Method and product of the method of making tubes having internal fins

693,009. Making heat exchangers. BROWN FINTUBE CO. July 6, 1950, No. 16905/50. Classes 83 (ii) and 83 (iv). [Also in Groups XI and XIII] Brazing.-A method of making heat exchange tubes comprises applying surface-extension elements 2 to the exterior surface of a supporting tube 1, introducing this structure into a closelyfitting outer tube 10, supplying bonding metal to the contiguous surfaces of the elements 2 and the outer tube 10, expanding the supporting tube 1 to provide firm contact between the outer tube and the elements 2 and then fusing the bonding metal. The elements 2 are of channelsection having longitudinally-extending fins 3 and 4 joined by a web 5 which is bonded to the supporting tube 1. The fins have flanges 7 and 8 which may be of arcuate cross-section. The flanges and/or the interior surface of the tube 10 are coated with the bonding metal, e.g. copper and its alloys, silver, solder, tin, &c. The tube 1 with the elements 2 attached, is inserted within the outer tube 10 and expanded by a mandrel. The bonding of the outer tube to the elements 2 is effected by passing the assembly through a high-frequency electrical induction block 14 and a stationary torch 16 located within the tube 1 in juxtaposition to the block 14 ensures an even expansion throughout that portion of the assembly within the block. Fuel gas is fed to the torch by a feed-pipe 17 which leads to peripheral ports 19a and 19b in a chamber 18. Close-fitting caps 21 are mounted on each end of the outer tube 10 and are held together by the rods 26. A reducing gas, e.g. hydrogen or partially burned natural gas, is admitted to the tube through an opening 22 in the cap 21 and a flexible conduit 24. Longitudinally extending channel members may be applied to the exterior of the outer tube 10 and these members may have flanges similar to the elements 2 so that the whole assembly may be inserted in another outer tube 31, as shown in Fig. 7, so forming a series of concentric tubes separated from each other by annular passages. In another embodiment external annular fins 49a, 49b, 49c, Fig. 9, are applied to the tube 10 either singly or in groups. As hitherto the bonding metal is applied to the elements 2 and to the inner portions of the fins and after assembling and expanding the inner tube 1 the bonding is effected by heating in an electric furnace. The fins may have a pentagonal, hexagonal, octagonal, &c. periphery and may be paired together by an outside web between adjacent fins. Spec