Study Of Membrane Heat Exchanger And Influence To The Lithium Bromide Absorption Refrigeration System

Lithium Bromide aqueous solution is a kind of environmentally friendly working medium,and Lithium Bromide absorption refrigeration system can effectively make use of low grade heat source,such as solar energy,waste heat and so on,which can relatively protect the environment and save energy.Thus,Lithium Bromide absorption refrigeration system has been paid more and more attention.However,how to improve its Coefficient of Performance(COP)has been a problem.As a significant component of the system,solution heat exchanger can recycle heat energy of solution from generator and absorber,which can improve the COP of the system to some extent.However,there is only heat transfer but no mass transfer in the solution heat exchanger.The application of membrane distillation to the solution heat exchanger can not only make heat transfer but also make water vapor molecules transfer from concentrated solution to dilute solution simultaneously.As a result,the temperature of dilute solution increases and its mass fraction decreases.What's more,the temperature of concentrated solution decreases and its mass fraction increases.The thermal load of generator and absorber can be further reduced and the application of membrane distillation to Lithium Bromide absorption refrigeration system seems to be a subject which is worthy to be researched.So far,there has been little study about the distribution of different parameters such as temperature and concentration alone the membrane module and little study about the influence of different working conditions and membrane structure to the COP of the system.This subject develops a mathematical model in the membrane heat exchanger on the basis of its heat and mass transfer.The specific contents and conclusions are as follows:(1)The application and classification of membrane distillation,material of the membrane and structure of the membrane module are introduced respectively.The principle of membrane heat exchanger is stated.Temperature polarization and concentration polarization phenomena,which are the main influence factors of heat and mass transfer,are specifically explained.(2)On the basis of the coupled effect of heat and mass transfer in the membrane heat exchanger,a mathematical model of both co-current and counter-current flow is developed and solved by MATLAB.The model takes temperature polarization phenomenon into account while ignores the concentration polarization phenomenon.Equations of the temperature,concentration and mass flow rate of the outlet solution are obtained.This model is verified by the experimental parameters and results in available literatures.(3)The thermodynamic calculation method of the Lithium Bromide absorption refrigeration system with membrane heat exchanger is stated,which is different with traditional Lithium Bromide absorption refrigeration system.Designing parameters,thermal load of each equipment and COP are calculated respectively.(4)In the co-current and counter-current conditions,the distribution of different parameters along the length of a single membrane tube is analyzed.The parameters include bulk temperature,membrane temperature and bulk concentration on both sides of the membrane,and loss of heat transfer driving force,membrane flux,heat transfer capacity.Simulation results show that:The heat transfer amount in counter-current module is greater than that in co-current module.What's more,mass transfer amount is also greater because water vapor molecules transfer from concentrated solution to dilute solution all the time.As a result,Lithium Bromide concentrated solution from the generator is further concentrated and Lithium Bromide dilute solution from the absorber is further diluted,which can further reduce the thermal load of generator and absorber.And finally the COP of the Lithium Bromide absorption refrigeration system can be effectively improved.(5)For the small Lithium Bromide absorption refrigeration system with a counter-current membrane module,the effect of different external conditions and membrane structures to the membrane distillation progress and the COP of the system are researched.External conditions include heat source temperature,cooling water inlet temperature and chilled water inlet temperature.Membrane structures include mean pore size and porosity of the membrane.Simulation results show that:In the case that other parameters are constant,the amount of heat and mass transfer are greater with higher heat source temperature and lower cooling water inlet temperature,which leads to the increase of COP.However,the amount of heat and mass transfer is almost unchanged with the increasing of chilled water inlet temperature.In addition,larger mean pore size and porosity are beneficial to the improvement of COP.