Merging History and Technology To centralize or not to centralize? That was the question answered when a college campus looked for boiler systems that would offer student comfort – and historic preservation.
This article was published in Consulting-Specifying Engineer.
“To centralize or not to centralize?” That was the question that Elon College in North Carolina began to ask in 1990.
A master plan drafted for the campus that year centered on tying together its historic and more modern halves by razing a central parking lot and building a new student center opposite the central administration building. These two buildings would be the focal points of a central “quad”, flanked by a renovated athletic center and a new library.
A significant stumbling block was a central steam-boiler plant in the middle of the would-be quad. To adhere to the master plan, the school would have to demolish its steam plant, the source of heating and domestic hot water for 22 buildings.
In 1996, the decision had to be made to move to a new system by the start of classes in 1998. The college had two choices: to construct an equivalent 1,200-hp central plant in a more suitable location, or to decentralize by utilizing smaller boiler plants scattered around the campus.
An Inefficient Central Plant
The central plant had three steam boilers: Two 30-year old 500-hp boilers did the lion’s share of work, while a newer 200-hp boiler operated mostly during low-load conditions. The older boilers were operating at 60-percent efficiency - 20% less than what is expected nowadays. A below-grade 10,000 gallon oil storage tank served as a secondary fuel service, as natural gas was contracted on an interruptible basis from a local utility company, saving an estimated $20,000 annually.
The below grade steam/condensate distribution system was originally an archtile-type installation and was re-built in 1965. The piping was adequately sized, but line losses were amplified by numerous typical leaks, and repairs were often complicated by the presence of asbestos containing insulation. A serious overhaul of the central boiler plant and campus distribution piping seemed likely by the year 2000.
Considering its circumstances, the school weighed its options:
Maintaining Centralization
At a minimum, the central boiler plant would have to be relocated. Because there was little, if any, salvage value in the old plant, a new centralized system would mean construction of a new building, installation of new equipment - boilers, pumps and oil tanks - and, for the time being, reliance upon the current piping system.
Replacing the boiler plant essentially meant maintaining the centralized approach to serving heating and hot-water needs, at a cost of as much as $2 million. On the other hand, improved boiler efficiency would save fuel costs of as much as $60,000 a year. Still , the school would have to replace steam and condensate piping five years down the road, costing as much as another $1 million - a large expense that made decentralized options look more attractive.
Decentralization
In contrast, de-centralization is often driven by aging infrastructure, high distribution-line losses and the operational problems of large boiler plants. Smaller boiler systems located at individual buildings generally can be fired at least as efficiently as central plants, eliminate distribution line losses, reduce ongoing maintenance requirements and eliminate routine manual operation and monitoring. Still, the decentralized approach does have drawbacks. To house the multiple boilers, pumps and related equipment, each building would need additional mechanical space, and possibly a chimney. This could affect facility aesthetics, not to mention create a loss of usable floor space. For Elon College, decentralization would also eliminate fuel oil as a secondary fuel source - natural gas was available at each building – meaning no added storage tanks or leak detection systems.
Replacing the central plant with multiple boiler systems would cost nearly $2 million. And while gas utility rates would rise, improved efficiencies could save up to $28,000 annually.
The Answer: Decentralize
Elon College elected to pursue decentralization, in part to avoid the expense of future distribution piping replacement and associated downtime and landscaping destruction. Key benefits would include improved system maintainability for available in-house staff and the pleasure of not having a central plant disrupt future campus plans.
To determine the best way to implement decentralization, locations and types of boiler systems were identified, and a plan for reworking gas-distribution lines was developed to improve utility rates. Finally, a plan was needed to install multiple boiler systems during two short summer breaks.
To meet scheduling and budget constraints, building heating systems such as air-handling-unit heating coils and radiators needed to be reused, which dictated the type of boiler system for each building and potential combined boiler services for groups of buildings. Of the 22 facilities heated by the old central plant, five use hot water (converted from central steam); 11 use low pressure steam (15 psi); and 6 buildings use medium pressure steam (40 psi).
