CA3076953A1 - Tube joining - Google Patents

Abstract

Joining thin heat exchanger tubes to manifold header tanks is improved by extruding thick tubes having channels or microchannels. Tube material is removed from spaced areas of central portions, leaving finned thin heat exchanger central portions and thicker header portions. The tubes are curved, twisted and tilted. The tubes are aligned horizontally, vertically or angularly, and the header portions are welded together. Tube material is removed from the ends of the header portions to form end faces with projections extending from the end faces and surrounding the channels or microchannels. End plates with openings to receive the projections are attached and sealed to the projections and end faces by welding or brazing. The face plates are attached and sealed to openings in the header tanks by welding or brazing. Tube ends are flattened. Rings are added. Ends and rings are friction welded, avoiding areas around channel openings, and polished.

Description

Tube Joining This application claims the benefit of U.S. Provisional Application No.
62/563,382 filed September 26, 2017, which is hereby incorporated by reference in its entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
Major problems exist in heat exchangers and air conditioning systems. Heat transfer tubes with added fins have many connections. Connecting fins to the tubes is one problem. Connecting ends of tubes to header tanks is a more serious problem.
When breaks in connections occur, leaks develop that exhaust internal fluid and kill the heat exchangers used in air conditioning, heat, power and refrigeration systems, for example. Vibrations and bimetallic cell corrosion cause breaks in the connections.
Ends of heat exchanger tubes are connected to openings in header tanks at the ends of the tubes. Connecting the tubes to the header tanks with friction welding is not a solution because the relatively thin tube ends deform. The friction temperatures needed to fuse and weld the tube ends and the headers distort and destroy the tube ends and ports/channels.
Needs exist for solutions to the heat exchange leakage problems and to problems of tube-to-header joining in heat exchangers.
SUMMARY OF THE INVENTION
The present invention solves the problems of leakage destruction in heat exchangers used in heat, power, air conditioning and refrigeration systems, for example.
The present invention provides tubes with relatively thin central sections to promote heat exchange and with large, thicker end sections for joining to the headers.
The large ends of the tubes are machined flat. Material is removed from the flat ends to leave projections having material surrounding the channels on the tubes.

2 Openings are made in the headers to receive the shapes of the material surrounding the channels in the tubes. In one embodiment the large tube ends fit together in the header face plates. The tube ends themselves are joined together and are joined with the header face plates by friction welding or by brazing. The projections of the tube ends are friction welded or brazed in the openings in the header plate. When the tube ends are joined together by brazing, the projections of the tube ends extend into the header chamber from the side of the header face plate.
The brazing extends from sides of the tube ends, between the flat surfaces of the tube ends and the header face plates and along the projections that extend through the openings in the header face plates.
When the tube ends and projections are welded to the header face plates, the tube ends are flush with and are welded to the header plate openings on the header side of the face plate. The tube ends and edges of the large flat tube ends are friction welded to the header face plates. The intersecting edges of the header face plates and the header body are friction welded together.
Important features of the new invention are and include:
= large ends on heat exchange tubes, = angular or circular patterns at tube material thickness transition points, when going from thick to thin portions of the tube and vice versa, = joining the large ends to heat exchanger face plates and/or, = joining large ends of the heat exchanger tubes to each other, = flattening end surfaces of the enlarged ends of heat exchanger tubes, = removing material from the flattened ends of the heat exchanger tubes to form projections surrounding the inner channels within the tubes, = inserting the projections into openings in heat exchanger header connector face plates, = joining the projections and the flat surfaces of the heat exchanger tubes to the heat exchanger header face plates,

