The present invention relates to a method and apparatus for producing metallic heat exchanger tubes having a heat exchange surface that is increased by ribs or fins. Helically wound or wrapped on the outer surface of a rotating tube is a strip that forms the fins. The strip is fed to the tube surface under tension, with the bottom edge of the strip being welded to the tube.
A number of methods and apparatus for producing ribbed or finned heat exchanger tubes are known. Common to all of these methods and apparatus is that a strip is unwound from a supply roll and is wound in an upright state upon the outer surface of a rotating tube. German Offenlegunggschrift No. 21 47 079 discloses such a method, where to begin with the tube is wound with the strip, whereupon the wound strip is welded securely to the tube with a welding torch. However, this method has the drawback that a lot of thermal energy is introduced during the welding, as a result of which deformations result and the heat exchanger tubes become discolored. In addition, with this heretofore known method the weld seam is not symmetrical and is relatively large, because the thermal energy must be conveyed from one side of the strip, through the strip and through the wall of the tube, to the welding joint. A weld seam that completely welds the bottom edge of the strip to the surface of the tube therefore extends deep into the strip and into the wall of the tube, and reduces the strength factors of the heat exchanger tube, and with alloys can lead to segregation. These problems occur not only during high frequency welding using a high frequency electrical current, but also during electrical resistance welding. Another drawback with the heretofore known methods is that the strip is subjected to a plastic deformation that increases from the bottom edge of the strip to the outer edge of the strip. In this connection, care must be taken that no wavelike deformations, which are caused by compressive forces, occur on the bottom edge of a strip, because when the heat exchanger tube is used, these deformations would promote the deposition of dirt and moisture. In addition, no cracks caused by stretching stress that is too great can be allowed to occur on the outer edge of the strip, because these cracks on the one hand adversely affect the service life of the tubes, and on the other hand can also lead to dirt and moisture deposits, which reduce the heat exchange effect. In order to increase the stability of the helically wound strip on the tube, it is known to provide the base edge of the wound strip with a short leg that projects at right angles and that increases the support surface of the strip upon the tube surface. Finally, published European application No. 0 259 607 proposes stretching the flat strip prior to winding it on the tube via two rollers in a conical roller gap in the longitudinal direction along the outer edge of the strip, and compressing the bottom edge of the strip, so that during the winding process neither deformations nor cracks are formed. With this method, the bottom edge of the strip is introduced into grooves that are previously formed in the tube surface, with the strip then being wedged in these grooves. With this expensive method, gaps are necessarily formed on the surface of the tube; these gaps promote gap corrosion.
These heretofore known methods and apparatus for avoiding deformations and cracks in the strip all have the drawback that on the one hand a significant structural effort has to be expended for securing the strip in position upon the surface of the tube, and on the other hand as a result of this the speed at which the strip can be wound upon the outer surface of the tube is limited, so that on the whole the cost for producing such heat exchanger tubes is high.
It is therefore an object of the present invention to provide a method and an apparatus for producing heat exchanger tubes of the aforementioned general type whereby it is possible to weld any metallic materials in the tube and strip via small fusion zones and via the introduction of only a small amount of heat, all without adversely affecting the winding speed.