The maximum inlet air temperature and air recirculation ratio of information technology machines are important in evaluating the air-conditioning efficiency of data centres. In this study, evaluation models for estimating air and heat distributions were developed based on the thermal equilibrium in data centres, assuming conventional computer room air-conditioning and local computer room air-conditioning systems. The computer room air-conditioning cooling efficiency ( η-index) was defined as the ratio of the effective cooling air volume of an information technology machine to the air volume of the computer room air-conditioning system. The recirculation ratio for an information technology machine ( γ-index) was defined based on the air volume and thermal equilibrium. Several factors affecting air-conditioning efficiency, such as the mounting positions and heat load of the information technology machines and the ratio of the computer room air-conditioning system air volume to that of the information technology machine ( V/Vm-index), were analysed using computational fluid dynamics. A mismatch was confirmed between the heat recirculation ratio ( γm-index) and air recirculation ratio ( γq-index), and the effect of heat transmission by the wall could be ignored in data centres. The mean γ values and inlet air temperatures ( θ0 m-index) could be reduced effectively by increasing the value of V/Vm and mounting the information technology machines at the bottom of each rack.
