These systems control the grinding chamber environment, helping to move bulk materials through the screen. For this reason, grinders with air assist systems will ordinarily be much more efficient than those without them, yielding higher throughput. Estimates of the improvement in efficiency range from 15%-40% or even greater, depending on the material, desired particle size and overall system design.

Since hammermill pulverisers move air in a manner similar to a fan, pressure is created inside. Aspiration systems create negative pressure (i.e. a vacuum) on the discharge side, thus causing extra airflow from the grinding chamber inlet through the screen perforations. In this way, pressure in the grinding chamber is reduced, hence the alternative name “air relief system”. Particularly in the case of light particles, the vacuum helps to overcome the force of the airflow produced by the hammermill’s rotor. It is not enough simply to vent the discharge, because without aspiration, air is forced out of the mill even on the inlet side.

Apart from increased efficiency, there are also some other advantages: aspiration can help produce more uniform particle sizes, reduce heating and limit dust in the area. The reduced heating will help to reduce condensation in downstream conveying and processing, making it possible to process materials with a somewhat higher moisture content than gravity-discharge mills. Improved dust control arises from the fact that all the air in the grinding chamber is pulled in one direction, so that swirling dust is not scattered everywhere. Depending on the kind of feeder used in the hammermill, heavy materials may also be better removed.

While some mills may not have an aspiration system, retrofits are usually possible. However, the mill may need to be modified, and the aspiration system must be designed and installed properly, otherwise the system will not perform adequately.

When installing an aspiration system, air and material flow patterns into and out of the mill, airflow volumes, and the question of separating the processed material from the air are all factors that must be taken into account. In order to ensure adequate airflow volumes and patterns, air has to be introduced together with the material to be processed. An inlet that is too small will cause the system to struggle to cope and lead to excessively high air velocities. The limit for inlet air velocity should ordinarily be 10-12.5 m/s. Higher airflow volumes and lower pressures are used for coarse grinding (with screens of 2 mm or greater) than for fine grinding.

Around 0.55 to 0.658 m³/min per square metre of screen area is normally considered adequate to help move material and for dust control, but requirements can be as high as 1.317 m³/min. The precise airflow required can be determined by consultation with the manufacturer of the hammermill or the aspiration system. Expected pressure drop varies with operating conditions, but is typically 498-1244 Pa.

After the air has passed through, fines must settle before the air is filtered. A settling chamber or plenum will allow this to happen. This device will reduce the air speed as much as possible at a location far enough from the air pickup point not to permit any turbulent entrained fines to enter. As a general rule, the air velocity should fall to less than bulk density multiplied by fifteen. The larger the plenum, the better the separation of fines from the air. The plenum size will be limited by practical considerations.

The airflow path must also be controlled. To prevent air from being drawn back through the discharge system rather than through the hammermill, the conveying system requires a seal such as a shroud or rotary airlock where material is discharged from the hammermill.

Choosing the correct fan is critical to the performance of the system. Using a poor quality or undersized fan for the aspiration system can lead the hammer mill to function at low capacity. Since the processed product usually goes through the fan, a material handling fan is typically used. It may be installed either as a separate component or as an attachment to the main mill shaft.