Energy consumption rates in a data center have increased year by year, and by this, the introduction of energy conserving cooling systems is rapidly emerging all over the world. In this study, a multi-stage outdoor air enabled (MOA) cooling system is proposed as the safer and more energy conserving system for a data center. An MOA cooling system is composed of the combination of water-side economizer, air side economizer and mechanical cooling. Effects of an MOA cooling system are investigated by energy simulation from various viewpoints. The results show that the MOA cooling system can save cooling energy consumption rates by about 21%, and if combined with a special rack enclosure, by about 33%. Introducing outdoor air flow into the MOA cooling system decreases energy use to about half of that of the air-side economizer, and if combined with the rack enclosure, by about 9%. Thus, the concerns about particulate and gaseous contamination can be reduced. Continuous operating time of the humidifier drops to about 3 days, and if combined with the rack enclosure, to '0' day. These results make it possible to remove the humidifier as part of the MOA cooling system. (C) 2016 Elsevier B.V. All rights reserved.