Performance Analysis And Optimization Of Air-water Direct Contact Heat Recovery Device

The heat and moisture transfer between air and water is a common way of air conditioning in HVAC systems.This paper analyzes heat and mass transfer performance of the spray packing and heat recovery module.The improvement ideas of the basic flow of system is put forward in view of the problems of conventional evaporative cooling and condensing dehumidification fresh air system driven by heat pump,which is helpful to optimize the performance of the air conditioning equipment and deepen the comprehension of the process of thermal and humid treatment.This paper establishes the mathematical model of heat and mass transfer between air and water based on Matlab.The reliability of the model is verified by the experimental data in the literature.Based on the mathematical model,the heat and mass transfer performance of air and water with different parameters is analyzed.The results show that:（1）the closer the air is to saturation,the smaller transfer loss.（2）The mass flow matching characteristics of heat and moisture transfer of different temperature are different.The higher the air temperature is,the larger the optimal mass flow ratio of air to water.（3）The higher the number of mass transfer units（NTU_m）of spray packing is,the larger the optimal mass flow ratio of air to water.（4）The multi-stage packing helpes to improve the heat and mass transfer efficiency,but the increased efficiency will be halved with per added stage of packing.The conventional evaporative cooling and condensing dehumidification fresh air system driven by heat pump is introduced,and the visual simulation model of the system process is set up.It is pointed out that the new air dehumidification load is large（combined heat and humidity treatment）;the heat recovery efficiency of return air is low;and the matching of heat and mass transfer process between air and water is poor,and so on.The following suggestions are proposed:（1）In order to reduce the dehumidification load of the heat pump cooler,a single-stage total heat recovery module consisting of a surface cooler and a spray filler is proposed to precooling the fresh air,then the improved process I is obtained.The simulation results show that:keeping total heat and mass transfer capacity of the system unchanged（the total number of mass transfer units（NTU_m）of the spray packing,the total heat transfer capacity of the surface cooler）,the system performance first increases and then decreases with the increase of the heat and mass transfer capacity of the total heat recovery module,under typical indoor and outdoor air conditions,the system optimal COP?_sysys is 0.33 higher than the basic process.Under this condition,the heat and mass transfer capacity of total heat recovery module accounts for about30%of the total heat and mass transfer capacity of the system.（2）In order to reduce the loss of exergy of heat recovery process and raise the supply air temperature,single stage of sensible heat recovery can be added to obtain the improved process II on the basis of the improved process I,the COP?_sys of the system is further increased by 0.07 with respect to the improved process I.The fresh air can be sent into the room directly without reheat.（3）Multi-stage total heat recovery helps to improve the air-water heat and mass transfer matching,but the improvement of the overall system performance is limited when the pump power consumption is taken into account.At last,an experiment table based on the improved process II was built,and the simulation values were verified.A new type of waste-heat recovery system based on spray tower is proposed.The core equipment of system is designed and calculated by the mathematical model of countercurrent heat and humidity transfer between air and water.The system is applied to the exhaust heat recovery project in a heating boiler.The preliminary operation results show that the system can not only recover the waste heat of exhaust smoke in the boiler,reduce the temperature and humidity of exhaust smoke,but also heat the fresh air to improve the effect of combustion,the boiler exhaust is heated by the surface cooler before entering the chimney,and the white mist can be reduced.The reliability of the model is verified by engineering measured data.In addition,the spray process absorbs a large amount of sulfur dioxide and nitrogen oxides in the flue gas,and the energy saving,environmental protection and economic benefits of the system is pretty good.