HOW TO OPTIMIZE YOUR PLATE HEAT EXCHANGER DESIGN
1.1 Improve heat transfer efficiency
The cold and hot fluids transfer heat through the plates of the heat exchanger. The plate heat exchanger is an inter-wall heat transfer heat exchanger. The fluid is in direct contact with the plate, and the heat transfer methods are heat conduction and convection heat transfer. The key to improving the heat transfer efficiency of the plate heat exchanger is to increase the heat transfer coefficient and the logarithmic average temperature difference. A
Reduce the thermal resistance of the dirt layer:
The cold and hot fluids transfer heat through the plates of the heat exchanger. The plate heat exchanger is an inter-wall heat transfer heat exchanger. The fluid is in direct contact with the plate, and the heat transfer methods are heat conduction and convection heat transfer. The key to improving the heat transfer efficiency of the plate heat exchanger is to increase the heat transfer coefficient and the logarithmic average temperature difference. A
Reduce the thermal resistance of the dirt layer:
① Improve the heat transfer coefficient of the heat exchanger
Only at the same time increase the external heat transfer coefficient of the cold and hot plates. Only use plates with high thermal conductivity to reduce the thickness of the plates to effectively increase the heat transfer coefficient of the heat exchanger.
Only at the same time increase the external heat transfer coefficient of the cold and hot plates. Only use plates with high thermal conductivity to reduce the thickness of the plates to effectively increase the heat transfer coefficient of the heat exchanger.
a. Improve the heat transfer coefficient of the plate
The external heat transfer coefficient is related to the geometric structure of the corrugation of the plates and the flow state of the medium. The waveform of the plate includes herringbone, straight, spherical and so on. After years of research and experiments, it has been found that due to the corrugation of the plate heat exchanger, the fluid can generate turbulent flow at a relatively low flow rate. When the Reynolds number is 150, a higher external heat transfer coefficient can be obtained. The shape of the corrugated cross-section is triangular and sinusoidal. The outer surface has the largest heat transfer coefficient, the pressure drop is small, and the stress distribution is uniform under compression. However, processing is difficult. The higher the medium velocity in the flow path between the plates, the greater the external heat transfer coefficient.
The external heat transfer coefficient is related to the geometric structure of the corrugation of the plates and the flow state of the medium. The waveform of the plate includes herringbone, straight, spherical and so on. After years of research and experiments, it has been found that due to the corrugation of the plate heat exchanger, the fluid can generate turbulent flow at a relatively low flow rate. When the Reynolds number is 150, a higher external heat transfer coefficient can be obtained. The shape of the corrugated cross-section is triangular and sinusoidal. The outer surface has the largest heat transfer coefficient, the pressure drop is small, and the stress distribution is uniform under compression. However, processing is difficult. The higher the medium velocity in the flow path between the plates, the greater the external heat transfer coefficient.
b. Reduce the thermal resistance of the dirt layer
The heat transfer coefficient is reduced by about 10%. Therefore, the key to reducing the thermal resistance of the fouling layer of the heat exchanger is to prevent the plate from scaling. When the scale thickness of the plate is 1mm. Care must be taken to monitor the water quality on both sides of the heat exchanger to prevent scaling of the plates and prevent debris from adhering to the plates. To prevent water theft and corrosion of steel parts, some heating units add chemicals to the heating medium, so it is necessary to pay attention to the water quality and the adhesive to cause the sundries to contaminate the heat exchanger plates. If there is sticky debris in the water, it should be treated with a special filter. When choosing pharmaceuticals, it is advisable to choose non-sticky pharmaceuticals.
The heat transfer coefficient is reduced by about 10%. Therefore, the key to reducing the thermal resistance of the fouling layer of the heat exchanger is to prevent the plate from scaling. When the scale thickness of the plate is 1mm. Care must be taken to monitor the water quality on both sides of the heat exchanger to prevent scaling of the plates and prevent debris from adhering to the plates. To prevent water theft and corrosion of steel parts, some heating units add chemicals to the heating medium, so it is necessary to pay attention to the water quality and the adhesive to cause the sundries to contaminate the heat exchanger plates. If there is sticky debris in the water, it should be treated with a special filter. When choosing pharmaceuticals, it is advisable to choose non-sticky pharmaceuticals.
c. Use plates with high thermal conductivity
The thermal conductivity is about 14.4W / mK high strength, the plate material can choose austenitic stainless steel, titanium alloy, copper alloy and so on. Stainless steel has good thermal conductivity. The stamping performance is good, it is not easy to be oxidized, the price is lower than that of titanium alloy and copper alloy, and it is most used in heating engineering, but its ability to resist chloride ion corrosion is poor.
