The ends of numerous tubes are connected to hollow headers so that they are in fluid communication with the headers. Slit shaped apertures are formed in the outside face of the headers in the direction of their circumference, and partititions are inserted through the apertures. Each partition is constructed out of two partition plates which fit into the header through the aperture in superimposed position. Alternatively, the partition plates may be connected integrally at one end so that their unconnected ends tightly contact the edges of the slit shaped aperture. The partitions are then braced to the header to become integral with it.