FIELD OF THE INVENTION
The present invention relates to heat exchanger tubes and specifically to heat exchangers for use in high pressure applications, such as hydraulic coolers, radiators, HVAC systems, and CO2 systems.
BACKGROUND OF THE INVENTION
There is a need for heat exchangers that do not swell under high pressure, that increase the heat transfer of normal tubes by adding indents to the inner surface of the tube, and that enable the use of a higher conductance material for high pressure tube technology.
There have been attempts to improve high pressure tubes by providing the tubes with thicker, heavier material for the bottom wall to reduce bulging.
Also, some tubes have been made out of aluminum and then brazed to create indents. Another type of tube includes an extruded micro-channel structure for strength.
Each of the above methods includes drawbacks. If the tubes have a thick bottom wall to prevent bulging, this causes the heat exchanger to be heavier than needed if it was made from a thin wall material.
The brazing material cannot be applied easily to certain materials and there are environmental issues with brazing. Also, the brazing process adds cost due to the extra raw materials and additional manufacturing steps.
The microchannel structure creates isolation between the channels that limits heat transfer enhancement.
SUMMARY OF THE INVENTION
The present invention meets the above-described needs and overcomes the drawbacks by providing a heat transfer tube and a method of forming a heat transfer tube with indented portions formed in the opposed walls. The indents may be cold welded or forge welded such that dimples or indentations meet in the middle of the tube. The bottom of a first indentation disposed on a first side of the tube is welded to the bottom of a second indentation formed in the opposite side of the tube.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which:
FIG. 1 is a perspective view of the tube of the present invention;
FIG. 2 is a schematic representation of a roll forming apparatus for forming tube;
FIG. 3 is a partial perspective view of an alternate embodiment of the tube of the present invention;
FIG. 4 is another partial perspective view of the tube of the present invention;
FIG. 5 is a schematic representation of a stamping process for forming indentations in the tube;
FIG. 6 is a schematic representation of a pair of rolls for forming the indentations in the tube;
FIG. 7 is a cross-sectional view taken along lines 7-7 of FIG. 1;
FIG. 8 is a plan view of a tube having indentations with various shapes;
FIG. 9A is a plan view of an alternate embodiment of the tube of the present invention;
FIG. 9B is a cross-sectional view taken along lines IXB-IXB of FIG. 9A;
FIG. 10 is a cross-sectional view of a tube of the present invention having a joint thickness equal to zero;
FIG. 11 is a cross-sectional view of an alternate embodiment of the bottom wall of the indentation; and,
FIG. 12 is a partial perspective view of a tube of the present invention having different indentations in different zones of the tube along the tube axis.
DETAILED DESCRIPTION
In FIG. 1, a tube 1 has major outer surfaces 200 formed on the outside of opposed walls 2a and 2b. Walls 2a and 2b are connected by curved end walls 8a and 8b. Walls 2a and 2b also have inner surfaces 202. The tube 1 may be constructed out of copper or other suitable materials. The tube 1 may be formed by several methods as will be evident to those of ordinary skill in the art. For example as shown in FIG. 2, tube 1 may be formed by taking flat stock 203 and roll forming it by turning up the edges gradually and then welding a seam 206 to join the edges. The seam is welded in a continuous process by an apparatus 209. Apparatus for continuous welding of tubes are generally known and therefore will not be described in detail herein. If the tube 1 is initially formed with a round profile, the tube may then be flattened by a press to form a flat tube having opposed walls 2a and 2b. Alternatively, the tube may be formed into a flat tube configuration and welded to shape. Also, other processes for forming the tube 1 such as by drawing or extruding may also be suitable as will be evident to those of ordinary skill in the art. Although the invention has been illustrated in connection with a tube having substantially flat opposed major surfaces, the present invention is also suitable for tubes having major surfaces that are non-flat. For example, as shown in FIG. 3, opposed major surfaces on tube 201 may include sections 212 curved inwardly. Other profiles are also included in the present invention.
In FIG. 4, the tube 1 may have a width W in the range of one-half inch to 5 inches. The height H of the tube may range from 1.5 mm to 5 mm.
Indentations 3a and 3b are formed from the outside through a cold weld or forge welding process. The cold weld may be formed by a stamping process. In the stamping process a pair of opposed dies 12 (FIG. 5) having raised surfaces 14 are arranged such that they stamp the major surfaces of the tube 1 in alignment on opposite sides of the tube 1. As a result, the opposed indents are welded together at the joint 17 where the bottom surface of each indentation meets. Alternatively, the tube 1 may be heated prior to stamping to provide a forge welding process. Also, electrical current may be passed through the tube 1 during the cold or forge welding process.
As yet another alternative shown in FIG. 6, the opposed indentations 3a and 3b may be welded by means of a pair of rolls 20 having raised surfaces 23 for embossing/deforming the tube 1. The raised surfaces 23 on the rolls 20 are aligned such that the indentations 3a and 3b on opposite major surfaces of the tube 1 are welded together at the joint 17 where the bottom surface of each indentation meets. The tube 1 is passed through the rolls 20 in the direction indicated by arrows 21. In the roll process described above, the tube 1 may also be heated prior to engaging with the roll 20. Also, electric current may be passed through the tube 1 during the roll process.
