Shandong heat exchanger maintenance 2. structural design process 2.1 selection steps of tubular heat exchanger: which fluid flows through the tube side of the heat exchanger and which fluid flows through the shell side, the following points can be used for reference (take fixed tube plate heat exchanger as an example)（ 1) The fluid that is unclean and easy to scale should be taken into the pipe to facilitate the cleaning of the pipe（ 2) Corrosive fluid should be taken into the pipe to avoid the shell and pipe being corroded at the same time, and the pipe is easy to clean and repair（ 3) The fluid with high pressure should be taken into the pipe to avoid the shell being under pressure. The flow rate can be increased by using multiple tube passes（ 4) For the liquid with high viscosity or small flow rate, it is advisable to walk between pipes. Because of the constant change of flow velocity and flow direction when the fluid flows in shell with baffle plate, turbulence can be achieved at low Re (re>100) to improve the convective heat transfer coefficient. When choosing the flow diameter, the pressure, corrosion prevention and cleaning requirements of the fluid are considered in the maintenance of the heat exchanger in Shandong Province, and then the convective heat transfer coefficient and pressure drop are checked. The design takes oil and circulating cooling water as the heat transfer medium. The water goes through the pipe and the oil goes through the shell. Because of the high pressure of water, the circulating cooling water is easy to scale and the flow rate needs to be increased. In order to facilitate the cleaning of scale, the circulating water should be taken into account in the pipeline and soybean oil shell. 2.2 the choice of fluid velocity increases the flow rate of the fluid in the heat exchanger, which will increase the convection heat transfer coefficient and reduce the possibility of fouling deposition on the pipe surface, that is, reduce the fouling thermal resistance and increase the total heat transfer coefficient, thus reducing the heat transfer surface product of small heat exchanger maintenance in Shandong heat exchanger. But the increase of velocity increases the resistance of fluid and the power consumption increases. Therefore, the appropriate velocity can be determined by economic balance. In addition, when choosing the flow rate, the structural requirements should be considered: choosing high velocity, reducing the number of tubes, and having to adopt longer tubes or increase the number of passes for a certain heat transfer area. The pipe is too long to be cleaned easily, and the average temperature difference will decrease when the single pass becomes multi pass. Because of the design of this heat exchanger, the total heat load is small, and it does not need too high convective heat transfer coefficient. Oil and water are liquid, and the decrease of average temperature affects the heat exchange, so 0.5m/s is selected in the common flow rate. 2.3 if the temperature of the cold and hot fluid in the heat exchanger is specified by the process conditions, there is no problem of determining the temperature at both ends of the fluid. If one of the fluids is known only of the inlet temperature, the outlet temperature shall be determined by the designer. For example, cooling a hot fluid with cold water, the inlet temperature of cold water can be estimated according to the local temperature conditions, and the cold water temperature at the outlet of the heat exchanger needs to be determined according to economic balance. In order to save water, the outlet temperature of water can be increased in Shandong heat exchanger maintenance, but the heat transfer area needs to be increased; In order to reduce the heat transfer area, the water quantity should be increased. The two are contradictory. The operating conditions of the design task book of chemical principles course give the temperature of the cold and hot fluid in the heat exchanger, so there is no problem of determining the temperature of both ends of the fluid. 2.4 when selecting pipe diameter, the flow rate shall be higher as possible, but generally it shall not exceed the flow rate range described above. The liquid with easy scaling and high viscosity should be of large pipe diameter for Shandong heat exchanger maintenance. In the current trial series standards of tubular heat exchanger in China, only oil and circulating cooling water are used as heat transfer medium. The water goes through the pipe and the oil goes through the shell. Because of the high pressure of water, the circulating cooling water is easy to scale and the flow rate needs to be improved. In order to facilitate the cleaning of scale, the circulating water should be taken into account in the pipeline and soybean oil shell. 2.2 the choice of fluid velocity increases the flow rate of the fluid in the heat exchanger, which will increase the convection heat transfer coefficient and reduce the possibility of fouling deposition on the surface of the pipe, that is, the fouling thermal resistance is reduced and the total heat transfer coefficient increases, thus reducing the heat transfer area of the heat exchanger. But the increase of velocity increases the resistance of fluid and the power consumption increases. Therefore, the appropriate velocity can be determined by economic balance. In addition, when choosing the flow rate, the structural requirements should be considered: choosing high velocity, reducing the number of tubes, and having to adopt longer tubes or increase the number of passes for a certain heat transfer area. The pipe is too long to be cleaned easily, and the average temperature difference will decrease when the single pass becomes multi pass. Because of the design of this heat exchanger, the total heat load is small, and it does not need too high convective heat transfer coefficient. The maintenance oil and water of Shandong heat exchanger are liquid, and the decrease of average temperature affects the heat exchange, so 0.5m/s is selected in the common flow rate.