By comparing the generalized results of the tests with a large group of power-related condensing heat exchanging apparatus (condensers of steam turbines, preheaters of feed and main water) with computational characteristics obtained with the use of earlier known techniques, it is shown that the available techniques of thermal calculation of heat exchanging apparatus do not take into account many aspects on the course of physical processes in different zones in the heat transfer surface of apparatuses. The advantages of calculation based on differentiated determination of heat transfer characteristics (heat transfer coefficients) of heat carriers are shown and this provides a possibility for a more complete account for the effect of different operational and design factors on the process of heat transfer. It also allows one to reflect the influence of various methods of increase in the thermal efficiency on the thermal characteristics of apparatuses.