Developments in scale-up procedures for industrial dryers

The aim of dryer scale-up is to increase batch size, or throughput of a continuous process, while maintaining final moisture content, product quality, end-use properties, safety and reliability. This article reviews and updates previously published material on scale-up processes of dryers in general and individual dryer types, particularly material from a Drying Technology special issue on scale-up in 1994. This is adapted to give a basic generic theory for scale-up, applying this to various dryers and noting specific exceptions. It is demonstrated that simplifying assumptions can often be reliably made for scale-up which reduce the level of complexity for modeling compared to equipment design and performance calculations. For example, prediction of drying rate and drying time in spray and flash dryers needs a kinetic model, but for scale-up a heat balance and scaling calculation usually suffices. Approximations such as a one-third power law for drying time are often effective. It is shown that there is some theoretical basis for empirical scale-up rules that have been used by dryer manufacturers. Scale-down is also important, selecting the most appropriate conditions for small-scale tests that will correspond to the desired performance at commercial scale. The individual concepts are not new, but they have not previously been unified into a simply stated general basis for scale-up.