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Factory. Township.; Advances in finned tube heat exchanger technology* Zhu Dongsheng, Li Xiaoxin, Key Laboratory of Heat Transfer Enhancement and Process Energy Saving, South China University of Technology, 510640 Guangzhou. Abstract: Heat exchangers are essential devices in industrial heat transfer processes. With the advancement of science and the development of new energy sources, there is an increasing need to design efficient and energy-saving heat exchangers. Finned tubes are the core components of various heat exchangers, and the quality of these tubes directly affects the performance of the heat exchangers. This article discusses the technological advancements in finned tube heat exchangers over recent years as well as their future development trends, and introduces a new type of finned tube – the integral aluminum finned tube (AIF). Keywords: Finned tube, Heat exchanger, AIF. Development of Technology on Finned Tube Heat Exchangers by Zhong Dongsheng, Liao Naon, Institute of Chemical Engineering, South China University of Technology, Guangzhou (510640). Abstract: Heat transfer is absolutely essential in the industry of heat transfer. With the progress of science and the exploration of new resources, there is an urgent need to design heat exchangers that are highly efficient and have low energy losses. Finned tubes are the most important components in various types of heat exchangers; their functionality determines the performance of these devices. This paper outlines the developments in finned tube heat exchangers over the past few years. Additionally, it provides an overview of a new type of finned tube – the AIF. As is well known, heat exchangers are indispensable equipment in industrial heat transfer processes, and they are widely used in various industries, especially in fields such as chemistry, metallurgy, power generation, transportation, aviation, and aerospace. In recent years, due to the development of new technologies and energy exploration, improving the performance of heat exchangers has received increasing attention from the industrial sector. New technologies and new processes are playing an increasingly important role in the field of modern industrial production. Continuously increasing the technical level of products, striving for high efficiency and energy savings, and maximizing economic benefits have become the basic principles guiding enterprises in their production activities. For many years, shell-and-tube heat exchangers have been widely used in various industrial sectors. With the advancement of science and technology, and particularly the rapid development of the industries mentioned above, there is a need for heat exchange equipment that is compact, lightweight, efficient, and miniaturized. However, conventional shell-and-tube heat exchangers fail to meet these requirements, which has led to efforts to develop more efficient heat exchangers. Among them, the finned tube heat exchanger is one of the most extensively studied types of heat exchangers. Finned tubes are the core components of various heat exchangers, and the quality of these tubes directly affects the performance of the heat exchangers. 2. Development of finned tubes 2.1. Various materials for heat exchangers should be selected based on the application and operating conditions of the heat exchanger. Commonly used materials currently include: pure aluminum, aluminum alloys, copper, brass, nickel, titanium, stainless steel, Inconel alloys, etc. Among them, aluminum and aluminum alloys are used the most. The basic requirements for the base material of fin-tube heat exchangers are: good solderability and formability, high mechanical strength, as well as excellent corrosion resistance and thermal conductivity. Aluminum and aluminum alloys not only meet these requirements but also possess the property that their ductility and tensile strength increase as the temperature decreases. Supported by the National Natural Science Foundation of China (59976009), the Guangdong Provincial Natural Science Foundation (011584), and the Ministry of Education’s Program to Support Outstanding Young Teachers (Document No. Jiao Ren Si (2002) 40), they are therefore widely used in compact heat exchangers around the world, especially in those operating at low temperatures. 1) Aluminum has a low density, and its strength can be enhanced to that of structural steel through alloying and heat treatment. Aluminum heat exchangers are used in various types of vehicles, especially small cars, which will surely reduce their weight and lower fuel consumption. 2) Aluminum has good corrosion resistance. The oxides formed on aluminum products under harsh conditions are non-toxic. Heat exchangers made of aluminum do not require excessive concern regarding failure due to oxidation by air or the liquid inside the tubes over time. 3) Aluminum has good thermal conductivity, making it particularly suitable for heat sinks, as well as evaporators and condensers used in heat exchange. 