Hydrocyclones were originally designed to promote solid-liquid separations but nowadays they are also used for solid-solid, liquid-liquid and gas-liquid separations. Although a hydrocyclone is a very simple equipment to build, the we of custom-made cyclones is not widely used. This is probably due to the lack of a simple procedure for hydrocyclone design. In the present work a procedure is presented which allows the design and performance prediction of hydrocyclones that follow Bradley and Rietema recommended geometries, that are the only two well-known families of geometrically similar hydrocyclones. The procedure proposed here resulted in little error, when the results were compared with experimental data. Additionally, a comparison of both families of hydrocyclones revealed that, for a given hydrocyclone diameter and at the same operational conditions, Bradley hydrocyclones provide higher efficiencies, while Rietema hydrocyclones give higher capacities. (C) 2000 Elsevier Science Ltd. All rights reserved.