A production mixing unit is usually not geometrically similar to the mixer used for process development. Such differences can make scale-up from the laboratory or pilot plant challenging. A solution to these problems is to systematically calculate and evaluate mixing characteristics for each geometry change.
Geometric similarity is often used in mixing scale-up because it greatly simplifies design calculations. Geometric similarity means that a single ratio between small scale and large scale applies to every length dimension (see figure). With geometric similarity, all of the length dimensions in the large-scale equipment are set by the corresponding dimensions in the small-scale equipment. The only remaining variable for scale-up to large-scale mixing is the rotational speed — one or more mixing characteristics, such as tip speed, can be duplicated by the appropriate selection of a large-scale mixer speed.