Chapter two presents the determination of heat transfer correlations on the air- and water-side Nusselt number in plate-fin and tube crossflow heat exchangers. A method for determining the correlations on a single tube and across the whole exchanger is developed. The method is compatible with the experimental determination of correlations for calculating the average heat transfer coefficient (HTC) on a single tube row or in the whole heat exchanger. The proposed method, based on CFD (Computational Fluid Dynamics) modelling of the heat exchangers, can replace the expensive experimental studies to determine correlations for Nusselt numbers in finned heat exchangers. Another inverse problem analysed in this chapter is the determination of the HTC, temperature and thermal stresses on the inner surface of a cylindrical pressure element, which is thermally insulated on its outer surface. Based on the measured transient wall temperature at a short distance from the inner surface of the cylindrical element, the distribution of temperature and thermal stresses across the wall thickness and the HTC on the inner surface are determined.