Entransy Analysis And Optimization For Horizontal-tube Falling Film Evaporator

The horizontal-tube falling film evaporator is major equipment in LT-MEE desalination system.It has the characteristics of compact structure,high heat transfer coefficient,good resistance to causticity and adaptability for small temperature difference heat transfer.Based on the second law of thermodynamics and entropy generation analysis,scholars had made a lot of research on the thermodynamic performance of LT-MEE desalination system.In recent years,the entransy dissipation analysis and the thermal resistance method were conceived,which emphasized the loss of heat transfer capacity of energy.Based on the theory of entransy dissipation,scholars had proposed relevant analysis methods for heat exchanger and heat exchange network,but there are few literatures about the analysis of entransy dissipation of heat exchanger with phase transformation in both cold and hot fluids.In this paper,the calculation program was compiled,and the evaporator was simulated and calculated by distributed parameter method.Based on calculation results,the calculation method of entransy dissipation in the preheating section and evaporation section of horizontal-tube falling film evaporator was proposed,and the influence of viscous heat on entransy dissipation was discussed.It is found that the preheating section is the main area where the loss of heat transfer capacity produces in evaporator,spraying density and temperature of feed seawater have a great influence on entransy dissipation of preheating section.Reducing the loss of heat transfer capacity and entransy dissipation in preheating section can improve the performance of evaporator.The heat transfer temperature difference of horizontal-tube falling film evaporator is small,thus subtle change of phase transition temperature of cold and hot fluids can't be ignored.The application of weighted equivalent temperature difference based on entransy dissipation in the performance analysis of horizontal-tube falling film evaporator was studied.The ratio of weighted equivalent temperature difference to apparent heat transfer temperature difference was defined as the parameter describing the overall thermal loss of evaporator.Based on concept of weighted equivalent temperature difference,the proportion of steam flow resistance inside the tube,vapor flow resistance outside the tube and thermal loss caused by bpe to overall thermal loss was studied.The influence of operating parameters on the weighted equivalent temperature difference was analyzed.The significance and influencing factors of equivalent thermal resistance of horizontal-tube falling film evaporator were analyzed.Based on the concepts of equivalent thermal resistance and overall thermal loss,the evaluation criteria of horizontal-tube falling film evaporator was proposed.According to above criteria,the optimal number of tube rows and the optimal flow rate of inlet steam were obtained when the apparent heat transfer temperature difference was constant.The effect of apparent heat transfer temperature difference on the performance of evaporator was studied.Under different working conditions,there is an optimal apparent temperature difference,which makes the overall thermal performance of the evaporator optimal.