In lines for high-temperature fluid solids (e.g., in fluid catalytic cracking units), it is sometimes necessary to calculate the metal temperature with an internal, insulating refractory lining. In this case, the metal temperature may be significantly less than the fluid temperature. The refractory lining resists erosion while at the same time permitting use of carbon steel with a fluid that would otherwise be prohibitively hot for its use. Note that the metal temperature is often deliberately kept warm, often above 150°C (300°F), to prevent condensation against the inside of the steel pipe (of gasses passing through refractory pores and cracks) and resultant corrosion (e.g., due to sulfuric acid and sulfurous acid in flue gas) of the metal under the refractory. The following equations and a chart of film coefficient are provided as means for calculating the metal temperature. Note that, for design temperature, conditions that maximize the metal temperature are assumed (zero wind and hottest ambient temperature).
The heat flow through the pipe, per square fool per hour, is given by
This is normally essentially the same as the temperature on the outside surface, because the conductivity of the pipe material is generally high relative to the refractory and outside film coefficient. #Little_PEng