Need a Boiler Fix? It's Elementary
Heating costs were mounting for the District of Columbia's school system, as were operation and maintenance problems attributed to the age and obsolescence of the district's boiler systems. Eventually, the school board realized that the problem had to be addressed, even though there wasn't enough funding to install new boilers in 13 target schools.
Heating costs were mounting for the District of Columbia's school system, as were operation and maintenance problems attributed to the age and obsolescence of the district's boiler systems. Eventually, the school board realized that the problem had to be addressed, even though there wasn't enough funding to install new boilers in 13 target schools. The answer for these facilities—many of which had 4
0-year-old boilers that were near or beyond their expected life cycle—was an innovative retrofit scheme that has resulted in improved efficiencies and lower operating costs.
This regeneration process began when the managing jurisdiction—in this case, the U.S. Army Corps of Engineer's Baltimore district—contracted Hayes, Seay, Mattern & Mattern to survey the district's schools. This job included coordinating with the school district's chief engineer to evaluate each building with the end goal of either replacing or re-tubing the existing boilers. Evaluations were also made regarding other alterations for refurbishing existing systems. Even though this was an economical-patch approach, it would meet money-saving goals until new schools would eventually be built.
Of the group of boilers, some units were gas-fired, fire-tube boilers that produced low-pressure steam, and others produced low-temperature hot water. A big problem, however, was the fact that many of the boiler tubes leaked, causing blockages that drastically reduced efficiency. Further, inadequate water treatment allowed severe scaling in some boilers.
The survey, however, involved considerably more than detailing the type and condition of boilers. The contract called for HSMM to produce design drawings, specifications, design analysis and a cost estimate to replace boiler room equipment including boilers, pumps, water heaters, boiler room piping, etc., in all 13 schools.
Fortunately, the Corps of Engineers provided a spreadsheet that calculated block-heating loads for the individual buildings, which helped determine boiler size. In order to utilize this software tool, the following data was collected:
Size and number of windows at each school.
External wall areas.
Interior space heights.
The number of fixtures in each rest room.
With each school's system sized according to the analysis and calculations, the next step was to evaluate best-cost alternatives. The issue, however, was complicated by the fact that a simple boiler re-tubing or replacement would only be a starting point. In other words, the total retrofit package needed to address:
Replacing scale-caked components and boiler room piping.
Replacing circulation pumps and domestic hot-water heaters.
Adding water softeners and water-treatment systems.
Providing peripheral improvements such as boiler controls and condensate systems.
Getting to work
The engineering analysis led to this conclusion: The schools would be best served by replacing many of the old boilers in favor of multiple-sectional, cast-iron, gasket-type steam boilers.
As it was in most of the schools, the two large boilers that had been heating each facility generally operated without any contemporary, energy-saving control systems. Each boiler was rated for 75% full load with modulated burners with turn-down rates of about 60%. This, of course, is in contrast with most modern boiler rooms which operate at full load less than 10% of the time, and most of that time at only 20% to 30% of capacity.
However, it did not make sense to try to make the schools fit such a scheme. Rather, replacing the central plant system with a series of distributed cast-iron sectional boilers offered a variety of advantages over replacing older boilers with newer, high-efficiency condensing units. The principal advantage was the ability to bring online only those boilers needed to meet demand. This strategy would allow each boiler to achieve best efficiencies. Furthermore, the multi-unit system increases reliability, allowing the master controller to turn on or off as demand dictates, and bypass any unit giving a fault alarm. In addition, each boiler can be isolated for service without disrupting the rest of the system.
Maintenance advantages have also accrued. The units are relatively small—3-ft. by 4-ft—and the sectional cast-iron boilers have proven to be more adaptable to changing conditions by adding, removing, repairing or replacing individual sections, and by ease of movement to a different site. Further, operation and maintenance for the sectional cast-iron units can be performed by typical mechanical and/or plumbing trade personnel, whereas the high-efficiency condensing boilers require more highly trained, specially certified maintenance personnel. In addition, condensing boilers use more complex control systems, again requiring more highly trained operators, and must have more specialized, confined space for cleaning.
With regards to efficiencies, the new sectional boilers are rated 85% efficient, compared to the older units that were estimated to operate at 50% to 60% efficiency.
Of the 13 schools, only seven were selected for complete boiler room equipment replacement, at a construction cost of $8 million. Boilers in the remaining six schools were selected for re-tubing, at a cost of $4 million.
While not a total solution, the selective retrofit strategy allowed the district to reap greater efficiencies with lower operating costs, thus achieving its goal, despite a limited budget.