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Various types of plate heat exchangers: 1. Spiral plate heat exchanger. Its structure is as shown in the figure. It is characterized by one end plate that is not connected to the housing, allowing it to expand and contract freely along the axial direction. This structure not only completely eliminates thermal stress, but also allows the entire tube bundle to be removed from the shell since the tube sheet at the fixed end is connected to the shell via flanges, facilitating cleaning and maintenance. Floating-head heat exchangers are widely used, but they have a complex structure and high costs. The diameter of spiral plate heat exchangers is generally within 1.6 m, the plate width ranges from 200 to 1200 mm, and the plate thickness is 2 to 4 mm. mhtml:file://C:\Documents and Settings\Administrator\Desktop\Untitled Document.mht! The distance between the two plates is controlled by spacers that are pre-welded to the plates; the distance between adjacent plates ranges from 5 to 25 mm. Common materials are carbon steel and stainless steel. The advantages of spiral plate heat exchangers are: ① High heat transfer coefficient: The fluid in the spiral flow channels reaches a turbulent state at lower Reynolds numbers due to the effect of centrifugal inertial force (turbulence generally occurs at Re=1400–1800), and higher flow velocities can be used (2 m/s for liquids, 20 m/s for gases), resulting in a high heat transfer coefficient. For heat transfer between water and water, the heat transfer coefficient can reach 2000–3000 W/(m2·°C), whereas in shell-and-tube exchangers it is generally 1000–2000 W/(m2·°C). ②Resistant to scaling and blockage: Since it is a single-channel system for each fluid flow, the flow velocity is high; coupled with the effect of centrifugal inertial forces, turbulence is intense. As a result, the particles suspended in the fluid do not tend to settle, so spiral plate heat exchangers are resistant to scaling and blockage, making them suitable for handling suspensions and fluids with high viscosity. ③Ability to utilize low-temperature heat sources: Due to the long length of the flow channels for fluid flow and the complete ability of the two fluids to flow in opposite directions, operation can be carried out at a small temperature difference, allowing for effective recovery of low-temperature heat sources. According to some available information, the temperature difference between the hot and cold fluid streams at the outlet can be as low as 3°C. ④Compact structure: The heat transfer area per unit volume is approximately 3 times that of the shell-and-tube type. The main disadvantages of spiral plate heat exchangers are: ① The operating pressure and temperature should not be too high: currently, the maximum operating pressure does not exceed 20 atm, and the temperature is below 400°C. ②Difficult to maintain: Since the commonly used spiral plate heat exchangers are welded together, repairs are difficult once they become damaged. 2. Plate heat exchangers: Plate heat exchangers (usually referred to as plate-type heat exchangers) are primarily composed of a set of rectangular thin metal plates with specific undulating ridges and grooves, which are arranged parallel to each other and assembled on a frame using sealing and clamping devices. The edges of two adjacent plates are lined with gaskets, which enable external sealing when compressed. During operation, it is required that the cold and hot fluids flow alternately in the channels between the plates; that is, one channel carries hot fluid, while the channels adjacent to it on both sides carry cold fluid. To this end, a circular hole is drilled at each of the four corners of each plate. By providing or not providing a straight circular gasket outside the round hole, it is possible to ensure that each inter-plate channel is connected to only two holes. The assembly process of the plate heat exchanger is shown in Figure (a). As can be seen from the diagram, the introduced fluid can flow in parallel into a set of inter-plate channels, while the sets are connected in series. The structure of the heat exchange plate is shown in Figure (b). The ridges and grooves on the plate can increase the turbulence of the fluid as well as enhance the rigidity of the plate. There are various forms of corrugations; the one shown in Figure (b) is a herringbone corrugated plate. mhtml:file://C:\Documents and Settings\Administrator\Desktop\Untitled Document.mht! The advantages of plate heat exchangers are: ① High heat transfer coefficient: Due to the corrugations on the plate surface, turbulence can be achieved at low Reynolds numbers (around Re=200), and since the thickness of the plates is small, the heat transfer coefficient is high. The heat transfer coefficient for heat exchange between hot and cold water can reach 1500–4700 W/(m2·℃). ②Compact structure: The typical plate spacing is 4–6 mm, and the heat transfer area that can be provided per unit volume of equipment is 250–1000 m2/m3 (whereas shell-and-tube exchangers have only 40–150 m2/m3). ③It features a detachable structure: the heat transfer area can be increased or decreased by adjusting the number of plates as needed. Therefore, maintenance and cleaning are quite convenient. The main disadvantages of plate heat exchangers are: ① The operating pressure and temperature are not very high: high pressures can lead to leaks, and the operating pressure should not exceed 20 atm. The operating temperature is limited by the heat resistance of the gasket material, generally not exceeding 250°C. ②Low processing capacity: Since the distance between the plates is only a few millimeters and the flow rate is not high, the processing capacity is low. 3. Plate-fin heat exchangers: Plate-fin heat exchangers are a type of heat exchanger that is more efficient, compact, and lightweight, and they are widely used. There are many structural configurations for plate-fin heat exchangers, but their basic structural elements remain the same: between two parallel thin metal plates, metal fins of wavy or other shapes are inserted, and the two sides are sealed off, thus forming a heat exchange unit. mhtml:file://C:\Documents and Settings\Administrator\Desktop\Untitled Document.mht! By stacking the various basic components in different ways and arranging them appropriately, and then fixing them with brazing, a co-current, counter-current, or cross-flow plate bundle (or core) can be created. Its structure is as shown in the figure. By welding a header with fluid inlet and outlet nozzles to the plate bundle, a plate-fin heat exchanger is formed. The commonly used fin types in our country currently include straight fins, serrated fins, and porous fins, as shown in the figure. The advantages of plate-fin heat exchangers are: ① High heat transfer coefficient and excellent heat transfer performance: The fins promote turbulence to varying degrees and disrupt the development of the heat transfer boundary layer, resulting in a high heat transfer coefficient. The heat transfer coefficient for forced air convection is 35–350 W/(m2·℃), while for forced oil convection it ranges from 115–1750 W/(m2·℃). mhtml:file://C:\Documents and Settings\Administrator\Desktop\Untitled Document.mht! Heat transfer between cold and hot fluids occurs not only through the flat partition as the heat transfer surface, but also mostly via fins (secondary heat transfer surfaces), thereby improving the heat transfer efficiency. ②Compact structure: The heat transfer area provided per unit volume of the equipment generally ranges from 2,500 to 4,300 m2/m3. ③Light and sturdy: Usually made of aluminum alloy, giving the panel a low weight. At the same heat transfer area, its weight is approximately 1/10 that of a shell-and-tube heat exchanger. Wave-shaped fins serve not only as heat transfer surfaces but also as supports between the two plates, hence they have high strength. ④High adaptability and wide operating range: Due to the high thermal conductivity of aluminum alloys, and their high ductility and tensile strength at temperatures below 0°C, they are suitable for use in low- and ultra-low temperature environments, thus offering a wide operating range. Furthermore, it can be used for heat exchange between two fluids as well as for heat exchange among multiple different media within the same device, thus offering strong adaptability. The disadvantages of plate-fin heat exchangers are: ① The flow channels in the equipment are very small, prone to blockage, and difficult to clean and maintain; therefore, the material should be clean or pre-treated. ②Since both the partitions and fins are made of thin aluminum sheets, it is required that the medium does not corrode aluminum. Last edited by The wise are free from confusion on 2009-2-19 10:12]