The thermal conductivity is about 14.4W / mK high strength, the plate material can choose austenitic stainless steel, titanium alloy, copper alloy and so on. Stainless steel has good thermal conductivity. The stamping performance is good, it is not easy to be oxidized, the price is lower than that of titanium alloy and copper alloy, and it is most used in heating engineering, but its ability to resist chloride ion corrosion is poor.
d. Reduce plate thickness
It is related to the pressure-bearing capacity of the heat exchanger. The plate is thickened, and the design thickness of the plate has nothing to do with its corrosion resistance. It can improve the pressure capacity of the heat exchanger. When using the combination of herringbone plates, the adjacent plates are inverted and the corrugations are in contact with each other to form a fulcrum with a high density and uniform distribution. The angled structure of the plates and the edge sealing structure have been gradually improved, making the heat exchanger very good Pressure endurance. The maximum pressure capacity of the domestic removable plate heat exchanger has reached 2.5MPa. The thickness of the plate has a great influence on the heat transfer coefficient. The thickness is reduced by 0.1mm. The total heat transfer coefficient of the symmetrical plate heat exchanger is increased by about 600W / mK. The model is increased by about 500W / mK¨ Under the premise of satisfying the pressure bearing capacity of the heat exchanger, the thickness of the plate should be selected as small as possible.
It is related to the pressure-bearing capacity of the heat exchanger. The plate is thickened, and the design thickness of the plate has nothing to do with its corrosion resistance. It can improve the pressure capacity of the heat exchanger. When using the combination of herringbone plates, the adjacent plates are inverted and the corrugations are in contact with each other to form a fulcrum with a high density and uniform distribution. The angled structure of the plates and the edge sealing structure have been gradually improved, making the heat exchanger very good Pressure endurance. The maximum pressure capacity of the domestic removable plate heat exchanger has reached 2.5MPa. The thickness of the plate has a great influence on the heat transfer coefficient. The thickness is reduced by 0.1mm. The total heat transfer coefficient of the symmetrical plate heat exchanger is increased by about 600W / mK. The model is increased by about 500W / mK¨ Under the premise of satisfying the pressure bearing capacity of the heat exchanger, the thickness of the plate should be selected as small as possible.
② Increase the logarithmic average temperature difference
The logarithmic average temperature difference is the largest when it is countercurrent, and the flow pattern of the plate heat exchanger is countercurrent, cocurrent and mixed flow. There are both countercurrent and cocurrent in the same operating conditions. It is the smallest when flowing downstream, and the mixed flow pattern is between the two. The method to increase the logarithmic average temperature difference of the heat exchanger is to adopt a counter-current or near-countercurrent mixed flow pattern as much as possible, increase the temperature of the hot-side fluid as much as possible, and reduce the temperature of the cold-side fluid.
The logarithmic average temperature difference is the largest when it is countercurrent, and the flow pattern of the plate heat exchanger is countercurrent, cocurrent and mixed flow. There are both countercurrent and cocurrent in the same operating conditions. It is the smallest when flowing downstream, and the mixed flow pattern is between the two. The method to increase the logarithmic average temperature difference of the heat exchanger is to adopt a counter-current or near-countercurrent mixed flow pattern as much as possible, increase the temperature of the hot-side fluid as much as possible, and reduce the temperature of the cold-side fluid.
③ Determination of the position of import and export pipes
For the convenience of maintenance, the plate heat exchanger is arranged in a single process. The fluid inlet and outlet pipes should be arranged as far as possible on the side of the fixed endplate of the heat exchanger. The greater the temperature difference of the medium, the stronger the natural convection of the fluid and the more obvious the effect of the formed retention zone. Therefore, the inlet and outlet positions of the medium should be arranged according to the hot fluid up and down, and the cold fluid up and down to reduce the impact of the retention zone To improve heat transfer efficiency.
