Abstract: A heat exchanger is illustrated in the form of a pair of juxtaposed chambers defining a first flow path for passing a heated fluid therethrough and a second flow path for passing a cooling medium therethrough. A heat exchanging structure extends between the first and second flow paths in communication therewith. The heat exchanging structure includes at least one composite of thermally conductive fibrous material laid up unidirectionally in a direction between the flow paths for transferring heat from the heated fluid in the first flow path for absorption by the cooling medium in the second flow path.