Research On Operation Performance Of A Large Capacity Natural Draft Dry Cooling System With One Tower For Two Units

The natural draft dry cooling system uses the ambient air to cool the circulating water in the air-cooled heat exchanger.Because of the great advantages of water saving,it is widely used in "Three Norths" area in China.In recent years,a number of large-scale natural draft dry cooling system have been established in northern China.The one tower for two units solution can greatly reduce the land space and initial investment.For the double-layer arrangement of air-cooled heat exchanger,the height of one-layer heat exchanger is limited within 15 meters.Compared with the traditional single-layer air-cooled heat exchanger layout,it can reduce the power consumption of the circulating water pump and improve the flow and heat transfer performances.Taking a 2×660 MW large-capacity natural draft dry system with one tower for two-units as the research object,the physical and mathematical models are developed in this paper.The commercial software FLUENT is adopted to simulate and analyze the flow and heat transfer performances under various working conditions.The heat transfer performance of each sector in the air-cooled heat exchanger is obtained,and the influences of the ambient wind on the flow and heat transfer of each sector are analyzed.The results show that the heat transfer capacity of the upper and lower sectors at different positions varies with the wind speed,but the lower heat exchanger has a better performance overall.Besides,the overall flow and heat transfer for two different arrangements of heat exchanger are compared,and it is found that the natural draft dry cooling system with double-layer air-cooled heat exchanger performs better,but the difference between them gets reduced as the wind speed increases.For the freezing risk of air-cooled heat exchanger of natural draft dry cooling system under low temperature conditions,the minimun outlet water temperature of the radiator should not be less than 0℃.Based on the turbine choking back pressure with the better energy efficiency,the effects of ambient temperature and wind speed,power output,the number of sectors in operation,and the circulating water flow rate are analyzed.The critical conditions for the safe operation of air-cooled heat exchanger under different working conditions have been determined,and the control logics of anti-freezing operation have been established for the natural draft dry cooling system in winter.