In terms of fin shape and structure, finned tubes can be categorized into square finned tube, spiral finned tube, longitudinal finned tube, helical serrated finned tube and inner finned tube etc.

        Based on whether the material of fin and the tube is the same, finned tubes can be divided into two types: monometallic finned tube and bimetallic composite finned tube. 

        With various processing technics, finned tubes can be categorized into extruded fin tube, welded finned tube, rolled finned tube and fitted finned tube.

 

        Served as a heat-exchanging element, the finned tube is usually long-termly employed in the terrible high-temperature working environment with smoke and gas. For example, the finned tube for boiler is usually used where the temperature and pressure are high and there is full of corrosive atmosphere. It is, therefore, required to possess the following performance.

        1>     Anti-corrosion

        2>     Anti-wear

        3>     Lower contact resistance

        4>     Higher stability

        5>     Dust accumulation-proof ability
 

        In the process of making fitted fin tubes, firstly a batch of individual fins are punched out by punching machine in advance, secondly fitting them on the external surface of tubes from a certain height (fin pitch) by interference manually or mechanically. It has been proved to be the earliest method applied to manufacture finned tubes. And still now, many factories prefer to use this method just because of its simple processing and technology, low cost of producing equipments and easy maintenance. As a labor-intensive processing, it suits ordinary small factories or township enterprises on the bases of their capital and technology.

        Manual-fitting, operated by labor power, is a technology of pressing in the fins one after another by labor power with the help of certain tools. However, the result is not so satisfactory because the manually-fitted fins are prone to loosen for the small magnitude of interference produced by limited pressure of labor power. Mechanical-fitting, on the contrast, is accomplished by fitting machines which can positively produce powerful impact force or fluid pressure. So large magnitude of interference can be adopted and the combination between the fins and tube is relatively solid and the fins will not loosen easily. All in all, the mechanical-transmission device enjoys a high productivity but has a bad reputation for its noise, poor security and bad working condition for workers. By comparison, hydraulic-transmission device does not have those above-mentioned problems. But its disadvantages lie in the higher price and requirements for skilled maintaining personnel and lower productivity. 

B. Mosaic Type Spiral Finned Tube 

        As for mosaic type spiral finned tube, a spiral groove with specified width and depth is made on the steel tube in advance, and then embed the steel belt in the tube on the lathe. Because of the pretightening force produced in the process of convolving, the steel belt can embed in the spiral groove closely; and the contact area between the belt and the tube can be assured. Additionally, in order to prevent the belt from rebounding and falling off the tube, the two ends of the steel belt must be welded on the tube. And a proper gap is also needed for a smooth embedding, because an over-narrow gap will cause interference which will hinder the embedding. Although rebounding which makes the steel belt and the bottom of the spiral groove fail to match perfectly is inevitable. Mosaic type spiral finned tube can be made in general devices with a relatively low cost. However, its complicated processing and lower productivity should not be ignored.

C. Brazed Spiral Finned Tube 

        It usually takes two steps to produce a brazed spiral finned tube. First, we wind the steel belt perpendicularly to the tube axis around the tube in a spiral-line way, then weld the two ends of the belt on the tube, and finally braze the steel belt and steel tube together so that the gap between them can be eliminated. But due to its high cost, another method is usually preferred, that is putting the tube enwound with steel belt into a spelter pot for hot galvanization. With this method, plating solutions can not totally permeate into the gap between fins and steel tubes, but it does form a complete galvanized layer on the external surface of the tube. This technology fails to provide the perfect combination between the fins and the tube because it is limited by the thickness of the layer (the thicker the layer, the more easily it will drop off), and as I mentioned above, plating solutions can not totally permeate into the gap. In addition, the heat-transfer capability of zinc is not as good as steel due to its smaller heat transfer coefficient which is about 78% of steel's. In conclusion, galvanized spiral finned tube is not suitable for making air preheater which is designed for boiler's flue gas and heat recovery, because zinc is vulnerable to be corroded in acid, alkali and sulfides. 

        High frequency welded spiral finned tube is one of the most widely used spiral finned tubes. Nowadays it is widely used for the pre-heating and recycling of cement industry as well as in power industry, metal industry and petrochemical industry etc. For high frequency welded spiral finned tube, when metal band twines the steel tube, heat the surface of metal band and steel tube until plastic state or melted state by skin effect and proximity effect of high frequency current, then complete welding under the pressure of metal band. This high frequency welding is actually a solid phase welding. Compared with mosaic, braze welding (pot galvanizing), high frequency welding is more advanced in product quality (the bonding rate of fins can reach 95%), productivity and degree of automation. 

        The production principle of three-roll diagonal-rolled integral welded spiral finned tube, as shown in figure 1.3, is firstly lining a core rod inside the plain tube, then driven by the rotation of formed bits of mill roller, fins can be made through the hole cavity composed by groove and core print on the external surface of seamless steel tube. The parent tube and outer fin made by this method is an organic whole so there is no contact resistance loss and heat transfer efficiency is high. Compared with other welding processes, this production line has high production efficiency, low raw materials consumption and the fin tubes have high rate of heat exchange.

        At present three-roll diagonal-rolled integral welded spiral finned tube skill has been applied successfully in making single finned tube or composite finned tube of copper or/and aluminum, steel low finned tube. Low finned tubes of integral steel finned tubes are most commonly seen in the market, while integral high finned tubes are often made by aluminum, copper and so on.  Besides usually they are formed by cold-rolling practice.