For the convenience of maintenance, the plate heat exchanger is arranged in a single process. The fluid inlet and outlet pipes should be arranged as far as possible on the side of the fixed endplate of the heat exchanger. The greater the temperature difference of the medium, the stronger the natural convection of the fluid and the more obvious the effect of the formed retention zone. Therefore, the inlet and outlet positions of the medium should be arranged according to the hot fluid up and down, and the cold fluid up and down to reduce the impact of the retention zone To improve heat transfer efficiency.
1. 2 Ways to reduce the resistance of the heat exchanger
It can improve the heat transfer coefficient and increase the average flow rate of the medium in the inter-channel flow path. Reduce the heat exchanger area. However, increasing the flow rate will increase the resistance of the heat exchanger and increase the power consumption and equipment cost of the circulating pump. The power consumption of the circulation pump is proportional to the third power of the medium flow rate, and it is not economical to increase the flow rate by obtaining a slightly higher heat transfer coefficient. When the flow of cold and hot medium is relatively large, the following methods can be used to reduce the resistance of the heat exchanger and ensure a higher heat transfer coefficient.
It can improve the heat transfer coefficient and increase the average flow rate of the medium in the inter-channel flow path. Reduce the heat exchanger area. However, increasing the flow rate will increase the resistance of the heat exchanger and increase the power consumption and equipment cost of the circulating pump. The power consumption of the circulation pump is proportional to the third power of the medium flow rate, and it is not economical to increase the flow rate by obtaining a slightly higher heat transfer coefficient. When the flow of cold and hot medium is relatively large, the following methods can be used to reduce the resistance of the heat exchanger and ensure a higher heat transfer coefficient.
① Use hot mixing board
According to the angle of the herringbone corrugation, the plate is divided into hard board H and soft board L. The angle is generally about 120 and greater than 90 is the hard board. The geometric structure of the corrugation of the two sides of the hot mixed board is the same. The included angle is generally around 70 and less than 90 is a soft board. The heat transfer coefficient of the hard plate of the hot mixing plate is high, and the fluid resistance is large, while the soft plate is the opposite. The combination of hard board and soft board can form a flow channel with three characteristics of high HH, medium HL, and low LL to meet the needs of different working conditions. The use of a hot mixing plate can reduce the plate area compared to a symmetric single-flow heat exchanger. The diameter of the corner holes on both sides of the hot and cold mixing plates is usually equal when the flow of cold and hot medium is relatively large. When the flow ratio of the cold and hot medium is too large, the pressure loss on the side of the cold medium is large. Also, it is difficult to achieve accurate matching of the hot-mix board design technology, which often results in limited board area savings. Therefore, when the flow rate of the cold and hot medium is too large, it is not suitable to use the hot mixing plate.
According to the angle of the herringbone corrugation, the plate is divided into hard board H and soft board L. The angle is generally about 120 and greater than 90 is the hard board. The geometric structure of the corrugation of the two sides of the hot mixed board is the same. The included angle is generally around 70 and less than 90 is a soft board. The heat transfer coefficient of the hard plate of the hot mixing plate is high, and the fluid resistance is large, while the soft plate is the opposite. The combination of hard board and soft board can form a flow channel with three characteristics of high HH, medium HL, and low LL to meet the needs of different working conditions. The use of a hot mixing plate can reduce the plate area compared to a symmetric single-flow heat exchanger. The diameter of the corner holes on both sides of the hot and cold mixing plates is usually equal when the flow of cold and hot medium is relatively large. When the flow ratio of the cold and hot medium is too large, the pressure loss on the side of the cold medium is large. Also, it is difficult to achieve accurate matching of the hot-mix board design technology, which often results in limited board area savings. Therefore, when the flow rate of the cold and hot medium is too large, it is not suitable to use the hot mixing plate.
