Views:0 Author:Site Editor Publish Time: 2020-05-09 Origin:Site
The plate heat exchanger is a partition heat transfer heat exchanger. The cold fluid transfers heat through the heat exchanger plates, and the fluid directly contacts the plates. The heat transfer method is heat conduction and convection heat transfer, which improves the heat transfer efficiency of the plate heat exchanger. The key is to improve the heat transfer coefficient and the logarithmic average temperature difference.
(1) Improve the heat transfer coefficient of the heat exchanger
Only increase the surface thermal coefficients of both sides of the plate at the same time, reduce the thermal resistance of the scale layer, select plates with high thermal conductivity, and reduce the thickness of the plates, in order to effectively improve the heat transfer coefficient of the heat exchanger.
① Improve the surface heat transfer coefficient of the plate
Because the corrugation of the plate heat exchanger can cause the fluid to generate turbulence at a lower 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. Waveforms include herringbone, straight, spherical, etc. After years of research and experiments, it has been found that the herringbone plate with a triangular corrugated cross-section has a higher surface heat transfer coefficient, and the greater the angle of the corrugation, the The higher the medium velocity in the flow channel, the greater the surface heat transfer coefficient;
② 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 the plate structure. When the thickness of the plate structure is 1mm, the heat transfer coefficient is reduced by about 10%. Therefore, care must be taken to monitor the water quality of the heat exchanger at both ends of the heat and cold, to prevent the plate structure, and to prevent debris in the water from adhering to the plate. To prevent water theft and corrosion of steel parts, add chemicals to the heating medium. Therefore, it is necessary to pay attention to the water quality and viscous chemicals to cause debris to contaminate the heat exchanger plates. If there are viscous impurities in the water, special filters should be used for treatment. When choosing pharmaceuticals, it is advisable to choose non-sticky pharmaceuticals;
③ Use high thermal conductivity plate
Plate material can choose austenitic stainless steel, titanium alloy, steel alloy, etc. Stainless steel has good thermal conductivity, thermal conductivity is about 14.4W / (mk), high strength, good stamping performance, not easy to be oxidized, the price is lower than titanium alloy and Copper alloy is low, but its ability to resist chloride ion corrosion is poor;
④ Reduce the thickness of the plate
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 thickened plates can improve the pressure-bearing capacity of the heat exchanger. When the chevron-shaped plates are combined, the adjacent plates are inverted The corrugations are in contact with each other, forming a dense and evenly distributed finger, and the sealing structure of the plate corners and edges has been gradually improved, so that the heat exchanger has a good pressure-bearing capacity. Should try to choose a smaller plate thickness;
(2) Increase the logarithmic average temperature difference
Plate heat exchangers have counter-current, co-current and mixed-flow types. Under the same operating conditions, the logarithmic average temperature difference during counter-current is extremely large, and the co-current is extremely small. The method of the average temperature difference of the logarithm of the device is to adopt a counter-current or near-counter-current mixed flow type 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 the 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 as far as possible on the side of the fixed end of the heat exchanger. The more obvious, the position of the medium inlet and outlet 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 and improve the heat transfer efficiency.
Increasing the average flow rate of the medium in the flow path between the plates can increase the heat transfer coefficient and reduce the area of the heat exchanger, but increasing the flow rate will increase the resistance of the heat exchanger, increase the power consumption of the circulation pump and the cost of equipment, by increasing the flow rate It is not economical to obtain 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) Adopt thermal mixing board
The geometric structure of the corrugation on both sides of the hot-mixed plate is the same. The plate is divided into a hard plate and a soft plate according to the angle of the herringbone corrugation. Generally around 79 °) is a soft board. The surface heat transfer coefficient of the hot mixed plate hard plate is high, the fluid resistance is large, and the soft plate is the opposite. The combination of the hard plate and the soft plate can form a flow channel with three characteristics of high, medium and low to meet the requirements of different working conditions.
(2) Asymmetrical plate heat exchanger
Symmetrical plate heat exchangers are composed of plates with the same corrugated geometric structure on both sides of the plate, forming plate heat exchangers with equal circulation cross-sectional areas of cold and hot runners. Pressure drop requirements, change the structure of the wave on both sides of the plate, to form a plate heat exchanger with different cross-sectional areas of the cold and hot runners, the larger the diameter of the side of the wide runner, the heat transfer coefficient of the asymmetrical plate heat exchanger When the drop is small, and the pressure drop is greatly reduced, when the flow of cold and hot medium is relatively large, the heat exchanger using an asymmetric single process can reduce the plate area by 15% to 30% compared to a heat exchanger using a symmetric single process.
(3) Multi-process combination
When the flow of cold and hot medium is large, a combination of multiple processes can be used. The small flow side uses more processes to increase the flow rate and obtain a higher heat transfer coefficient. The large flow side uses a smaller process to reduce Heat exchanger resistance. Multi-flow combinations have mixed flow patterns, and the average heat transfer temperature difference is slightly lower. The fixed end plate and the movable end plate of the plate heat exchanger with multi-process combination are both taken over, and the workload is large during maintenance.
(4) With heat exchanger bypass
When the flow of cold and hot medium is relatively large, a bypass pipe can be provided between the heat exchanger outlets on the side of the large flow to reduce the process of entering the heat exchanger and reduce the resistance. For easy 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, to ensure that the temperature of the cold medium after the heat exchanger merges can meet the design requirements, and the heat exchanger bypass pipe can ensure that the heat exchanger has a high The heat transfer coefficient reduces the heat exchanger resistance, but the adjustment is slightly more complicated.
(5) The choice of plate heat exchanger
The average velocity of the medium in the flow channel between the heat exchanger plates should be 0.3-0.6m / s, and the resistance should not be more than 100kPa. According to the flow ratio of cold and hot media, different types of plate heat exchangers can be used for reference. Symmetrical plate heat exchangers have a flow channel cross-sectional area ratio of 2. Symmetrical or asymmetrical, single-flow or multi-flow plate heat exchangers can be used. Heat exchanger bypass tubes can be provided, but detailed thermal calculations should be performed.
The above is the method for improving the thermal efficiency of the plate heat exchanger introduced by Yuquan Environmental Editor. The plate side heat exchanger can be used to condense the primary side inlet medium and the secondary side inlet medium. The medium on one side is water or steam with higher temperature. After entering the heat exchanger, it is condensed by the heat exchanger to convert the high temperature water or water The temperature of the steam is condensed to the set output temperature, and the water whose temperature has been reduced is output from the pipeline for people to use. After people use it, after a round of circulation, it will come back through the secondary side and enter the heat exchanger to reach The purpose of the cycle is conducive to the recycling of energy without waste, so the heat exchanger is also environmentally friendly equipment.