Research On Heat Transfer And Antifreeze Characteristics Of Direct Air-Cooling Unit Under Variable Working Conditions

Direct air-cooling technology has been widely used in coal-fired power plants in coal-rich and water-poor areas because of its superior water-saving performance.Although the whole machine of direct air-cooling unit has the characteristics of small space occupation and good performance,the air-cooling island in the unit is susceptible to environmental factors.In cold areas,lower temperatures in winter often reduce the heat transfer efficiency of condensers and lead to safety accidents.Air leakage will also seriously affect the performance of the condenser.A single air-cooled finned tube was studied in this paper.The heat transfer performance of the air side and the steam side of the air-cooled finned tube is studied by using the actual operation data of a 2×350MW direct air-cooled unit.It provides a good guidance for the optimal operation of air condenser.At the same time,the influence of different finned tube sizes on the heat transfer performance of finned tube was studied,which provided reference for the design and selection of the size of air-cooled finned tube.After studied The influence of non-condensable gas on air-cooled finned tube,It was found that when the air content was 5%,the average heat transfer coefficient of the steam side was reduced by about 31%.At the same time,when the air content was 5%,the average thickness of the liquid film was reduced by 5.6%.Under rated working conditions,the heat exchange of the countercurrent unit is 57.4% lower than that of the downstream unit,and the average temperature is about 15.2℃ lower than that of the downstream unit.Based on the calculation of heat transfer performance,the antifreeze problem of the unit is studied by combining the operation parameters of the whole unit and the measurement data of air-cooling island temperature measurement system.It is found that the antifreeze temperature should be about 4℃lower than the antifreeze warning temperature selected based on the temperature measurement point.In addition,the minimum anti-freezing steam flow rate,anti-freezing back pressure and critical value of variable working conditions of the anti-freezing fan were also obtained.It was found that when the ambient temperature was reduced to-20℃,the critical speed of the fan was only 21.93% of the full load.