Optimum Design Direction of Plate Heat Exchanger
In recent years, the technology of plate heat exchanger is becoming more and more mature. It has high heat transfer efficiency, small size, light weight, low dirt coefficient, easy disassembly, many kinds of plates and wide application range, and has been widely used in the heating industry. Plate heat exchangers can be divided into detachable type, welding type, brazing type, plate-shell type and so on. Because the removable plate heat exchanger is easy to be disassembled and cleaned, and the area of heat exchanger is flexible, it is widely used in heating engineering. Removable plate heat exchanger is limited by the heat resistance temperature of rubber gasket, which is suitable for water-water heat transfer. In this paper, the optimization design for improving the efficiency of removable plate heat exchanger is studied.
Optimum Design Method of Plate Heat Exchanger
Improving the efficiency of plate heat exchanger is a comprehensive economic benefit problem, which should be determined through technical and economic comparison. In order to improve the heat transfer efficiency of heat exchanger and reduce the resistance of heat exchanger, the material of plate and rubber sealing gasket and installation method should be selected reasonably to ensure the safe operation of the equipment and prolong the service life of the equipment.
1. Improve heat transfer efficiency
(1) To improve the heat transfer coefficient of heat exchanger, only by simultaneously increasing the surface heat transfer coefficient on both sides of the plate, reducing the thermal resistance of the fouling layer, choosing the plate with high thermal conductivity and reducing the thickness of the plate, can the heat transfer coefficient of heat exchanger be effectively improved.
A. Improving the surface heat transfer coefficient of the plate
Because the ripple of plate heat exchanger can make the fluid turbulent at a lower flow velocity (Reynolds number 150), it can obtain a higher surface heat transfer coefficient, which is related to the geometrical structure of plate ripple and the flow state of medium. The waveforms of plates include herringbone, flat, spherical and so on. After many years of research and experiments, it is found that the shape of corrugated section is triangular (sinusoidal surface has the largest heat transfer coefficient, smaller pressure drop, uniform stress distribution under pressure, but difficult to process? The herringbone plate has higher surface heat transfer coefficient, and the bigger the angle of corrugation, the higher the velocity of medium flow in the inter-plate channel, the bigger the surface heat transfer coefficient.
B. Reducing thermal resistance of fouling layer
The key to reduce the thermal resistance of fouling layer in heat exchanger is to prevent plate fouling. When the thickness of scale is 1 mm, the heat transfer coefficient decreases by about 10%. Therefore, attention must be paid to monitoring the water quality on both sides of the heat exchanger, to prevent plate scaling and to prevent impurities from adhering to the plate. In order to prevent water theft and corrosion of steel parts, some heating units add chemicals to the heating medium. Therefore, it is necessary to pay attention to the water quality and sticky agent causing sundries to contaminate the heat exchanger plates. If there are viscous impurities in water, special filters should be used to treat them. When choosing medicines, it is advisable to choose non-sticky medicines.
C. Selection of plates with high thermal conductivity
Austenitic stainless steel, titanium alloy, copper alloy and so on can be selected for sheet material. Stainless steel has good thermal conductivity, thermal conductivity of about 14.4 W/(m K), high strength, good stamping performance, not easy to be oxidized, lower price than titanium and copper alloys. It is used most in heating engineering, but its resistance to chloride ion corrosion is poor.
D. Reducing sheet thickness
The design thickness of the plate has nothing to do with its corrosion resistance, but with the pressure bearing capacity of the heat exchanger. Thickening plate can improve the pressure-bearing capacity of heat exchanger. When the herringbone plate combination is adopted, the adjacent plates are inverted and the corrugates contact each other, forming the fulcrum of high density and uniform distribution. The corner L and edge sealing structure of the plate have been gradually improved, which makes the heat exchanger have good pressure bearing capacity. The maximum pressure-bearing capacity of domestic detachable plate heat exchangers has reached 2.5 MPa. The thickness of the plate has a great influence on the heat transfer coefficient. The thickness decreases by 0.1 M M. The total heat transfer coefficient of the symmetrical plate heat exchanger increases by about 600 W/(m K) and that of the asymmetrical plate heat exchanger by about 500 W/(m K). On the premise of meeting the pressure-bearing capacity of heat exchangers, the smaller plate thickness should be chosen as far as possible.
(2)Increasing logarithmic mean temperature difference
The flow patterns of plate heat exchangers are counter-flow, counter-flow and mixed flow patterns (both counter-flow and counter-flow). Under the same conditions, the logarithmic mean temperature difference is the largest in countercurrent and the smallest in downstream, and the mixed flow pattern is between the two. The method to increase the logarithmic mean temperature difference of heat exchanger is to adopt the mixed flow pattern of countercurrent or near countercurrent as far as possible, to increase the temperature of hot side fluid as possible, and to reduce the temperature of cold side fluid.
(3) Determination of the location of import and export pipes
For the plate heat exchanger with single flow arrangement, in order to facilitate maintenance, the fluid inlet and outlet pipes should be arranged as far as possible on the side of the fixed end plate of the heat exchanger. The bigger the temperature difference of medium is, the stronger the natural convection of fluid is, and the more obvious the influence of the detention zone is. Therefore, the position of the inlet and outlet of medium should be arranged according to the hot fluid's up-down and the cold fluid's down-up-out, so as to reduce the influence of the detention zone and improve the heat transfer efficiency.
