What kinds of close connection methods for fin tube string technology?
The manufacturer of laser welded stainless steel finned tube Murphy answers: because the finned tube is connected to the tube one by one, the tube is often not round (especially welded steel tube), so there is inevitably a gap between the fin and the tube wall. If there is a gap, there is a gap thermal resistance. For this reason, the following measures are often adopted.
First, apply a layer of flux on the cleaned and polished tube wall by resistance welding method, then put the fins on the tube, then connect the power supply at both ends of the fin tube, melt the metal in the flux with a large current, and weld the tube and the fins together.
Secondly, zinc and tin are the main coating metals, followed by cadmium. Pipes and fins must be chemically treated to remove oxide and oil scale from their surfaces before immersion. 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; copper fins are cleaned with dilute sulfuric acid only.
After the fin tube is cleaned and degreased, it is treated with ammonium chloride solution containing stannous chloride (except for copper fin). It can be put into the molten zinc (or tin) bath (the melting point of zinc is 400 ℃, and that of tin is 230 ℃). Due to capillary action, zinc (or tin) can fill all gaps between fins and tubes evenly. After solidification, the two can form a firm connection, and a protective film of 20-50 μ m thick is formed on the surface of fins and pipes. However, this method consumes a lot of metals and costs a lot, and consumes about 1.25-1.30kg zinc per square meter of heat exchange area.
Third, contact welding adopts special electric welding machine and seam welding machine to weld fins to pipes while stringing (or winding) fins. In order to avoid pipe deformation during welding, the mandrel is usually inserted. In this process, the hole of the fin is usually folded for welding. This method is generally accepted as an ideal method, but its disadvantage is that it requires more complex special equipment.
Fourth, connect the finned tube in series to the special pressure equipment by expanding the tube, and expand the tube channel by hydraulic method (generally pressurized to more than 30MPa), so that the finned tube is tightly clamped on the tube. The expansion method is relatively simple, and can only be used for non-ferrous metal finned tubes such as copper and aluminum with good ductility. Rarely used in steel pipes.
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