Because the finned tube is to string the fins on the tube one by one, because the tube is often not round (especially welded steel pipe), there is a gap between the fin and the tube wall. If there is a gap, there will be a gap thermal resistance. For this reason, the following close connection measures are often used.
First, apply a layer of flux to the cleaned and polished tube wall of the resistance welding method, and then string (or wind) the fins on the tube, and then turn on the power supply at both ends of the fin tube, and use a larger current to make the flux The metal in the metal melts, and the tube and the fin are welded together.
Second, immersion plating method The plating metal is mainly zinc and tin, followed by cadmium. Before immersion plating, the tubes and fins must be chemically treated to remove oxides and grease on the surface. Chromium-nickel-molybdenum alloy steel is washed with hydrochloric acid, sulfuric acid and oxidant (such as nitric acid); carbon steel is washed with cold hydrochloric acid and hot dilute sulfuric acid solution; copper fins are only washed with dilute sulfuric acid. After the finned tube has been cleaned and greased as necessary, it is treated once with an ammonium chloride solution containing stannous chloride (except for copper fins) before it can be placed in the molten zinc (or tin) tank (the melting point of zinc) The temperature is 400°C, and that of tin is 230°C.) Due to capillary action, zinc (or tin) can evenly fill all the gaps between the fin and the tube. After condensing, the two can form a stable connection, and a layer of 2050 micron thick egg wing membrane is formed in the details of the fin and the tube. However, this type of configuration consumes more metals and costs more, and consumes about 1.251.30 kg of zinc per square meter of heat exchange area.
Contact welding Use a common electric welder and seam welder to weld the fins to the pipe while stringing the pieces (or winding the pieces). To avoid deformation of the pipe during welding, the individual should be inserted into the mandrel. When using this process, the perforation of the fin must be folded to facilitate welding. This type of style is recognized as an ideal way, but its disadvantage is that it requires more complex special equipment.
Tube expansion method connect the stringed finned tube to the public pressurizing equipment, and use the hydraulic method (individual pressurization to over 30MPa) to expand the tube channel, so that the finned tube is tightly clamped on the tube. The tube expansion method is relatively simple, and the individual can only be used for non-ferrous metal fin tubes such as copper and aluminum with good ductility. It is rarely used for steel pipes.
