A heat transfer segment for a curved heat exchanger, wherein the heat transfer segment comprises a plurality of enclosure bars that at least partially define two opposite ends of the heat transfer segment, wherein the two opposite ends define respective general planes, characterised in that: the enclosure bars are shaped and arranged such that the general plane of one end of the heat transfer segment is not parallel with the general plane of the other end of the heat transfer segment, such that when said heat transfer segment is joined to an adjacent heat transfer segment in an end-to-end fashion the enclosure bars the heat transfer segment may join to corresponding enclosure bars of the adjacent heat transfer segment.