The hottest way to improve the heat exchange effic

2022-08-15
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Methods to improve the heat exchange efficiency of plate heat exchanger

in recent years, the technology of plate heat exchanger has become increasingly mature, with high heat transfer efficiency, small volume, light weight, low fouling coefficient, convenient disassembly, many varieties of plates and a wide range of applications. It has been widely used in the heating industry. Plate heat exchangers can be divided into detachable, welded, brazed, plate and shell types according to different assembly methods. Because the removable plate heat exchanger is easy to disassemble and clean, and the area of the heat exchanger is flexible to increase or decrease, it is widely used in heating projects. The presidential career should be the beginning of bending over and listening. The detachable plate heat exchanger is limited by the heat-resistant temperature of the rubber gasket, which is suitable for water-water heat transfer

improving the efficiency of plate heat exchanger is a comprehensive economic benefit problem, which should be determined after technical and economic comparison. Improving the heat transfer efficiency of the heat exchanger and reducing the resistance of the heat exchanger should be considered at the same time, and the plate material and rubber gasket material and installation method should be reasonably selected to ensure the safe operation of the equipment and prolong the service life of the equipment

2. Optimization design method of plate heat exchanger

2.1 improve heat transfer efficiency

plate heat exchanger is an intermediate wall heat transfer heat exchanger, and the angle of cold and hot fluid: change the angle of collet rotation, heat transfer through the plate of heat exchanger, the fluid is in direct contact with the plate, and the heat transfer mode is heat conduction and convective heat transfer. The key to improve the heat transfer efficiency of plate heat exchanger is to improve the heat transfer coefficient and logarithmic average temperature difference

(1) to improve the heat transfer coefficient of the heat exchanger, only by simultaneously improving the surface heat transfer coefficient of both cold and hot sides of the plate, reducing the thermal resistance of the dirt layer, selecting the plate with high thermal conductivity and reducing the thickness of the plate, can the heat transfer coefficient of the heat exchanger be effectively improved

a. improve the surface heat transfer coefficient of the plate

because the ripple of the plate heat exchanger can make the fluid produce turbulence at a small flow rate, a higher surface heat transfer coefficient can be obtained. The surface heat transfer coefficient is related to the geometric structure of the plate ripple and the flow state of the medium. The waveforms of plates include herringbone, straight, spherical, etc. After years of research and experiments, it is found that herringbone plates with triangular corrugated cross-section have higher surface heat transfer coefficient, and the larger the included angle of corrugated, the higher the medium flow rate in the channel between plates, and the greater the surface heat transfer coefficient

b. reduce the thermal resistance of the fouling layer

the key to reduce the thermal resistance of the fouling layer of the heat exchanger is to prevent the scaling of the plate. When the fouling thickness of the plate is 1mm, 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 scaling of the 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, so attention must be paid to the water quality and viscosity, which cause impurities to contaminate the heat exchanger plates. If there are viscous impurities in the water, special filters should be used for treatment. When choosing agents, it is advisable to choose non viscous agents

c. select plates with high thermal conductivity

the materials of plates can be austenitic stainless steel, titanium alloy, copper alloy, etc. Stainless steel has good thermal conductivity, thermal conductivity of about 14.4 w/(m k), high strength, good stamping performance, and is not easy to be oxidized. Its price is lower than that of titanium alloy and copper alloy. It is most used in heating projects, but its ability to resist chloride ion corrosion is poor

d. reduce the plate thickness

the design thickness of the plate has nothing to do with its corrosion resistance, and is related to the pressure bearing capacity of the heat exchanger. The plate thickening can improve the pressure bearing capacity of the heat exchanger. When using herringbone plate combination, the adjacent plates are inverted and the ripples are in contact with each other, forming a fulcrum with high density and uniform distribution. The sealing structure of plate angle and edge has been gradually improved, so that the heat exchanger has a good pressure bearing capacity. The maximum pressure bearing capacity of domestic removable plate heat exchanger has reached 2.5 MPa. The thickness of the plate has a great influence on the heat transfer coefficient. If the thickness decreases by 0.1mm, the total heat transfer coefficient of the symmetrical plate heat exchanger increases by about 600w/(m k), and that of the asymmetric plate heat exchanger increases by about 500 w/(m k). On the premise of meeting the pressure bearing capacity of the heat exchanger, try to choose a smaller plate thickness

(2) increase the logarithmic average temperature difference

the flow patterns of plate heat exchangers include countercurrent, downstream and mixed flow patterns (both countercurrent and downstream). Under the same working 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 the heat exchanger is to adopt the mixed flow pattern of countercurrent or close to countercurrent 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 the plate heat exchanger with single process arrangement, for the convenience of maintenance, the fluid inlet and outlet pipes should be arranged on the side of the fixed end plate of the heat exchanger as far as possible. The greater the temperature difference of the medium is, the stronger the natural convection of the fluid is, and the more obvious the influence of the formed detention zone is. Therefore, the inlet and outlet of the medium should be arranged according to the upper inlet and lower outlet of the hot fluid and the lower inlet and upper outlet of the cold fluid, so as to reduce the influence of the detention zone and improve the heat transfer efficiency

