How to improve heat exchange efficiency of plate heat exchanger & reduce thermal resistance
Frist Improve heat transfer efficiency
Plate heat exchanger is a wall heat transfer heat exchanger, the cold fluid heat transfer through the heat exchanger plate, the fluid is in direct contact with the plate, the heat transfer mode is heat conduction and convection heat transfer. The key to improving the heat transfer efficiency of plate heat exchangers is to increase the heat transfer coefficient and the logarithmic mean temperature difference.
(1) increase heat transfer coefficient of heat exchanger
Only improve the surface thermal coefficient on both sides of the plate cold and hot, reduce the thermal resistance of the scale layer, select the plate with high thermal conductivity, reduce the thickness of the plate, can effectively improve the heat transfer Heat transfer coefficient of the device.
1 increase the surface heat transfer coefficient of the plate
Because the corrugated plate heat exchanger can cause turbulence of fluid at a small flow rate, it can obtain a high surface heat transfer coefficient, surface heat transfer coefficient and the geometry of the plate corrugation The flow state of the medium is related. The waveform of the plate includes a herringbone shape, a straight shape, a spherical shape, and the like. After years of research and experiments, it is found that the herringbone plate with triangular shape of corrugation has a high surface heat transfer coefficient, and the larger the angle of the corrugation, the higher the medium flow velocity in the flow channel between the plates, the greater the surface heat transfer coefficient. .
2 reduce the thermal resistance of the fouling layer
The key to reducing the thermal resistance of the fouling layer of the heat exchanger is to prevent the structure of the plate. When the thickness of the sheet structure is 1 mm, the heat transfer coefficient is reduced by about 10%. Therefore, it is necessary to pay attention to monitoring the water quality at both ends of the heat exchanger, preventing the structure of the plate and preventing the inclusion of impurities in the water on the plate. Some heating units add chemicals to the heating medium in order to prevent water theft and corrosion of steel parts. Therefore, it is necessary to pay attention to the water and viscous agents causing debris to contaminate the heat exchanger plates. If there is viscous debris in the water, it should be treated with a special filter. When using a drug, it is advisable to choose a non-stick agent.
3Select a plate with high thermal conductivity
Plate material can choose austenitic stainless steel, titanium alloy, steel alloy and so on. Stainless steel has good thermal conductivity, thermal conductivity of about 14.4W/(mk), high strength, good stamping performance, and is not easily oxidized. The price is lower than that of titanium alloy and copper alloy, but its resistance to chloride ion corrosion is poor.
4Reduce 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 plate is thickened to improve the pressure bearing capacity of the heat exchanger. When the herringbone plate is used, the adjacent plates are inverted and the corrugations are in contact with each other, forming a dense and uniform distribution finger. The plate corner and edge sealing structure have been gradually improved, so that the heat exchanger has a good pressure. ability. Under the premise of satisfying the pressure bearing capacity of the heat exchanger, a smaller thickness of the plate should be selected as much as possible.
(2) Improve the logarithmic mean temperature difference
Plate heat exchanger flow patterns have countercurrent, downstream and mixed flow patterns. Under the same working conditions, the logarithmic mean temperature difference is large in the countercurrent, small in the downstream, and the mixed flow is in between. The method of increasing the logarithmic mean temperature difference of the heat exchanger is to adopt a mixed flow pattern of countercurrent or near countercurrent as much as possible, to increase the temperature of the hot side fluid as much as possible, and to reduce the temperature of the cold side fluid.
(3) Determination of the locations of the inlet and outlet pipes
For a single-flow plate heat exchanger, for the convenience of maintenance, the fluid inlet and outlet pipes should be placed on the fixed end side 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 formed retention zone. Therefore, the inlet and outlet of the medium should be moved in and out according to the hot fluid, and the cold fluid is placed in and out to reduce the influence of the retention zone. Improve heat transfer efficiency.
