Abstract: A heat exchanger, especially recuperator for high temperature reactors, comprising a prismatic extending jacket and a multiplicity of elongated hexagonal elements arranged within the jacket and essentially parallel to the primary direction or extent thereof. Each hexagonal element has a central tube and heat transfer surfaces arranged about each central tube and extending axially up to the outer contour or outline of the corresponding hexagonal element. Both ends of the heat transfer surfaces are connected to collectors seated upon the central tube. Each hexagonal element is connected by spacers with its neighboring elements and with analogous spacers spatially fixedly arranged at the region of the jacket laterally bounding the heat exchanger. The spacers have play so that the spacing between the elements and between the elements and the spatially fixedly arranged spacers can alter by the amount of such play.