Empirical models are an important tool for the selection of appropriately sized hydrocyclones for particular tasks, as well as for estimation of the performance of these classifiers. These models are usually specific to the conditions under which these classifiers are operated and can therefore not be used for prediction of anomalous conditions, such as choking of hydrocyclones. In this paper a simplified model of flow in hydrocyclones is proposed, based on elementary particle interaction theory and basic fluid flow principles. In contrast with other models published in the literature, the resulting semi-empirical model correctly simulates hydrocyclone (small laboratory scale) operation at low and high feed concentrations.