② Use asymmetrical plate heat exchanger
Plate heat exchangers with equal cross-sectional areas of cold and hot runners are formed. Asymmetrical type unequal cross-sectional area plate heat exchanger According to the heat transfer characteristics of cold and hot fluids and pressure drop requirements, symmetrical plate heat exchangers consist of plates with the same geometrical structure on both sides of the plate. Change the wave geometry structure of the two sides of the plate to form a plate heat exchanger with different cross-sectional areas of the hot and cold runners. The angle L on the side of the wide runner is larger. The heat transfer coefficient of the asymmetrical plate heat exchanger drops slightly, and the pressure drop decreases greatly. When the flow of cold and hot medium is relatively large, the use of an asymmetric single-flow heat exchanger can reduce the plate area by 15% to 30% compared with a symmetric single-flow heat exchanger.
Plate heat exchangers with equal cross-sectional areas of cold and hot runners are formed. Asymmetrical type unequal cross-sectional area plate heat exchanger According to the heat transfer characteristics of cold and hot fluids and pressure drop requirements, symmetrical plate heat exchangers consist of plates with the same geometrical structure on both sides of the plate. Change the wave geometry structure of the two sides of the plate to form a plate heat exchanger with different cross-sectional areas of the hot and cold runners. The angle L on the side of the wide runner is larger. The heat transfer coefficient of the asymmetrical plate heat exchanger drops slightly, and the pressure drop decreases greatly. When the flow of cold and hot medium is relatively large, the use of an asymmetric single-flow heat exchanger can reduce the plate area by 15% to 30% compared with a symmetric single-flow heat exchanger.
③ Adopt multi-process combination
Multi-flow combination arrangement can be adopted when the flow of cold and hot medium is large. On the small flow side, more processes are used to increase the flow rate and obtain a higher heat transfer coefficient. Less flow is used on the large flow side to reduce heat exchanger resistance. The combination of multiple processes presents a mixed flow pattern with a slightly lower average heat transfer temperature difference. The fixed endplate and the movable endplate of the plate heat exchanger with multi-process combination are both taken over, and the workload is large during maintenance.
Multi-flow combination arrangement can be adopted when the flow of cold and hot medium is large. On the small flow side, more processes are used to increase the flow rate and obtain a higher heat transfer coefficient. Less flow is used on the large flow side to reduce heat exchanger resistance. The combination of multiple processes presents a mixed flow pattern with a slightly lower average heat transfer temperature difference. The fixed endplate and the movable endplate of the plate heat exchanger with multi-process combination are both taken over, and the workload is large during maintenance.
④ Heat exchanger bypass pipe
A bypass pipe can be installed at the inlet and outlet of the heat exchanger on the side of the large flow when the flow of cold and hot medium is relatively large. Reduce the flow into the heat exchanger and reduce the resistance. To facilitate the adjustment, a regulating valve should be installed on the bypass pipe. This method should adopt counter-current arrangement to make the temperature of the cold medium out of the heat exchanger higher, and to ensure that the temperature of the cold medium after the heat exchanger merges can meet the design requirements. The heat exchanger bypass tube can ensure that the heat exchanger has a higher heat transfer coefficient and reduce the resistance of the heat exchanger, but the adjustment is slightly more complicated.
⑤ The choice of plate heat exchanger
The resistance should be no more than 100kPa. According to different cold and hot medium flow ratios, the average flow velocity of the medium in the flow channel between the heat exchanger plates should be 0.30.6ms.
A bypass pipe can be installed at the inlet and outlet of the heat exchanger on the side of the large flow when the flow of cold and hot medium is relatively large. Reduce the flow into the heat exchanger and reduce the resistance. To facilitate the adjustment, a regulating valve should be installed on the bypass pipe. This method should adopt counter-current arrangement to make the temperature of the cold medium out of the heat exchanger higher, and to ensure that the temperature of the cold medium after the heat exchanger merges can meet the design requirements. The heat exchanger bypass tube can ensure that the heat exchanger has a higher heat transfer coefficient and reduce the resistance of the heat exchanger, but the adjustment is slightly more complicated.
⑤ The choice of plate heat exchanger
The resistance should be no more than 100kPa. According to different cold and hot medium flow ratios, the average flow velocity of the medium in the flow channel between the heat exchanger plates should be 0.30.6ms.
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