Modeling And Heat Analysis Of Vertical Tube Falling Film Evaporator

Falling film evaporator is widely used in pharmaceutical,chemical,food,light industry and other industries.It has many advantages,such as strong evaporation ability,high heat transfer efficiency,low energy consumption,low operating cost and so on.At the same time,the equipment is operated continuously under the condition of vacuum and low temperature,and it can ensure the invariability of the material in the evaporation process,especially for the heat sensitive material.In the design of falling film evaporator,the calculation of heat transfer coefficient is very important.However,compared with the complexity of its internal flow and heat transfer,the existing engineering design and application still rely on empirical values,so the theoretical research needs to be more in-depth.First of all,the heat and mass transfer process of falling film evaporator involves evaporation and boiling.Compared with no phase change heat transfer process,the theoretical support is less,and the mathematical model established by numerical simulation is simplified too much which is far from the real situation.So it is necessary to study the basic theory of phase change heat transfer.Secondly,the evaporation boiling process is not only closely related to the structural parameters,but also closely related to the physical properties and operating parameters of the working fluid used.Therefore,it is necessary to carry out experimental research and mathematical modeling for specific working fluids under practical operating conditions.Combining the theoretical,experimental,and numerical simulation results with a computerized system analysis will benefit the application and optimization of falling film evaporators.In this paper,the mathematical model of the liquid film flow inside and outside the falling film evaporator was established.The heat transfer model was used to analyze the heat transfer mechanism.The total heat transfer coefficient and heat transfer capacity were obtained,and the results were compared with the experiment which provided basic data and theoretical guidance for heat transfer performance prediction and operation optimization of evaporator.Firstly,a physical model was established for the heat transfer process that the saturated ethanol water liquid in the tube was heated by shell saturated steam in falling film evaporator.A mathematical model of heat transfer was established for the fluid flow inside and outside the tube.According to the flow of condensate out of the tube,the thickness of the liquid film at different axial positions and the total condensate were obtained.The velocity distribution and temperature distribution of the condensate were obtained by numerical simulation with MATLAB.Considering the influence of different composition of liquid film on the heat transfer during the flow process,the average thickness,velocity distribution,temperature distribution and composition distribution of the liquid film in the tube were obtainedby numerical simulation.At the same time,this study used a self-designed,installed and commissioned vertical tube falling film evaporator small test device as the research object.The phase variables and the corresponding temperatureinside and outside the evaporator tube were obtained under different operating conditions such as pressure,ethanol content,feed flow rate and saturated steam volume.And the ethanol content of the evaporated gas in the tube was measured.In order to adapt to the engineering application,combined with the experimental conditions and the physical and chemical properties of the raw material,a quasi-correlation equation of the phase-to-phase heat transfer coefficient of the evaporation tube was fitted and brought into the heat transfer model.The formula for calculating the total heat transfer coefficient of falling film evaporator was obtained.The total heat transfer coefficient and heat transfer quantity in the evaporative pipe were calculated,and the results showed that the total heat transfer coefficient of the equipment increased obviously with the increase of feed flow and ethanol content in the device tube,but the total heat transfer coefficient decreased with the increase of heat transfer temperature difference inside and outside the tube.The total heat transfer of the device increased gradually with the increase of feed flow rate,ethanol content and temperature difference inside and outside the tube.The change of saturated water vapor had no significant effect on the total heat transfer coefficient and heat transfer.Comparing the total heat transfer obtained by the calculation and experiment,the trend of change under each operating variable was similar and the errors obtained were within 18%.