Research On Thermodynamic Characteristics And Optimization Of Two-stage Mechanical Vapor Re-compression System

The single-stage mechanical vapor re-compression system is widely used in various evaporation and concentration filed.It is an effective form of energy saving and consumption reduction.It has good applicability and energy-saving properties for materials with low boiling points and high evaporation capacity.When the temperature rise of the boiling point of the material is high,the energy consumption of water vapor compressor is significantly increased,resulting in a huge reduction in the energy efficiency of the single-stage system.In order to effectively solve this problem,the paper use Mg SO4 solution as a research object and design a two-stage mechanical vapor compression system.The solution with a lower initial temperature and a higher boiling point temperature is passed through the first-stage evaporator for pre-evaporation.And the solution will be evaporated to saturation in the secondary evaporator,achieving further energy saving in the evaporation and concentration process.This article mainly includes the following contents:(1)The numerical simulation of the heat and mass transfer process of a vertical tube falling film evaporator.Taking the falling film evaporation process of Mg SO4 solution in vertical tube as the research object.Taking into account the influencing factors of heat transfer in the mass transfer process,the paper studies the heat and mass transfer of the falling tube evaporator in the two-stage system.Based on simplifying the process of heat and mass transfer,a numerical solution is used to solve the temperature filed and concentration filed distribution of liquid film in the tube.This paper analysis the effect of the wall temperature,heat transfer tube length,the concentration of imported solution on the heat transfer performance.(2)Study on the thermodynamic characteristics of water vapor compressor and vapor balance.Comparing and analysis the effect of R245 fa and vapor on the structural design and energy consumption under different working conditions of MVR system,this paper proposes a heat pump system suitable for low temperature rise and constructs a mathematics model for energy analysis of MVR compressor.Based on the analysis of physical properties,the heat balance and vapor balance of the MVR system is studied.The paper obtains the effects of evaporation pressure,heat transfer temperature difference,material outlet concentration and injected saturated water volume.(3)Mathematical model of two-stage mechanical vapor re-compression system.By analyzing the actual requirements in the process of evaporation,concentration and crystallization of Mg SO4,a two-stage MVR system process is established.On the basis of thermodynamic analysis,the paper constructs a two-stage MVR system thermalmodel and every heat transfer calculation models.The energy consumption of secondary forced circulating pump can not be ignored,and the mathematics model of pump is established.(4)Optimized design of two-stage MVR system.By analyzing the thermal model,six key parameters such as the evaporation pressure in the first-stage evaporator and the concentration of the first-stage evaporator outlet solution under actual working conditions are selected as the optimization variables.The total annual cost of system is selected considering the energy saving and economy of the system.The two-stage MVR system will be optimized through a genetic algorithm,and the minimum total cost is1.222 million yuan annually.