Heat exchanger is to transfer part of the heat of hot fluid to cold fluid equipment, also known as heat exchanger. Heat exchanger plays an important role in chemical industry, petroleum, power, food and many other industrial production. In chemical production, heat exchanger can be used as heater, cooler, condenser, evaporator and reboiler. How to improve the heat exchange efficiency in the application of heat exchanger? What factors will affect it
Increase the logarithmic mean temperature difference
The flow patterns of plate heat exchanger include counter current, forward flow and mixed flow (both counter current and forward flow). Under the same 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 improve the logarithm mean temperature difference of heat exchanger is to adopt the mixed flow pattern of countercurrent or close to countercurrent as far as possible, increase the temperature of hot side fluid as much as possible, and reduce the temperature of cold side fluid.
Determination of inlet and outlet pipe position
For the plate heat exchanger with single flow arrangement, the inlet and outlet pipes of fluid should be arranged on the fixed end plate side of the heat exchanger for the convenience of maintenance. 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 retention zone is. Therefore, the location of the medium inlet and outlet should be arranged according to the upper and lower inlet and lower outlet of the hot fluid, so as to reduce the influence of the retention zone and improve the heat transfer efficiency.
To improve the heat transfer efficiency, the plate heat exchanger is a wall heat exchanger. The cold and hot fluid transfer heat through the plates of the heat exchanger, and the fluid contacts with the plate directly. The heat transfer mode is heat conduction and convection heat transfer. The key to improve the effect of plate heat exchanger is to improve the heat transfer coefficient and logarithmic average temperature difference.
To improve the heat transfer coefficient of heat exchanger, the heat transfer coefficient of heat exchanger can be effectively improved only by increasing the surface heat transfer coefficient on both 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.
a. Improving the surface heat transfer coefficient of plate
Because the ripple of plate heat exchanger can make the fluid produce turbulence at low velocity (Reynolds number – 150), it can obtain higher surface heat transfer coefficient. The surface heat transfer coefficient is related to the geometry structure of plate ripple and the flow state of medium.
The wave shape of the plate includes herringbone, flat shape and spherical shape. After years of research and experiment, it is found that the shape of corrugated cross-section is triangular (sinusoidal shape, the surface heat transfer coefficient is the largest, the pressure drop is small, and the stress distribution is uniform under pressure, but the processing difficulty of herringbone plate has higher surface heat transfer coefficient, and the greater the angle of ripple, the higher the medium flow velocity in the channel between plates, the greater the surface heat transfer coefficient.
b. Reduce the thermal resistance of dirt layer
The key to reduce the thermal resistance of fouling layer in heat exchanger is to prevent the plate from scaling. 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 impurities from adhering to the plate.
In order to prevent water stealing and steel corrosion, some heating units add chemicals to the heating medium. Therefore, attention must be paid to the contamination of heat exchanger plates by water quality and viscous chemicals. If there is viscous debris in the water, it should be treated with a special filter. When choosing the medicament, it is better to choose the non viscous medicament.
c. Select the plate with high thermal conductivity
The plate material can be austenitic stainless steel, titanium alloy, copper alloy, etc. Stainless steel has good thermal conductivity, thermal conductivity of 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 widely used in heat supply engineering, but its ability to resist chloride ion corrosion is poor.
d. Reduce plate thickness
The design thickness of the plate has nothing to do with its corrosion resistance, but has something to do with 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 are in contact with each other, forming a fulcrum with high density and uniform distribution. The plate corner piece L and edge sealing structure have been gradually improved, which makes the heat exchanger have a good pressure bearing capacity.
The maximum pressure bearing capacity of the removable plate heat exchanger made in China has reached 2.5MPa. The thickness of the plate has a great influence on the heat transfer coefficient. The thickness of the plate 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 500W / (M? K).
On the premise of meeting the pressure bearing capacity of heat exchanger, the smaller plate thickness should be selected as far as possible.