How to improve the efficiency 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 volume, light weight, low dirt coefficient, easy disassembly, many kinds of plate, wide application range, and has been widely used in the heating industry. The plate heat exchanger can be divided into detachable, welded, brazed and shell type according to the assembly method. Because the removable plate heat exchanger is easy to be disassembled and cleaned, and the area of the heat exchanger is flexible, it is widely used in heating engineering. The detachable 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 to improve the efficiency of detachable plate heat exchanger is studied.
Improving the efficiency of plate heat exchanger is a problem of comprehensive economic benefit, which should be determined after technical and economic comparison. In order to improve the heat transfer efficiency and reduce the resistance of the heat exchanger, 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.
Optimization design method of plate heat exchanger
Improve heat transfer efficiency
Plate heat exchanger is a kind of heat exchanger with wall to wall heat transfer. The cold and hot fluid transfers heat through the plate of heat exchanger, and the fluid contacts with the plate directly. The heat transfer mode is heat conduction and convection. The key to improve the heat transfer efficiency of plate heat exchanger is to increase the heat transfer coefficient and logarithmic mean temperature difference.
① In order to improve the heat transfer coefficient of the heat exchanger, it is necessary to increase the surface heat transfer coefficient of the cold and hot sides of the plate at the same time, reduce the thermal resistance of the fouling layer, select the plate with high thermal conductivity, and reduce the thickness of the plate, so as to effectively improve the heat transfer coefficient of the heat exchanger.
a. Improving the surface heat transfer coefficient of plate
Because the ripple of plate heat exchanger can make the fluid turbulent at a small flow rate, it can obtain a higher surface heat transfer coefficient, which is related to the geometry of the plate ripple and the flow state of the medium. The waveforms of the plate include herringbone, straight, spherical and so on. After years of research and experiments, it is found that the shape of corrugated section is triangle (sinusoidal shape, the surface heat transfer coefficient is the largest, the pressure drop is small, and the stress distribution is uniform under pressure, but it is difficult to process?) The larger the angle between the corrugations is, the higher the flow velocity is, and the higher the surface heat transfer coefficient is.
b. Reduce the thermal resistance of fouling layer
The key to reduce the thermal resistance of fouling layer of heat exchanger is to prevent the plate from scaling. When the thickness of fouling is 1 mm, the heat transfer coefficient decreases about 10%. Therefore, it is necessary to monitor the water quality on both sides of the heat exchanger to prevent the plate from scaling and prevent the water debris from adhering to the plate. In order to prevent water stealing and corrosion of steel parts, some heating units add chemicals into the heating medium. Therefore, it is necessary to pay attention to the contamination of heat exchanger plates caused by water quality and stickies. If there are viscous impurities in the water, special filters should be used for treatment. When choosing the medicament, it is better to choose the medicament without viscosity.
c. Select the plate with high thermal conductivity
Plate material can choose austenitic stainless steel, titanium alloy, copper alloy, etc. Stainless steel has good thermal conductivity, about 14.4w / (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 engineering, but its resistance to chloride ion corrosion is poor.
d. Reduce the plate thickness
The design thickness of the plate is not related to its corrosion resistance, but to the pressure bearing capacity of the heat exchanger. Plate thickening can improve the pressure bearing capacity of heat exchanger. When the herringbone plate combination is adopted, the adjacent plates are inverted and the corrugations contact each other, forming the fulcrum with high density and uniform distribution. The plate angle and edge sealing structure have been gradually improved, which makes the heat exchanger have good pressure bearing capacity. The maximum pressure bearing capacity of domestic detachable plate heat exchanger has reached 2.5MPa. The thickness of the plate has a great influence on the heat transfer coefficient. When the thickness decreases by 0.1 mm, the total heat transfer coefficient of the symmetrical plate heat exchanger increases by 600 W / (m · K), and that of the asymmetric plate heat exchanger increases by 500 W / (m · K). On the premise of meeting the pressure bearing capacity of heat exchanger, the plate thickness should be as small as possible.
② Increase logarithmic mean temperature difference
The flow patterns of plate heat exchanger are counter flow, forward flow and mixed flow (both counter flow and forward flow). Under the same conditions, the logarithm mean temperature difference is the largest in countercurrent flow and the smallest in downstream flow. The method to increase the logarithm mean temperature difference of heat exchanger is to adopt counter current or near counter current mixed flow pattern as far as possible, increase the temperature of hot side fluid as far as possible, and reduce the temperature of cold side fluid.
③ Determination of inlet and outlet pipe position
For the plate heat exchanger with single flow 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 larger the temperature difference of the medium is, the stronger the natural convection of the fluid is, and the more obvious the influence of the stagnant zone is. Therefore, the inlet and outlet of the medium should be arranged according to the arrangement of 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 stagnant zone and improve the heat transfer efficiency.
