A data center cooling system comprising: at least one air-to-liquid heat exchanger adapted to cool air from a hot aisle of at least one room of a data center; a liquid-to-liquid heat exchanger fluidly connected to the at least one air-to-liquid heat exchanger; a first chiller fluidly connected to the liquid-to-liquid heat exchanger; a second chiller fluidly connected to the first chiller; wherein the first and second chillers are vapor-compression chillers including a refrigerant loop inside which a refrigerant flows; at least one first pump fluidly connected to the at least one air-to-liquid heat exchanger for pumping in a first coolant loop a first coolant from the at least one air-to-liquid heat exchanger, the first coolant flowing selectively to the liquid-to-liquid heat exchanger, and to the first and second chillers prior to flowing back to the at least one air-to-liquid heat exchanger; a free cooling unit fluidly connected to the liquid-to-liquid heat exchanger for cooling the first coolant flowing therethrough and fluidly connected to the first and second chillers for condensing refrigerant circulated therein; and at least one second pump fluidly connected to the free cooling unit for pumping in a second coolant loop a second coolant from the free cooling unit, the second coolant flowing selectively to the liquid-to-liquid heat exchanger, and to the first and second chillers prior to flowing back to the free cooling unit; wherein the at least one air-to-liquid heat exchanger, the liquid-to-liquid heat exchanger, the first chiller and the second chiller are fluidly connected in series; wherein the free cooling unit, the liquid-to-liquid heat exchanger, the first chiller and the second chiller are fluidly connected in series; wherein when the first coolant flows to the liquid-to-liquid heat exchanger and to the first and second chillers, the first coolant flows sequentially from the at least one air-to-liquid heat exchanger, to the liquid-to-liquid heat exchanger, to the first chiller, to the second chiller, and back to the at least one air-to-liquid heat exchanger in the first coolant loop; wherein when the second coolant flows to the liquid-to-liquid heat exchanger and to the first and second chillers, the second coolant flows sequentially from the liquid-to-liquid heat exchanger, to the second chiller, to the first chiller, to the free cooling unit, and back to the liquid-to-liquid heat exchanger in the second coolant loop; wherein respective valves (44, 62, 66, 82, 84, 86) are provided to selectively bypass each of the liquid-to-liquid heat exchanger, the first chiller, and the second chiller. at least one air-to-liquid heat exchanger adapted to cool air from a hot aisle of at least one room of a data center; a liquid-to-liquid heat exchanger fluidly connected to the at least one air-to-liquid heat exchanger; a first chiller fluidly connected to the liquid-to-liquid heat exchanger; a second chiller fluidly connected to the first chiller; wherein the first and second chillers are vapor-compression chillers including a refrigerant loop inside which a refrigerant flows; at least one first pump fluidly connected to the at least one air-to-liquid heat exchanger for pumping in a first coolant loop a first coolant from the at least one air-to-liquid heat exchanger, the first coolant flowing selectively to the liquid-to-liquid heat exchanger, and to the first and second chillers prior to flowing back to the at least one air-to-liquid heat exchanger; a free cooling unit fluidly connected to the liquid-to-liquid heat exchanger for cooling the first coolant flowing therethrough and fluidly connected to the first and second chillers for condensing refrigerant circulated therein; and at least one second pump fluidly connected to the free cooling unit for pumping in a second coolant loop a second coolant from the free cooling unit, the second coolant flowing selectively to the liquid-to-liquid heat exchanger, and to the first and second chillers prior to flowing back to the free cooling unit; wherein the at least one air-to-liquid heat exchanger, the liquid-to-liquid heat exchanger, the first chiller and the second chiller are fluidly connected in series; wherein the free cooling unit, the liquid-to-liquid heat exchanger, the first chiller and the second chiller are fluidly connected in series; wherein when the first coolant flows to the liquid-to-liquid heat exchanger and to the first and second chillers, the first coolant flows sequentially from the at least one air-to-liquid heat exchanger, to the liquid-to-liquid heat exchanger, to the first chiller, to the second chiller, and back to the at least one air-to-liquid heat exchanger in the first coolant loop; wherein when the second coolant flows to the liquid-to-liquid heat exchanger and to the first and second chillers, the second coolant flows sequentially from the liquid-to-liquid heat exchanger, to the second chiller, to the first chiller, to the free cooling unit, and back to the liquid-to-liquid heat exchanger in the second coolant loop; wherein respective valves (44, 62, 66, 82, 84, 86) are provided to selectively bypass each of the liquid-to-liquid heat exchanger, the first chiller, and the second chiller.