Fluted tubes can enhance the condensing heat transfer, which have obtained the commercial application abroad. In 1954, Gragalig proposed a method to improve heat transfer on the tubes surface by utilizing surface tension. The outer surface of tube has vertical flutes. Because of the curvature of surface, the liquid pressure of the crest is higher than vapor pressure. The surface of crest and wave are combined to build a surface tension field, which allows the liquid to flow down from the crest surface to the wave zone. Therefore, there is high condensing rate on the surface of the crest, and the wave part will work as a liquid exclude channel, which quickly discharges condensing liquid. Webb, Panikdl and Bell, have put forward a general comprehensive theory on such condensing surface, to determine the most appropriate flute geometry structure; they have also put forward the structure equation of flute. The overall heat transfer coefficient on this structure is much higher than that of a bare tube.
At present, our company can batch produce the enhanced tubes, the common specifications are carbon steel, copper nickel alloy and aluminum tubes with outside diameter of 25 and 32 mm. There are about 40 longitudinal flutes on the outer surface of the tube; length of plane end is 200 mm, which can facilitate tube sheet welding. The length of heat transfer tube is 6000-9000 mm.
