Study On Fouling Characteristics And Heat Transfer Performance Of Finned Tube Heat Exchangers

The finned tube heat exchanger is one kind of the heat exchangers.The finned tube heat exchanger has the advantages of large heat transfer coefficient,strong heat shock resistance,compact structure,easy to clean grime,good temperature resistance,low pressure drop and strong corrosion resistance.So it is widely used in the evaporator and condenser of HVAC systems.However,there will be fouling problem on the surface of the finned tube heat exchanger after long time use.The presence of dirt will affect the heat exchange of the finned tube heat exchanger,increase the pressure drop,and also cause corrosion on the heat exchange surface,thus shortening the service life of the equipment,resulting in huge energy waste and economic loss.In this paper,the mixed dirt formed by Aspergillus Niger and Bacillus Subtilis in the condensed water on the fin surface of the finned tube heat exchanger and the dust dirt formed on the fin surface by dust in the air were studied.The finned tube heat exchanger is simulated by computer.This provides a theoretical basis for the heat transfer performance of finned tube heat exchanger and the scale removal and inhibition of finned tube heat exchanger.The main research work and research results are as follows:(1)Field investigation was conducted in several local factories to investigate the fouling on fins of finned tube heat exchangers after they have been used for a certain number of years.The dirt samples were collected to detect the microbial species in the dirt,and two representative microorganisms were selected——Aspergillus Niger and Bacillus Subtilis.Aspergillus Niger and bacillus subtilis were prepared in a sterile environment to prepare for the experiment of inoculating microorganisms into the condensed water on the fin of finned tube heat exchanger under the condition of desiccation.(2)Under the operating conditions of water inlet flow rate of 0.7m/s,water inlet temperature of 7?,air inlet wind speed of 0.9m/s,air inlet temperature of 25?,dust concentration of 2g/m~3 and air moisture content of 80%,the fouling deposition process and the time needed to generate different fouling thickness on the surface of the finned tube heat exchanger were recorded,and the fouling thermal resistance with different fouling thickness was calculated.It is concluded that the heat transfer resistance when the fouling thickness on the fin is 0.5mm is about 2.35 times of the heat transfer resistance when there is no fouling on the fin.At this time,the heat exchanger's heat transfer efficiency has been reduced a lot,and the fouling heat resistance value also provides a numerical basis for the numerical simulation of the finned tube heat exchanger.In the experiment,it was found that the cooling capacity of the finned tube heat exchanger when the fouling thickness was 0.2mm was larger than that when there was no fouling.The analysis showed that the reason was that the fouling particles increased the disturbance of airflow on the finned surface and increased the heat transfer.(3)Under the conditions of different water inlet velocity,different air inlet wind speed and different air inlet temperature,experiments were conducted respectively to record the time needed to generate different fouling thicknesses,and to see the influence of external conditionson the characteristics of microbial fouling.It is concluded that under the operating conditions of dust concentration of 2g/m~3 and moisture content of air of 80%,the velocity of water inlet is 0.7m/s,the temperature of water inlet is 7?,the wind speed of air inlet is 0.9m/s and the temperature of air inlet is 25?,and the rate of fouling on fin is the slowest.(4)The numerical simulation analysis of finned tube heat exchanger is carried out,and then the comparison and analysis with the experimental results show that the difference in temperature between inlet and outlet of air side with 0.2mm thick dirt increases by 3.52%compared with that without dirt.(5)The relationship between the air temperature of the finned tube heat exchanger and time was studied by numerical simulation and experiment;relationship between surface fouling and cooling capacity of finned tube heat exchanger with different thickness;the influence of different inlet flow rates on the cooling capacity of finned tube heat exchanger under the condition that the inlet temperature of the cooling medium water is 7?;the effect of different air inlet wind speed on the cooling capacity of finned tube heat exchanger;the temperature distribution law of the cold medium water in finned tube heat exchanger under different conditions.