The present investigation is concerned with the development of scale-up equations for the oxygen transfer coefficient, k, and the power number, P-O, for unbaffled circular tank surface aerators under geometrically similar conditions, with the purpose of designing energy efficient aerators. It has been found for the present aerators that k and P-O are uniquely related to a parameter, X, governing the theoretical power per unit volume and which is defined as (FrRe1/3)-Re-4/3, where Fr and Re are the impellers' Froude and Reynolds numbers, respectively. Empirical correlations between k and X, as well as P-O with X are developed for the range of experiments conducted. Based on such experimental results, procedures to design energy efficient aeration systems have been demonstrated and it was found that smaller sized aerators are more energy efficient and economical when compared to bigger sized tanks, while aerating the same volume of water, by maintaining the same input power to the tanks irrespective of their size. It has been also demonstrated that substantial amounts of energy can be saved if the aeration tanks run at higher input power.