2. Ways to Reduce Heat Exchanger Resistance
Increasing the average velocity of the medium in the inter-plate channel can increase the heat transfer coefficient and reduce the area of the heat exchanger. However, increasing the flow rate will increase the resistance of heat exchanger, increase the power consumption of circulating pump and equipment cost. The power consumption of the circulating pump is proportional to the third power of the medium flow rate. It is not economical to obtain a slightly higher heat transfer coefficient by increasing the flow rate. When the flow rate of hot and cold medium is large, the following methods can be used to reduce the resistance of heat exchanger and ensure a higher heat transfer coefficient.
(1) Using hot mixing plate
The geometrical structure of corrugation on both sides of the plate is the same. The plate is divided into hard plate (H) and soft plate (L) according to the angle of the herringbone corrugation, and the angle is 120. About) greater than 90. For hard board, angle (generally 70). About) less than 90. For soft boards. The surface heat transfer coefficient of the hot mixing plate is high, and the fluid resistance is high, while that of the soft plate is opposite. The combination of hard plate and soft plate can form three kinds of runners with high (HH), medium (HL) and low (LL) characteristics to meet the needs of different working conditions.
When the flow rate of hot and cold medium is large, the plate area can be reduced by using hot mixing plate instead of symmetrical single-flow heat exchanger. The diameters of the corner holes on both sides of the hot and cold mixing plate are usually equal. When the flow ratio of the hot and cold media is too large, the angular L pressure loss on the side of the cold medium is very large. In addition, the design technology of thermal mixing plate is difficult to achieve accurate matching, which often leads to limited area saving. Therefore, when the flow ratio of hot and cold media is too large, the hot mixing plate should not be used.
(2) Using asymmetric plate heat exchanger
The symmetrical plate heat exchanger consists of plates with the same geometrical structure on both sides of the plate, forming a plate heat exchanger with the same cross-section area of flow in the cold and hot runners. According to the heat transfer characteristics and pressure drop requirements of cold and hot fluids, the asymmetric plate heat exchangers change the waveform geometry on both sides of the plate to form plate heat exchangers with different cross-section areas of cold and hot runners. The corner L diameter on one side of the wide runner is larger. The heat transfer coefficient of asymmetric plate heat exchanger decreases slightly, and the pressure drop decreases sharply. When the flow rate of hot and cold medium is large, the area of plate can be reduced by 15% to 30% by using asymmetric single-flow heat exchanger compared with symmetric single-flow heat exchanger.
(3) Using multi-process combination
When the flow rate of hot and cold media is large, multi-process combination arrangement can be adopted, and more processes can be used on the side of small flow rate to improve the flow rate and obtain higher heat transfer coefficient. In order to reduce the resistance of heat exchanger, less flow is used on the side of large flow rate. Mixed flow pattern appeared in multi-process combination, and the average temperature difference of heat transfer was slightly lower. Fixed end plate and movable end plate of plate heat exchanger with multi-process combination have nozzles, which cause heavy workload in overhaul.
(4) Setting up bypass tubes for heat exchangers
When the flow rate of hot and cold medium is relatively large, bypass pipes can be installed at the inlet and outlet of heat exchanger on the side of large flow rate to reduce the flow rate into heat exchanger and reduce resistance. For easy adjustment, the regulating valve should be installed on the bypass pipe. The counter-current arrangement should be adopted to make the temperature of the outlet of the cold medium heat exchanger higher, so as to ensure that the temperature of the outlet of the heat exchanger after confluence can meet the design requirements. The bypass tube of heat exchanger can ensure that the heat exchanger has a higher heat transfer coefficient and reduce the resistance of heat exchanger, but the adjustment is a little complicated.
(5) Selection of Plate Heat Exchanger Form
The average velocity of medium in the inter-plate channel of heat exchanger is 0.3-0.6m/s, and the resistance is less than 100 kPa. According to different flow ratios of heat and cold media, different types of plate heat exchangers can be selected according to Table 1, and the cross-sectional area ratio of asymmetric plate heat exchangers is 2. With symmetrical or asymmetrical, single or multi-process plate heat exchangers, heat exchanger bypass tubes can be installed, but they should be calculated in detail.
3. Material and Installation Method of Rubber Seal Gasket
(1) Selection of Material
In the water-water heat exchanger, the hot and cold media are not corrosive to the rubber gaskets. The key to choose the material of rubber gasket is its temperature resistance and sealing performance. The material of rubber gasket can be selected according to the literature.
(2) Selection of Installation Mode
Rubber gaskets are usually installed by bonding and clipping. Adhesive type is to bond rubber gasket with glue in plate sealing groove when heat exchanger is assembled. The clip type is to fix the rubber sealing gasket in the plate sealing groove by using the clip structure of the rubber sealing gasket and the edge of the plate when the heat exchanger is assembled. Because of the small installation workload, the damage rate of rubber gasket is low when the heat exchanger is disassembled, and there is no chloride ion in the glue which may cause the corrosion of the plate, so it is used more.
4. Rational Selection of Sheet Material
The corrosion failure of stainless steel sheet may occur in some corrosion, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion and so on. The incidence of stress corrosion is higher.
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