A heat exchanger extends between a first end and a second end and includes: a central core; and a heat exchange section; wherein the heat exchange section comprises: a primary flow inlet; a secondary flow inlet; a primary flow outlet; a secondary flow outlet; a plurality of primary flow tubes for conveying a primary flow from the primary flow inlet to the primary flow outlet; and a plurality of secondary flow tubes for conveying a secondary flow from the secondary flow inlet to the secondary flow outlet. The primary flow tubes and the secondary flow tubes are grouped together to form at least one strand; and wherein the at least one strand is helically wrapped around the central core.