Advanced impeller design: Anti-ragging impeller, ARI2

The waste water treatment plant anoxic denitrification process employs agitated vessels which contain process fluids contaminated with fibrous, rag-like materials that have a tendency to collect on impeller blades and adversely affect mixing performance. The existing Philadelphia Mixing Solutions Ltd. anti-ragging impeller made from fiberglass reinforced plastic is not cost effective for this low torque application. The challenge was to design an impeller that replicated the performance of the original impeller but utilized common industrial manufacturing processes to reduce cost. The flexible construction technology chosen was sheet metal fabrication. The design effort produced a highly skewed, forward raked, hydrofoil impeller with applications that extend beyond the waste water treatment industry. The performance of the new impeller was evaluated by comparing it to the fiberglass impeller as well as three other common impeller types used in stirred tanks. Impeller pumping capacity was measured using 2-D Particle Image Velocimetry and impeller power draw was measured using a load cell and tachometer. The characteristic dimensionless impeller power and flow number, Np and Nq, were computed to be 0.17 and 0.42, respectively.