A: Jack Osborn, Airdusco, says:
Achieving optimal performance from your hammermill is related to different factors. Having experience with many hammermill applications, I’ve learned that the following operational conditions are critical to achieve successful milling results.
Control the raw material feedrate into the mill. Often a hammermill supplier will prefer to use their company’s rotary feeder or a similar device to meter the raw material into the hammermill. However, a rotary feeder is a volumetric feeding device and its actual feedrates can only be inferred. Using a weighbelt or loss-in-weight feeder or a similar device is recommended to provide an actual weight-based feedrate to the hammermill. This controlled-rate equipment will assist significantly in optimizing your hammermill’s performance.
For most applications, use a hammermill with a controlled airflow. This airflow is necessary to assist in the milling process by maintaining material temperature control, mitigating material accumulations on the screen’s low-pressure side, and providing conveying air to carry the sized material through the hammermill’s screens. This is an induced airflow through slots or similar openings in the rotary feeder housing, the hammermill body, or both. Most of the induced airflow must enter the hammermill with the raw material feed (actual percentage can vary).
Some materials are sensitive to heat loading and can blind the internal screens and build up on internal hammermill surfaces. Thus, the induced airflow’s temperature is critical to optimizing hammermill operation. Providing humidity and temperature-controlled airflow into the hammermill may also be necessary.
Ensure proper screen sizing. Having a supplier test the material to determine what screen size will be best to produce the desired material results is key. The testing is also beneficial for determining proper operating parameters such as induced airflow and operating temperature. For existing operations, experimentation may be necessary to determine the optimal screen sizing.
Pneumatically convey material to an air-material separator. Once the raw material is milled, it’s necessary (unless the hammermill discharges directly into storage) to pneumatically convey the material to an air-material separator (AMS). Typically, the airflow volume induced through the hammermill isn’t sufficient to convey the milled material’s mass. Therefore, additional airflow is required to provide the airflow volume needed to convey the material to the desired discharge location. Maintaining the overall airflow volume (air mass) is important. Low airflow volume will result in material accumulations within the conveying line, resulting in an upset condition.
Select the proper AMS type. This is typically a dust collector, filter-receiver, cyclone, or combination of a cyclone and dust collector. Too often the dust collector’s filter area is insufficient to provide long-term and steady-state operation. Being conservative in your AMS selection rarely introduces negative performance factors. Ensuring first-in, first-out continuous material discharge is also imperative. The fan or blower package used for inducing the airflow must also be carefully selected for the application.
The device selected must allow for all operational conditions, such as differential pressure loss through the dust collector or cyclone, required vacuum at the feed to the mill, inducing additional conveying airflow beneath the mill among others.
Provide proper continuous monitoring of operational conditions. Operational factors to monitor for optimal hammermill performance include the raw material feedrate, fan or blower performance and the differential pressure across the AMS. The hammermill’s inlet and outlet temperatures, drive motor amperage, and the mill’s inlet and discharge differential pressure are also important to monitor.
Don’t miss Jack Osborn’s upcoming webinar “Dust collection hood design for engineers and nonengineers.” To learn more and register, click here.
Jack Osborn is the engineering manager at Airdusco.