In a heat exchanger having two header tubes and a number of heat exchange elements (23) extending between the header tubes, each heat exchange element (23) is a substantially flat tube. The flat sides (21. 22) of the flat tubes (23) are inwardly concave (24) so that the side walls (21, 22) are closer together along the axial length of the tube (23) in a central region of the side walls (21, 22), substantially mid-way between the curved shorter side wall portions (25, 26) of the flat tube (23) or than the spacing of the flat sides (21, 22) in the immediate vicinity of the end or edge walls (25, 26). This modified tube structure improves the ability of the element to sustain overpressure without permanent deformation.
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