Rotor-stator devices are increasingly important for process intensification applications but the understanding of their operation and scale-up is poor. Consequently a new expression is presented for the calculation of the delivered power. which consists of three terms. Firstly a term reflecting the power required to rotate the shaft in response to the resistance of the fluid. Secondly a term reflecting the convection of power away from the mixing chamber clue to the flow of product. Thirdly, a term reflecting losses. A constant is required for each term and the expression describes the power consumption over the range 5-20 kW with a RMSE of 0.5 kW. (C) 2008 Elsevier B.V. All rights reserved.