Comparison between flow velocity profiles in conical and cylindrical hydrocyclones

Both conical and cylindrical cyclones are used in the size classification and density separation of particles. Conical hydrocyclones have a more general use and have been thoroughly studied concerning both flow velocity profiles and their classification performance. Cylindrical cyclones with peripheral underflow discharge have been used less and studied less, but there has recently been renewed interest. This concerns particularly density separation as well as size classification in two-stage cylinder-cyclone systems. Tangential velocity profiles differ in conical and cylindrical cyclones. In conical cyclones, they approach the theoretical free vortex profile (with a forced vortex region confined near the axis) ; in cylindrical cyclones with peripheral underflow discharge they approach the forced vortex profile in a large part of the vessel. These results have been obtained by making Laser Doppler Velocimetry (LDV) measurements on 100-mm diameter cylindrical cyclones with peripheral and axial discharges under different flow conditions and with different ratios between overflow and underflow flow rates. It has been demonstrated that the occurrence of the one or the other vortex type does not depend on the vessel shape alone but on the inward radial flow patterns inside the vessel. This depends on how the fluid is fed and discharged as well as on the vessel shape. Finally, the paper discusses the influence that the different tangential and axial velocity profiles in conical and cylindrical cyclones have on separation sharpness, in both size classification and density separation processes.