Cyclones are equipments used for gas-solid separation in many applications at industrial level, however, they are considered low-efficiency devices and their design have been simplified to basic models without further improvements. This paper presents a numerical study by Computational Fluid Dynamics (CFD) for gas and gas-solid flows in two designs of High-Efficiency (H.E.) cyclones: the commonly known Stairmand cyclone and one proposed by L´opez, Trujillo and Quintana (LTQ) that showed better performances according to preliminary calculations. The turbulence of the gas flow was obtained by the Reynolds Stress Model (RSM). The pressure and velocity fields of both geometries were analyzed. The results indicate that the proposed design has lower pressure drops and higher collection efficiency for the given operation conditions, allowing savings in operating costs