The Design of Draft Tube Circulators

Author:

John A. Shaw

The article discusses the design, operation, and optimization of draft tube circulators as a more efficient alternative to Pachuca air-lift agitators for slurry agitation in large tanks, particularly in the alumina industry. It covers critical design parameters such as draft tube/tank diameter ratio, impeller type and efficiency, hydraulic power requirements, and tank bottom configuration, as well as operational considerations like resuspension, startup, and intertank connections. Practical examples and historical developments illustrate how optimized circulator systems reduce energy consumption, improve process stability, and enhance overall system performance.

Key Learnings

• Draft tube circulators provide a more energy-efficient alternative to Pachuca air-lift agitators for large slurry tanks.
• Optimal draft tube/tank diameter ratio is critical for minimizing power requirements and ensuring proper flow.
• Impeller design, including aerofoil blades and progressive pitch, significantly impacts hydraulic efficiency and stability.
• Tank design, particularly flat bottoms with fillets, improves flow distribution and reduces solids deposition.
• Resuspension vents and slots help prevent shutdown issues and ensure stable operation after power interruptions.
• Accurate matching of impeller, draft tube, and speed reducer is essential for energy savings and system performance.
• System efficiency depends on hydraulic design, impeller efficiency, and speed reducer performance.
• Practical applications include alumina precipitation, gold leaching, wastewater treatment, and other high-volume slurry processes.
• Early designs required higher motor power, but modern optimized designs reduce operating costs and first costs through improved hydraulics.
• Proper intertank connections and careful control of liquid levels prevent short-circuiting and ensure consistent circulation.
• Material selection and impeller construction influence longevity, maintenance, and repair feasibility.
• Specific speed considerations help in determining optimal impeller rotation for desired flow and head without excessive energy use.