2.2 methods to reduce the resistance of heat exchanger

increasing the average flow rate of medium in the channel between plates can improve the heat transfer coefficient and reduce the area of heat exchanger. 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 circulating pump is proportional to the third power of the medium flow rate. It is uneconomical to obtain a slightly higher heat transfer coefficient by increasing the flow rate. 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 high heat transfer coefficient

(7.1.1 product inspection is divided into factory inspection and type inspection 1) the corrugated geometry on both sides of the hot mixed plate is the same. The plate is divided into hard plate (H) and soft plate (L) according to the included angle of herringbone ripple. If the included angle (generally about 120 degrees) is greater than 90 degrees, it is hard plate, and if the included angle (generally about 70 degrees) is less than 90 degrees, it is soft plate. The surface heat transfer coefficient of the hard 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 high (Hh), medium (HL) and low (LL) characteristics to meet the needs of different working conditions

when the flow of cold and hot medium is relatively large, the plate area can be reduced by using hot mixing plate than by using symmetrical single process heat exchanger. The diameter of the corner holes on both sides of the hot and cold mixing plate is usually equal. When the flow ratio of the hot and cold medium is too large, the English Financial Times article on the cold medium side says that the pressure loss of the corner L is very large. In addition, the thermal mixing plate design technology is difficult to achieve accurate matching, which often leads to limited plate area savings. Therefore, when the flow ratio of cold and hot medium is too large, the hot mixing plate should not be used

(2) adopt asymmetric plate heat exchanger

symmetric plate heat exchanger is composed of plates with the same corrugated geometric structure on both sides of the plate, forming a plate heat exchanger with equal flow cross-sectional area of cold and hot channels. According to the heat transfer characteristics and pressure drop requirements of the cold and hot fluid, the asymmetric (unequal cross-sectional area) plate heat exchanger changes the waveform geometry on both sides of the plate to form a plate heat exchanger with different flow cross-sectional areas of the cold and hot channels, and the corner L on the side of the wide channel has a larger diameter. The heat transfer coefficient of asymmetric plate heat exchanger decreases slightly, and the pressure drop decreases significantly. When the flow of cold and hot medium is relatively large, the plate area of asymmetric single process heat exchanger can be reduced by 15% - 30% than that of symmetric single process heat exchanger

(3) adopt multi process combination

when the flow of cold and hot medium is large, multi process combination arrangement can be adopted, and more processes can be adopted on the side of small flow to improve the flow rate and obtain higher heat transfer coefficient. Less flow is used on the side with large flow to reduce the resistance of the heat exchanger. The mixed flow pattern appears in the multi process combination, and the average heat transfer temperature difference is slightly lower. The fixed end plate and movable end plate of the plate heat exchanger with multi process combination have nozzles, so the workload during maintenance is large

(4) set the bypass pipe of the heat exchanger

when the flow of cold and hot medium is relatively large, a bypass pipe can be set between the inlet and outlet of the heat exchanger on the side of 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 countercurrent arrangement to make the temperature of the cold medium out of the heat exchanger higher and ensure that the temperature of the cold medium after the confluence at the outlet of the heat exchanger can meet the design requirements. Setting the bypass pipe of the heat exchanger can ensure that the heat exchanger has a high heat transfer coefficient and reduce the resistance of the heat exchanger, but the adjustment is slightly complicated

(5) selection of the form of plate heat exchanger

the average flow rate of the medium in the channel between the plates of the heat exchanger should be 0.3-0.6m/s, and the resistance should not be greater than 100KPA. If symmetrical or asymmetrical, single process or multi process plate heat exchangers are used, the bypass pipe of the heat exchanger can be set, but detailed thermal calculation shall be carried out

2.3 rubber gasket material and installation method

(1) material selection

in the water-water heat exchanger, the cold and hot media are not corrosive to the rubber gasket. The key to choose the rubber gasket material is the temperature resistance and sealing performance. The rubber gasket material can be selected according to the literature

(2) selection of installation methods

the common installation methods of rubber gaskets are adhesive and snap. Adhesive type is to glue the rubber gasket into the plate sealing groove when the heat exchanger is assembled. Snap type is to fix the rubber gasket in the plate sealing groove by using the rubber gasket and the snap structure at the edge of the plate when the heat exchanger is assembled. Because the workload of snap on installation is very small, the damage rate of the rubber gasket during the disassembly of the heat exchanger is low, and there is no chloride ion that may be contained in the glue to cause corrosion to the plate, so it is used more

2.4 reasonable selection of plate material

stainless steel plates may cause corrosion failure, including pitting corrosion, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion, etc., and the occurrence rate of stress corrosion is high

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