Plate heat exchanger
Thread Content
As is well known, chlor-alkali production is an energy-intensive industry. In recent years, as the production scale of chlor-alkali plants has continued to expand, their total energy consumption has also increased. Since the energy consumption and investment associated with heat exchangers account for a significant portion of the costs in caustic soda production, the use of new types of heat exchangers becomes increasingly important in the expansion of chlor-alkali plants. A plate heat exchanger is a compact and highly efficient heat exchange device. Compared to a shell and tube heat exchanger, its floor area is only about 20% of that of a shell and tube heat exchanger. Due to the increased perturbation, heat transfer was enhanced, and the heat transfer efficiency improved by 4 to 5 times. It features flexible assembly, low heat loss, and no need for insulation; it is also lightweight, easy to disassemble and maintain, and is widely used in various industrial applications.
1. Introduction to plate heat exchangers A plate heat exchanger is a new type of efficient heat exchanger constructed by stacking a series of metal sheets with a certain wavy shape. Thin rectangular channels are formed between the various plates, through which heat exchange takes place via the half-plates. Compared with conventional shell-and-tube heat exchangers, it has a much higher heat transfer coefficient under the same flow resistance and pump power consumption, and shows a tendency to replace shell-and-tube heat exchangers within its applicable range. The main types of plate heat exchangers are frame-type (removable) and brazed types. The plate configurations include herringbone corrugated plates, horizontally flat corrugated plates, and bulged plates. 1.1 Basic structure of plate heat exchangers Plate heat exchangers are mainly composed of two major parts: a frame and plates. The plates are thin sheets made of various materials, which are shaped into waves using different types of grinding tools; corner holes are provided at the four corners of the plates to serve as channels for the medium to flow through. The perimeter of the plate and the corner holes are sealed with rubber gaskets. The frame consists of a fixed pressing plate, a movable pressing plate, upper and lower guide rods, and clamping bolts, etc. A plate heat exchanger consists of plates that are stacked between a fixed compression plate and a movable compression plate, and then secured together with clamping bolts. 1.2 Characteristics of plate heat exchangers (Comparison between plate heat exchangers and shell-and-tube heat exchangers)
a. High heat transfer coefficient: Due to the interposition of various corrugated plates, complex flow channels are formed. This causes the fluid to undergo three-dimensional rotational flow within these channels. Turbulence can be generated at relatively low Reynolds numbers (typically Re = 50–200). As a result, the heat transfer coefficient is high; it is generally considered to be 3–5 times that of shell-and-tube heat exchangers. b. Large logarithmic mean temperature difference, small terminal temperature difference: In shell-and-tube heat exchangers, the two fluids flow separately in the tube side and the shell side, resulting in a cross-flow pattern; hence, the logarithmic mean temperature difference correction factor is low. In plate heat exchangers, the fluids generally flow in parallel or counterflow, and the correction factor is usually around 0.95. Additionally, in plate heat exchangers, the cold and hot fluids flow parallel to the heat exchange surface without any bypass flow, which results in a small terminal temperature difference – it can be less than 1°C for heat exchange with water, whereas in shell-and-tube heat exchangers it is typically around 5°C.
