Energy-efficient Anti-freezing Operation Of Natural Draft Dry Cooling System

Natural draft dry cooling system(NDDCS)has become popular in northern areas of China because of the abundant coal storage but the inadequate water resource.However,the northern areas usually suffer from quite cold weather in winter,which may cause the freezing risks of the finned tube bundles of the air-cooled heat exchanger,so there is an urgent need of the safe operation of NDDCS in power plants.Currently,the foreign and domestic researches focus rarely on the thermo-flow characteristics of NDDCS in those cold regions,and the empirical adjustment of back pressure is generally adopted for anti-freezing operation,which leads to the higher turbine back pressure but lower thermal efficiency of the power generating unit.In this research,the freezing mechanism and also the corresponding anti-freezing measures of NDDCS have been deeply disclosed on basis of the typical NDDCS in those cold regions.This research may present valuable theoretical guidelines and engineering applications.The coupled heat load models of cold end system which incorporates the air-cooled condenser and natural draft dry cooling system are developed based on the heat matching principle of turbine exhaust steam,circulating water,and ambient air.The transport performances of NDDCS under low atmospheric temperature are analyzed,the outlet water temperatures of air-cooled sector and cooling delta are revealed,and then the heat exchangers of some certain sectors which may have potential freezing risks are discovered.The circulating water redistribution effects on the energy-efficient operation of NDDCS are thoroughly explored followed by the obtained anti-freezing water flow rates of various sectors under various ambient conditions.The results show that,the frontal sector avoids freezing risks only at the low wind speeds by increasing the water flow rate,while at the high wind speed,it still confronts with severe freezing dangers.The frontal and middle rear sectors can free from the freezing risk with the increased circulating water,implying that the antifreezing could be approached by strengthening the water-side heat load.At low wind speeds,the circulating water of the middle and middle rear sectors should not be decreased too much in case of the freezing dangers.However,at high wing speeds the two sectors still show potential anti-freezing capability even with the excessively decreased water flow rate.Consequently,when redistribute the circulating water of air-cooled sectors at the low wind speeds,the water flow rate should be appropriately increased for the frontal sector but decreased for the middle and middle rear sectors.While at the high wind speeds,the circulating water should be increased as much as possible for the frontal sector but decreased for the middle and middle rear sectors.By means of circulating water distribution of various sectors,the cooling efficiency of NDDCS could be improved on the premise of the anti-freezing operation.The anti-freezing effects on NDDCS from the decreased heat transfer area of air-cooled heat exchanger,namely switching off a certain sector,are discussed in this research.With various sector off cases,the air-side thermo-flow performances and outlet water temperatures of the heat exchanger columns as well as sectors are analyzed and compared with the all sector on case.The results indicate that,the inlet water temperature of the air-cooled heat exchanger could be decreased properly for lifting the thermal efficiency of the power generating unit as long as the ant-freezing is satisfied.However,at the wind conditions,the air-side cooling capabilities of the operating sectors present non-uniform distributions,therefore the inlet water temperature should be increased to intensify the water-side heat load,which can match successfully with the highest air-side heat load.By switching off the air-cooled sector,the water-side heat capacity is increased evidently for enhancing the anti-freezing capability,besides when dismissing the front sector,the required water-side heat capacity is maximally lowered.In addition,the middle and middle rear sectors face no freezing dangers with or without the sector off.What's more,the turbine back pressure reduces with the sector off cases,thus the energy-efficient anti-freezing operation could be achieved for the cold end system.Following the present empirical anti-freezing strategies of NDDCS in power plants,this research provides the simultaneous adjustment of the circulating water and louvers.With such practical anti-freezing strategy,the anti-freezing water flow rate and opening degree of louvers are obtained,which provide the anti-freezing guidelines for NDDCS in cold winter.Furthermore,for the working condition of the chocking back pressure,the optimized anti-freezing operation is acquired.The results illustrate that,at the not very low ambient temperatures,the cooling capability of NDDCS should be preferentially utilized,so the louvers have a complete opening degree.In such a case,if the air-side heat load exceeds the water-side for a certain heat exchanger,the circulating water should be increased independently to avert the freezing dangers.However,if the ambient temperature falls continuously,the louvers should be lowered down to restrain the air-side heat load which should be smaller than the maximum water-side heat capacity.When the ambient temperature changes from-5? to-10?.the turbine back pressure should not be lower than the anti-freezing back pressure,while if it varies from-15? to-30?,the working condition of the chocking back pressure should be adopted for the most energy-efficient anti-freezing operation of NDDCS in power generating unit.