A relationship describing the suspension of particulate solids in a propeller loop reactor is derived and applied by analogy to the corresponding process in stirred tanks. According to this equation, the ratio of minimum power input for suspension to settling power under the conditions of laminar flow, is inversely proportional to the tank diameter. In contrast, the power ratio remains constant for fully turbulent flow. The observations presented explain the fact that numerous authors have reported various scale-up rules which range between these two extremes. The scale-up procedure is demonstrated for a 21 m(3) full-scale tank.