Limitation in the performance of fine powder separation in a turbo air classifier

The deflector wheel classifier is a widely used device for the separation of fine powders in different industrial applications. The primary objective of the separation process is to achieve high-quality separation of fine powders characterized by a narrow particle size distribution and high separation sharpness. Theoretically, the reduction in the cut size is accomplished by decreasing the gas flow rate or increasing the rotational speed of the classifier, which amplifies the centrifugal forces compared to the drag forces exerted on the particles. This behavior is, indeed, observed in many cases, but it cannot be extrapolated arbitrarily. At their performance limit, classifiers may, against expectation, show an increase in cut size and, in addition, a reduction in the sharpness of the separation process. The limitation in the reduction in the cut size and in the improvement in the separation sharpness arises due to an imbalance between the operating rotational speed and flow rate, which results in a non-uniform flow field in the classifier. If the balance conditions are fulfilled, an optimum separation with a high separation sharpness can be achieved. In this work, CFD simulations validated by some experimental results are employed to represent this limitation, which is obtained by varying the operating parameters using different material densities with particles ranging from one to ten microns.