Multi-stage outdoor air enabled (MOA) cooling strategy integrated with the merits of air-side economizers and water-side economizers was established as a safer and energy conserving system for data centers. The MOA cooling system combined with a rack enclosure delivers a power reduction of 32.6% compared with the standard mechanical cooling system. There is a significant reduction in operating time of the humidifier, which leads to the reasoning that it should be possible to remove the humidifier from the MOA cooling system. The supplied outdoor air volume in the investigated MOA cooling system decreases largely.