4) Aluminum has a high yield strength, is resistant to stamping, and is easy to process into shapes. 2.2 Type of fins: Tubes are generally divided into two types – tubes with internal fins and tubes with external fins, among which tubes with external fins are the most commonly used. Tubes with external fins have fins formed on their outer surface through mechanical processing, with a certain height, spacing between fins, and thickness. 2.2.1, Spiral fins: Spiral fins consist of steel strips that are wrapped around the outer surface of the tube in a spiral pattern, with the plane of these strips being perpendicular to the axis of the tube. When forming spiral fins, the concave panel edges inside the fin’s neutral line are compressed, while the convex panel edges outside the neutral line are stretched. When the tensile stress in the m-plane reaches the material’s strength limit, steel strips with a smaller thickness lose stability and develop wrinkles. The steel strips used in spiral fin coolers are not thick (usually 0.6–2.0 mm), and instability often occurs. It can be said that wrinkling at the root of the fins is a common problem in spiral finned tubes. Ruffling at the root of the fin can make the surface of the fins at the root uneven. If tubes of this kind are used as heat exchangers, there are many drawbacks: the protruding parts increase air flow resistance, while the recessed parts accumulate dirt, making cleaning difficult. Figure 1: Hot-dip galvanized coiled elliptical steel finned tube 22.2, fin set. The finning process involves first manufacturing a series of individual fins using a punch press, and then, manually or mechanically, fitting these fins onto the outer surface of the tube at regular intervals, using interference fit. It was one of the earliest methods for processing finned tubes. Due to the simple manufacturing process, low technical requirements, inexpensive equipment, and ease of maintenance, many factories still use this method to this day. This process is a labor-intensive approach, suitable for the financial and technical capabilities of ordinary small factories or township enterprises. We know that there are mainly three requirements regarding the quality of finned tubes: first is the fin spacing; second is the fin height; third is the degree of tightness of the connection between the fins and the outer surface of the bare tube. The fewer gaps resulting from incomplete connections, the better, which in turn means a lower thermal resistance. Generally speaking, spiral fins and assembled fins are easier to meet the first two requirements, but problems often arise with the third one. This is precisely the fatal flaw of these pipes. The poor contact between the tube and the fins results in high thermal resistance, which affects heat transfer. 123. Roll-formed finned tubes: In recent years, roll-formed finned tubes, which have seen rapid development, are a type of efficient and energy-saving heat transfer finned tube (referred to as high-fin tubes). Cold-extruded finned tubes are created by passing thick-walled tubes through multiple sets of extrusion rollers; during this extrusion process, the excess metal material forms fins on the outer surface of the tube. The height of the rolled fins is generally -13 mm; the fin spacing is generally 2–3.5 mm, and the fin thickness is generally... .2-0.5mm。 At the same time, this cold extrusion processing method can also be used to roll composite tube-fin tubes. For the fins of commonly used composite pipes, the base pipe (i.e., the inner tube) is typically made of carbon steel, stainless steel, titanium, copper, and other materials. The outer tube, that is, the outer casing of the extruded fins, is commonly made of aluminum or copper tubes. In rolled finned tubes, since the fins and the tube wall form a single unit, there is no contact thermal resistance nor electrochemical corrosion, which enables improved and stable heat transfer performance as well as an extended service life for the finned tubes. Finned tubes manufactured by high-wing sheet rolling machines feature fins that are tightly bonded to the base tube, eliminating the need for brazing. Parameters such as fin spacing, fin height, and fin thickness can be controlled by adjusting the operating conditions of the rolling machine. The fins produced have a smooth surface, clear patterns, and precise pitch. Heat exchangers assembled from these fins have been used by various power generation companies, demonstrating good overall economic performance. Generally speaking, the rolled finned tube has a compact structure, low flow resistance, low thermal conduction resistance, high heat transfer efficiency, a small size, and excellent overall performance. However, it requires specialized tube rolling machines, and the joints between the finned tubes and the end plates are prone to leakage. The composite tube structure in rolled finned tubes involves