Abstract: A heat exchanger for fluids such as liquids, steam, gases, etc. comprises a tank (10) with two separated closed systems for the fluids, between which heat is to be exchanged.The tank (10) has an insert (18)consisting of longitudinally curved, preferably semicircularly shaped elements (12). The elements (12) are assembled in a sequence with their concave surfaces contacting the edges of the adjoining element. Along said edges the elements are assembled fluidtight. Between two elements (12), thus, a channel (16) is formed open to two opposite sides of the insert (18). The inlet and outlet means (20) are respectively connected to these sides, so that every second fluid channel (16) is connected to the same inlet and outlet means (20) respectively, the fluid thus being adapted to be supplied to every second channel from the corresponding inlet and outlet means.