3 = friction welding edges of the projections and edges of the large ends to corresponding edges of the heat exchanger header face plates, = when brazing, extending the projections through and beyond the face plate and brazing the surfaces of the projections and of the flat large ends of the tubes to the heat exchanger face plates, = joining all enlarged ends together in a fixed pattern, = wherein the fixed pattern is circular.
The invention provides tubes having one or more channels and having long, thin central heat exchange portions and shorter terminal header portions. The terminal header portions are thicker than the thin central heat exchange portions.
Having angular or circular patterns at tube material thickness transition points. End faces of the terminal portions are machined or oblated to provide outward projections extending around and protecting the one or more channels. The outward projections extend from smooth, flat end faces on the thick terminal header portions.
End plates have inner openings for receiving the outward projections. The end plates have inner faces lying against, attached and sealed on the end faces of the terminal header portions. The inner openings of the end plates are attached and sealed to the outward projections of the end surfaces of the terminal portions.
The end plates and the outward projections are co-extensive from the end faces of the tubes in one embodiment. The end plates are welded to the end faces and to the outward projections on the end faces of the terminal header portions.
In another embodiment the outward projections extend beyond the face plates and the face plates are brazed to the outward projections. Extension of the projections beyond the face plates prevents migration of the brazing material into the tube channels.
The tubes are extruded, and opposite surfaces of the middle portions of the tube are machined or ablated to remove sections of the tube material and to leave spaced fins integrally formed on the thinned middle portion. The fins are slanted with respect to

