Slurry mixing with impellers: Part 1, theory and previous research

Abstract

Mixing is essential for effective aerobic or anaerobic treatment of farm slurries, but can add significantly to the fixed and variable costs of such treatments. Rotating impellers are frequently used for mixing but their efficiency depends strongly on the relationships between their diameter, shape and speed. Previous research studies have attempted to explore these relationships with agricultural slurries in laboratory and pilot-scale mixing experiments, but none has used volumes greater than 1 m3. Hence substantial scale-up factors are required to apply the results to full-size systems which may be 1000 m3 or more, and therefore slurry mixer design based on present research findings is susceptible to large errors. To provide a better basis for mixer scale-up, the appropriate theories of impeller design are discussed in this paper, and various alternative expressions for scale-up of impellers are compared. The problems in determining the impeller Reynolds number are also discussed and the most appropriate method is identified for large-scale mixing experiments. Also, a novel expression is derived to assess the effectiveness of slurry mixers.