An analysis of the efficiency of power-generating plant heat exchangers under operational conditions reveals the differences between experimental and theoretical data. One of the possible reasons for these differences, in the authors' opinion, is the unimproved method of thermal calculation for heat exchangers, in particular the method of making allowance for velocity of the approach flow. As shown by calculations carried out according to the Guidelines b the zone-by-zone method, in vertical condensing heat exchangers there are zones of condensation for practically stationary steam and zones with high steam velocities.