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Rotary airlocks function as pressure boundaries as much as metering devices. The rotor tip clearance governs particle attrition at the nip zone and the rate of fine-particle bypass under differential pressure. As clearance widens through wear, the downstream particle size distribution shifts in a pattern frequently misread as upstream process variation.

Both the Hausner Ratio and Carr Index reduce bulk and tapped density to a single number. The mechanism behind that number, whether aeration, particle shape, or surface cohesion, determines whether it supports a process decision or actively misleads one.

The same screw feeder at the same speed can deliver consistently on one powder and drift by several percent on another. The difference lies in compressibility, wall friction, and cohesion, and in how bulk density shifts as the hopper empties.

Process analytical technology combined with machine learning is changing residual moisture control and yield prediction in pharmaceutical and food spray drying. The gains are real but bounded, and the off-line test panel has not been replaced.