The apparatus (1), which combines heat transfer between a liquid (8) and a heat-exchanging medium (7) with static mixing of the liquid, comprises internal fittings (2) inside a casing (3). The casing extends longitudinally between a head end (4) and a base end (5). The internal fittings form a heat-transfer and mixing structure. The heat-exchanging medium can be carried from the base end to the head end as an inner stream inside tubes (21, 22) of the internal fittings. The liquid can be carried from the head end to the base end as an outer stream. Reinforcing elements (6, 6', 6'') are provided, which stabilize the internal fittings in the longitudinal direction with respect to pressure gradients produced by the liquid. In a main region, the internal fittings are connected by the reinforcing elements to form a non-expandable part-structure (2a); and in a secondary region that is complementary to the main region they remain at least partially unreinforced, as a part-structure (2b) which can expand in the longitudinal direction.