Research On Heat Transfer Characteristics Of Shell Heat Exchanger Under Influence Of Rubber Ball On-line Cleaning

The problems caused by building energy consumption are becoming increasingly serious,and the air conditioning system is significant for energy saving and emission reduction.The shell-and-tube heat exchanger will generate fouling after long-term operation.However,the heat transfer performance will be effectively improved in regularly cleaning.The rubber ball on-line cleaning system is popular due to the advantages of convenient operation and low cost,but there is still a gap in the actual operating parameter settings.Therefore,in order to provide scientific guidance for engineering applications,this paper aims to study the influence of rubber ball on-line cleaning system on the heat transfer characteristics of the shell-and-tube heat exchanger.The shell-and-tube condenser in the lithium-bromide refrigeration machine was used as experimental subjects.By measuring the relevant parameters,the fouling resistance,the friction coefficient and the COP were obtained.Precisely,the fouling resistance characterized the heat transfer effect of the shell-and-tube heat exchanger,the friction coefficient characterized the fluid resistance within the heat exchange tube,and the COP of the refrigeration machine characterized the energy efficiency.The effects of ball percentage,running time and start-stop ratio on the heat transfer characteristics of shell-and-tube heat exchangers were discussed respectively,in order to optimize cleaning mode.The operation aspect of the refrigeration machine shows that relative error of the thermal balance was within ±20%,which indicated that the test results were highly reliable.Then,in the test of fouling growth characteristics,it was found that the fouling resistance began to rise rapidly on the 16 th day and gradually approached a certain value,and the COP decreased from 1.98 to 1.07.After using the rubber ball on-line cleaning system,the fouling resistance was reduced by 26.39%,indicating that the descaling effect was excellent.In the subsequent comparative experiments,it was found that when the ball percentage was 10%-30%,the growth rate of fouling resistance decreased from 32.8% to 8.9%,which showed that the more the number of rubber balls,the better the descaling effect.Nevertheless,incremental rubber balls were also likely to cause fluid resistance and blockage.When the operating cycle was shortened,the frequently running rubber balls caused fouling hardly adhere to the pipe wall.Particularly,when the running time was 20 s,the fouling resistance was even lower due to the rubber ball reinforced disturbance.Besides,the coefficient of friction changed periodically with the on-off state of the rubber ball pump.At last,when the start-stop ratio was 1:1,1:2,and 1:3,the fouling resistance increased by 16.1%,23.0%,and 29.0%,respectively,which manifested the shorter cleaning interval resulted in the better descaling effect.Among the above three parameters,the ball percentage has the greatest influence on the heat transfer characteristics.Considering the running cost and the service life of the rubber ball,it is most reasonable to choose the ball percentage of 20%,the running time of 40 s and the start-stop ratio of 1:2.