Fluoropolymer composite coating for condensing heat exchangers: Characterization of the mechanical, tribological and thermal properties

• A fluoropolymer composite coating was developed for condensing heat exchangers.
• The coating has excellent corrosion resistance and good adhesion strength.
• The thermal conductivity of the composite is much higher than virgin fluoropolymer.
• The coefficient of friction of the composite coating is very low.
• The heat transfer rate of the coated tube is greatly increased by using the composite.

Condensing heat exchangers constantly work in extremely corrosive environments because of the strong acids formed from the condensing flue gases. Cost effective metallic materials, e.g. carbon steel, aluminum, copper, stainless steel, etc., cannot survive in such environments. In this research, a fluoropolymer based coating material with improved thermal conductivity was developed by post-blending graphite particles into perfluoroalkoxy (PFA) powder. The composite was coated onto metallic substrates commonly used for heat exchanger manufacturing (e.g. stainless steel, aluminum, etc.), and the coating shows excellent surface smoothness (without pinholes) and corrosion resistance, very low friction coefficient and good adhesion to the substrate. The surface mechanical and tribological properties of the coating were characterized by nano-indentation, scratch and wear tests. The thermal diffusivity, specific heat and thermal conductivity of the composite material were measured using laser flash and differential scanning calorimetry (DSC) techniques, and significant improvement of thermal conductivity was observed in the graphite-filled PFA. The overall heat transfer behavior of coated tubes was evaluated using a thermal contact resistance model, and substantial increase of heat transfer rate was anticipated by improving the thermal conductivity of the composite coating, reducing the coating thickness, and increasing the real contact area at the polymer/tube interface. The composite coating provides an alternative material solution for condensing heat exchangers used in low-grade waste heat recovery.