c. Small footprint: Plate heat exchangers have a compact structure, with a heat exchange area per unit volume that is 2 to 5 times that of shell-and-tube heat exchangers. Moreover, there is no need to reserve space for removing the tube bundles for maintenance, as in shell-and-tube heat exchangers. Therefore, to achieve the same heat transfer capacity, plate heat exchangers require only about 1/5 to 1/10 of the space needed by shell-and-tube heat exchangers. d. It is easy to change the heat exchange area or the combination of processes; by adding or removing a few plates, it is possible to increase or decrease the heat exchange area ； By changing the arrangement of the plates or replacing a few of them, it is possible to achieve the desired combination of processes and adapt to new heat exchange conditions, whereas it is almost impossible to increase the heat transfer area of shell-and-tube heat exchangers. e. Light weight: The thickness of the plates in plate heat exchangers is only 0.4–0.8 mm, whereas the thickness of the heat exchange tubes in shell-and-tube heat exchangers is 2.0–2.5 mm. The shell of a shell-and-tube heat exchanger is much heavier than the frame of a plate heat exchanger; generally, the weight of a plate heat exchanger is only about 1/5 that of a shell-and-tube heat exchanger. f. Low cost: Using the same materials and at the same heat exchange area, the price of plate heat exchangers is about 40%~60% lower than that of shell-and-tube heat exchangers. g. Easy to manufacture: The heat transfer plates of plate heat exchangers are produced through stamping; they have a high degree of standardization and can be mass-produced. In contrast, shell-and-tube heat exchangers are generally manufactured manually. h. Easy to clean: For frame-type plate heat exchangers, simply loosening the clamping bolts allows the plate bundle to be released, enabling the plates to be removed for mechanical cleaning. This is extremely convenient for heat exchange processes where frequent equipment cleaning is required. i. Low heat loss: In plate heat exchangers, only the outer shell plates of the heat transfer plates are exposed to the atmosphere; therefore, heat loss is negligible, and no insulation measures are required. Shell-and-tube heat exchangers have high heat losses and require insulation. j. The capacity is smaller, at 10%~20% of that of a shell-and-tube heat exchanger. k. High pressure loss per unit length: Due to the small gaps between the heat transfer surfaces and the irregularities on these surfaces, the pressure loss is higher compared to that of conventional smooth tubes. l. It is resistant to scaling: Due to the strong turbulence inside, scaling does not occur easily; its scaling coefficient is only 1/3 to 1/10 that of shell-and-tube heat exchangers.
m. The operating pressure should not be too high, nor should the temperature of the fluid be too high, as this may lead to leaks. Plate heat exchangers use gaskets for sealing, and the operating pressure generally should not exceed 2.5 MPa; the fluid temperature should also be below 250°C, otherwise leaks may occur. n. Prone to clogging: Due to the narrow channels between the plates, which are usually only 2–5 mm wide, the channels can easily become blocked when the heat exchange medium contains large particles or fibrous substances. 1.4 Application scenarios of plate heat exchangers a. Refrigeration: Used as condensers and evaporators. b. HVAC: Intermediate heat exchangers used in conjunction with boilers, intermediate heat exchangers for high-rise buildings, etc. c. Chemical industry: soda ash industry, synthetic ammonia, alcohol fermentation, resin synthesis cooling, etc. d. Metallurgical industry: heating or cooling of aluminate mother liquors, cooling in steelmaking processes, etc. e. Machinery industry: cooling of various quenching fluids, cooling of reducer lubricants, etc. f. Power industry: cooling of high-voltage transformer oil, cooling of generator bearing oil, etc. g. Paper industry: heat recovery in bleaching processes, heating pulp wash water, etc. h. Textile industry: cooling of viscose fiber in alkaline solution, cooling of boiled nitrocellulose, etc. i. Food industry: sterilization and cooling of fruit juices, heating and cooling of animal and vegetable oils, etc. j. Fat processing: soap-based normal-pressure drying, heating or cooling of liquids used in various processes. k. Centralized heating: District heating using waste heat from thermal power plants to heat water for bathing. l. Others: petroleum, pharmaceuticals, ships, seawater desalination, geothermal utilization. 1.5 Issues to be noted when selecting plate heat exchangers 1.5.1 Selection of plate type The type or corrugation pattern of the plates should be determined based on the actual requirements of the heat exchange application. For situations with high flow rates and a tolerance for low pressure drops, a plate type with low resistance should be selected; conversely, a plate type with high resistance should be chosen. Based on the fluid pressure and temperature conditions, determine whether to choose a detachable or brazed type. When determining the plate type, it is advisable not to choose plates with too small a surface area, so as to prevent an excessive number of plates, low flow velocity between plates, and a excessively low heat transfer coefficient. This issue deserves particular attention in the case of larger heat exchangers. 1.5.2 Selection of flow paths and channels A flow path refers to a set of parallel channels within a plate heat exchanger in which a fluid flows in the same direction, whereas a channel is the flow path for the fluid formed by two adjacent plates within a plate heat exchanger. Generally, several flow channels are connected in parallel or series to form various combinations of cold and hot medium pathways. The process combination form should be determined based on heat transfer and fluid resistance calculations, while meeting the requirements of the process conditions. Try to make the convective heat transfer coefficients in the cold and hot water channels equal or similar, thereby achieving the best heat transfer performance. Because the heat transfer coefficient attains a higher value when the convective heat transfer coefficients on both sides of the heat transfer surface are equal or similar. Although the flow velocities between the plates of a plate heat exchanger vary, the average flow velocity is still used in heat transfer and fluid resistance calculations. Since the nozzles of the single \"U\"-shaped process are all fixed on the pressing plate, they are easy to install and remove. 1.5.3 Pressure drop verification In the design and selection of plate heat exchangers, certain requirements are generally imposed on the pressure drop; therefore, it is necessary to conduct a verification thereof. If the verified pressure drop exceeds the allowable value, redesign and selection calculations must be carried out again until the process requirements are met.
