The large data center electricity consumption is a growing global concern. To be environment-friendly and to enhance energy efficiency in operation, data center cooling systems adopt a variety of advanced cooling and renewable energy technologies such as free cooling. However, these free cooling systems are not optimally designed in field practices, and their energy efficiencies are much lower than that of the ideal case. In this study, optimal designs in water piping, pumps and equipment sequencing control are introduced to maximize the cooling efficiency of free cooling systems. It finds that the use of distribution headers around cooling towers and pumps, the maximization of the number of operating cooling towers, the minimization of the number of operating pumps and the mixed use of large and small single-speed pumps can reduce the system's power consumption by 60% under certain operating conditions. The results also show that the designs can reduce the annual energy consumption by 3-15% depending on the climate conditions. (C) 2019 Elsevier Ltd. All rights reserved.