What types of finned tube heat exchangers are there?

What types of finned tube heat exchangers are there?

Finned tube

   A finned tube is a type of heat exchanger tube that is designed to increase the heat transfer efficiency, usually by adding fins to the surface of the heat exchanger tube to increase the external surface area (or internal surface area) of the heat exchanger tube, thereby increasing the heat transfer efficiency.

   The main performance requirements of finned tubes

   As a heat exchange element, finned tubes are required to have high performance indicators.

   1) Anti-corrosion performance

   2) Abrasion resistance

   3) Low contact thermal resistance

   4) High stability

   5) Anti-dusting capability

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  The current state of finned tube production technology

Cased fins

    The set fin process is to pre-process a batch of individual fins with a press, and then manually or mechanically set the fins on the outer surface of the tube at a certain pitch (fin pitch) by overfilling. It is one of the earliest methods of processing finned tubes. Due to the simplicity of the set process, the low technical requirements, the reasonable price of the equipment used and the ease of maintenance, there are still many factories using it. This process is a labour-intensive process solution and is suitable for the financial and technical conditions of small factories or township enterprises in general.

The manual method of kitting is called hand kitting. It is done with the help of tools and relies on human strength to press the fins in one by one. In this method, the fins are pressed in with limited force, so the set has a small interference and the fins are prone to loosen.

    The mechanical set of fins is carried out on a fin setter. As the fins are pressed in by mechanical impact or liquid pressure, the press-in force is high and, therefore, a larger amount of interference can be used. The bonding strength between fin and tube is high and not easy to loosen. The productivity of the mechanically driven setter is high, but the noise level is high, the safety is poor and the working conditions of the workers are not good. Although the hydraulic drive does not have the above problems, the equipment is more expensive, the technical requirements for the use of maintenance personnel are higher, and its productivity is lower.

Inlaid spiral fins

     The inlaid spiral finned tube is pre-processed on the steel pipe with a certain width and depth of spiral groove, and then the steel strip is inlaid on the steel pipe on the lathe. During the winding process, the steel strip is tightly strung in the spiral groove due to a certain pre-tightening force, thus ensuring a certain contact area between the steel strip and the steel pipe. In order to prevent the strip from springing back off, the ends of the strip are welded to the steel pipe. In order to facilitate inlay, there should be a certain side gap between the steel strip and the spiral groove. If the side clearance is too small, forming an overfill, the inlay process will be difficult to carry out smoothly. In addition, there is always a certain amount of springback in the wound steel strip, which results in a poor bond between the strip and the bottom of the spiral groove. Fin inlay can be carried out on common equipment at low cost, but the process is complex and inefficient.

Brazed Spiral Finned Tubes

     The brazed spiral finned tube is processed in two steps. Firstly, the steel strip is wound on the outer surface of the tube in a spiral manner perpendicular to the tube axis and the ends of the strip are welded to the steel tube to secure it.

     This method is expensive, so another method is often used, which is to wrap the steel tube into the zinc bath for the whole hot-dip galvanising instead. Although the overall hot-dip galvanising solution does not necessarily penetrate well into the tiny gap between the fins and the steel tube, a complete galvanised layer is formed on the outer surface of the fins and the outer surface of the steel tube. Using the overall hot-dip galvanized spiral finned tube, because of the limitations of the thickness of the galvanized layer (when the galvanized layer is thick, the zinc layer is poorly firm and easy to fall off), coupled with the fact that the zinc solution is unlikely to penetrate all the way into the gap, so the bonding rate between the fins and the steel pipe is still not high. In addition, the heat transfer coefficient of zinc is smaller than that of steel (about 78% of steel), so the heat transfer capacity is low. Zinc is highly susceptible to corrosion in acids and alkalis and sulphides, therefore, galvanised spiral finned tubes are not suitable for making air preheaters (recovery of boiler flue gas waste heat).

High frequency welded spiral fins

       High-frequency high-frequency welded spiral finned tubes are one of the most widely used spiral finned tubes, and are now widely used for waste heat recovery in the electric power, metallurgical and cement industries, as well as in the petrochemical industry.