In-line rotor-stators are used in a wide range of industrial applications—primarily for dispersion processes such as emulsification, deagglomeration. Three rotor-stator heads have been used to investigate their performance in breaking up of nanoparticle clusters within a large project. This article reports the findings of a part of this study aimed at investigating the flow and power characteristics in single phase to highlight the differences of three different mixer heads. Power characteristics are determined using the calorimetry allowing the characteristic power numbers for these devices to be obtained. These are also compared with CFD calculations. Flow characteristics are studied through numerical simulations.