a. High heat transfer coefficient: Due to the interposition of various corrugated plates, complex flow channels are formed. This causes the fluid to undergo three-dimensional rotational flow within these channels. Turbulence can be generated at relatively low Reynolds numbers (typically Re = 50–200). As a result, the heat transfer coefficient is high; it is generally considered to be 3–5 times that of shell-and-tube heat exchangers. b. Large logarithmic mean temperature difference, small terminal temperature difference: In shell-and-tube heat exchangers, the two fluids flow separately in the tube side and the shell side, resulting in a cross-flow pattern; hence, the logarithmic mean temperature difference correction factor is low. In plate heat exchangers, the fluids generally flow in parallel or counterflow, and the correction factor is usually around 0.95. Additionally, in plate heat exchangers, the cold and hot fluids flow parallel to the heat exchange surface without any bypass flow, which results in a small terminal temperature difference – it can be less than 1°C for heat exchange with water, whereas in shell-and-tube heat exchangers it is typically around 5°C.
c. Small footprint: Plate heat exchangers have a compact structure, with a heat exchange area per unit volume that is 2 to 5 times that of shell-and-tube heat exchangers. Moreover, there is no need to reserve space for removing the tube bundles for maintenance, as in shell-and-tube heat exchangers. Therefore, to achieve the same heat transfer capacity, plate heat exchangers require only about 1/5 to 1/10 of the space needed by shell-and-tube heat exchangers. d. It is easy to change the heat exchange area or the combination of processes; by adding or removing a few plates, it is possible to increase or decrease the heat exchange area ； By changing the arrangement of the plates or replacing a few of them, it is possible to achieve the desired combination of processes and adapt to new heat exchange conditions, whereas it is almost impossible to increase the heat transfer area of shell-and-tube heat exchangers. e. Light weight: The thickness of the plates in plate heat exchangers is only 0.4–0.8 mm, whereas the thickness of the heat exchange tubes in shell-and-tube heat exchangers is 2.0–2.5 mm. The shell of a shell-and-tube heat exchanger is much heavier than the frame of a plate heat exchanger; generally, the weight of a plate heat exchanger is only about 1/5 that of a shell-and-tube heat exchanger. f. Low cost: Using the same materials and at the same heat exchange area, the price of plate heat exchangers is about 40%~60% lower than that of shell-and-tube heat exchangers. g. Easy to manufacture: The heat transfer plates of plate heat exchangers are produced through stamping; they have a high degree of standardization and can be mass-produced. In contrast, shell-and-tube heat exchangers are generally manufactured manually. h. Easy to clean: For frame-type plate heat exchangers, simply loosening the clamping bolts allows the plate bundle to be released, enabling the plates to be removed for mechanical cleaning. This is extremely convenient for heat exchange processes where frequent equipment cleaning is required. i. Low heat loss: In plate heat exchangers, only the outer shell plates of the heat transfer plates are exposed to the atmosphere; therefore, heat loss is negligible, and no insulation measures are required. Shell-and-tube heat exchangers have high heat losses and require insulation. j. The capacity is smaller, at 10%~20% of that of a shell-and-tube heat exchanger. k. High pressure loss per unit length: Due to the small gaps between the heat transfer surfaces and the irregularities on these surfaces, the pressure loss is higher compared to that of conventional smooth tubes. l. It is resistant to scaling: Due to the strong turbulence inside, scaling does not occur easily; its scaling coefficient is only 1/3 to 1/10 that of shell-and-tube heat exchangers.
