Improve heat transfer efficiency of plate heat exchanger
1. Optimal design direction of plate heat exchanger
In recent years, the plate heat exchanger technology has become increasingly mature, with high heat transfer efficiency, small size, light weight, low fouling coefficient, easy disassembly, a wide variety of plates, and a wide range of applications. It has been widely used in the heating industry. Plate heat exchangers are divided into detachable type, welded type, brazed type, plate-shell type, etc. according to the assembly method. Because the detachable plate heat exchanger is easy to disassemble and clean, it is flexible to increase or decrease the area of the heat exchanger, and it is used more in heating projects. The detachable plate heat exchanger is limited by the heat-resistant temperature of the heat exchanger gasket, and is suitable for water-to-water heat transfer. This paper studies the optimization design to improve the efficiency of the detachable plate heat exchanger.
Improving the efficiency of the plate heat exchanger is a comprehensive economic benefit issue, which should be determined after technical and economic comparison. Improve the heat transfer efficiency of the heat exchanger and reduce the resistance of the heat exchanger should be considered at the same time, and the material of the heat exchanger plate and the material of the heat exchanger gasket and the installation method should be selected reasonably to ensure the safe operation of the equipment and extend the use of the equipment life.
2. Optimal design method of plate heat exchanger
2.1 Improve heat transfer efficiency
The plate heat exchanger is a wall-to-wall heat exchanger. The hot and cold fluid transfers heat through the heat exchanger plates, and the fluid directly contacts the heat exchanger plates. The heat transfer method is 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.
① To improve the heat transfer coefficient of the heat exchanger can only increase the surface heat transfer coefficient on both sides of the plate at the same time, reduce the thermal resistance of the fouling layer, select heat exchanger plates with high thermal conductivity, and reduce the heat exchanger plate The thickness can effectively improve the heat transfer coefficient of the plate heat exchanger.
a. Improve the surface heat transfer coefficient of heat exchanger plates
Because the corrugation of the plate heat exchanger can make the fluid produce turbulence at a small flow rate (Reynolds number-150), it can obtain a higher surface heat transfer coefficient, the surface heat transfer coefficient and the geometry of the heat exchanger plate corrugation The structure is related to the flow state of the medium. The waveforms of the heat exchanger plates include herringbone, straight, spherical and so on. After years of research and experiments, it has been found that the corrugated cross-section shape is triangular (the heat transfer coefficient of the sinusoidal surface is the largest, the pressure drop is small, the stress distribution is uniform under pressure, but the processing is difficult?) The herringbone plate has a higher surface transmission. Thermal coefficient, and the greater the angle of the corrugation, the higher the flow velocity of the medium in the flow channel between the plates, and the greater the surface heat transfer coefficient.
b. Reduce the thermal resistance of the dirt layer
The key to reducing the thermal resistance of the fouling layer of the heat exchanger is to prevent fouling of the heat exchanger plates. When the thickness of the heat exchanger plate fouling is 1 mm, the heat transfer coefficient is reduced by about 10%. Therefore, care must be taken to monitor the water quality on both sides of the heat exchanger to prevent fouling of the heat exchanger plates and prevent debris in the water from adhering to the plates. In order to prevent water theft and corrosion of steel parts, some heating units add chemicals to the heating medium. Therefore, attention must be paid to water quality and adhesives that cause debris to contaminate heat exchanger plates. If there are viscous debris in the water, special filters should be used for treatment. When choosing medicaments, it is advisable to choose non-sticky medicaments.
c. Use heat exchanger plates with high thermal conductivity
The plate material of the heat exchanger can be selected from austenitic stainless steel, titanium alloy, copper alloy, etc. Stainless steel has good thermal conductivity, with a thermal conductivity of about 14.4 W/(m•K), high strength, good stamping performance, and is not easy to be oxidized. The price is lower than that of titanium alloy and copper alloy. It is used most in heating engineering, but its Poor resistance to chloride ion corrosion.
d. Reduce the thickness of the heat exchanger plate
The design thickness of the heat exchanger plate has nothing to do with its corrosion resistance, but is related to the pressure bearing capacity of the heat exchanger. The heat exchanger plate is thickened, which can improve the pressure bearing capacity of the plate heat exchanger. When the herringbone plate combination is adopted, the adjacent heat exchanger plates are turned upside down, and the corrugations are in contact with each other, forming a fulcrum with high density and uniform distribution. The device has a good pressure bearing capacity. The maximum pressure-bearing capacity of the detachable plate heat exchanger has reached 2.5 MPa. The thickness of the heat exchanger 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/(m•K), and the asymmetrical type is increased by about 500 W/( m •K). On the premise of meeting the pressure-bearing capacity of the heat exchanger, the thickness of the heat exchanger plate should be as small as possible.
② Increase the logarithmic mean temperature difference
The flow patterns of plate heat exchangers are counter-current, co-current and mixed flow (both counter-current and co-current). Under the same working conditions, the logarithmic average temperature difference is the largest in the countercurrent flow and the smallest in the downstream flow, and the mixed flow pattern is somewhere between the two. The method to increase the logarithmic mean temperature difference of the heat exchanger is to use countercurrent or close to countercurrent mixed flow as much as possible, increase the temperature of the fluid on the hot side as much as possible, and reduce the temperature of the fluid on the cold side.
