Segmental Heat Transfer Algorithm Of Shell-and-tube Exchanger

At present,the common design and calculation method of shell-and-tube heat exchanger is to determine the physical properties and estimate the heat transfer coefficient according to the average temperature of the inlet and outlet of the shellside and tubeside.However,when phase change heat transfer occurred,this method may lead to wrong calculated results.Therefore,it is necessary to study the segmental heat transfer method.The temperature correction coefficient F=F(p,R)is generally used to correct the logarithmic average temperature difference in the thermodynamic calculation of multipass heat exchangers,This method is also not applicable to segmental calculation.In addition,in the process of heat transfer,the temperature cross occurs commonly in the last tubepass of the multipass heat exchanger,and then the calculation cannot continue,the real heat transfer data in the heat exchanger cannot be obtained,either.Therefore,this study carried out in-depth research of heat transfer in cross-flow heat exchanger and helical baffle heat exchanger from three parts: Heat transfer mechanism analysis,Numerical algorithm research,Programming and verification.(1)Heat transfer mechanism analysis: Introduce the boiling and condensation heat transfer zones in tubes,summarize the calculation methods of critical points in each zone,heat transfer coefficient and pressure drop correlation.The classical shell-side heat transfer models and shell-side heat transfer correlations of helical baffles were introduced.(2)Numerical algorithm: The segmental heat transfer algorithm was proposed and the static one-dimensional heat transfer calculation equation was established.Based on the differential method,the wall temperature equation was developed.The integral iteration differential heat transfer model of the cross-flow heat exchanger and helical baffle heat exchanger were established respectively,and the temperature cross problem of the multipass heat exchanger was studied.(3)Programming and verification: According to the algorithm,programs of cross-flow heat exchanger and helical baffle heat exchanger were developed by Fortran 95 language.The results of cross-flow heat exchanger calculated by the program were compared with those calculated by HTRI software.The heat transfer results of helical baffles heat exchanger were validated by the engineering data,and the heat transfer and resistance performance in each segment of heat exchanger were studied.For the thermodynamic calculation of multipass heat exchanger,The F factor correction for logarithmic mean temperature difference was not needed,due to the algorithm was based on the iteration of temperature in the end of each tube pass.The algorithm can do the correction computation after temperature crosses,so the problem caused by temperature crosses was also solved.The temperature and heat transfer data of each segment along the shell and tube in the heat exchanger can be obtained.The relative errors between the calculated results of the program and those calculated by HTRI software and the data obtained from the company were within 9% and 13%,respectively.The calculated value of the program was proved to have high accuracy,and it also provided detailed data for the design and optimization of industrial heat exchanger.