An Evaluation of the Impact of Surface Coatings on the Heat Transfer in High Temperature Ceramic Recuperators

Abstract

Engineering ceramics, particularly silicon carbide (SiC), are increasingly being used as materials in high temperature recuperators for preheating combustion air from furnace exhaust gases. As typical flue gases from these furnaces may contain sodium, potassium, halides, etc. that may attack SiC, protective coatings, such as alumina, zirconia and others, have been investigated as a means of increasing the life and reliability of these SiC recuperators. The objective of this work was to determine the effect of coating properties, such as emissivity and thermal conductivity, on the heat transfer performance of these high temperature ceramic recuperators. The approach was to formulate a simple rectangular cross section flow model to determine the amount of heat transferred by radiation from H2O and CO2 in the combustion gases to the coating surface as well as by convection to the same surface and by conduction through the coating and plate material to the combustion air on the other side. The results indicated that the thermal conductivity of the coating and coating thickness had a negligible effect on the overall heat transfer coefficient. Further, increasing the surface emissivity from 0.5 to 0.8 resulted in an increase of 10% in the overall heat transfer coefficient in typical waste heat recovery applications. It was concluded that by combining enhancement techniques on the air side with high emissivity materials on the gas side, one obtains optimal performance in a gas to gas heat exchanger.