③ Determination of the position of inlet and outlet pipes
For plate heat exchangers arranged in a single process, for easy maintenance, the fluid inlet and outlet pipes should be arranged on the side of the fixed end plate of the heat exchanger as much as possible. The greater the temperature difference of the medium, the stronger the natural convection of the fluid, and the more obvious the influence of the stagnation zone. Therefore, the inlet and outlet positions of the medium should be arranged in accordance with the hot fluid up and down, and the cold fluid in and out to reduce the influence of the stagnant zone. , Improve heat transfer efficiency.
2.2 Methods to reduce the resistance of plate heat exchangers
Increasing the average flow velocity of the medium in the flow channel between the plates of the heat exchanger can increase the heat transfer coefficient and reduce the area of the heat exchanger. However, increasing the flow rate will increase the resistance of the heat exchanger and increase the power consumption of the circulating pump and the 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 increase the flow rate to obtain a slightly higher heat transfer coefficient. When the flow of cold and hot media is relatively large, the following methods can be used to reduce the resistance of the plate heat exchanger and ensure a higher heat transfer coefficient.
① Adopt thermal mixing plate
The geometric structure of the corrugation on both sides of the heat mixing plate is the same. The heat exchanger plates are divided into hard plates (H) and soft plates (L) according to the angle of the herringbone corrugation. The angle (usually 120. About) is greater than 90. It is a rigid board, and the included angle (usually 70. About) is less than 90. For soft board. The surface heat transfer coefficient of the hard plate of the thermal 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 high (HH), medium (HL), and low (LL) runners to meet the needs of different working conditions.
When the flow of cold and heat medium is relatively large, the use of a heat mixing plate can reduce the plate area than a symmetrical single-process heat exchanger. The diameters of the holes on the hot and cold sides of the hot mixing plate are usually the same. When the flow ratio of the cold and hot medium is too large, the pressure loss of the holes on the cold medium side is large. In addition, it is difficult to achieve precise matching with thermal mixing plate design technology, which often results in limited plate area saving. Therefore, it is not suitable to use a hot mixing plate when the flow ratio of the cold and heat medium is too large.
② Adopt asymmetrical plate heat exchanger
The symmetrical plate heat exchanger is composed of plates with the same corrugated geometry on both sides of the heat exchanger plates, forming a plate heat exchanger with equal cross-sectional areas of the cold and hot runners. Asymmetric (unequal cross-sectional area) plate heat exchangers change the wave geometry of the two sides of the plate according to the heat transfer characteristics and pressure drop requirements of the cold and hot fluids to form a plate heat exchanger with unequal cross-sectional areas of the cold and hot runners , The inlet diameter on the side of the wide runner is larger. The heat transfer coefficient of the asymmetrical plate heat exchanger decreases slightly, and the pressure drop is greatly reduced. When the flow of cold and heating medium is relatively large, the use of an asymmetric single-process heat exchanger can reduce the area of the heat exchanger plate by 15% to 30% compared with a symmetrical single-process heat exchanger.
③ Adopt multi-process combination
When the flow rate of the cold and heat medium is large, a combination of multiple processes can be used, and more processes are used on the side of the small flow rate to increase the flow rate and obtain a higher heat transfer coefficient. The large flow side adopts fewer processes to reduce the resistance of the plate heat exchanger. Mixed flow patterns appear in the combination of multiple processes, and the average heat transfer temperature difference is slightly lower. Both the fixed end plate and the movable end plate of the plate heat exchanger adopting multi-process combination take over, which requires a lot of work during maintenance.
④ Set the bypass pipe of the heat exchanger
When the flow of cold and heat medium is relatively large, a bypass pipe can be installed between the inlet and outlet of the heat exchanger on the side of the large flow to reduce the flow into the heat exchanger and reduce the resistance. In order to facilitate adjustment, a regulating valve should be installed on the bypass pipe. This method should adopt a countercurrent arrangement to make the temperature of the cold medium exiting the plate heat exchanger higher, and to ensure that the temperature of the cold medium after the heat exchanger outlet merges can meet the design requirements. The bypass pipe of the heat exchanger 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 Selection of plate heat exchanger form
The average flow velocity of the medium in the flow channel between the heat exchanger plates is preferably 0.3 to 0.6 m/s, and the resistance is preferably not more than 100 kPa. According to different flow ratios of cold and heat media, different forms of plate heat exchangers are selected.
2.3 Heat exchanger gasket material and installation method
① Selection of material
In the water-water plate heat exchanger, the cold and hot media are non-corrosive to the heat exchanger gasket. The key to selecting the material of the heat exchanger gasket is temperature resistance and sealing performance. The material of the heat exchanger gasket can be selected according to the literature.
② Choice of installation method
The commonly used installation methods of heat exchanger gaskets are bonding type and snap-in type. The bonding type is when the plate heat exchanger is assembled, the heat exchanger gasket is glued into the sealing groove of the heat exchanger plate. The snap-in type is to use the heat exchanger gasket and the snap structure on the edge of the plate to fix the heat exchanger gasket in the heat exchanger plate sealing groove when the plate heat exchanger is assembled. Due to the small workload of the snap-in installation, the heat exchanger gasket damage rate is low when the plate heat exchanger is disassembled, and there is no chloride ion that may be contained in the glue to cause corrosion to the heat exchanger plates, so it is used more.
2.4 Reasonable selection of heat exchanger plate material
The phenomenon of corrosion failure of stainless steel plates may cause bit corrosion, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion, etc., and the incidence of stress corrosion is relatively high.