m. The operating pressure should not be too high, nor should the temperature of the fluid be too high, as this may lead to leaks. Plate heat exchangers use gaskets for sealing, and the operating pressure generally should not exceed 2.5 MPa; the fluid temperature should also be below 250°C, otherwise leaks may occur. n. Prone to clogging: Due to the narrow channels between the plates, which are usually only 2–5 mm wide, the channels can easily become blocked when the heat exchange medium contains large particles or fibrous substances. 1.4 Application scenarios of plate heat exchangers a. Refrigeration: Used as condensers and evaporators. b. HVAC: Intermediate heat exchangers used in conjunction with boilers, intermediate heat exchangers for high-rise buildings, etc. c. Chemical industry: soda ash industry, synthetic ammonia, alcohol fermentation, resin synthesis cooling, etc. d. Metallurgical industry: heating or cooling of aluminate mother liquors, cooling in steelmaking processes, etc. e. Machinery industry: cooling of various quenching fluids, cooling of reducer lubricants, etc. f. Power industry: cooling of high-voltage transformer oil, cooling of generator bearing oil, etc. g. Paper industry: heat recovery in bleaching processes, heating pulp wash water, etc. h. Textile industry: cooling of viscose fiber in alkaline solution, cooling of boiled nitrocellulose, etc. i. Food industry: sterilization and cooling of fruit juices, heating and cooling of animal and vegetable oils, etc. j. Fat processing: soap-based normal-pressure drying, heating or cooling of liquids used in various processes. k. Centralized heating: District heating using waste heat from thermal power plants to heat water for bathing. l. Others: petroleum, pharmaceuticals, ships, seawater desalination, geothermal utilization. 1.5 Issues to be noted when selecting plate heat exchangers 1.5.1 Selection of plate type The type or corrugation pattern of the plates should be determined based on the actual requirements of the heat exchange application. For situations with high flow rates and a tolerance for low pressure drops, a plate type with low resistance should be selected; conversely, a plate type with high resistance should be chosen. Based on the fluid pressure and temperature conditions, determine whether to choose a detachable or brazed type. When determining the plate type, it is advisable not to choose plates with too small a surface area, so as to prevent an excessive number of plates, low flow velocity between plates, and a excessively low heat transfer coefficient. This issue deserves particular attention in the case of larger heat exchangers. 1.5.2 Selection of flow paths and channels A flow path refers to a set of parallel channels within a plate heat exchanger in which a fluid flows in the same direction, whereas a channel is the flow path for the fluid formed by two adjacent plates within a plate heat exchanger. Generally, several flow channels are connected in parallel or series to form various combinations of cold and hot medium pathways. The process combination form should be determined based on heat transfer and fluid resistance calculations, while meeting the requirements of the process conditions. Try to make the convective heat transfer coefficients in the cold and hot water channels equal or similar, thereby achieving the best heat transfer performance. Because the heat transfer coefficient attains a higher value when the convective heat transfer coefficients on both sides of the heat transfer surface are equal or similar. Although the flow velocities between the plates of a plate heat exchanger vary, the average flow velocity is still used in heat transfer and fluid resistance calculations. Since the nozzles of the single \"U\"-shaped process are all fixed on the pressing plate, they are easy to install and remove. 1.5.3 Pressure drop verification In the design and selection of plate heat exchangers, certain requirements are generally imposed on the pressure drop; therefore, it is necessary to conduct a verification thereof. If the verified pressure drop exceeds the allowable value, redesign and selection calculations must be carried out again until the process requirements are met.
I’m not sure if it’s correct; please help to correct it
Thank you for the correction. I will continue to strive in the future
The reason why plate heat exchangers are considered energy-saving products becomes evident when looking at the chlor-alkali industry; their high heat transfer efficiency indeed allows for energy savings. Therefore, we often compare it with other heat exchangers. The cost-effectiveness of plate heat exchangers should take energy consumption into account, rather than simply looking at the price alone.