What is worse, a number of different, even contrary, results were obtained during the past years. For instance, some argued that the “fish- hook effect”, that is, an inflexion in the separation efficiency curve indicating a dip at sub sieve sizes, was of physical origin, whereas others believed that it was just random and sporadic phenomenon caused by the measurement errors. Most researchers be- lieved that the feed pressure did not have significant effect on the se- paration performance of hydrocyclones, and it was not need to use high feed pressure (> 150 kPa) to enhance hydrocyclone separation. Nevertheless, by using a hydrocyclone operating at high feed pressure up to 6 MPa, Neesse et al. realized the solids classi- fication in submicron range, which cannot be achieved by utilizing hydrocyclones with lower feed pressure.