Abstract: A heat exchanger comprising a casing wherein a bank of tubes is arranged and provided with means for conducting a first medium through the tubes of the bank from a supply opening for said medium in the wall of the casing, and means for conducting a second medium from another supply opening in the wall of the casing around the tubes of the bank towards a discharge opening in the wall of the casing, whereby the means for conducting the second medium through the apparatus comprise a pump impeller arranged centrally at the bottom of the casing, said impeller being connected to a pump shaft projecting upwardly within a cylindrical inner jacket disposed centrally in the casing and which is driven above the casing by a motor arranged in situ, while the tube bank for the first medium is an annular bank which is arranged around the cylindrical jacket, whereby the tubes of the bank are tubes of the bayonet type.