4 longitudinal directions of the middle portions of the tubes, and the one or more channels are one or more microchannels.
Plural similar extruded tubes have channels within the tubes. Material is removed from the middle portions of the tubes, and spaced fins are left on the thin middle portions. Thick terminal header portions are left on ends of the plural tubes having angular or circular patterns at tube material thickness transition points. The plural tubes are aligned horizontally or vertically or are curved, twisted and sloped with the twisted and tilted tubes in parallel helical relationship. The terminal header portions of the plural tubes are aligned and joined together by welding before machining or ablating ends of the plural joined terminal header portions to create plural end faces and plural outward projections on the plural end faces. The plural outward projections extend around and protect the channels. The plural outward projections are joined to either flat end plates or inner and outer perimeter rings. The end plates have plural inner openings receiving the plural outward projections on the plural end faces of the joined terminal header portions. The plural inner openings of the end plates are welded or otherwise joined to the plural outward projections. The end plates are welded or otherwise joined to the plural end faces.
The new method includes extruding wide and thick tubes with channels in the tubes, and then machining or oblating central portions of the tubes, removing material in spaced sections from the central portions, thinning the central portions, forming fins across the tubes and leaving thicker terminal header portions at ends of the tubes, having angular or circular patterns at tube material thickness transition points, arranging the tubes parallel to each other, and welding or otherwise joining the terminal portions of adjacent tubes.
Machining or oblating ends of the joined terminal header portions forms end faces with outward projections extending around and protecting the channels.
Providing end plates with openings for receiving the outward projections, and welding or otherwise joining the end plates to the end faces and to the projections readies the end plates to be joined to header tanks at the ends of the tubes, or providing inner and outer perimeter rings and welding or otherwise joining the inner and outer perimeter rings readies the rings to be joined to header tanks at the end of the tubes.
Extending the outward projections through and even with a thickness of the end plates readies the end plates for welding to the end faces and the projections by friction stir welding or hybrid friction diffusion welding.
Extending the outward projections through and beyond the end plates readies the end plates and the projections for brazing. The end plates may be brazed or welded to the end faces.
Joining thin heat exchanger tubes to manifold header tanks is improved first by extruding thick tubes having channels or microchannels. Tube material is removed from spaced areas of central portions, leaving fins on thinner heat exchanger central portions and thicker terminal header portions. The finned central portions of the tubes are curved, sloped, tilted and twisted. The tubes are aligned horizontally, vertically or angularly, and the terminal header portions are welded together. Tube material may be removed from the ends of the header portions to form end faces with projections extending from the end faces and surrounding the channels or microchannels.
End plates with openings to receive the projections are attached and sealed to the projections and end faces by welding or brazing. The face plates are attached and sealed to openings in the header tanks by welding or brazing, solving the problem of thin heat exchanger tubes joined to manifold header tanks.
Tubes have one or more channels extending through the tubes and have an elongated central portions and shorter header portions. The header portions are thicker than the central portions. In one embodiment, end faces of the header portions have outward projections extending around the one or more channels. The outward projections extend outward from the end faces of the thick terminal header portions.
The one or more channels are one or more microchannels.
End plates have inner openings receiving the outward projections. The end plates have inner faces lying against and attached to the end faces of the header portions. The inner openings of the end plates are attached to the outward projections of the end surfaces of the terminal portions.
In some embodiments the end plates and the outward projections are co-extensive from the end faces of the tube, and the end plates are welded to the end faces and to the outward projections on the end faces of the header portions.
In other embodiments the outward projections extend beyond the face plates and the face plates are brazed to the outward projections.
The tubes are extruded, and opposite parallel surfaces of the middle portions of the tubes are machined, removing material of the tube and leaving spaced fins integrally formed on the middle portions. In some embodiments the fins are slanted with respect to a longitudinal direction of the middle portion of the tube.
Plural similar extruded tubes have channels in the tubes and have material removed from the middle portions and spaced fins left on middle portions of plural similar tubes and thick header portions left on ends of the plural tubes. The header portions of the tubes are aligned and welded or otherwise joined together before machining ends of the plural joined header portions of the tubes and creating plural end faces and plural outward projections on the plural end faces. The plural outward projections extend around the channels in the tubes. End plates have plural inner openings receiving the plural outward projections on the end faces of the joined header portions. The plural inner openings of the end plates are welded, brazed or otherwise joined to the plural outward projections, and the end plates are welded or otherwise joined to the plural end faces.
The plural tubes are aligned horizontally, vertically, or are aligned in curved, sloped, twisted and tilted parallel helical relationship.
The tubes are curved, at least partially twisted and sloped and at least partially wound and twisted and are adapted for form at least part of a helical structure in an overall cylindrical structure having a cylindrical shape. The fins are curved, at least partially twisted and sloped and at least partially wound and twisted and are adapted for form at least part of a helical structure in an overall cylindrical structure having a cylindrical shape.
In some embodiments the tubes have curved middle sections and end sections, and the fins on at least one side of the tube are formed slightly concave with center parts of the fins offset inward from outer parts of the fins. The fins on one other side of the tube are slightly convex with center sections of the fins offset outward from outer sections of the fins.
The invention provides a method of extruding wide, thick tubes with channels in the tubes, machining central portions of the tubes, thereby removing material in spaced sections from the central portions, forming fins across the tubes. The method provides thinner central portions of fins, leaving header portions at ends of the tubes and having angular or circular patterns at tube material thickness transition points.
The tubes are arranged parallel to each other and are joined at the thicker portions of adjacent tubes. Machining ends of the joined terminal header portions forms end faces. Some end faces have outward projections extending around the channels.
Some end plates have openings for receiving the outward projections. The end plates are then joined to the end faces and some embodiments to the projections.
Extending the outward projections through and even with a thickness of the end plates, the joining of the end plates with the end faces and the projections, if any, is by friction stir welding or hybrid friction diffusion welding.
The outward projections extend through and beyond the end plates when the joining of the end plates to the projections is by brazing. The joining of the end plates to the end faces is by welding.
In one form, the method of the invention curves the tubes longitudinally after forming the thinner central portions with fins, curves the thinner curved tubular central portions and curves the thicker curved tubular end portions. The projections from the end faces are curved, and curved openings in the end plates receive the curved projections. The end plates are welded to the end faces and welding or brazing joins the end plates to the projections.

