Abstract: A heat exchanger assembly of the counterflow type having a vertically extending longitudinal configuration is constructed from a plurality of individual tube nests, with each of the nests formed of a plurality of individual tubes adapted to have a first heat exchange medium flow therethrough. The upper and lower ends of the tubes are connected to tube plates and a guide shell is provided surrounding each of the tube nests for effecting therein flow of a secondary heat exchange medium around the individual tubes of each nest. Each of the tube nests is formed by its associated guide shell and tube plates as an individual box with the boxes thus formed when viewed in a plane extending transversely to the longitudinal direction of the heat exchanger assembly being arranged as a hexagonal grid.