An aspirator (also called an air classifier) separates lighter material — husks, chaff, dust and other low-density impurities — from heavier grain or ingredient particles by passing a controlled stream of air through or across the material flow, lifting and carrying away the lighter fraction while allowing the denser, target material to fall through and continue along the process line. Unlike screening, which separates by particle size, aspiration separates primarily by density and aerodynamic drag, making it effective at removing contaminants that may be similar in size to good product but considerably lighter.

In feed mills, aspirators are most commonly used during ingredient cleaning at receiving, removing chaff, light foreign material and dust from incoming grain before it proceeds to storage or grinding, improving both product quality and protecting downstream equipment from unnecessary wear caused by abrasive foreign material.

Aspirator designs range from simple gravity-fed units, where material falls through a cross-current of air in a relatively short vertical drop, to more elaborate multi-stage designs incorporating additional separation chambers or recirculating air systems intended to improve separation efficiency for difficult-to-clean ingredient streams.

Air volume and velocity through the aspirator are the primary tunable parameters affecting separation performance: insufficient airflow fails to lift and remove lighter contaminants effectively, while excessive airflow risks carrying away an unacceptable proportion of good, marketable product along with the intended waste fraction, representing a real yield loss if not properly balanced.

Aspirated material removed from the main product stream is typically captured via a cyclone or bag filter system connected to the aspirator's exhaust air, both to prevent the removed material (and any associated dust) from simply being vented into the working environment, and in some cases to allow recovery or appropriate disposal of the separated material depending on its composition and value.

Aspirators are often installed in combination with screening equipment as part of a broader ingredient cleaning system, since the two technologies are complementary — screening efficiently removes material that differs in size from the target product, while aspiration efficiently removes material that may be similarly sized but differs in density, together providing more thorough cleaning than either technology alone.