| Fouling is a common problem in finned tube heat exchangers to deteriorate the heat transfer performance.Due to outdoor arrangement of the air-cooled platform of the DC transmission system’s converter valve,fouling is more likely to occur on the finned tube wall,which can impact the safe and reliable operation of the converter valve.This study focuses on the finned tube heat exchanger in the air-cooling system for the converter station in Yinchuan East,Ningxia.Based on the fan-heat exchanger resistance characteristic model,the actual air flow,rate flow resistance,and heat transfer performance of the fouled finned tube heat exchanger were studied thoroughly under different inlet air velocities and different fouling thicknesses.The specific research work and results are as follows:(1)The influence of fouling on the air-side flow resistance and air flowrate of the finned tube heat exchangers was studied.As the fouling thickness increases,the airflow through the finned tube gradually decreases due to the increasing pressure loss caused by fouling clogging the flow path.When the fouling thickness is 0.5mm,the pressure loss increases by 11.7% and the air flowrate decreases by 17.6% compared to the clean condition.(2)Under actual working conditions at a constant fan speed,the influence of fouling thickness on the heat transfer capacity of the heat exchanger was studied.As the fouling thickness increases,the temperature difference between the inlet and outlet decreases,and the heat transfer capacity also decreases due to the decrease in inlet air flowrate.Compared with the clean condition of the tube bundle,when the fouling thickness is 0.5mm and the fouling thermal conductivity is 0.1W/(m·K),the temperature difference decreases by 2.29℃,and the heat transfer capacity decreases from 843 k W to 349 k W,which is only 41% of the clean condition.(3)The influence of fouling thickness on the air-side convective heat transfer capacity under different inlet air velocities was studied.The factor of comprehensive flow and heat transfer performance is introduced for assessment.When the inlet air velocity is 6.5m/s and the fouling thickness is 0.5mm,compared with the clean condition,Nu increases by 23%,and the resistance coefficient increases by 66%.The factor of comprehensive flow and heat transfer performance decreases 7%.(4)The Orthogonal experiment method was used to explore the degree of influence of fouling thermal conductivity,inlet air velocity,and fouling thickness on the heat transfer capacity of the heat exchanger.Fouling thermal conductivity is the dominant factor affecting the heat transfer performance of the finned tube heat exchanger,and the smaller the fouling thermal conductivity,the greater the impact on the heat transfer performance.When the fouling thermal conductivity is 0.1 W/(m·K)and the fouling thickness is 0.5mm,the convective heat transfer coefficient and total heat transfer coefficient of the finned tube decrease by 21.4% and 62%,respectively,compared with the clean condition.(5)The HTRI design software combined with on-site monitoring data are combined to analyze heat transfer performance under different water scale thickness and thermal conductivity conditions.There is an optimal number of increase units of heat exchangers under the same inner tube flow condition.When the inner and outer water scale thickness is 1 mm and the fouling thermal conductivity is 2.9 W/(m·K),increase of 4 heat exchanger units can achieve the maximum increase in heat transfer capacity,reaching a heat transfer capacity of 4748 k W and a growth rate of 6.1%.Fouling can significantly reduce the heat transfer capacity of heat exchangers.Therefore in engineering practice,expanding the heat transfer area by increasing the heat exchanger units is a common method to compensate for the loss of heat transfer caused by fouling.It is necessary to monitor the thickness and thermal conductivity of fouling,and consider economic factors to select the appropriate number of expansion units. |
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