A modular cooling system configured to treat IT air generated by a data center includes a frame and a plurality of cooling sub-system modules supported by the frame. The plurality of cooling sub-system modules are configured to operate in parallel to achieve total cooling effect or a lesser cooling effect with some level of redundancy within the data center. Each cooling sub-system module includes a housing configured to support cooling equipment, an air-to-air heat exchanger supported by the housing to cool IT air generated by the data center, the air-to-air heat exchanger having at least one tube configured to direct IT from one end of the air-to-air heat exchanger to an opposite end of the air-to-air heat exchanger and configured so that outdoor air circulates around the at least one tube, and a mechanical cooling system supported by the housing. The mechanical cooling system is configured to receive IT air treated by the air-to-air heat exchanger and to provide further cooling to the treated IT air. Other embodiments of the cooling system and methods of cooling are further disclosed.