In another form, the method of the invention curves the tubes longitudinally after forming the thinner central portions with fins, curved the thinner curved tubular central portions and curves the thicker curved tubular end portions. The projections from the end faces are curved and curved inner and outer perimeter rings receive the curved projections. The inner and outer perimeter rings are welded to the end faces and welding or brazing joins the inner and outer perimeter rings to the projections.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a tube extrusion with micro channels.
Figure 2 shows making perpendicular fins by cutting away material.
Figure 3A shows making angular fins by cutting away material and thereby forming a monoblock finned tube with square pattern at tube material thickness transition points.
Figure 3B shows making angular fins by cutting away material and thereby forming a monoblock finned tube with angled pattern at tube material thickness transition points.
Figure 3C shows making angular fins by cutting away material and thereby forming a monoblock finned tube with circular pattern at tube material thickness transition points.
Figure 4 shows an extrusion of a tube having a single port.
Figure 5 shows the extrusion of the tube shown in Figure 4 after cutting away material and forming perpendicular fins.
Figure 6 shows the extrusion after cutting away material to form angular fins.

Figure 7 shows arranging tubes horizontally between flat vertical header connector plates.

Figure 8 shows arranging tubes vertically between flat horizontal header connector plates.
Figure 9 shows arranging the tubes cylindrically between flat annular header plates.
Figure 10 shows cylindrically arranged curved tubes.
Figure 11 shows curved, twisted, tilted, helically arranged tubes with sloped ends.
Figure 12 shows joining adjacent tube ends by friction stir welding or hybrid friction diffusion bonding.
Figure 13 shows machining projections on tube ends.
Figure 14 shows a flat header connector plate with openings for receiving tube end projections.
Figure 15 shows joined tube ends with projections.
Figure 16 shows tube ends with projections frictionally welded in the plate.
Figures 17A and 17B show flattening ends at angles with respect to the tube slope.
Figure 18 shows joining flattened angular ends.
Figures 19A and 19B show machining joined flat ends to form projections.
Figures 20A and 20B show a flat ring plate with openings for receiving the projections.
Figures 21A and 21B show joining end projections within the flat ring plates by friction welding.
Figure 22 shows a tube with a large end.
Figure 23 shows machining a projection on a tube end for brazing.
Figure 24 shows a flat header connector plate for receiving machined ends with the projections.
Figure 25 shows tubes with the large ends and projections ready to be mounted on and in the header connector plate.

Figure 26 shows brazing and end in the plate after insertion of the projections through the plate.
Figure 27 shows a brazed assembly to compare to fusion welding.
Figure 28 shows fusion welding compared with brazing in Figures 26 and 27.
Figure 29A shows the flattening steps of Figures 17A, 17B and 18.
Figure 29B shows an embodiment in the form of the curved, sloped, tilted and twisted tubes in a helical cylindrical arrangement of many tubes without machining.
Figure 30 shows inner and outer machined perimeter sections of the tube ends already welded.
Figure 31 shows a helical coil assembly with machined upper and bottom perimeters for connecting corresponding rings.
Figure 32 shows the end rings attached to the header portions and welded areas.
Figures 33A and 33B show upper and bottom solid state coil polished flat surfaces for further attachment to header tanks or vessels.
DETAILED DESCRIPTION
Figure 1 shows a tube extrusion with micro channels. The solid extrusion 10 has multiple through ports or channels 11 and sloping sides 13 with side connecting walls 15.
Figure 2 shows making perpendicular films by cutting away material.
Material portions 17 have been removed to leave perpendicular fins 19. The resulting tubes 20 have relatively smaller middle portions 21 and larger ends 23.
Figure 3A shows making angular fins by cutting away material and thereby forming a monoblock finned tube with square pattern at tube material thickness transition points.
Figure 3B shows making angular fins by cutting away material and thereby forming a monoblock finned tube with angled pattern at tube material thickness transition points.

