Heat exchanger (100, 120) having a tube bundle configured to achieve a heat exchange between a couple of fluids, the heat exchanger comprising: - the tube bundle (80, 80') inside which a first fluid of the couple of fluids flows, said first fluid passing through the tube bundle or through its portions according to a variable number of passes, - a shell inside which the tube bundle (80, 80') and the second fluid of the couple of fluids are contained, the second fluid flowing outside of the tube bundle (80, 80'), said shell provided with at least an inlet nozzle and at least one outlet nozzle of said second fluid, - at least a head (70, 70', 70") for the distribution of the first fluid inside the tubes of the tube bundle according to a variable number of passes, said head comprising for this purpose a variable number of separating baffles, fixed or movable, and at least one inlet nozzle and at least one outlet nozzle of said first fluid, wherein said heat exchanger is configured so as to be able to vary the number of passes of the first fluid without having to open the at least one head and/or remove one of the two fluids from the heat exchanger, wherein the variation of the passes number is obtained by means of the movement of at least a movable separating baffle (A, B, A', B', D, E, F, 17, 61) inside of the heads (70, 70', 70") or by varying the opening and closing of a plurality of valves external to the heat exchanger, the heat exchanger (100, 120) being characterized in that: when the variation of the passes number is obtained by means of the movement of at least a movable separating baffle (A, B, A', B', D, E, F, 17, 61) inside of the heads (70, 70', 70"), - the movable separating baffles (A, B) are hinged near the head center and rotate from a first position, in which said separating baffles are overlapping and defining a two passes heat exchanger, to a second position rotated of 90° in which said separating baffles are opposite arranged along a straight line, so as to realize a four passes heat exchanger, or when the variation of the number of passes is obtained by varying the opening and closing of a plurality of valves external to the heat exchanger, the heat exchanger comprising: - a left head, divided into four portions, having a couple of inlet nozzles for the first fluid and a couple of output nozzles for the same fluid, and provided with a first valve in fluid connection with the two inlet nozzles, a second valve in fluid connection with an inlet nozzle and an outlet nozzle, angularly adjacent to each other and a third valve in fluid connection with the two outlet nozzles, - a right head, divided into two portions, provided with a fourth valve in fluid connection with said two portions of the right head. - the tube bundle (80, 80') inside which a first fluid of the couple of fluids flows, said first fluid passing through the tube bundle or through its portions according to a variable number of passes, - a shell inside which the tube bundle (80, 80') and the second fluid of the couple of fluids are contained, the second fluid flowing outside of the tube bundle (80, 80'), said shell provided with at least an inlet nozzle and at least one outlet nozzle of said second fluid, - at least a head (70, 70', 70") for the distribution of the first fluid inside the tubes of the tube bundle according to a variable number of passes, said head comprising for this purpose a variable number of separating baffles, fixed or movable, and at least one inlet nozzle and at least one outlet nozzle of said first fluid, - the movable separating baffles (A, B) are hinged near the head center and rotate from a first position, in which said separating baffles are overlapping and defining a two passes heat exchanger, to a second position rotated of 90° in which said separating baffles are opposite arranged along a straight line, so as to realize a four passes heat exchanger, or when the variation of the number of passes is obtained by varying the opening and closing of a plurality of valves external to the heat exchanger, the heat exchanger comprising: - a left head, divided into four portions, having a couple of inlet nozzles for the first fluid and a couple of output nozzles for the same fluid, and provided with a first valve in fluid connection with the two inlet nozzles, a second valve in fluid connection with an inlet nozzle and an outlet nozzle, angularly adjacent to each other and a third valve in fluid connection with the two outlet nozzles, - a right head, divided into two portions, provided with a fourth valve in fluid connection with said two portions of the right head.