A heat exchanger includes a pressurized shell and a tube bundle with exchanging tubes between flexible tubesheets. The flexible tubesheets are reciprocally interconnected by tie rods in a central zone of the flexible tubesheets which is devoid of exchanging tubes. The exchanging tubes in the tube bundle are arranged around the tie rods. The heat exchanger may further include conveying diaphragms arranged along the tube bundle. The conveying diaphragms may be shaped, alternately, as discs and rings.