Packaged outdoor copper-fin-tube boilers were selected for hot-water applications because they don’t need chimneys or interior mechanical space. Exterior screen walls matching adjacent architecture were constructed to hide these boilers.
Forced-draft, sectionalized cast-iron boilers were selected for low pressure steam use, in part because they use a smaller flue than atmospheric equipment. Also, Elon College was already familiar with sectionalized boilers, which can be taken apart for installation in tight quarters. Finally, forced-draft, wet-back fire-tube boilers were used to provide medium-pressure steam.
Boiler sizes were established after estimating steam and hot-water loads for each building. Although not always feasible, the use of multiple boilers at each building was preferred in order to offer some level of redundancy. The same size unit was specified for each boiler type, allowing the school to be as familiar as possible with the equipment and to maintain a common stock of replacement parts.
Location, Location, Location
Choosing boiler locations was the core effort of decentralization. It was determined that the 22 buildings on the central system should be served by nine smaller boiler systems. Five buildings needing hot water varied in function - a dormitory, classrooms, library and theater - and were too spread out to be combined. In each case, the existing steam-to-hot water converter was removed and the hot-water side was repiped and extended to the new boilers. Existing pumping was replaced only when necessary. Where domestic water usage was very low, small stand-alone gas or electric water heaters were specified. At a residence hall, outdoor boilers were designed to deliver domestic hot water via a new water-to-water heat exchanger and an existing storage tank.
Locating low-pressure-steam boilers was somewhat difficult, but three systems were designed to serve 11 buildings. One new boiler system would serve a residence hall and dining hall that already shared steam distribution lines. An existing steam-to-hot water converter and an auxiliary kitchen steam boiler were replaced with a single stand-alone boiler. A domestic water coil was inserted to supply hot water via existing storage tanks to both the residence halls and kitchen.
A second low-pressure boiler system serves the campus student center and three residence halls. The student center’s boiler room was large enough for conversion to steam production, and two new low-pressure-steam boilers were installed. The existing flue was enlarged, a new steam-to-hot water converter was added for the student center, and new steam and condensate lines were piped to the residence halls.
A third low-pressure system serves three dormitories and two classroom buildings, mainly due to a lack of available mechanical space on the historic side of the campus. Existing site distribution piping was in good shape and reutilized, and the plant was located in the basement of a 70-year-old classroom building after lowering the basement floor by about three feet. An architecturally designed chimney was constructed to vent the boilers.
Most challenging, however, was the medium-pressure steam system serving six historic buildings. To preserve the campus aesthetic, a below-grade boiler plant was proposed, which was very appealing to administrators. Structural engineers designed a large concrete box below grade with a removable plank top designed for future access. A patio with loose brick pavers was laid over the top, replete with cast-iron side railings, an access ramp and plenty of new sod and plantings.
Energy cost issues also impacted the project design. During design development, it was discovered that the attractive gas rate offered for the central boiler plant would no longer be available after the local utility separated its service into nine delivery points. To qualify for the commercial rate, Elon College had to consolidate to two gas meters, adding $140,000 for piping, regulators and subgrade boring for gas distribution. Projected energy cost savings, however, would total about $40,000 per year - a very appealing 3.5-year payback.
Scheduling and Little Fanfare
To schedule the decentralization work, the project demanded preplanning, prepurchasing and well-coordinated construction phasing. The boilers were the longest lead item, and were pre-purchased after competitive bidding and then stored in a nearby warehouse. Preplanning involved contractors completing piping fit-ups off site in advance of the official start date, and construction phasing helped contend with summer college activities. Gas piping and hot-water systems were installed during the first summer, and new interior boiler spaces were built the next year.
Today, ten years after the master plan was drafted, a central boiler plant is gone and a new quad is complete. Few people walking across the quad will ever realize what made it all possible – and why should they, as long as the students can take hot showers, attend warm classes and relax on a new brick patio that, for reasons unknown, melts snow in the winter?