Figure 3C shows making angular fins by cutting away material and thereby forming a monoblock finned tube with circular pattern at tube material thickness transition points.
Figure 4 shows an extrusion of a tube having a single port. A solid extrusion block 40 cut from a continuous extrusion has a single central port or channel 41, sloping side walls 43 and laterally extending side connection walls 45.
Figure 5 shows the extrusion of the tube shown in Figure 4 after cutting away material and forming perpendicular fins. Portions 47 extending across the extrusion have been removed to leave perpendicular fins 49 extending across the resulting tube 50. Tube 50 has large ends 33 around a smaller middle 51.
Figure 6 shows the extension after cutting away material to form angular fins.

Material has been removed from portions 67 of a solid extrusion 40 to leave fins 69 at an angle to dimensions of the resulting tube 70.
Figure 7 shows arranging tubes horizontally with the flat vertical header plates.
Tubes 50 have large ends 53 connected to vertical mounting plates 80 which in turn are connected to header tanks, vessels, shells and lids to create header chambers (not shown).
Figure 8 shows arranging tubes vertically with flat horizontal header plates.
Vertically arranged tubes 50 have large ends 53 connected to horizontal mounting plates 82 which are connected to header chambers.
Figure 9 shows arranging tubes in a circle with flat circular header plates.
Vertically arranged tubes 50 have large ends 53 connected to upper and lower annular mounting plates 84 which are connected to header chambers.
Figure 10 shows curved tubes circularly arranged tubes. Vertically arranged curved tubes 90 have thinner curved tubular central sections 91 and thicker curved end sections 93. The curved openings 97 appear in the upper and lower annular mounting plates 95.
Figure 11 shows curved, twisted, tilted, helically arranged tubes with sloped ends. Twisted, tilted helical tubes 100 have larger ends 103 of similar shapes.

Smaller middle portions 101 of tubes 100 have the continued curved, twisted, tilted and helical shape of the remainder of the tubes 100. The outer header rings 105 are flat for connection to header vessels.
Figure 12 shows joining adjacent tube ends by friction stir welding or hybrid friction diffusion bonding. Large ends 53 of tubes 50 are joined 110 together by friction stir welding (FWS) or hybrid friction diffusing bonding (HFDB) 112.
Figure 13 shows machining projections on tube ends. Material 121 is removed from ends 125 of large ends 53 of tubes 50 which have been joined 110 to leave projections 127.
Figure 14 shows a flat plate with openings for tube end projections.
Figure 15 shows joined tube ends with projections.
Flat plates 130 have openings 137 to receive and hold the projections 127 from the ends 125.
Figure 16 shows tube ends with projections frictionally welded in the plate.
Ends 129 of the projections are friction welded 139 to edges of the openings 137 in the flat plates. The welding is done on the chamber-facing side 131 of the flat plates.
Figure 17A shows a single curved, twisted, tilted and helically shaped tube.
Tubes 100 are aligned and parts of ends 103 are removed so that the ends 107 are angular to the tubes and are aligned parallel in the opposite end plates.
In Figure 17B each curved, twisted, tilted and helical formed tube 100 has enlarged ends 103 and fins 101. Enlarged ends are to be cut or machined in order to flatten them horizontally.
Figure 18 shows joining flattened angular ends. Aligned angular end faces 109 of wide end parts 103 of tubes 101 are friction welded 140 so that complete cylinders of welded tube ends 103 are joined together.
Figures 19A and 19B show machining joined flat ends. Parts of material 141 from welded together ends are removed to provide projections 145 as shown in the two figures. The angular removal of parts of the ends and the projections surrounding the openings of the channels in the tubes provide large communication openings 146 that reduce refrigerant pressure drop.
Figures 20A and 20B show a ring with openings for receiving the projections.
Flat ring 150 has openings 155 to receive the projections 145.
Figures 21A and 21B show joining end projections within the rings by friction welding. The inside edges 157 of the openings 155 and the end edges 147 of the projections 145 are friction welded 159 on the vessel side 151 of connecting flat ring 150.
Figures 22-27 show joining of tubes and header connector plates by brazing compared with fusion welding in Figure 28.
Figure 22 shows a tube with an enlarged end. Tube 200 is formed with large ends 203 and a relatively thin 201 central section. Fins may be formed in the central section by removing material from the extruded tube.
Figure 23 shows machining a projection on a tube end for brazing. Sections 204 are removed from the ends to leave a projection 205 extending from surfaces 207.
Figure 24 shows a flat plate for receiving machined ends.
Figure 25 shows brazing tubes on and in the plate.
The header connector plate 210 has openings 215 to receive the projections 205 at the large ends 203 of tubes 200.
Figure 26 shows brazing and end in the ring after insertion of the projections in the ring, the projections 205 extend beyond the header vessel side 211 of the header connector plate. Brazing is continuous between surfaces 207, projections 205, complementary outer surfaces 217 and surfaces of openings 215.
Figure 27 shows a brazed assembly to compare to fusion welding.
Figure 28 shows fusion welding to compare with brazing.
The protrusion 209 of the projection 205 as shown in Figures 26 and 27 is necessary to prevent the filler material from travelling and entering the micro ports of the tubes.

