Experimental Investigation Of Heat Transfer Characteristics Of Natural Gas Flow Condensation In Shell And Tube Heat Exchanger With Helical Baffles

Shell and tube heat exchanger is widely used as the precooling condenser in natural gas liquefaction plants.Shell and tube heat exchanger is the key equipment in the liquefaction process.In natural gas precooling process,low temperature sea water flows in tube side,natural gas refrigerant flows in shell side.In the shell side of heat exchanger,baffles are used to increase flow velocity and enhance flow disturbance so as to enhance shell side heat transfer.Comparing the typical segmental baffles,helical baffles can effectively reduce pressure drop,minimize flow dead zone and alleviate vibration.Hense shell and tube heat exchanger with helical baffles are usually used as the LNG sea-water precooling condersor.In view of mixed refrigerant and pure refrigerant heat transfer characteristic differences,the study on shell side flow condensation of shell and tube heat exchanger with helical baffles has the vital significance on optimize heat exchanger structure as well as prevent design failure.The experimental study and numerical simulation were performed for investigating the two phase flow condensation characteristics of natural gas refrigerant in shell side of shell and tube heat exchanger with helical baffles.The main research contents in the present study are introduced as follows:Firstly,experimental apparatus for studying flow condensation in shell side of shell and tube heat exchanger with helical baffles were built.The experimental apparatus can control the influence of inlet vapor quality,heat flux and mass flux of test section,which has the similar operating conditions to actual natural gas liquefaction plants.The experimental test sample is a horizontal type helical baffle shell and tube heat exchanger.Structure parameters of the test section,such as tube diameter,tube layout and helical baffle angle are based on actual sea water condensers.High pressure sight glass was installed on the shell to observe the flow patterns of the test section.Through experimental data processing and error analysis,it is concluded that the relative error of heat transfer coefficient is less than ±10.5%.Secondly,in the experimental process,droplet mode flow,droplet-column mode flow,and column mode flow was observed with the increasing of mass flux and vapor quality.The experimental study on investigation of pure propane and ethane/propane(0.5/0.5)mixture flow condensation on shell side of helical baffle shell and tube heat exchanger was conducted.The influence of vapor quality,heat flux and mass flux on heat transfer characteristics of condensation in shell side is analyzed.Under experimental operating condition,the influence of vapor quality,heat flux and mass flux on heat transfer characteristics for two kinds of refrigerant has similarities.The heat transfer coefficient in shell side initially increases and then decreases with the increasing of vapor quality,and it reaches up to highest as vapor quality is 0.9.The heat transfer coefficient in shell side increases with the increasing of heat flux and increases rate gradually smaller.The heat transfer coefficient in shell side increases with the increasing of mass flux.Compared with the pure propane,ethane/propane mixture heat transfer coefficient decreases 35%~65%.Furthermore,new correlations based on existing heat transfer correlations of flow condensation on tube bundle were proposed respectively for pure propane and ethane/propane mixture for flow condensation in shell side of heat exchanger with helical baffles.The new correlations can agree with the 95% propane experiment data within deviation of ±20% and agree with the 92% ethane/propane mixture experiment data.Finally,for better investigation of the gas-liquid distribution in shell side of helical baffle heat exchanger,CFD method is used to simulate the shell side two-phase flow.VOF multi-phase model is chosen as the calculation model.And the gas-liquid distribution,path line,velocity vector diagrams in shell side is figured out under experimental conditions.The simulated flow patterns are consistent with the observed ones in experiment.