METHOD OF MANUFACTURE AN ENHANCED HEAT TRANSFER SURFACE AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract: An apparatus and method for producing a high performance evaporator tube (10) having a subsurface channel (16) formed between adjacent helical fins (12) with the subsurface channels (16) having alternating closed portions and open pores above the subsurface channels (16). The fins (12) of the tube (10) are rolled-over toward the adjacent fin (12) and then con­ tacted with a notched disc (28) to form the alternating closed portions and open pores.

Description: This invention relates to a method and an apparatus of forming a heat exchanger tube: according to the features as indicated in the precharacterising portions of claims 1 and 3. Tubes manufactured in accordance with the present invention are used in a heat exchanger of the evaporator type wherein a fluid to be cooled is passed through the tubing and a boiling liquid, usually refrigerant, is in contact with the exterior of the tubing whereby heat is transferred from the fluid in the tubing to the boiling liquid. A method and an apparatus according to the precharacterizing portions of claims 1 and 3 are disclosed in US-A-4,425,696 and US-A-4,438 807 according to which an enhanced evaporator tube having subsurface channels communicating with the surroundings of the tube through openings located above an internal rib is manufactured according to a method whereby a grooved mandrel is placed inside an unformed tube and a tool arbor having a tool gang thereon is rolled over the external surface of the tube. The unformed tube is pressed against the mandrel to form at least one internal rib on the internal surface of the tube. Simultaneously, an external fin convolution is formed on the external surface of the tube by the tool arbor with the tool gang. The external fin convolution has depressed sections above the internal rib where the tube is forced into the grooves of the mandrel to form the rib. A smooth roller-disc on the tool arbor is rolled over the external surface of the tube after the external fin is formed. The smooth roller disc is designed to bend over the tip portion of the external fin to touch the adjacent fin convolution only at those sections of the external fin which are not located above an internal rib. The tip portion of the depressed sections of the external fin, which are located above the internal rib, are bent over but do not touch the adjacent convolution thereby forming a pore which provides fluid communication between the surroundings of the tube and the subsurface channels of the tube. In US-A-4,313,248 a method is disclosed for forming the heat transfer surface for a heat transfer tube whereby a finning disc forms fins on the surface of a tube and a roller disc compresses the top surface of adjacent fins downwardly to form a narrow gap between adjacent shoulders of adjacent fins. The creation of high performance heat exchanger tubes has been pursued because it has been found that the transfer of heat to a boiling liquid is enhanced by the creation of vapor entrapment sites or cavities. It is theorized that the provision of vapor entrapment sites assist nucleate boiling. According to this theory the trapped vapor forms the nucleus of a bubble, at or slightly above the saturation temperature, and the bubble increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid refrigerant enters the vacated volume trapping the remaining vapor and another bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the boundary layer of superheated liquid refrigerant, which covers the vapor entrapment sites, results in improved heat transfer. Also, it is known that excessive influx of liquid from the surroundings can flood or deactivate a vapor entrapment site. In this regard, a heat transfer surface having a continuous gap between adjacent fins reduces the performance of the tube. Further, enhanced tubes as disclosed in US-A-4,425,696 and US-A-4,438 807 having subsurface channels communicating with the surroundings through surface openings or pores having a specified "opening ratio", although they may prevent flooding of the subsurface channel, are generally limited to having openings for the cavities only at those locations above an internal rib or depression in the external surface of the tube. The performance of enhanced tubes is critically dependent on the size of the subsurface channels and pores above the subsurface channels, and the number of and spacing between the pores. It is therefore important to manufacture externally enhanced tubes having consistent subsurface channels and pores around the circumference of the tube. It has been determined that in order to improve the performance of enhanced tubes the quantity of pores must be much higher than presently obtained by using an internal rib to form the pores thereabove. A higher quantity of pores can be provided by the method and tool disclosed in JP-A-61/291895 according to which, after rolling the fin, notches are cut into the tip portion thereof and the notched fin convolutions are then rolled over to contact adjacent convolutions between the notches, with the notches providing the open pores. However, cutting the notches can deform the fin convolutions and result in irregular gaps around the circumference of the tube. Thus, there is a clear need for a high performance tube having an enhanced outer surface with a plurality of subsurface channels communicating with the outside space through an increased number of evenly spaced fixed size surface pores that will, to a large extent, overcome the inadequacies that have characterized the prior