Methods to reduce the resistance of heat exchanger
Increasing the average velocity of the medium in the inter plate channel can improve 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, 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, and it is uneconomical to obtain slightly higher heat transfer coefficient by increasing the flow rate. When the flow rate of cold and hot 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
According to the angle of herringbone wave, the plate can be divided into hard plate (H) and soft plate (L), and the angle of herringbone wave is 120. About 90. It is a hard board with an included angle (generally 70. About 90. It is a soft board. The results show that the surface heat transfer coefficient of the hard plate is high and the fluid resistance is high, while that of the soft plate is on the contrary. The combination of hard plate and soft plate can form three kinds of flow channels with high (Hh), medium (HL) and low (LL) characteristics to meet the requirements of different working conditions.
When the flow rate of hot and cold medium is large, the plate area of the heat exchanger with hot mixing plate is smaller than that with symmetrical single flow. The diameter of corner holes on both sides of hot and cold mixing plate is usually equal. When the flow ratio of hot and cold medium is too large, the pressure loss of corner holes on one side of cold medium is large. In addition, it is difficult to achieve accurate matching of hot mix plate design technology, which often leads to limited plate area. Therefore, it is not suitable to use the hot mixing plate when the flow ratio of cold to hot medium is too large.
② Asymmetric plate heat exchanger is adopted
The symmetrical plate heat exchanger is composed of plates with the same corrugated geometry on both sides of the plate, forming a plate heat exchanger with equal cross-sectional area of cold and hot flow channels. According to the requirements of heat transfer characteristics and pressure drop of cold and hot fluid, the asymmetric plate heat exchanger (unequal cross-section type) changes the wave shape geometry on both sides of the plate to form a plate heat exchanger with unequal cross-section areas of cold and hot flow channels. The angle l diameter on one side of the wide flow channel is larger. The heat transfer coefficient of asymmetric plate heat exchanger decreases slightly, and the pressure drop decreases greatly. When the flow rate of hot and cold medium is large, the plate area of the heat exchanger with asymmetric single flow can be reduced by 15% – 30% compared with that with symmetric single flow.
③ Adopt multi process combination
When the flow rate of hot and cold medium is large, multi flow combination arrangement can be used, and more flow can be used on the side with small flow rate to improve the flow rate and obtain higher heat transfer coefficient. In order to reduce the resistance of the heat exchanger, less flow is used on the high flow side. The results show that 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 movable end plate of plate heat exchanger with multi process combination have connecting pipes, so the maintenance work is heavy.
④ By pass pipe of heat exchanger
When the flow rate of hot and cold medium is relatively large, a bypass pipe can be set between the inlet and outlet of the heat exchanger on the side with large flow rate to reduce the flow rate into the heat exchanger and reduce the resistance. In order to facilitate adjustment, a regulating valve should be installed on the bypass pipe. In this way, counter current arrangement should be adopted to make the temperature of cold medium out of the heat exchanger higher, so as to ensure that the temperature of cold medium after combined flow at the outlet of the heat exchanger can meet the design requirements. The bypass tube of heat exchanger can ensure high heat transfer coefficient of heat exchanger and reduce resistance of heat exchanger, but the adjustment is a little complicated.
⑤ Selection of plate heat exchanger
The average velocity of medium in the channel between plates of heat exchanger should be 0.3 ～ 0.6m/s, and the resistance should be no more than 100KPA. According to different flow ratio of cold and hot medium, different types of plate heat exchanger can be selected according to table 1. The cross-sectional area ratio of asymmetric plate heat exchanger in the table is 2. When using symmetrical or asymmetrical plate heat exchanger, single flow or multi flow plate heat exchanger, the bypass pipe of heat exchanger can be set, but the detailed thermal calculation should be carried out.
Material and installation method of rubber gasket
① Material selection
In the water water heat exchanger, the cold and hot medium have no corrosion to the rubber gasket. The key to select the rubber gasket material is the temperature resistance sealing performance, and the rubber gasket material can be selected according to the literature.
② Selection of installation mode
The common installation methods of rubber gasket are adhesive type and snap type. Adhesive type is to glue the rubber gasket in the plate sealing groove during the assembly of heat exchanger. Snap type is to fix the rubber gasket in the plate sealing groove by using the snap structure of rubber gasket and plate edge when assembling the heat exchanger. Because the workload of snap fit installation is very small, the damage rate of rubber gasket is low when the heat exchanger is disassembled, and there is no chloride ion in the glue that may cause corrosion to the plate, so it is widely used.
Reasonable selection of plate material
Pitting corrosion, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion and so on may cause corrosion failure of stainless steel plate, and the occurrence rate of stress corrosion is high.
Source: China Heat Exchanger Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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