wrapping a rolled finned tube around a steel tube; this structure combines the advantages of both steel finned tubes and aluminum finned tubes. It has been successfully developed in China, but the manufacturing process is complex. Therefore, more advanced technologies still need to be studied. Figure 2 Bimetallic rolled tube 2.2.4. There are many structural configurations for plate-fin heat exchangers, but their basic structural units are similar; they all consist of fins, partitions, seals, and guide vanes. A basic unit is formed by placing fins on a metal plate (i.e., the so-called secondary heat transfer surface), and then adding another metal plate on top of it, with the edges sealed using seals. The two metal plates at the top and bottom serve as partitions. The core of a plate-fin heat exchanger is composed of many basic units. Hot and cold fluids flow in the channels of adjacent basic units, exchanging heat through fins and partitions integrated with the fins. By overlapping and arranging the various channels in different ways and brazing them together, the most commonly used plate-fin heat exchanger plate packs with counterflow, cross-flow, and mixed countercross-flow configurations can be obtained. In addition to performing the main heat transfer function, the fins also serve to reinforce the space between the two partitions. Therefore, despite the thinness of both the fin and partition materials, it has high strength, allowing it to withstand high pressures. For ordinary heat exchangers, plate-fin heat exchangers have the following advantages: high heat transfer efficiency; compact structure; light weight yet strong; wide adaptability; and good cost-effectiveness. However, these heat exchanger fins have a small spacing, which makes them prone to clogging; once blocked, they are difficult to clean, resulting in an **increase in resistance. If scaling is not removed, it will affect the safe and stable operation of the system. Figure 3: Diagram of the finned plates in a plate-fin heat exchanger; basic structural diagram of a plate-fin heat exchanger. Plate-fin heat exchangers are primarily used as oil coolers and compressed air coolers in compressors, and they come in both air-cooled and water-cooled versions. Whether it is an air-cooled or water-cooled cooler, high-temperature compressed air will produce condensation water during the cooling process, which accumulates on the fins to form \"water bridges\" that prevent air flow. This leads to an increase in air pressure and a decrease in heat exchange efficiency; a similar situation occurs in the plate-fin heat exchangers used in air conditioners. Although aluminum and its alloys have good corrosion resistance, condensed water that remains on their surface for long periods absorbs oxygen, sulfur, nitrogen, and other elements from the air, forming corrosion cells on the aluminum surface and accelerating corrosion. The accumulation of corrosion products on the surface of aluminum fins will reduce the heat exchange rate. For water coolers, corrosion issues also exist on the water side, and long-term operation will shorten the lifespan of aluminum plate-fin heat exchangers as well. Plate-fin heat exchangers operating under high pressure are prone to fatigue due to cyclic pressure changes, which can cause cracks in the diaphragms and lead to leaks; therefore, fatigue failure must be taken into account in their structural design. 2.3 Development directions As can be seen from the above analysis and comparison, the future development of finned tubes is moving in the direction of **enhancing the heat exchange surface area and integrating the fins with the base tube. Furthermore, production costs have dropped significantly, and production is becoming more scaled up. 3. Special applications and designs of finned tubes 3.1 Design of air coolers: Discussions on air cooling and water cooling methods have been going on for a long time, and economic analyses and debates between the two continue to this day. However, the advantages of air cooling are attracting increasing attention, and the trend of using air cooling instead of water cooling is still on the rise. _The cooling of air in dead air systems is carried out in air coolers, with air serving as the cooling medium; this method can be used for the cooling and condensation of various fluids. Since the specific heat of air is low (about 1.005 kJ/kg·°C), which is only one-fourth of that of water, if the amount of heat to be transferred is the same and the temperature rise of the cooling medium is the same, the amount of air required will be four times that of water. Given that the density of air is much lower than that of water, the volume of an air cooler is significantly larger compared to a water cooler. Furthermore, the heat transfer coefficient on the air side is very low, at around 50–60 W/(m·°C). This results in a low overall heat transfer coefficient for the air-cooled exchangers; it is about 10–30 times lower than that of water-cooled exchangers. To compensate for the low heat transfer coefficient on the air side, air-cooled exchangers typically use finned tubes with expanded surfaces, where the fin ratio is generally between 10 and 24. Finned tubes are the core and key components of air-cooling exchangers, and their performance directly affects the efficiency and effectiveness of these exchangers. In fact, it was the advent of finned tubes that enabled the development of air coolers. Except that longitudinal fins inside the tubes can be used to cool the solid media, all air coolers use transverse fins outside the tubes. 