The assembled fin technology involves pre-processing a batch of individual fins using a press brake, then manually or mechanically assembling the fins onto the outer surface of the tube at a specific pitch (fin spacing) by interference fit. It is one of the earliest methods for manufacturing finned tubes. Due to its simplicity, low technical requirements, and inexpensive equipment, this process is still widely used for maintenance purposes. As a labor-intensive manufacturing method, it is suitable for small factories or rural enterprises with limited capital and technical resources.
Hand-assembled kits are those assembled manually. This process involves using tools to individually press the fins into place with human effort. Due to the limited pressing force on the fins, the fit of the assembly is less tight, leading to potential loosening of the fins.
The mechanical fin sets are assembled on a fin set machine. Since the fins are inserted by mechanical impact force or liquid pressure, the insertion force is high, allowing for a larger interference fit. The bond between the fins and the tube is strong and does not loosen. Machines with mechanical transmission have high production rates but are noisy, have poor safety, and offer poor working conditions for workers. Although hydraulic transmission does not have the aforementioned issues, the equipment is expensive, requires highly skilled operators for maintenance, and has a lower production rate.
Inscribed helical fins
The embedded spiral fin tube is pre-machined with a certain width and depth of spiral grooves on the steel pipe, and then the steel strip is embedded in the steel pipe on a lathe. During the winding process, due to the pre-tension, the steel strip is tightly clamped in the spiral grooves, ensuring a certain contact area between the steel strip and the steel pipe. To prevent the steel strip from rebounding and detaching, the ends of the steel strip must be welded to the steel pipe. To facilitate embedding, there should be a certain lateral gap between the steel strip and the spiral groove. If the lateral gap is too small, it will create an interference fit, making the embedding process difficult. Additionally, the wound steel strip will always have some rebound, which results in poor fit between the steel strip and the bottom of the spiral groove. The embedding of fins can be done on general-purpose equipment at a low cost, but the process is complex and production efficiency is low.
Soldered Spiral Fin Tubes
The manufacturing process of brazed spiral fin tubes consists of two steps. Firstly, the steel strip is wrapped around the outer surface of the tube in a spiral manner, perpendicular to the tube's axis, with its ends焊接 onto the steel pipe to secure it in place. Then, to eliminate the gaps at the contact points between the steel strip and the steel pipe, brazing is used to join the two together.
This method is often replaced with an alternative due to its high cost, which involves placing steel-belted pipes into a zinc bath for full hot-dip galvanizing. While the galvanizing solution may not fully penetrate the gaps between the fins and the steel pipes, it forms a complete galvanized layer on both the outer surface of the fins and the steel pipes. However, the combination rate of the fins with the steel pipes remains low for spiral finned tubes that undergo full hot-dip galvanizing, as the thickness of the galvanized layer limits its durability (thicker layers are less secure and prone to flaking). Additionally, the zinc's thermal conductivity is lower than that of steel (about 78%), resulting in reduced heat transfer capacity. Zinc is susceptible to corrosion in acidic, alkaline, and sulfide environments, thus, galvanized spiral finned tubes are not suitable for use in air preheaters (for recovering excess heat from boiler flue gases).
