the outer tube is regarded as a long horizontal cylinder that exchanges heat by natural convection with ambient air at 18 °C. The calculation of ℎ 2 is performed using the same correlation as Zuritz , while ℎ 𝑎 is calculated based on the correlation suggested by Morgan . Conductive resistance is neglected. When using the correlation for ℎ 𝑎 , the air properties are calculated at the film temperature, i.e., at the arithmetic mean between the air temperature and the wall temperature of the outer tube. The latter is assumed as the arithmetic mean of 𝑇 2 between the inlet and the outlet of the TTHE (calculated iteratively).