Temperature Design Requirements of Shell-and-tube Heat Exchanger and Selection of Heat Exchange Tubes

- Jun 01, 2018-

When designing a shell-and-tube heat exchanger, consider the effect of temperature on its performance and pay special attention to this. Under normal circumstances, the cooling water outlet temperature should be controlled within 60 °C to prevent serious scaling. Moreover, the temperature difference at the high temperature end of the shell-and-tube heat exchanger must not be lower than 20°C, and the temperature difference at the low temperature end should not be less than 5°C.


It should be noted that when using a shell-and-tube heat exchanger, if the heat exchange between the two process fluids occurs, the temperature difference at the low temperature end should not be less than 20°C. If multi-pass, single-shell heat exchanger equipment is used and water is used as coolant, the coolant outlet temperature must not exceed the process stream outlet temperature. Moreover, when cooling or condensing the process stream, the population temperature of the coolant should be higher than the freezing point of the frozen components in the process fluid, which is generally higher than 5°C.


In addition, when designing, the design temperature of the shell-and-tube heat exchanger should be greater than its maximum use temperature, and the general requirement is 15°C higher. At the same time, the choice of heat exchange tubes in this heat exchanger is also very important. Heat exchange tubes can use light tubes, threaded tubes, spiral grooved tubes, etc. However, the choice of pipe size must meet the relevant requirements.


First, the diameter of the heat exchange tubes used in shell-and-tube heat exchangers must be reasonable. Under normal circumstances, the smaller the diameter of the heat exchanger is, the more compact and cheaper it is, and the better the ratio of convective heat transfer coefficient and drag coefficient can be obtained. However, the smaller the pipe diameter, it may also cause its pressure drop to increase. Therefore, in the case of ensuring that the pressure drop is appropriate, it is recommended to choose 19 pipes.


Second, we must pay attention to reasonable control of the length of the tube, the arrangement of the tubes and the distance between the cores. For shell-and-tube heat exchangers, when there is no phase change heat, the heat transfer coefficient increases when the tube is longer. In the case of the same heat transfer area, the use of a long tube results in a small flow cross-sectional area, a large flow rate, and a small number of tube passes. The tube spacing is the distance between the centers of two adjacent tubes in a shell-and-tube heat exchanger. The smaller the pipe spacing, the more compact the equipment. The general range is (1.25 ~ 1.5) d0, and d0 is the outer diameter of the pipe.