A heat exchanger comprising: a first straight section, a second straight section and a bent section or elbow linking the first straight section and the second straight section, each straight section (9, 10) comprising a part of an external cylindrical shell; a bundle of parallel U-bent tubes having each a first and a second straight part respectively located in said first straight section and second straight section of the exchanger and a 180°-bent part located in said bent section or elbow of the exchanger, so that, in use, a first fluid to be heated and vaporized is flowing in the tubes, said external cylindrical shell being provided respectively at one end with an inlet and at another end with an outlet for a second fluid which is a hot thermal fluid; and baffles configured to guide the second fluid, wherein the bundle of parallel U-bent tubes is connected via a connection to a first end and to a second end; characterised in that each straight section (9, 10) further comprises a part of an internal cylindrical shell, said external cylindrical shell and said internal cylindrical shell cooperating to form an intershell space enclosing the bundle of parallel U-bent tubes, so that, in use, said second fluid is flowing according to an annular flow path within the intershell space and cooling down by exchanging heat with the first fluid flowing in the tubes; and wherein the first end comprises a first hemispherical bonnet for distributing the first fluid to the tubes of the first straight section, and the second end comprises a second hemispherical bonnet for collecting the first fluid under the form of liquid, vapor or a mixture liquid /vapor from the tubes of the second straight section, said intershell space enclosing the baffles, each said connection being made of a tube sheet (11, 12) which is designed to withstand the difference between the second fluid low pressure inside said intershell space and the first fluid high pressure inside the respective bonnet, wherein the tube sheet comprises a circular plate having a central circular orifice and wherein the tube sheet further comprises a hemispherical shell located over said orifice and tightly connected to said circular plate, so as to make a physical separation between the first fluid and the second fluid. a first straight section, a second straight section and a bent section or elbow linking the first straight section and the second straight section, each straight section (9, 10) comprising a part of an external cylindrical shell; a bundle of parallel U-bent tubes having each a first and a second straight part respectively located in said first straight section and second straight section of the exchanger and a 180°-bent part located in said bent section or elbow of the exchanger, so that, in use, a first fluid to be heated and vaporized is flowing in the tubes, said external cylindrical shell being provided respectively at one end with an inlet and at another end with an outlet for a second fluid which is a hot thermal fluid; and baffles configured to guide the second fluid, wherein the bundle of parallel U-bent tubes is connected via a connection to a first end and to a second end; characterised in that each straight section (9, 10) further comprises a part of an internal cylindrical shell, said external cylindrical shell and said internal cylindrical shell cooperating to form an intershell space enclosing the bundle of parallel U-bent tubes, so that, in use, said second fluid is flowing according to an annular flow path within the intershell space and cooling down by exchanging heat with the first fluid flowing in the tubes; and wherein the first end comprises a first hemispherical bonnet for distributing the first fluid to the tubes of the first straight section, and the second end comprises a second hemispherical bonnet for collecting the first fluid under the form of liquid, vapor or a mixture liquid /vapor from the tubes of the second straight section, said intershell space enclosing the baffles, each said connection being made of a tube sheet (11, 12) which is designed to withstand the difference between the second fluid low pressure inside said intershell space and the first fluid high pressure inside the respective bonnet, wherein the tube sheet comprises a circular plate having a central circular orifice and wherein the tube sheet further comprises a hemispherical shell located over said orifice and tightly connected to said circular plate, so as to make a physical separation between the first fluid and the second fluid.