Study On The Heat And Mass Transfer Process And Water-saving,high-efficiency Strategy For Inlet Air Pre-cooling Of Closed Cooling Towers

Due to design defects,etc,the cooling capacity of closed cooling towers is insufficient when tower spray is off while the water consumption is high when tower spray is on during operation.It is urgent to find a water-saving,high-efficiency strategy for closed cooling towers.Therefore,this paper investigates the feasibility of the pre-cooling technology for inlet air of closed cooling tower,aiming to seek water-saving,high-efficiency scheme for inlet air precooling of closed cooling tower.The research of this paper is helpful to guide low-carbon and water-saving operation of closed cooling tower,and possesses certain theoretical significance and applicable value.Firstly,the numerical simulation model for inlet air pre-cooling of the closed cooling towers with direct/indirect evaporative technology are established;Secondly,the factors are studied of thermal performance for inlet air pre-cooling of closed cooling towers with direct/indirect evaporative technology,including media types,environmental meteorological parameters and fan powers,etc,and the feasible operating ranges(i.e.,environmental parameter ranges)for inlet air pre-cooling of closed cooling towers with direct/indirect evaporative technology are obtained;Then,feasibility analysis is conducted for inlet air pre-cooling of closed cooling towers with direct/indirect evaporative technology based on meteorological parameters in typical dry-hot area(Hami,Xinjiang);Finally,comparison of inlet air pre-cooling by direct and indirect evaporative technology considering the fan powers and water consumption,and the recommended scheme for inlet air pre-cooling of closed cooling tower is obtained,which provides theoretical and technical guidance for water-saving and highefficiency operation of the closed cooling towers.Through the research,the main conclusions are as follows:(1)The numerical simulation models of mechanical draft closed cooling tower when tower spray on/off and inlet air precooling of the closed cooling towers with direct/indirect evaporative technology when tower spray on/off in this paper have been verified and the relative error is within 10%;(2)With the ambient humidity of 20%and temperature of 0～40℃,for the four types of wetted medium(i.e.,CELdek7060,CELdek7090,CELdek5090,and PVC1200)direct evaporative inlet air precooling system and plate indirect evaporative inlet air pre-cooling system studied,the cooling performance of closed cooling tower is not improved when tower spray is on while the water consumption of tower is reduced.Besides,the cooling performance of closed cooling tower is improved only ambient temperature reaches a certain range when tower spray is off;(3)When tower spray is off,the feasible range of inlet air pre～cooling of closed cooling tower with direct evaporative technology is:ambient temperature of 30～40 ℃ and relative humidity 0～80%,ambient temperature of 20～30℃ and relative humidity 0～60%,ambient temperature of 15～20℃and relative humidity 0～40%and ambient temperature of 11～15℃ and relative humidity 0～20%while the feasible range of inlet air pre～cooling of closed cooling tower with indirect evaporative technology is:ambient temperature of 35～40℃ and relative humidity 0～80%,ambient temperature of 30～35℃ and relative humidity 0～60%,ambient temperature of 25～30℃and relative humidity 0～40%and ambient temperature of 20～25℃ and relative humidity 0～20%;(4)With the aim of water-saving and high-efficiency,the recommended operating mode for closed cooling tower is:only dry cooling when the ambient temperature is lower than 0℃,inlet air pre-cooling of closed cooling tower while tower spray is off when the ambient temperature is 0～15℃,tower spray is on when the ambient temperature is higher;(5)After comprehensively considering the heat transfer performance,water loss and fan power of the inlet air pre-cooling of closed cooling tower with direct/indirect evaporative technology,it is found that inlet air pre-cooling using direct evaporative system is superior to that of indirect evaporative system during hot periods when tower spray is off.In July,the hottest month in 2022 in Hami,Xinjiang,comparing inlet air pre-cooling using direct evaporative system with that using indirect evaporative system,when tower spray is off,the heat rejection rate increased by 31.06%,the temperature drop reduced by 1.12℃,the water evaporation rate per MW of heat rejection rate decreased by 15.35%and the fan power per MW of heat removal rate decreased by 22.51%.In conclusion,it is recommended to adopt inlet air pre-cooling scheme of medium direct evaporative technology for closed cooling tower in hot-dry area when tower spray is off.The inlet air pre-cooling system should be off when the tower is on.To achieve water-saving,highefficiency operation of tower,it is suggested to set temperature and humidity sensors to monitor and control the operating mode of inlet air pre-cooling of closed cooling tower.