art. The object of the present invention is to overcome the foregoing difficulties and shortcomings experienced in the prior art and to provide a method and a tool for forming a heat exchanger tube having an improved heat transfer performance. In accordance with the invention to achieve this, there is provided a method of forming a heat exchange tube having a subsurface channel between adjacent convolutions of at least one radially extending helical fin and a plurality of closed sections over the subsurface channel around a circumference of the tube and alternating with open pores therebetween, comprising the steps of rolling said at least one radially extending helical fin in the exterior tube surface along the longitudinal axis of the tube while backing the interior tube surface, and rolling over said fin convolutions toward an adjacent convolution of the fin to form said subsurface channel between adjacent convolutions of the rolled over fin convolutions, with the tip portions of the adjacent rolled over convolutions remaining spaced from one another at least at circumferentially spaced locations to define said open pores, characterized in that in the step of rolling said at least one fin said backing is carried out so that all fin convolutions are prevented from being depressed by providing the interior tube surface with a smooth shape or with internal ribs closely spaced to prevent the portions of said at least one external fin located above the ribs from being depressed, that in the step of rolling over said fin convolutions adjacent fin convolutions are rolled over so as to remain spaced from one another at their tip portions throughout the circumferential extent thereof, and that after the step of rolling over the fin convolutions the tip portions of the rolled over fin convolutions are depressed at a plurality of spaced contact points around the circumference of the tube until the rolled over fin convolutions contact an adjacent convolution to form said closed sections. In further accordance with the invention there is provided an apparatus for use in forming a heat exchange tube according to the method steps of claim 1, comprising a) a mandrel adapted to be placed inside an unformed tube for backing the same; b) a tool gang comprising in succession 1.) an external fin forming means including a plurality of discs for rolling at least one radially extending helical fin in the exterior surface along the longitudinal axis of the tube; 2.) a fin rolling means for rolling over said radially extending fin to form a subsurface channel between adjacent convolutions of the rolled over fin; and characterized by 3.) a tooth-like notched disc means having a plurality of alternating protrusions and V-shaped notches about the circumference of said disc means for matingly engaging adjacent convolutions of the fin at contact points between said protrusions and the rolled over fin forming said closed sections at said contact points and said open pores below said V-shaped notches; said mandrel having a smooth outer surface or a grooved outer surface formed to prevent the external fin from being depressed. With the method and apparatus according to the invention the number of pores can be substantially increased as open pores are not only formed at those locations where the external fin in the tube is forced into the grooves of the mandrel to form the internal rib. An average number of between about seventy five and eighty open pores can be formed about a circumference of the tube. Cutting of notches into the fin is not required. The method and tool of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same, and in which: Figure 1 is a side elevation view of a tube, a smooth mandrel, and a tool arbor having a tool gang thereon for rolling the tube on the mandrel to form the heat transfer tube manufactored in accordance with the present invention; Figure 2 is a fragmentary sectional view on an enlarged scale showing a typical tube being finned, rolled over, and depressed by the tool gang arrangement of the present invention; Figure 3 is a side elevational sectional view on an enlarged scale of the high performance evaporator tube manufactored in accordance with the present invention with internal ribs; Figure 4 is a 10X photograph of the surface of the high performance evaporator tube manufactored in accordance with the present invention; Figure 5 is an elevational sectional view of the final notched roller of the tool gang of the present invention forming the enhanced surface shown in Figure 4; Figure 6 is an enlarged view of the teeth of the final notched roller as shown in Figure 5; and Figure 7 is a graphical representation of the boiling performance of the high performance evaporator tube manufactored in accordance with the present invention in comparison with a prior enhanced tube. The high performance enhanced tubes manufactored in accordance with the present invention are designed for use in an evaporator of a refrigeration system having a fluid to be cooled passing through heat transfer tubes and having refrigerant, which is vaporised, in contact with the external surface of the tubes. Typically, a plurality of heat transfer tubes are mounted in parallel and connected so that several tubes form a fluid flow circuit and a plurality of such parallel circuits are provided to form a tube bundle. Usually, all of the tubes of the various circuits are contained within a single shell wherein they are immersed in the refrigerant. The heat transfer capabilities of the evaporator are largely determined by the average heat transfer characteristics of the individual heat transfer tubes. The