Knowledge on heat exchanger selection

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The characteristics and application scenarios of several types of partitioned heat exchangers are described to facilitate comparison in the design and selection of heat exchangers. Keywords: Heat exchanger design, selection, energy use. Energy is one of the major challenges facing humanity today; the development, conversion, and utilization of energy have become important issues for various countries. Heat exchangers are essential devices in the process of energy utilization, and they are used in almost all industrial fields, with particularly widespread application in sectors such as chemistry, metallurgy, power generation, transportation, aviation, and aerospace. In recent years, due to the development of new technologies and the utilization of new energy sources, various types of heat exchangers have received increasing attention from the industrial sector. Heat exchangers are crucial devices in energy-saving measures; therefore, whether it is for industrial development or for the efficient use of energy, proper design, manufacturing, selection, and operation of heat exchangers are of great importance. 1 Classification of Heat Exchangers 1.1 Direct heat transfer heat exchangers: These are heat exchangers that do not require a heat transfer surface, as heat exchange takes place through direct contact between the cold fluid and the hot fluid. Such heat exchangers are commonly used in industrial production. 1.2 Heat exchangers with a partition for heat transfer. A heat exchanger is a device in which cold and hot fluids exchange heat through wall surfaces such as tubes or plates; it is the most common and widely used type of heat exchanger. Both the cold and hot fluids are in liquid form, and can be air, flue gas, steam, or water. This is the type of heat exchanger that is primarily discussed in this article. 　　 1. 3 　Regenerative heat exchanger. It is intermittent heat transfer, and it represents a practical and effective method for recovering waste heat in waste heat regenerators. It is often used to recover the waste heat from combustion gases, as well as as a means of regulation when the demand for steam and similar substances varies. 　　 2 　Features of several types of heat exchangers and their application: In actual design and selection processes, the inlet and outlet temperatures of the high-temperature fluid and the low-temperature fluid are usually known. When carrying out process design and selection, it is necessary to achieve the highest possible heat transfer rate with the smallest possible heat transfer area, as well as lower costs for the equipment and installation. Additionally, the design and selection of heat exchangers should take into account the convenience of operation, running, and maintenance cleaning. 　　Basic equation for heat transfer: Q = UAΔt 　　　　　Kcal/h; where: U is the heat transfer coefficient, Kcal/m2·h·°C; A is the heat transfer area, m2; Δt is the average temperature across the boundary layers of the two fluids. The heat transfer coefficient h, in Kcal/m2·h·°C, and the flow velocity u, in m/s, are related as follows: For the heat transfer coefficient in the tube side, ht: In the laminar flow region (Re ≤ 2100), ht ∝ u^0.33t; in the transitional flow region (2100 ≤ Re ≤ 10000), ht ∝ u^0.33–0.8t; and in the turbulent flow region (Re ≥ 10000), ht ∝ u^0.8t. For the heat transfer coefficient on the shell side, hg: Since the fluid on the shell side flows vertically through the tube bundle, its flow pattern is more chaotic, and there is no clear distinction between laminar and turbulent regions; hence, ht ∝ u^0.55g. The corresponding pressure drop ΔP, in kg/cm2, is roughly the same for both the tube side and the shell side: In the laminar and transitional flow regions, ΔP ∝ u^1.0t; in the turbulent flow region, ΔP ∝ u^1.8t. As can be seen from these equations, at a given flow velocity, the higher the Reynolds number, the greater the heat transfer coefficient, and simultaneously, the greater the pressure drop. 2.1 Shell and tube heat exchangers Shell and tube heat exchangers are the most commonly used type of heat exchanger structure. They include fixed-tube-sheet heat exchangers, U-tube shell and tube heat exchangers, heat exchangers with expansion joints, floating-head heat exchangers, segmented heat exchangers, and double-pipe heat exchangers, among others. 　　Fixed-tube-sheet heat exchangers have advantages such as simple structure, light weight, and low cost; the disadvantage is that the tubes bend due to thermal expansion. The U-tube shell and tube heat exchanger overcomes this drawback by shaping the tubes into a “U” shape, with one end fixed and the other end movable, thereby allowing the heat exchanger to remain unaffected by expansion. It has a simple structure and is lightweight; however, its disadvantages include the inability to clean it mechanically, difficulty in replacing the tubes, and low heat transfer capacity per unit volume and per unit mass. It is suitable for applications with large temperature differences and where the fluid inside the tubes is relatively clean. 　　Heat exchangers with expansion joints can address the issue of expansion; thanks to their design featuring expansion joints, they are suitable for fluids with large temperature differences as well as high-pressure fluids. Since the joints can be removed for cleaning, they can be used with fluids prone to scaling. However, they are not suitable for low-pressure gases, and their drawback is their complex manufacturing process. 　　In a floating-head shell and tube heat exchanger, the floating head is not connected to the shell and can move freely, which not only solves the problem of thermal expansion but also facilitates cleaning; during maintenance, the tube core can be removed. For fixed-tube-sheet, shell-and-tube, and double-pipe heat exchangers, the heat transfer area is approximately 30–40 m² per 1 m³ of shell volume. For U-tube shell and tube heat exchangers and floating-head heat exchangers, when the volume of each shell is 1 m3, their heat transfer area is around 70 m2. 2.2 Plate heat exchangers: Since it is possible to create uneven drainage channels on the heat transfer surface of plate heat exchangers, turbulence can occur even at lower Reynolds numbers; as a result, their heat transfer coefficient is high, typically ranging from 3,000 to 5,000 Kcal/m2·h·°C. Compared with shell-and-tube heat exchangers operating under the same flow rates, this value is about 3 to 5 times higher. Although the resistance of plate heat exchangers is somewhat greater in such cases, when considering the same level of energy input, their heat release coefficient is roughly twice that of shell-and-tube heat exchangers. 　　Due to their compact structure and small gaps, plate heat exchangers have a larger heat transfer area per unit volume. Their installation area is approximately 1/2 to 1/3 of that of shell-and-tube heat exchangers, which helps to save space and construction costs. For every 1 m3 of volume of the housing, the heat transfer area is around 80 m2. Additionally, it is easy to increase or decrease the heat transfer area in plate heat exchangers. In contrast, when shell-and-tube heat exchangers need to handle more liquid, it is almost impossible to increase their heat transfer area, whereas it is easy to do so with plate heat exchangers, thereby increasing their processing capacity. Moreover, in plate heat exchangers only the housing plates that contain the heat transfer plates are exposed to the atmosphere; therefore, heat loss is negligible, and no insulation measures are required. 　　One of the advantages of plate heat exchangers in terms of operation and maintenance is their ease of installation and removal; it is even possible to avoid completely disassembling them – simply loosening the clamping bolts allows the plates to be taken out for cleaning, the gaskets to be replaced, or even the plates themselves to be replaced. This is particularly important for materials in which heat transfer media tend to form deposits. 　　Regarding the allowable temperatures and pressures: In plate heat exchangers, gaskets are used between every two plates to prevent leakage of the fluid; as a result, the total length of the sealing perimeter is quite large. Preventing leakage from the gaskets is an important aspect of plate heat exchangers, and the gaskets must be able to withstand