Impeller Design Choice Is Key to Stock Agitator Efficiency

Author:

Tom C. Devries

The paper reviews the importance of impeller design in stock agitator efficiency for papermaking, focusing on a comparison between axial-flow and radial-flow designs in side-entering applications. It argues that a truly axial-flow impeller, like the Lightnin A312, is both more process-efficient (higher flow per horsepower) and mechanically sound than radial or "quasi-axial" alternatives, which often require larger diameters and shafts to compensate for inefficiencies and higher mechanical loads. The paper concludes that proper evaluation of agitator performance must prioritize the axial-flow component and consider the mechanical demands imposed by different impeller design choices.

Key Learnings

• Impeller design is the most critical factor for successful stock agitator performance, determining both process results (blending/storage) and energy efficiency.
• Axial-flow impellers are mechanically sound and energy-efficient for side-entering stock agitation, while radial-flow impellers create throttled, inefficient, and unacceptable flow patterns in this mode and should be avoided.
• The goal is a truly axial-flow impeller; "quasi-axial" designs contain a radial-flow component that reduces flow efficiency (flow per horsepower) and creates additional mechanical loads.
• An agitator is considered "energy-efficient" if it requires less horsepower to do the work, meaning it has a higher flow per horsepower.
• Two paths to energy efficiency exist: developing a more efficient design (like the Lightnin A312) or increasing the impeller diameter to lower the required horsepower.
• While increasing diameter can reduce horsepower, it significantly increases the mechanical demands on the agitator system (e.g., higher bending stress and deflection), necessitating a larger shaft to maintain mechanical integrity.
• The axial-flow component (Qₐ) is the only part of the flow that contributes to stock motion and should be the value used to evaluate an agitator's pumping capacity, not the total resultant flow (Q).
• Proper agitator mounting requires the impeller to be located no less than one-half of its diameter from the wall to ensure it draws the proper horsepower and pumps the necessary flow.