size of the subsurface channels and the size, number, and configuration of the pores on the surface of the tubes are particularly critical for R-11 applications. Moreover, the creation of a high performance evaporator tube that can be manufactured from a commercial prime tube in a single pass on a conventional tube finning machine is preferred since it permits more rapid operation and is more cost effective. Referring now to the drawings, Figure 1 shows the relationship between a tube 10 being enhanced and a tool arbor 20 spaced thereabout and a mandrel 30 inserted therein. Normally, a finning machine contains a plurality of tool arbors, e.g., three spaced 120° apart, but only one tool arbor is shown for clarity. The mandrel 30 is of sufficient length that the interior surface of the tube 10 is supported beneath the tool arbor 20. The mandrel 30 may either be smooth (as shown in Figure 1) or grooved to form internal ribs (as shown in Figure 3). However, if the mandrel forms ribs in the tube it is important that the ribs are closely spaced to prevent the external fins located above the ribs from being depressed. The tool arbor 20 with a tool gang 22 is used to form the external fin convolutions 12. The tool gang 22 comprises a plurality of fin forming discs 24 which are used to displace the material of the tube wall 14 of tube 10 to form the helical external fin convolutions 12, and a plurality of roller-like discs 26 to contact the formed fins. A tooth-like notched disc 28 is the last roller-like disc to contact the tube 10. As shown in Figure 2 the external fin convolution 12 is formed by the fin forming discs 24. Subsequently, the smooth roller-like discs 26 roll over the tip portion 13 of the fin convolution 12 toward the adjacent convolution to form subsurface channels 16 but with the tip portion 13 of the fin remaining spaced along the complete length thereof from an adjacent fin convolution 12. The high performance evaporator tube can be easily manufactured with the apparatus and method as shown in Figures 1 and 2. Accordingly, in operation, an unformed tube 10 is placed over the mandrel 30. The mandrel 30 is of sufficient length that the interior surface of the tube 10 is supported beneath the tool arbor 20. The tool gang 22 on the tool arbor 20 is brought into contact with the tube 10 at a small angle relative to the longitudinal axis 11 of the tube 10. This small amount of skew provides for tube 10 being driven along its longitudinal axis as tool arbors 20 are rotated. The fin forming discs 24 displace the material of the tube wall 14 to form the external fin convolution 12 having a root portion 17 and a tip portion 13 while at the same time depressing the tube 10 against the mandrel 30. Generally, the discs 24 form between seventeen and twenty three fins per cm (forth-five and sixty fins per inch) along the longitudinal axis of the tube for maximum performance. When the tube mandrel 30 is grooved, depressing the tube 10 against the grooved mandrel will displace the tube wall 14 into the grooves of the mandrel to form internal ribs 15. Figure 3 illustrates the configuration of a tube formed with a grooved mandrel after the fin forming discs 24, roller-like discs 26, and tooth-like notched disc 28 are rolled over the exterior of the tube 10 to form subsurface channels 16 and surface pores 18, and the ribs 15 are formed on the internal surface. The internal ribs 15 are closely spaced to prevent undulations from being formed on the exterior surface of the tube. A generally smooth exterior surface provides for fins of constant height or outer diameter, thereby insuring that the roller discs and notched disc contact the fins evenly. As clearly shown in Figure 4, the tool arbor 20 creates a pattern of helical subsurface channels 16 having cavity openings or pores 18 alternating with closed sections 19, on the exterior of the tube 10. For the tubes shown in Figures 1-4, with a smooth internal wall or internal ribs (as shown in Figure 3), the enhanced surface area pattern is generally similar because the initial height of the fin convolutions 12 formed on the surface of the tube is generally equal along the entire length of the tube. A typical mandrel having either a smooth surface or a surface with greater than 36 grooves about its circumference and used with a tool gang to form more than 15 fins per cm (40 fins per inch) along the longitudinal axis of the tube creates a pattern of open sections, corresponding to the pores 18 and closed sections 19 as a result of the final tooth-like notched disc 28 contacting the roller over fins. This alternating open pore and closed section provides improved performance when there are generally eighty pores around the circumference of the tube along a subsurface channel. Referring now to Figures 5 and 6, the general construction details of the final tooth-like notched disc 28 are shown. Accordingly, in operation of the preferred embodiment, e.g. having a tool arbor 20 as shown in Figure 1, the rolled over fins remain initially spaced from one another at their tip portions along the complete circumferential extent thereof, the notched disc 28 then contacts the previously rolled over fin convolutions 12 and forms closed sections 19. The notched disc 28 has a plurality of alternating projections or tooth-like protrusions 29 and V-shaped notches 27 about the circumference of the disc. A typical notched disc 28 has between 190 and 220 protrusions. Thus, the notched disc 28 depresses the rolled over fins at the location contact is made between the rolled over fin and the protrusion 29. The contact between the tube 10 and the notched disc 28 