Friction stir welding (FSW) or hybrid friction diffusion bonding (HFDB) does not require the extension of the projections 225 beyond the vessel side 231 of the header connecting plate 230.
In one embodiment of the invention the tubes are made without protrusions in end faces. The step of machining of end faces to produce protrusions by removing material from the end faces is avoided.
Figure 29A shows the flattening steps of Figures 17A, 17B and 18. Tubes 100 are aligned and parts of ends 103 are removed so that the ends 107 are angular to the tubes and are aligned parallel in the opposite end plates. Each twisted, tilted and helical curved tube 100 has enlarged ends 103 and fins 101. Aligned angular end faces 109 of wide end parts 103 of tubes 101 are friction welded 140 so that complete cylinders of welded tube ends 103 are joined together.
Figure 29B shows such an embodiment in the form of curved, sloped, tilted and twisted tubes 300 in a helical tube cylindrical arrangement 350, as shown in Figures 11, 17A, 17B and 21B. Header portions 333 are shown without the machining and flattening.
Figure 30 shows machined inner perimeter 442 and machined outer perimeter 444. Welding areas 440 are shown between the machined perimeter areas 442 and 444 of the flattened headers 446.
Figure 31 shows a helical coil arrangement 600 of curved, twisted, tilted and sloped tubes 400 with machined inner perimeters 450 and 451, machined outer perimeters 452 and 454 together with their corresponding upper connecting end rings 462 and 462 and the bottom connecting end rings 474 and 476.
Figure 32 shows the end rings 462, 464, 474 and 476 attached respectably to the header portions 480 and 481, the upper welding areas 482 between adjacent upper tube headers 480 and the upper perimeter welding areas 484 and 486 between the upper end rings 462 and 464 to the upper tube headers 480. The outer areas 478 are shown extended outward from the lower joined headers 481.

Figure 33A and 33B show a welded solid state helix coil assembly 700 with re-machined and/or polished upper and bottom surfaces 500 and 502 respectably to further be connected to coil header tanks or vessels.
Story Telling Charts pages 1, 2, 3, 4, 5 on the following pages 18-22 describe by illustration and labels the forming and joining of tube ends by steps of clamping and flattening headers, welding adjacent headers in areas spaced from channels, machining header faces, use of header end plates and polishing ends of said solid state robust structures.
While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.

Claims (21)

I claim:

1. Apparatus comprising:
a heat exchanger tube having:
a relatively thin central portion, relatively thick header portions on ends of the relatively thin central portion, fins extending outward from the central portion, end faces on ends of the header portions remote from the central portion, and at least one channel extending through the tube.

2. The apparatus of claim 1, further comprising progressive transition portions between the relatively thin central portions and the relatively thick header portions.

