Fined tube is a kind of heat exchange element. In order to improve the heat exchange efficiency, fins are usually added on the surface of the heat exchange tube to increase the external surface area (or internal surface area) of the heat exchange tube, so as to achieve the purpose of improving the heat exchange efficiency.
The machining of brazed spiral Fined tube is divided into two steps. Firstly, the steel strip plane is perpendicular to the pipe axis and wound on the outer surface of the pipe in a spiral way, and both ends of the steel strip are welded on the steel pipe to fix. Then, in order to eliminate the gap between the steel strip and the steel pipe, the steel strip and the steel pipe are welded together by brazing.
Because of its high cost, this method is often used, that is, the pipe wrapped with steel strip is put into the zinc bath for overall hot galvanizing instead. Although the bath can not penetrate into the very small gap between the fin and the steel pipe, a complete zinc coating is formed on the outer surface of the fin and the steel pipe. Due to the limitation of the thickness of the zinc coating (when the thickness of the zinc coating is thick, the zinc layer is poor in firmness and easy to fall off), and the liquid zinc can not completely penetrate into the gap, so the bonding rate between the fin and the steel pipe is still not high. In addition, the heat transfer coefficient of zinc is smaller than that of steel (about 78%), so the heat transfer capacity of zinc is low. Zinc is easy to be corroded in acid, alkali and sulfide. Therefore, it is not suitable to make air preheater with galvanized spiral Fined tube.