3.2, Automotive air conditioning: To date, more research and exploration have been conducted on air conditioning technologies for buildings. In comparison, less research has been done on the air conditioning technologies for vehicles such as cars, trains, ships, as well as special mobile devices that operate in high-temperature environments. Cars, trains, ships, as well as construction vehicles and machinery that operate in high-temperature environments can, in a sense, be regarded as special mobile \"buildings\". Therefore, the special air conditioning systems installed in such “buildings” share similarities with those in conventional land-based buildings, but they also have their own particularities. Simply put, car air conditioners require heat exchangers that are small in size and light in weight, so as to save space and reduce energy consumption. Furthermore, considering the operating conditions of vehicle air conditioning systems, they must possess a certain level of shock resistance. Thus, only efficient finned tubes can meet its requirements. 3.3 Refrigeration compressors: In small and medium-sized piston refrigeration compressors, finned tubes have traditionally been used in the form of sleeve fins and wound fins; these finned tubes are primarily used in various types of condensers and evaporators. Finned tubes are used primarily to enhance heat transfer by altering the geometric shape or surface condition of the heat exchange surface; this allows for an effective improvement in the condensation heat transfer outside the tubes, thereby increasing the overall heat transfer coefficient of the heat exchanger. 3.4 Condensers: As for condensers, land-based ones generally use copper tube finned tubes or copper-aluminum composite finned tubes. Marine condensers, being cooled by seawater, have a structure similar to that of land-based ones. However, due to the corrosive effects of seawater, the finned tubes in the areas in contact with seawater are generally made of red copper or aluminum brass; evaporators, on the other hand, are mostly constructed using copper tubes wrapped around aluminum sheets. 3.5, Intercoolers for diesel engines: On diesel engines in some large vehicles, due to the need for high driving speeds and constraints imposed by space limitations, high specific power turbocharged intercooled diesel engines are generally used. Selecting and designing an appropriate intercooler can greatly improve the efficiency and reliability of diesel engines. 4. From the analysis above regarding the development of finned tubes, it is evident that it is necessary to develop highly efficient and energy-saving finned tubes, with improvements required in both their economic performance and operational performance. Compared with the various types of finned tubes mentioned above, the overall aluminum finned tube (AIF tube) under study has the following advantages: Figure 4 shows the overall aluminum finned tube (AIF). (1) The heat exchange area of the finned tube increases significantly, both on the outside and inside of the tube; (2) The fins and the pipeline for fluid flow are integrated into a single structure, with a 100% integration rate and zero thermal resistance due to gaps, which thereby improves heat transfer efficiency and eliminates the risk of thermal corrosion; (3) The cross-sectional shape of the pipeline for fluid flow has been optimized, resulting in changes in the shape and number of the tube cavities. With the mass flow rate of the inlet medium (compressor outlet flow) remaining constant, the heat transfer coefficient increases due to a reduction in the cross-sectional area or equivalent diameter of the pipe. (4) Finned tubes have high strength, are resistant to thermal shock and mechanical vibrations, exhibit good thermal expansion properties, have a reliable structure, and a long service life. (5) The fins are smooth without spikes or folds; they do not accumulate dust or scale easily, are simple to clean and allow water on their surface to drain away. This results in low fluid resistance, enabling them to maintain good heat transfer performance over a long period of time. (6) The base material is aluminum, making it very light in weight. Aluminum is a material with a low density, good heat transfer properties, and high corrosion resistance, making it easy to process into heat exchange tubes with complex shapes.