3. The apparatus of claim 1, wherein the heat exchanger tube is extruded with the thickness of the header portions and with the at least one channel extending through the tube, and wherein material is removed from sides of the tube in spaced sections of the central portion, leaving the fins.

4. The apparatus of claim 1, wherein the tube is curved longitudinally.

5. The apparatus of claim 1, wherein the tube is sloped, twisted and tilted.

6. The apparatus of claim 5, wherein the fins are angular with respect to a longitudinal direction of the tube.

7. The apparatus of claim 1, further comprising plural tubes arranged adjacent to each other with a tight pressed fit between the header portions of the tubes, wherein ends of the header portions are flattened by cutting off or machining away ends of the header portion, and wherein flattened ends are joined.

8. The apparatus of claim 7, wherein header rings are tightly placed on inner and outer perimeter surfaces of the flattened ends and are joined with the flattened ends by friction stir welding.

9. The apparatus of claim 8, wherein the ends of the header portions are joined by friction welding away from areas surrounding the openings of the at least one channel in ends of the header portions.

10. The apparatus of claim 1, wherein the flattened joined ends of the header portions of the tubes and the header rings are polished and ready to be used as a heat exchanger.

11. The apparatus of claim 6, further comprising plural tubes arranged adjacent to each other with a tight pressed fit between the header portions of tubes, and wherein the ends of the tubes are flattened by machining or cutting and by further machining of the flattened end projections of the ends and continuous flat end face surfaces are formed around the projections on the ends of the tubes.

12. The apparatus of claim 11, further comprising face plates having openings receiving the projections joined to the flat end faces by welding.

13. The apparatus of claim 12, wherein the projections extended through and beyond the openings in the face plates, the openings in the face plates and the projections are sealed by brazing.

14. A method comprising forming heat exchanger tubes by extruding relatively thick flat tubes having at least one relatively thin channel extending through the tubes, machining or ablating spaced sections in central portions, leaving relatively thin central portions with spaced fins extending across the tubes and relatively thick header portions at ends of the tubes with openings of the channels at the ends of the header portions.

15. The method of claim 14, further comprising arranging plural tubes close together and pressing the header portions together in a tightly pressed fit between the header portions, and holding the header portions together.

16. The method of claim 15, further comprising initially curving the tubes.

17. The method of claim 15, further comprising initially forming the fins on an angle to the tubes, and curving, sloping, tilting and twisting the tubes before pressing the header portions together, flattening ends of the tubes by cutting or machining ends of the tubes, placing rings tightly around the flattened ends of the tubes, joining the rings and the flattened ends by friction welding and joining adjacent ends of tubes by friction welding, leaving areas around the channels free of the welding, and polishing the joined ends of the tubes and the joined end surfaces of the rings.

18. The method of claim 15, further comprising initially forming the fins at an angle to the tubes, and curving, sloping, tilting and twisting the tubes before pressing the header portions together, flattening ends of the tubes by cutting or machining the ends of the tubes to unitary flat surfaces, further machining ends of the tubes to form and leave projections of the tube material around the openings of the channels at the tube ends and leaving end faces of the header portions around the projections, providing end plates having openings for receiving the projections, placing the end plates on the end faces and friction welding the end plates to the end faces.

19. The method of claim 18, further comprising friction welding the openings in the end plates to sides of the projections.

20. The method of claim 18, further comprising extending the projections through the openings in the end plates and brazing the projections to the openings in the end plates.

21. Apparatus comprising a heat exchanger having plural identical monoblock coils formed of extruded flat tubes with channels, opposite spaced side sections removed from central sections leaving angled fins on a relatively thin central section and relatively thick header sections at ends of the tubes, the tubes being sloped, tilted and twisted and having central sections aligned and header sections pressed together in tight fit with machined flat end faces and joined with welds between adjacent end faces and welded areas spaced from areas surrounding tubes and end surfaces polished.