Device for heat transfer between a first fluid and a second fluid, comprising an assembly of - tube elements (3, 3c, 3d) for the conduction of the first fluid, which in each instance are developed with a first, nondeformed region (15c, 15d) and at least one second, deformed region (16c, 16d), disposed at an end of the tube element (3, 3c, 3d), - at least one tube plate with passage apertures as well as at least one sealing element with passage apertures, wherein the sealing element is in each instance disposed between an outer surface of the second region (16c, 16d) of a tube element (3, 3c, 3d) and an edge of a rim of the passage opening of the tube plate, wherein the tube elements (3, 3c, 3d) as flat tubes - are developed with flow channels for the conduction of the first fluid, wherein the flow channels are separated from one another by at least one internal structure element, and are as well - developed of a metal and the cross section of the tube elements (3, 3c, 3d) are flared in the second region (16c, 16d) in a plane oriented perpendicularly to a longitudinal direction (x) of each tube element, characterised in that the tube elements comprise at least one groove extending in the longitudinal direction (x) on an upper side as well as on a lower side, and that the internal structure element is developed by at least one welded connection in a connection region (14c, 14d), and wherein the connection region (14c, 14d) is developed so as to terminate at a predetermined distance from the at least one end of the tube element (3c, 3d) such that the second region (16c, 16d) has a planar upper side (10c, 10d) as well as a planar lower side (11c, 11d). - tube elements (3, 3c, 3d) for the conduction of the first fluid, which in each instance are developed with a first, nondeformed region (15c, 15d) and at least one second, deformed region (16c, 16d), disposed at an end of the tube element (3, 3c, 3d), - at least one tube plate with passage apertures as well as at least one sealing element with passage apertures, wherein the sealing element is in each instance disposed between an outer surface of the second region (16c, 16d) of a tube element (3, 3c, 3d) and an edge of a rim of the passage opening of the tube plate, wherein the tube elements (3, 3c, 3d) as flat tubes - are developed with flow channels for the conduction of the first fluid, wherein the flow channels are separated from one another by at least one internal structure element, and are as well - developed of a metal and the cross section of the tube elements (3, 3c, 3d) are flared in the second region (16c, 16d) in a plane oriented perpendicularly to a longitudinal direction (x) of each tube element, characterised in that the tube elements comprise at least one groove extending in the longitudinal direction (x) on an upper side as well as on a lower side, and that the internal structure element is developed by at least one welded connection in a connection region (14c, 14d), and wherein the connection region (14c, 14d) is developed so as to terminate at a predetermined distance from the at least one end of the tube element (3c, 3d) such that the second region (16c, 16d) has a planar upper side (10c, 10d) as well as a planar lower side (11c, 11d).