Heat Transfer Process Enhancement And Optimization Design For LNG Cold Energy Utilization Heat Exchanger

The rapid development of LNG industry is the internal requirement for China to reduce energy consumption per unit GDP,alleviate environmental pollution and improve energy structure,and low-temperature enhanced heat transfer of LNG heat exchanger is the key to reduce the cost of LNG production and improve the efficiency of cold energy utilization.At present,there exists too many problems,just like the problems of low adaptability of LNG low-temperature heat exchange medium,low heat transfer efficiency,high cost and difficult control,the scope of LNG cold energy recycling is greatly limited,and its effect is not ideal.Adding non-condensable gas to the low temperature heat transfer medium can effectively enhance its adaptability,but also affect its condensation heat transfer process.Therefore,it is of great theoretical and practical significance to study the performance of the heat exchange system between the non-condensable gas refrigerant and LNG and to explore the enhancement of heat transfer technology to improve its condensation performance.In this paper,FLUENT software is used to simulate the propane refrigerant heat transfer system with non-condensable gas.The simulation results show that the film thickness of the thermal boundary layer increases with the increase of the concentration of the non-condensable gas,but when the concentration increases to 3%（volume percentage）,the effect of the concentration on the film begins to slow down;the lower the wall temperature is,the thicker the gas film is.The thickness of the gas film increases 46%when the wall temperature is 120K compared with 200K;when the speed increases,the thickness of the gas film decreases,but when the speed is more than 1.5m/s,the effect of the speed on the gas film begins to slow down,the impact is no longer significant.Using the data obtained from the simulation,the condensation heat transfer coefficients of the refrigerant C1-C4 system,N₂-C4system,C1-C3 system and N₂-C3 system containing non condensable gas are fitted,and a series of correlation equations are obtained,which can provide reference for the design of this kind of heat exchanger.According to the heat transfer process of propane with nitrogen,physical and mathematical models are established to study the heat transfer and flow characteristics of the fluid in smooth circular tube and spiral flat tube.The results show that with the increase of Re,the degree of turbulence increases and the heat transfer coefficient increases.At the same time,it is found that the condensation heat transfer coefficient of the spiral flat tube is 1.3¹.4 times that of the smooth round tube.In order to verify the reliability of simulation results of heat transfer between propane refrigerant and LNG in spiral flat tube,an experimental platform for heat transfer of spiral flat tube was established to study the heat transfer between steam with air and water,air and water,oil and water,which exist in and out of the spiral flat tube.Comparing the experimental results with the simulation data,the results show that for different working pairs,the heat transfer efficiency of the spiral flat tube is higher than that of the smooth tube,and the heat transfer enhancement factor is about 1.18¹.42.Through the simulation and optimization analysis of the heat transfer process of the refrigerant system and heat exchange equipment,a new type technology of propane refrigerant containing N₂ is designed,and the spiral flat tube is used as the heat exchange tube to enhance the heat exchange process.The feasibility of the process scheme is analyzed by the mixed process,which contains between enhanced heat exchange process and the cold energy utilization system of LNG satellite station.The gasification capacity of the LNG satellite station is 4000²0000Nm3/h,the of gasification is 3⁵atm,and the temperature of gasification is-162℃¹5℃.Compared the new propane system containing N₂ with the original R404A system in terms of operation safety performance,monitoring sensitivity,manufacturing cost and operation cost,the results show that the new system has great improvement in operation safety and monitoring sensitivity compared with the original system.At the same time,compared with the original R404A system,the manufacturing cost and operation cost of the new system are reduced.The whole system can save about 39%of the investment and 44%of the annual power operation cost.The research results of this paper can provide reference for the design and manufacture of LNG low temperature heat exchange equipment and the optimization and upgrading of LNG cold energy utilization system.