Connection between cooled pipe and uncooled pipe in a double-pipe heat exchanger

A union connection between uncooled pipe and cooled double-wall pipe in a heat exchanger comprising a double-wall pipe (12) comprising in turn an internal pipe (15) traveled by a fluid to be cooled and an external pipe (14) defining with the internal pipe an air space (19) traveled by a cooling fluid with one end of the double-wall pipe (12) being connected to an inlet duct (30) of the fluid to be cooled through a connection part (16) also forming a bottom wall (18) of the air space virtually transversal to the double-wall pipe extension and characterized in that the connection part (16) has an annular form with U cross section to define two annular shanks (20, 21) extending longitudinally to the pipe (12) with each shank being welded to one end of one of the two pipes (14, 15) of the double-wall pipe (12) and in that the end (22) of the inlet duct (30) is welded to the connection part at said bottom wall (18) of the air space.