𝑈 1 is the overall heat transfer coefficient between the fluid in the inner annular section of the TTHE and the fluid in the outer annular section, based on the area associated with 𝑅 1,𝑖 -analogous to the overall heat transfer coefficient 𝑈 1,𝑖 between the conveyor fluid and the flue gas in the MBHE-. 𝑈 2