A heat exchanger comprising: a shell with an inner chamber through which a first coolant fluid circulates, a core comprising at least a bundle (2.1) of tubes through which a second fluid to be cooled circulates, and where the shell houses the at least one bundle (2.1) of tubes, where between the bundle (2.1) of tubes and the wall of the inner chamber of the shell there is a perimetral gap in at least one section of the length of the bundle (2.1) of tubes, a deflector located in the perimetral gap, where said deflector comprises: - a tubular body (3.1) adapted for surrounding at least one section of the length of the bundle (2.1) of tubes, - a first expansion section (3.2) arranged at one end of the tubular body (3.1) such that the tubular body (3.1) is extended longitudinally by means of the first expansion section (3.2) where: o the first expansion section (3.2) covers a larger perimeter than the perimeter covered by the tubular body (3.1) to reduce the perimetral gap between the bundle (2.1) of tubes and the wall of the inner chamber of the shell, o between the first expansion section (3.2) and the tubular body (3.1) there is a transition such that a first coolant fluid flow passage restriction is established for restricting it to the space between the tubes of the bundle (2.1) of tubes, characterized in that o the first expansion section (3.2) is elastically deformable to assure support against the wall of the inner chamber of the shell. a shell with an inner chamber through which a first coolant fluid circulates, a core comprising at least a bundle (2.1) of tubes through which a second fluid to be cooled circulates, and where the shell houses the at least one bundle (2.1) of tubes, where between the bundle (2.1) of tubes and the wall of the inner chamber of the shell there is a perimetral gap in at least one section of the length of the bundle (2.1) of tubes, a deflector located in the perimetral gap, where said deflector comprises: - a tubular body (3.1) adapted for surrounding at least one section of the length of the bundle (2.1) of tubes, - a first expansion section (3.2) arranged at one end of the tubular body (3.1) such that the tubular body (3.1) is extended longitudinally by means of the first expansion section (3.2) where: o the first expansion section (3.2) covers a larger perimeter than the perimeter covered by the tubular body (3.1) to reduce the perimetral gap between the bundle (2.1) of tubes and the wall of the inner chamber of the shell, o between the first expansion section (3.2) and the tubular body (3.1) there is a transition such that a first coolant fluid flow passage restriction is established for restricting it to the space between the tubes of the bundle (2.1) of tubes, characterized in that o the first expansion section (3.2) is elastically deformable to assure support against the wall of the inner chamber of the shell. - a tubular body (3.1) adapted for surrounding at least one section of the length of the bundle (2.1) of tubes, - a first expansion section (3.2) arranged at one end of the tubular body (3.1) such that the tubular body (3.1) is extended longitudinally by means of the first expansion section (3.2) where: o the first expansion section (3.2) covers a larger perimeter than the perimeter covered by the tubular body (3.1) to reduce the perimetral gap between the bundle (2.1) of tubes and the wall of the inner chamber of the shell, o between the first expansion section (3.2) and the tubular body (3.1) there is a transition such that a first coolant fluid flow passage restriction is established for restricting it to the space between the tubes of the bundle (2.1) of tubes, characterized in that o the first expansion section (3.2) is elastically deformable to assure support against the wall of the inner chamber of the shell. o the first expansion section (3.2) covers a larger perimeter than the perimeter covered by the tubular body (3.1) to reduce the perimetral gap between the bundle (2.1) of tubes and the wall of the inner chamber of the shell, o between the first expansion section (3.2) and the tubular body (3.1) there is a transition such that a first coolant fluid flow passage restriction is established for restricting it to the space between the tubes of the bundle (2.1) of tubes, characterized in that o the first expansion section (3.2) is elastically deformable to assure support against the wall of the inner chamber of the shell.