Armstrong S-57-3 3-Phase AB Bronze In-Line Circulators are apart of the Series S. The 3/4 HP bronze circulator pump is suitable for hydronic heating and cooling, domestic water systems, multi-stage zoning and general industrial service applications. Built using a standard three-piece design that features a radially-split body, oversized shaft, centrifugal impeller, positive mechanical seal and modular construction, Armstrong Pumps Series S Circulators are a trusted, durable product that have been used by HVAC professionals for decades.