Cyclones are equipment for gas-solid separation which are used in industry for many applications, however, they are considered low-efficiency equipment and their design has been simplified to basic models without further improvement. This paper presents a numerical study by computational fluid dynamics in single phase flow for two designs of high efficiency cyclones: the Stairmand commonly used in industry and one proposed by Lopez and Trujillo that according to preliminary calculations showed better performances. The turbulence of the gas flow was obtained by the Reynolds stress model. The flow fields and pressure obtained for both geometries were analysed. The results showed that the proposed design had lower pressure drops and therefore savings in operating costs.