A heat exchanger for the transfer of heat between a first fluid, a hot fluid, and a second fluid, a cold fluid, wherein said heat exchanger comprises: - a heat exchange tube bundle extending according to a longitudinal direction (X-X') between an inlet for the first fluid into the tube bundle and an outlet for the first fluid from the tube bundle, wherein the heat exchange tube bundle is intended for driving the flow of the first fluid, - a shell housing the heat exchange tube bundle and extending according to the longitudinal direction (X-X') at least between the inlet for the first fluid and the outlet for the first fluid, wherein the shell is intended for driving the second fluid through the space defined between the tubes of the heat exchange tube bundle and the shell, - a tubular inlet segment for the entry of the second fluid into the shell and a tubular outlet segment for the exit of the second fluid from inside the shell, wherein - the exchanger comprises at least one first manifold (3, 4) located, according to the longitudinal direction (X-X'), close to either the inlet for the first fluid or else the outlet for the first fluid, and positioned: ∘ either between the first tubular inlet segment and the shell, ∘ between the second tubular outlet segment and the shell, ∘ or else in both locations; - the shell has a slot (6.2) for the passage of the second fluid between the inside of the first manifold (3, 4) and the inside of the shell in which the tube bundle of the heat exchanger is housed, this slot (6.2) being spaced from the corresponding inlet/outlet (I, 0) for the first fluid in which said first manifold (3, 4) is located, - the shell has an extension (6.1) into the first manifold (3, 4) such that this prolongation (6.1) establishes a partial closure in the fluidic communication between the inside of the first manifold (3, 4) and the inside of the shell in which the tube bundle of the heat exchanger is housed, characterized in that - the slot (6.2) has an essentially elongated configuration according to a main direction and has a variable width. - a heat exchange tube bundle extending according to a longitudinal direction (X-X') between an inlet for the first fluid into the tube bundle and an outlet for the first fluid from the tube bundle, wherein the heat exchange tube bundle is intended for driving the flow of the first fluid, - a shell housing the heat exchange tube bundle and extending according to the longitudinal direction (X-X') at least between the inlet for the first fluid and the outlet for the first fluid, wherein the shell is intended for driving the second fluid through the space defined between the tubes of the heat exchange tube bundle and the shell, - a tubular inlet segment for the entry of the second fluid into the shell and a tubular outlet segment for the exit of the second fluid from inside the shell, - the exchanger comprises at least one first manifold (3, 4) located, according to the longitudinal direction (X-X'), close to either the inlet for the first fluid or else the outlet for the first fluid, and positioned: ∘ either between the first tubular inlet segment and the shell, ∘ between the second tubular outlet segment and the shell, ∘ or else in both locations; ∘ either between the first tubular inlet segment and the shell, ∘ between the second tubular outlet segment and the shell, ∘ or else in both locations; - the shell has a slot (6.2) for the passage of the second fluid between the inside of the first manifold (3, 4) and the inside of the shell in which the tube bundle of the heat exchanger is housed, this slot (6.2) being spaced from the corresponding inlet/outlet (I, 0) for the first fluid in which said first manifold (3, 4) is located, - the shell has an extension (6.1) into the first manifold (3, 4) such that this prolongation (6.1) establishes a partial closure in the fluidic communication between the inside of the first manifold (3, 4) and the inside of the shell in which the tube bundle of the heat exchanger is housed, characterized in that - the slot (6.2) has an essentially elongated configuration according to a main direction and has a variable width.