In FIG. 7, the indentations 3a, 3b formed on the opposite side of the tube 1 are disposed in registry such that the bottom of each indent 3a is cold welded to the bottom of the indent 3b to form a plurality of column portions 4. Due to the column portions 4, turbulence of refrigerant occurs, thereby improving the heat transfer capability.
The tube 1 has first wall 2a and second wall 2b defining major surfaces 200 which are substantially parallel to each other and disposed in spaced apart relation. A refrigerant path 23 is formed in the space surrounded by the first and second walls. A plurality of indentations 3a and 3b are formed by protruding relevant portions from the outside of each of the opposed first and second walls 2a and 2b, thus, a plurality of protrusions 25 corresponding to the indents 3a, 3b are formed at the refrigerant path 23 side.
In a plan view, each indent has an elliptical shape, the major axis of the ellipse being along the longitudinal axis 29 of tube 1. As shown in FIG. 7, the heads of the opposed indents 3a and 3b are made to contact each other in weld area 220, so that column portions 4 are formed between the first and second walls 2a and 2b, and each has an elliptical cross-sectional shape. The cross-sectional shape of the column portions 4 is not limited to an ellipse, but circles, ovals or the like are also possible. Turning to FIG. 8, the shape or geometry of the joint 17 may also include, but is not limited to, other shapes such as diamond 30, triangle 33, teardrop 36, double teardrop 39, polyhedral 40 and oval 41. For arbitrary shapes of the joint 17, the equivalent diameter De=4 A/C (where A equals the total area of the joint and C equals the length of a line encircling the joint). For the present invention, the equivalent diameter is in the range of 0.5 mm to 30 mm.
The area of the joint 17 (1×w as shown in FIG. 4) is in the range of 0.5 mm2 to 1,000 mm2. As shown in FIG. 9, substantially round joints 42 resulting from the indents 3a are formed in offset rows disposed along the longitudinal axis 29 of tube 1. For either type of joints, the percent of the major surface 200 that is occupied by joints 42 may range between 2-80%.
The joint thickness ranges from about 180% of the tube wall thickness down to zero. In FIG. 10, the wall thickness ranges down to zero because the opposed indentation 3a, 3b may cut all the way through the tube 1 such that a slug 50 is separated from the tube 1. When this occurs, the opposed walls seal around the opening 53 where the slug 50 has been removed.
The pitch P (FIG. 1) for the indentations 3a, 3b varies between 2 mm to 1 inch in the direction of the tube axis 29.
The joint density varies between 0.1-100 joints per square inch.
Turning to FIG. 11, the opposed surfaces at the bottom of the indentations 3a and 3b where the welds occur may be provided with either flat or non-flat surfaces. For example, the opposed indents may be provided with ridged surfaces 65 having complementary shapes for increasing the surface area where the opposed indentations 3a, 3b engage for welding. Other forms including male and female surfaces or the like are also suitable.
The inside surface of the tube 1 may be smooth or it may be provided with internal enhancements such as fins disposed either axially or helically and with or without cross-hatching.
The tube shown in FIG. 3 has a consistent pattern of joints 42. Alternatively, as shown in FIG. 12 the major surfaces of the tube may be divided into successive regions 70, 73, 76 and 79 along the longitudinal or tube axis 29. Each individual region may be provided with a different arrangement for the joints as described above in order to alter the heat transfer characteristics of the tube 1. In particular, the arrangement of the joints may be varied along the axis 29 in order to provide for varying vapor quality along the length of the tube 1. Joints 100 are disposed in region 70. Joints 103 are disposed in region 73. As shown, joints 100 are larger than joints 103. Also, the joint density for region 73 is greater than for region 70. Continuing in the axial direction, region 76 has joints 106 which are larger and less dense than joints 103. Region 79 has joints 109 which are the smallest and most dense out of any of the regions.
By way of example only, the joint density may increase, decrease or alternate along the successive regions. Also, the joint size may increase, decrease or alternate in similar fashion. Also, the joint shapes may vary in the successive regions in the axial direction.
As will be evident to those of ordinary skill in the art, there are several factors that contribute to the weld. These factors include the surface cleanliness (the surface should be clean and without oxidation), the rate of deformation at the joint, the total deformation for the joined surfaces, the surface area prior to joining, and the localized deformation within each joint. In connection with the stamped cold welding process, the impact range, the quantity of force and the speed of the force appear to be the most significant factors.
It has been determined that an internally grooved copper tube with a welded seam performs particularly well with respect to cold welding, however, other types of heat transfer tubes are also suitable for the present invention.
The above-described methods and apparatus provide joints formed by opposed indentations. The joints improve the heat transfer performance of high pressure HVACR tubes and provide performance in both phases while maintaining acceptable pressure drop to maximize heat transfer and flow performance. The joints may be formed by any type of cold welding as will be evident to those of ordinary skill in the art. The joints may also be forge welded. The joints are formed by metalworking processes that do not require metal cutting, brazing or the addition of filler materials.
The term “welded” as used in this specification refers to a coalescence of metals characterized by a permanent deformation at the interface.
The present invention has been described primarily in connection with a flat copper tube, however, the invention applies equally to other materials and other profiles of tubing.
While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
Patent Application
20050067156