creates a pattern of surface pores 18 and closed sections 19, where adjacent fins contact each other, above subsurface channel 16. For the notched disc 28, a typical V-shaped notch 27 is truncated and has an inclusive angle 25 between 35° and 45° as shown in Figure 6. Referring now to Figure 7, there is graphically shown a comparison of length-based heat transfer coefficient and length-based heat flux between tube "A", embodying a tube manufactored in accordance with the present invention, and tube "B", embodying an enhanced evaporator tube of the prior art. To obtain the measured length-based heat transfer coefficient of the present invention, a 19 mm (three-fourths inch) copper tube was enhanced with a mandrel having forty-eight grooves about its circumference, a plurality of roller-like discs forming sixteen fins per cm (forty-two fins per inch), and a notched disc having one hundred ninety-two protrusions with an inclusive angle of 40° about the circumference of the disc. The sample tube manufactored in accordance with the present invention was an enhanced tube with the internal fin convolutions having a 30° helix angle, and having sixteen external fins per cm) (forty-two external fin turns per inch), and having an internal rib pattern of forty-eight starts with a distance of approximately 1.7-2.2 mm (0.070-0.090 inches) between grooves, and having surface pores on the order of 0.05-0,12 mm (0.002-0.005 inches). Tests have shown that a high performance tube should have at least fourteen internal fins and have at least twenty external fins per cm (thirty-six internal fins and fifty-three external fins per inch). As graphically shown in Figure 7, a tube manufactored in accordance with the present invention was compared, using R-11 at 15.6°C (60°F), with that of a sixteen fin per cm (forty-two fin per inch) "TURBOCHILL" tube manufactured by the Wolverine Tube Company. As can be seen by the comparison, the high performance evaporator tube "A" manufactored in accordance with the present invention exhibits an average of approximately 300% performance improvement over the length-based heat transfer coefficient of the enhanced tube "B".

Claims: Method of forming a heat exchange tube having a subsurface channel (16) between adjacent convolutions (12) of at least one radially extending helical fin and a plurality of closed sections (19) over the subsurface channel (16) around a circumference of the tube (10) and alternating with open pores (18) therebetween, comprising the steps of: rolling said at least one radially extending helical fin in the exterior tube surface along the longitudinal axis of the tube while backing the interior tube surface, and rolling over said fin convolutions (12) toward an adjacent convolution of the fin to form said subsurface channel (16) between adjacent convolutions (12) of the rolled over fin convolutions, with the tip portions (13) of the adjacent rolled over convolutions (12) remaining spaced from one another at least at circumferentially spaced locations to define said open pores (18), characterized in that in the step of rolling said at least one fin said backing is carried out so that all fin convolutions (12) are prevented from being depressed by providing the interior tube surface with a smooth shape or with internal ribs closely spaced to prevent the portions of said at least one external fin located above the ribs from being depressed, that in the step of rolling over said fin convolutions (12) adjacent fin convolutions (12) are rolled over so as to remain spaced from one another at their tip portions (13) throughout the circumferential extent thereof, and that after the step of rolling over the fin convolutions (12) the tip portions (13) of the rolled over fin convolutions (12) are depressed at a plurality of spaced contact points around the circumference of the tube (10) until the rolled over fin convolutions (12) contact an adjacent convolution (12) to form said closed sections (19). Method according to claim 1, characterized in that an average number of between about seventy-five and eighty open pores are formed around a circumference of the tube (10). Apparatus for use in forming a heat exchange tube according to the method steps of claim 1, comprising: a) a mandrel (30) adapted to be placed inside an unformed tube for backing the same; and b) a tool gang (22) comprising in succession: 1.) an external fin forming means (24) including a plurality of discs for rolling at least one radially extending helical fin in the exterior surface along the longitudinal axis of the tube; 2.) a fin rolling means (26) for rolling over said radially extending fin to form a subsurface channel (16) between adjacent convolutions (12) of the rolled over fin; and characterized by: 3.) a tooth-like notched disc means (28) having a plurality of alternating protrusions (29) and V-shaped notches (27) about the circumference of said disc means (28) for matingly engaging adjacent convolutions (12) of the fin at contact points between said protrusions (29) and the rolled over fin forming said closed sections (19) at said contact points and said open pores (18) below said V-shaped notches (27); said mandrel (30) having a smooth outer surface or a grooved outer surface formed to prevent the external fin from being depressed. Apparatus according to claim 3, characterized in that said tooth-like notched disc means (28) has between 190 and 220 tooth-like protrusions (29) around the outside circumference of said disc means (28).

Inventors: ZOHLER, STEVEN R.
Applicants: CARRIER CORPORATION
Application year: 1988
Main classification: F28F13
IPC codes: B21D53, B21C37, F28F1