The paper presents optimization model of the chilled water based data center cooling system. The optimization procedure includes system technological and mathematical model, limiting conditions and optimization criterion, which in this case is annual power consumption minimum. The cooling system model is defined by constant parameters and decision variables and consists of aircooled chiller, independent external freecooling heat exchanger (drycooler), computer room air handling unit (CRAH) and constant flow chilled water system with circulation pump. Influence of the server racks architecture (open or closed rack aisle), server inlet temperature and chilled water regime on the annual power consumption of the cooling system has been shown. Case study calculation based on the described model has been presented, including optimum variant designation and power usage effectiveness ratio of mechanical system calculation.