Second, method of reducing heat exchanger resistance
Increase the average flow rate of the medium in the flow channel between the plates, can improve the heat transfer coefficient and reduce the heat exchanger area. However, increasing the flow rate will increase the resistance of the heat exchanger, increase the power consumption of the circulating pump and the cost of the equipment, and it is uneconomical to obtain a slightly higher heat transfer coefficient by increasing the flow rate. When the flow rate of the hot and cold 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.
(1)Using a hot mixing plate
The plate of the hot mixing plate has the same corrugated geometry on both sides, and the plate is divided into a hard plate and a soft plate according to the angle of the herringbone corrugation, the angle is greater than 90 & deg; (generally 120 & deg; left and right) is hard Plate, the angle is less than 90 & deg; (generally 79 & deg; left and right) is a soft board. The surface of the hot-mixed plate has a high heat transfer coefficient, a large fluid resistance, and a soft plate. The hard board and the soft board are combined to form a flow path with high, medium and low characteristics to meet the requirements of different working conditions.
When the flow of hot and cold medium is relatively large, the use of a hot mixing plate can reduce the area of the plate compared to a heat exchanger with a symmetrical single process. The diameters of the corner holes on both sides of the hot and cold plates are generally equal. When the flow ratio of the hot and cold medium is too large, the pressure loss on the side of the cold medium is large. In addition, the hot mix plate design technology is difficult to achieve accurate matching, often resulting in limited plate area. Therefore, it is not advisable to use a hot mixing plate when the flow ratio of the hot and cold medium is too large.
(2) using asymmetric plate heat exchanger
Symmetrical plate heat exchanger consists of plates with the same corrugated geometry on both sides of the plate, forming a plate heat exchanger with the same cross-sectional area of the hot and cold runners. Asymmetric plate heat exchangers change the structure of the two sides of the plate according to the heat transfer characteristics and pressure drop requirements of the hot and cold fluids, forming a plate heat exchanger with different cross-sectional areas of hot and cold flow channels, and one side of the wide flow channel. The angular diameter is large. The heat transfer coefficient of the asymmetric plate heat exchanger is reduced slightly, and the pressure drop is greatly reduced. When the flow rate of the hot and cold medium is relatively large, the asymmetric single flow can reduce the plate area by 15% — 30% compared with the symmetric single flow heat exchanger.
When the flow of hot and cold medium is large, multi-flow combination arrangement can be adopted, and more flow is adopted on the small flow side to increase the flow rate and obtain a higher heat transfer coefficient. A smaller flow is used on the side of the large flow to reduce heat exchanger resistance. The mixed flow pattern appears in the multi-flow combination, and the average heat transfer temperature difference is slightly lower. The fixed end plate and the movable end plate of the plate heat exchanger adopting the multi-process combination are taken over, and the workload is large during maintenance.
(4) Set the heat exchanger bypass pipe
When the flow rate of the hot and cold medium is relatively large, a bypass pipe can be arranged between the outlets of the heat exchanger on the large flow side to reduce the flow into the heat exchanger and reduce the resistance. For ease of adjustment, a regulating valve should be installed on the bypass pipe. The method should adopt a reverse flow arrangement to make the temperature of the cold medium heat exchanger higher, and ensure that the temperature of the cold medium after the heat exchanger outlet is merged can meet the design requirements. The heat exchanger bypass pipe can ensure the heat transfer coefficient of the heat exchanger and reduce the heat exchanger resistance, but the adjustment is slightly complicated.
(5) Choice of plate heat exchanger form
The average flow velocity of the medium in the flow path between the heat exchanger plates is preferably 0.3 & mdash; 0.6 m / s, and the resistance is preferably not more than 100 kPa. According to different flow ratioses of cold and heat medium, different types of plate heat exchangers can be selected, and the cross-sectional area ratio of the asymmetric plate heat exchanger is 2. Heat exchanger bypass pipes can be used with symmetrical or asymmetrical, single-flow or multi-flow plate heat exchangers, but detailed thermal calculations should be performed.