4. Multiple connection points are provided along the circumferential direction, where adjacent protruding portions in the axial direction of the pipe are connected by recessing them radially inward. The aforementioned connection point is a narrow section in the axial direction of the pipe where the cross-sectional area of ​​the cavity is smaller than the cross-sectional area of ​​the area outside the connection point, and the cavity is divided into a plurality of small sections along the circumferential direction by the narrow section. The boiling type heat transfer tube according to claim 2.

5. The aforementioned connecting points are provided across multiple rows of the aforementioned cavities. The boiling type heat transfer tube according to claim 3.

6. The aforementioned connecting points are provided across multiple rows of the aforementioned cavities. The boiling type heat transfer tube according to claim 4.

7. The aforementioned platform portion is arranged at equal intervals along the circumferential direction on the bottom surface of the cavity portion. A boiling-type heat transfer tube according to any one of claims 1 to 6.

8. A method for manufacturing a boiling-type heat transfer tube according to claim 1, By pressing the outer circumferences of multiple rolling discs against the outer surface of the tube, the outer surface of the tube is plastically deformed, thereby forming multiple rows of fins that protrude radially outward from the outer surface of the tube in a spiral shape along the central axis of the tube. A cutting disc with cutting blades formed on its outer circumference is inserted between adjacent fins in the axial direction of the tube body, and the sides of the fins are cut away with the cutting blades to form the recess and the extension piece. After inserting the cutting disc, a pressing disc is inserted between the fins to push the extension into the bottom of the groove between the fins, thereby forming the step at the bottom of the groove. After inserting the aforementioned pressing disc, the tips of the fins are pushed radially inward to form a pair of the aforementioned protruding portions. A method for manufacturing boiling-type heat transfer tubes.

9. 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 extends radially from the recess in the direction of the pipe axis. The aforementioned pressing disc cuts off a portion of the protruding part. A method for manufacturing a boiling-type heat transfer tube according to claim 8.

10. The aforementioned cutting disc is a gear disc with spur teeth formed on its outer circumference. A method for manufacturing a boiling-type heat transfer tube according to claim 8.

11. The aforementioned cutting disc is a gear disc with spur teeth formed on its outer circumference. A method for manufacturing a boiling-type heat transfer tube according to claim 9.

12. The protruding portions adjacent to each other in the axial direction of the pipe 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 axial direction of the pipe is smaller than the cross-sectional area of ​​the areas other than the connection points, and the cavity is divided into multiple small sections along the circumferential direction. A method for manufacturing a boiling-type heat transfer tube according to any one of claims 8 to 11.

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