Tandem tube is to string the fin piece by piece to the pipe, because the pipe is often not round (especially welded steel pipe), so there is inevitably a gap between the fin and the pipe wall. There is a gap, there is a gap thermal resistance, so the following sealing measures are often used.
First, the resistance welding method after cleaning the polished tube wall first coated with a layer of flux, and then the fin string (or around) to the tube, and then the finned tube at both ends of the power supply, with a large current to make the metal flux melt, the tube and fin welding together.
Two, dip plating method coating metal is mainly zinc and tin, followed by cadmium. The tubes and fins must be chemically treated to remove oxide and grease from their surfaces before dipping. Chromium-nickel-molybdenum alloy steel is cleaned with hydrochloric acid, sulfuric acid and oxidant (such as nitric acid). Carbon steel is cleaned with cold hydrochloric acid and hot dilute sulfuric acid solution. The copper fins were cleaned with only dilute sulfuric acid. After the finned tube has been cleaned to remove grease, it shall be treated once with an ammonium chloride solution containing stannous chloride (except for copper fins). Can be put into the melting of zinc (or tin) trough (zinc melting point of 400℃, tin of 230℃). Due to capillarity, zinc (or tin) uniformly fills all the Spaces between the fins and the tube. After solidification, the two can form a firm connection and form a protective film 20-50 micron thick on the surface of the fin and tube. However, this method costs a lot of metal and costs a lot. The heat transfer area of each square meter consumes about 1.25 ~ 1.30kg of zinc.
Three, the contact welding USES the special electric welding machine and the seam welding machine side string piece (or wrap piece) side will weld the fin to the pipe. In order to avoid pipe deformation in the process of welding, generally insert the mandrel. In this process, the finned perforations are usually flanged for welding. This method is generally accepted as an ideal one, but its disadvantage is that it requires more complex specialized equipment.
Four, tube expansion method will be connected to the series finned tube on the special pressure equipment, with hydraulic method (generally pressurized to 30 MPa or more) to expand the tube channel, so that the finned tube on the tube. The expansion tube method is simple and can only be used in copper, aluminum and other nonferrous metal finned tubes with good ductility. Seldom used in steel pipe.