A New Beginning for an Old DevelopmentIn 1855, when Samuel Colt built the American Repeating Arms Co. on the banks of the Connecticut River, the City of Hartford was a rising star on the New England industrial landscape. However, in recent decades, the former industrial powerhouse and “insurance capital of the world” began a precipitous fall into the ranks of America’s fading urban centers. To jump-start the rejuvenation of the city’s commercial prospects, real estate development corporation Homes For America Holdings Inc. has partnered with the City of Hartford to embark upon a major urban development project – the historic renovation of the old Colt factory – a major symbol of the city’s industrial glory days. Aptly named Colt Gateway, this $110-million development project will convert the 11 antiquated buildings of the American Repeating Arms Co. into 2,200 residential apartments, condos, offices, and a school. This planned live/work campus on the banks of the Connecticut River will offer downtown living in a park-like setting with an all-new, state-of-the-art infrastructure.Design engineers at the Maguire Group (Foxboro, MA) and mechanical contractors for the Harry Grodsky Co. (Springfield, MA) faced many challenges in transforming the old Colt factory into an energy-efficient, mixed-living community. Reconstructing the original building, which housed the complex’s central heating plant, was the first phase of the 5-year project. The Largest Water-Source Heat Pump System in New EnglandOriginally, a separate coal-fired boiler plant that utilized an extensive network of underground tunnels to convey steam to 11 separate buildings heated the entire 70-acre complex. This was later updated to an oil-fired steam boiler plant that operated in a similar fashion. Design engineers at the Maguire Group and the mechanical contractors from the Harry Grodsky Co. set out to overhaul the entire system, gutting the original plant in favor of an energy-efficient design. The project, which showcases one of the largest heat pump systems in New England, totaling over 2,000 tons of cooling, operates at a loop temperature between 65 degrees F. and 95 degrees F. A natural gas-fired boiler plant was specified to ensure that loop temperature is maintained at a minimum of 65 degrees F. during cold weather. The boiler plant will also be tied into Johnson Control’s Metasys energy management system.One of the early challenges was to design a boiler plant that could not only operate at conventional 180-degree-F. temperatures, but also simultaneously at heat pump loop temperatures of 65 degrees F. Thus, the boiler equipment selected during the first phase of construction had to support the new water–source heat pump systems as well as the existing buildings, which required high-temperature hot water to maintain heat to the other buildings on the property as the 5-year project unfolds. Key to Reliability: Ability to Withstand Low Return Water Temperature and High System Operating Pressures.“At completion, the Colt-Gateway complex will utilize hundreds of water-source heat pumps throughout the complex,” said Tom Bryda, vice president of the Maguire Group. “We needed a boiler that could tolerate both low entering water temperatures of 55 degrees F. and high operating pressures in the 125 psig range.” The fully modulating AERCO Benchmark boilers selected for the project were unique in their ability to support the demands of the water-source heat pump design. A 20:1 turndown ratio, unmatched in the industry, enables each 2.0 million BTU/hour Benchmark boiler to run on as little as 100,000 BTU input before cycling off. From there, output will increase in precise 1-percent increments – without overshoot – in response to 4-20 mA signals from the Johnson Control energy management system. The Benchmark units maintain a perfect balance between output and real-time demand based on the exact needs of the ever-changing environment. And, the Benchmark’s efficiency actually increases at part-load to further enhance the energy economy of the overall heat pump design. Key to Efficiency: Condensing Operations By turning more of the boiler’s fuel into usable heat, effective condensing in the heat exchanger has been shown to increase the efficiency of the boiler by as much as 11 percent to12 percent. Benchmark’s stainless steel heat exchanger was specifically constructed to withstand the corrosive impact of condensing operations and reaches peak operating efficiency at the lowest incoming water temperature. Choosing Benchmark also helped to streamline the central plant because the units could sit directly on the primary loop with no auxiliary heat exchangers, mixing valves, etc., needed to protect the boilers. Key to a 5-Year Transformation: Modular Design “We couldn’t approach the project completely from scratch,” said Bryda. “Although the new central plant would ultimately support the heat pump system design throughout the complex, initially it also had to support several tenant-occupied buildings on the property. What’s more, as each of the buildings was restored throughout the 5-year project, they too would need to be seamlessly transitioned to the heat pump loop. “To make things easier, we initially converted the old steam system to a hydronic loop employing the same Benchmark model that was specified to supplement the heat pump system,” said Bryda. “At the same time the central plant building was gutted, most of the steam infrastructure connecting it to the older outer buildings was also replaced with modern hydronic piping.”Inside the renovated central building, a 620-horsepower boiler plant composed of twelve 2.0 million BTU/hour Benchmark boilers is arranged in two rows. The six boilers in the first row support the cool temperature heat pump loop, which so far extends only to the newly constructed school. The six units in the second row support the existing buildings via a separate “transition loop,” which utilizes an outdoor reset schedule to maintain 180-degree-F. supply water temperatures. “Not only will this approach make it easy to transition capacity – on a boiler-by-boiler basis – to the heat pump loop as each outer building is renovated, it also enhances the profitability of the overall project,” said Kevin Riley, project manager with Harry Grodsky Co. “The modular approach delivered plenty of back-up capacity to the project without oversizing the plant, and the “transition loop” utilized a high temperature differential to reduce the cost of piping across the 70-acre complex. In addition, the condensing operation of the Benchmark boilers will also cut fuel consumption on the “transition loop” over the course of the 5-year project.”
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