A system for the refrigeration of a liquid comprising a primary liquid cooling circuit suitable for circulating said liquid between at least one inlet and at least one outlet of said a primary liquid cooling circuit, at least a first air-cooled refrigeration unit having at least a first air-cooled cooling coil inserted on the primary liquid cooling circuit, at least a second refrigeration unit with a cooling cycle having a refrigerant fluid circuit with at least one evaporator inserted on the primary liquid cooling circuit in cascade with the first air-cooled cooling coil, means for shutting off the first air-cooled cooling coil from the primary liquid cooling circuit inserted on the primary circuit, forced air ventilation means suitable for acting on at least the first air-cooled cooling coil and on at least a second air-cooled cooling coil provided for at least one condenser of the second refrigeration unit for condensing the refrigerant fluid operating in the cooling cycle, said first and second air-cooled cooling coils (6, 10) being side by side and sharing said forced ventilation means, and a secondary liquid circuit comprising a circulation pump on which at least the first air-cooled cooling coil and at least one secondary exchanger for subcooling the refrigerant fluid leaving the second air-cooled cooling coil are inserted, the system being configured to operate in a free cooling mode, a chiller mode and a mixed mode; characterized in that: the first air-cooled cooling coil has a first a second and a third manifolds (11, 12); wherein the secondary circuit is provided with valve means comprising a valve with a first a second and a third passageways (17a, 17b, 17c) and a shut-off valve applied on the manifold (11, 12) which acts as an outlet manifold in the free-cooling and mixed operating modes; wherein in the free-cooling and mixed operating modes the first air-cooled cooling coil has the following configuration: said first and second (11, 11) manifolds (11, 12) act as liquid feed manifolds and said third manifold acts as a liquid outlet manifold; and the valve has the first and the second passageways (17a and 17b) open and the third passageway closed, whereas the shut-off valve is open to attribute to the first and second manifolds (11, 11) the function of feed manifolds and to the third manifold the function of outlet manifold; wherein in the chiller mode the first air-cooled cooling coil has the following configuration: said liquid outlet manifold is closed and one of said first and second feed manifolds has the function of outlet manifold;and the valve has the second and the third passageways (17b and 17c) open and the first passageway closed, whereas the shut-off valve is closed to close the manifold that acted as an outlet manifold in the previous status and attribute the function of outlet manifold to one of the manifolds that acted as a feed manifold in the previous status. the first air-cooled cooling coil has a first a second and a third manifolds (11, 12); wherein the secondary circuit is provided with valve means comprising a valve with a first a second and a third passageways (17a, 17b, 17c) and a shut-off valve applied on the manifold (11, 12) which acts as an outlet manifold in the free-cooling and mixed operating modes; wherein in the free-cooling and mixed operating modes the first air-cooled cooling coil has the following configuration: said first and second (11, 11) manifolds (11, 12) act as liquid feed manifolds and said third manifold acts as a liquid outlet manifold; and the valve has the first and the second passageways (17a and 17b) open and the third passageway closed, whereas the shut-off valve is open to attribute to the first and second manifolds (11, 11) the function of feed manifolds and to the third manifold the function of outlet manifold; wherein in the chiller mode the first air-cooled cooling coil has the following configuration: said liquid outlet manifold is closed and one of said first and second feed manifolds has the function of outlet manifold;and the valve has the second and the third passageways (17b and 17c) open and the first passageway closed, whereas the shut-off valve is closed to close the manifold that acted as an outlet manifold in the previous status and attribute the function of outlet manifold to one of the manifolds that acted as a feed manifold in the previous status.