Anti-fouling Study Of Ni-P-PTFE Composite Coating Plated On Copper Tubes

Heat pump water heater system is considered to be economical and environment friendly, which transfers the heat from solar, atmosphere, underground water and so on. It can be used to supply heat and hot water efficiently in daily life. However, fouling adhesion occurs frequently on the heat exchanger surface of heat pump. It should cause a decline of heat transfer efficiency, an instable running and a decrease of lifetime of the heat exchangers. An urgent solution to the fouling is needed in the development of heat pumps.An electroless composite plating was utilized to reduce the surface free energy of the copper tubes in order to mitigate the fouling adhesion on the heat exchange. Experiments of Ni-P-PTFE electroless composite plating were carried out under different Polytetrafluoroethylene(PTFE) concentrations while other technological parameters were fixed.8test tubes covered by Ni-P-PTFE composite coating were ready for the anti-fouling test and the measurement of thermal conductivities.The tested tubes were heated in a simulate solution under the constant temperature in order to investigate the anti-fouling capacity of Ni-P-PTFE composite coatings compared to the surface without composite coating. Test results demonstrated that an increase of fouling adhesion occurred on the surface with an increasing temperature. It was witnessed that a much smaller quantity of fouling adhesion occurred on the surface of Ni-P-PTFE composite coating compared to copper and obvious differences of anti-fouling capacities among the coatings appeared under75℃. During the experiments, the fouling period on the surface of test tubes included induction period, deposition process and removal process. It was considered that the anti-fouling capacities of the Ni-P-PTFE composite coatings mainly depended on the PTFE contents in the coating. With an increase of PTFE content, the induction period was lengthened and less fouling formed on the surface of the coating. The thermal resistances of fouling adhesion on composite coatings were only6.47×10-4～1.97×10-3m2·K/W, which were71.9%-94.3%down compared to the surface of copper. The Ni-P-PTFE composite coating effectively mitigated the impact of fouling adhesion on thermal conduction of the test tubes.A thermal resistance method was utilized to measure the heat transfer coefficient of copper tubes with Ni-P-PTFE composite coatings. The thermal conductivities of these Ni-P-PTFE composite coatings were gained by means of Wilson plot in order to avoid the influence of measuring wall temperature. The results showed the thermal conductivities of these Ni-P-PTFE composite coatings depended on the PTFE and C contents in the coatings, which were weakened by the increases of PTFE, but enhanced when the C content increased. Although the thermal conductivities of the composite coatings were lower, their thermal resistances in the total heat transfer process were from9.91×10-5to1.6×10-4m2·K/W due to their small thickness. Therefore, the tubes with Ni-P-PTFE composite coatings still kept high thermal conductivities314.88-357.55W/(m-K), only reduced by11.77%-20.88%.Defined an integrated performance index of the anti-fouling efficiency and thermal conduction for test tubes, the sample formed at the PTFE concentration11ml/L in the solution behaved the best integrated performance of anti-fouling capacity and thermal conductivity under the conditions in the thesis, which owns an anti-fouling efficiency94.79%and a thermal conductivity356.89W/(m-K), reduced by13.43%compared to copper tubes.