RexCorp Plaza, a giant glass structure book-ended by two 15-story oblong towers, is located in Uniondale, New York. When it opened in 1984, construction on the Shoreham Nuclear Power Plant, in Shoreham, Long Island, was also nearing completion. Since the plant was expected to provide cheap electricity, the building's mechanical design employed an electrical space heating plant. However, this cheap power source never came to fruition, as the Shoreham Plant was abandoned shortly after construction was completed in 1985, due to intense public opposition.
To address the significant drain on the Plaza's financial bottom line, the owner began a project to overhaul its space heating system in 1999 with the goal of switching to a much cheaper gas-fired alternative. The Plaza chose engineers from Ettinger Engineering Associates (formerly Robert Ettinger Associates) to spearhead the challenging renovation.
"The owners didn't want to abandon their electrical heating plant investment. Instead they chose to establish a gas-powered plant elsewhere on the premises," said Eric Ettinger, president of Ettinger Associates. "The gas-powered plant would serve as the primary heating system, with the electric structure functioning as a backup. Although there would be two independent power plants, the gas-fired boiler plant would need to support the building's existing hydronic loop to minimize project costs."
This situation presented engineers with several challenges. The challenges were met by AERCO Benchmark boilers.
Finding space for a second mechanical room in a fully occupied building was almost impossible. Engineers needed to fit enough equipment into a small room in the basement and decided to design the system using a high temperature differential (ΔT) of 62°F to increase the output that could be produced by a small heating plant.
The system utilized eight 2.0M BTU/hr. gas-fired AERCO Benchmark boilers. The Benchmark was chosen because it fit the parameters of the room size and the system into which it would be integrated. Not only is the Benchmark space-efficient - each stainless steel unit fits through a 30-inch doorway and occupies a footprint of 12 sq. ft. with zero side-wall clearance - but it is also designed to run efficiently in systems utilizing high ΔTs.
Because of Benchmark's condensing capabilities, the low hot water temperature of the hydronic loop was an asset that enabled the AERCO boiler plant to operate more efficiently. Not only did the equipment fit the system, it did so in a way that would increase fuel savings and lower operating costs.
"Our existing electric boilers had no problem handling low temperatures," said Ettinger. "But in the absence of gas-fired combustion there is no opportunity to condense. This opportunity to reclaim energy that would otherwise be lost in the combustion process added a new dimension to the efficiency and cost effectiveness that the right gas-fired plant could deliver."
The Benchmark's combustion chamber and heat exchanger are constructed specifically to operate in condensing mode. Condensing within the heat exchanger increases efficiency by as much as 12%. The process occurs naturally when the water vapor, created as a by-product of the combustion process, cools below its dew point (≈135°F) as a result of exposure to cooler return water temperatures, which are typically around 100°F. The boiler's ability to accept low-return water temperatures fit perfectly into the Plaza's heating loop - where temperatures ranged from 105°F to 42°F.
However, as the latent heat is extracted from the water vapor, an acid condensate remains on the heat exchanger. While this acid can severely damage most boilers, the Benchmark units are constructed with the highest quality materials and are designed to drain freely. As a result, RexCorp reaps the economic benefit of the condensing operations without worrying about durability and longevity.
With the introduction of a gas-fired heating plant, RexCorp Plaza suddenly had venting to contend with, which was especially challenging due to the complex's all-glass design. A dark smoke plume would have been an unacceptable eyesore; the Ettinger team had to engineer a solution that would produce no visible plume.
To solve this problem, the engineers installed an air economizer which filtered out the waste that would travel up the flue, thus eliminating the thick smoke. The positive pressure/natural draft design of the Benchmark supported breach venting of all eight boilers to the single, shared air economizer and up a single flue. "The standard flue for a boiler plant of this type would be 48 inches," said Ettinger. "The AERCO plant utilized a passageway that was less than half that size."
While the overall switch from electric to gas promised inherent savings, the Benchmark units have helped alleviate the financial burden even more due to its unsurpassed modulating ability. With 20:1 turndown, each 2.0M BTU/hour unit can support a mere 100,000 BTU load (5% of total capacity) before cycling off. From there, boiler output increases in precise 1% increments to prevent temperature overshoot. Besides using less fuel by reducing wasteful on/off cycling and eliminating overshoot typical of conventional gas-fired equipment, the Benchmark units actually run more efficiently in part-load situations.
To fully leverage this capability, the eight Benchmark units function as a single 16.0M BTU/hour system under AERCO's Boiler Management System (BMS). The BMS regulates the units to operate in tandem, providing them with precise firing instructions that ensure maximum efficiency.
"Beyond simply switching our fuel source from electric to natural gas, the project that started as a series of challenges culminated in an outstanding example of a high-efficiency space heating design," said Ettinger. "The AERCO units have performed brilliantly. Even the simplicity of their design has been a plus. Not much can go wrong with them, and because of this, maintenance has not extended beyond the routine."
As for the electric plant, even though it has been kept in operation, it is an untapped asset. "Since the completion of the properly sized gas-fired plant, the energy demand of the massive structure has never exceeded the supply provided by those eight AERCO Benchmark boilers," added Ettinger. "Not a single electric unit has switched on."
For more information, visit www.aerco.com.