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Specifications and materials of heat exchange tubes: The size and shape of heat exchange tubes have a significant impact on heat transfer. The smaller the pipe diameter, the larger the heat transfer area per unit volume of the equipment, which means the equipment is more compact, its volume decreases, and the convective heat transfer coefficient is higher. But manufacturing is more troublesome, and the small tubes tend to scale up, making them difficult to clean mechanically. Therefore, it is advisable to use small tubes for clean fluids, while for liquids with high viscosity or prone to scaling, a larger tube diameter can be used. Currently, in the series of standards for shell-and-tube heat exchangers in our country, the seamless steel tube specifications used are mainly 19 mm x 2 mm and 25 mm x 2.5 mm, which are suitable for general fluids. In addition, there are also 38 mm x 2.5 mm and ф57 mm x 2.5 mm. The specifications for stainless steel acid-resistant pipes are M25 mm x2 mm, ф38 mm x2.5 mm. The selection of pipe length should take into account the rational use of the piping material as well as ease of cleaning. The recommended lengths for heat exchange tubes in China’s \"Design Specifications for Steel Shell and Tube Heat Exchangers\" are 1500 mm, 2000 mm, 2500 mm, 3000 mm, 4500 mm, 6000 mm, 7500 mm, 9000 mm, and 12000 mm. Additionally, the tube length (L) should be appropriate in relation to the shell diameter (D); generally, a L/D ratio of 4 to 6 is suitable (it can be higher for heat exchangers with smaller diameters). Heat exchange tubes are generally made of plain tubes, which offer a simple structure and are easy to manufacture, but have a low convective coefficient. To enhance heat transfer, tubes with more complex structures are sometimes used, such as shaped tubes, finned tubes, and threaded tubes. The material of the pipe is determined by the operating pressure, temperature, and corrosivity of the fluid medium; steel is generally used, but alloy steel, aluminum, copper, plastics, graphite, etc. are also used sometimes as needed.