Does the coating affect heat transfer in the heater? What is the thermal conductivity of the coating?
Thermal conductivities for the metal, coke, and coating are 54, 5, and 7 W/mK. Given that the coating is so thin (<1 micron) relative to the wall (8,000-10,000 microns), the heat resistance of the coating is insignificant to the thickness of the coke considering they are both insulators. The thickness of coke can exceed 1" or 25,400 microns.
