Because the string tube is to string the fins to the tube piece by piece, because the tube is often not round (especially welded steel pipe), there will inevitably be a gap between the fin and the tube wall. If there is a gap, there is a gap thermal resistance, so the following close connection measures are often used.

First, resistance welding method on the cleaned and polished tube wall first coated with a layer of flux, and then the fin string (or wind) to the tube, and then connect the power 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.

Second, the dipping method The coated metals are mainly zinc and tin, followed by cadmium. Both the tube and the fins must be chemically treated before dipping to remove oxides and oil scale from their surfaces. 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 only with dilute sulfuric acid. After the fin tubes have been cleaned as necessary to remove the grease, they are treated once with ammonium chloride solution containing stannous chloride (except copper fins). It can be placed in a molten zinc (or tin) tank (zinc melting point is 400 °C, tin is 230 °C). Due to capillary action, zinc (or tin) evenly fills all gaps between the fin and the tube. After solidification, the two can form a firm connection, and form a protective film 20~50 microns thick on the surface of the fin and tube. However, this method consumes higher metal and costs more, and the heat exchange area per square meter consumes about 1.25~1.30kg of zinc.

Third, contact welding using a special electric welding machine and seam welding machine while stringing (or winding) while welding the fin to the tube. In order to avoid the deformation of the tube during welding, a mandrel is generally inserted. In this process, the perforation of the fin is usually hemmed for welding. This method is recognized as a relatively ideal method, but its disadvantage is that it requires more complex special equipment.

Fourth, the expansion method The string fin tube is connected to the special pressurized equipment, and the pipe channel is expanded by hydraulic method (generally pressurized to more than 30 MPa) to make the fin tube tightly clamped. The expansion method is relatively simple, and can generally only be used for non-ferrous finned tubes such as copper and aluminum with good ductility performance. It is rarely used for steel pipes.