Graef was commissioned in 1999 to design a new parking structure for General Mitchell International Airport in Milwaukee, but found the best location was on top of the existing chilled water and hot water power plant.

By Peter D. Zak, PE, Graef-USA Inc., Milwaukee

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General Mitchell International Airport (GMIA) experienced a tremendous increase in the number of air travelers during the late 1990s. Although there were plans for additional ticketing, baggage, and departure facilities, the real need in 1999 was for a parking structure.

The airport hired Graef-USA to design the new structure and identify the most cost-effective client friendly site. After multiple attempts to locate the parking structure without disturbing possible future concourse locations, a site was located. Unfortunately, it was right on top of the existing chilled water (CHW) and hot water power plant. Using this as an opportunity to upgrade the plant’s efficiency, reduce operating costs, and plan for future expansion, GMIA moved forward with a new plant design located about ¼ mile from the existing plant. This presented several challenges, such as matching new state-of-the-art heating and cooling systems with existing distribution systems in the concourse areas.

Graef relied on a previous master plan and study of the existing mechanical systems. The study identified flow deficiencies, along with corrective actions in both the power plant and the airport facility. This data defined current and future needs, along with system diversity factors that helped reengineer the piping distribution system.

The new plant was commissioned in 2001. It consists of four 500-ton centrifugal chillers and three 15,000 MBH hot water boilers. The hot water, CHW, and condenser water systems incorporate primary variable flow pumping strategies. The plant was designed to accommodate two additional 500-ton chillers and one additional 15,000 MBH boiler.

The tertiary pumping systems within the airport terminal building were removed, which equated to approximately 500 hp of pumps and motors. The distribution piping was reconfigured and the appropriate flow control sensors were installed.

Operating costs after one year of operation showed an annual cost savings of approximately $250,000. The plant is poised to efficiently accept the challenges of the next facility expansion—whenever it occurs.