The Perfect Pair: Chopper Pumps and High-Velocity Nozzles

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The Perfect Pair: Chopper Pumps and High-Velocity Nozzles

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Digester mixing is a dirty job, but someone — or something — has to do it. As treatment plant operators know, the absence of pit mixing leads to thick sludge and difficult-to-pump slurries. To solve this problem, some chopper pump manufacturers offer mechanical hydraulic mixing systems. For example, the Vaughan Company’s Rotamix system pairs a chopper pump and a set of high-velocity nozzles to mix the tank while handling any tough solids. The result is a homogenous-state pit.

The Vaughan chopper pump is the heart of the Rotamix system. These centrifugal pumps chop all incoming solids prior to pumping. In the mixing pit, the chopper pump continuously chops solids in order to eliminate nozzle clogging, enhance sludge quality and increase digester efficiency by further reducing solid sizes and increasing surface contact.

The system’s mixing power is supplied by fixed nozzle assemblies installed at a factory-specified angle and permanently tightened so no additional adjustments are required. These high-velocity nozzles create a “dual-zone” mixing pattern that combines uniform and vortical flow fields. In a uniform flow field, the entire contents rotate as a solid unit with the highest velocity at the outside. In a vortical flow field, fluid velocities are the greatest at the center, thus creating a vertical-axis vortex. With dual-zone mixing, average velocities are higher and steadier, preventing solids from settling in the center.

Vaughan offers nozzle assemblies for Rotamix in single and double nozzle configurations, depending on tank diameter and depth. All fittings are ductile cast iron and glass lined for abrasion resistance, reduced friction and to eliminate struvite adhesion. The tapered nozzle barrels are 1 inch thick for long-term wear protection. The entire assembly is white metal sandblasted and powder coated with 3M Scotchkote Fusion Bonded Epoxy Coating, a one-part, heat-curable, thermosetting epoxy coating for superior surface protection. 

To see what this system can do and learn about the Rotamix system of mechanical hydraulic mixing, visit


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