A unique combination of traditional and contemporary architectural elements, the Sam Nunn Federal Center in downtown Atlanta, GA, houses 5,000 employees for dozens of federal agencies. The facility, one of the largest federal office buildings on the East Coast, encompasses 1.87 million sq ft of space. The center combines four distinct structural elements —including a remodeled 1924 department store — in a U-shape that straddles a busy downtown street and sits atop an underground train tunnel that is part of Atlanta’s transit system, MARTA.
The design architect chosen for the facility was the California firm of Kohn, Peterson, Fox, and Associates. Newcomb & Boyd, an Atlanta firm, was tapped as the contracting-construction team and the project engineer on this job. Southeastern Facility Management Inc. is contracted by the General Services Administration (GSA) to operate this facility.
Design RequirementsOne or more of the 1,310-ton chillers is to operate, depending on the building load, between the hours of 6 a.m. and 6 p.m. After 6 p.m., the 400-ton chiller is to carry all computer rooms and miscellaneous building loads.
Besides continuous occupancy, Atlanta has significant cooling loads for much of the year and high humidity as well, so the hvac system design goal was to ensure complete comfort in the building around the clock, year-round.
To achieve this, significant emphasis was placed on humidity control with a central chilled-water plant, air handlers for each area, and a zone-controlled vav air-delivery system.
Adept at EfficiencyThe Trane Company supplied the chillers, air-handling units, and vav terminal units for the facility. Bill Faulkner, sales engineer for the Trane Atlanta terminal units commercial sales office, worked with Newcomb & Boyd and with the owners in specifying equipment and systems to meet the efficiency, temperature, and humidity control standards.
The chilled-water system chosen was a central, electric, hermetic centrifugal chiller plant, with several large air handlers on each floor and more than 1,300 terminal units, either fan-powered or variable air volume (vav) units throughout the occupied areas.
The system uses four chillers rated at 1,310 tons, plus one machine rated at 400 tons, which are located in a basement mechanical room. The plant also features a plate-and-frame heat exchanger to allow economizer cooling without having to operate the chillers when outdoor conditions permit.
The building is run in economizer mode with outdoor ambient temperatures up to 46Â°F. The economizer’s range of operation is increased by an upward reset of supply air temperatures of as much as 10Â° during winter and mild weather.
Another efficiency approach employed was the use of a low-flow design for the cooling tower condenser water flow. The chillers were selected with cooling water flows of 2 gpm/ton to improve overall plant efficiency and allow lower first cost in condenser piping, cooling towers, and condenser water pumps. In the center design, for example, the piping from the basement chiller plant to the roof-mounted cooling towers 25 stories above was reduced from 24- to 18-in. pipe.
The facility managers strive to maintain building relative humidity at 40%, even with the extensive mixing of outside ventilation air. To accomplish this, they rely on the centrifugal chillers to deliver 42Â° chilled water for enhanced dehumidification.
The maintenance staff indicated that the lower relative humidity level they were able to achieve essentially eliminated any mold or mildew problems in the facility.
Regular Cleaning: an Important IAQ StrategyThe building facility management team puts a high priority on IAQ, and modular air handler design is an important feature for achieving and maintaining it.
The system in the building uses the mechanical room itself as an inlet and mixing source for the air handlers. The air handlers have a free return. Building air travels through a four-ply prefilter module, then through a bag filter section before going through the fan and coil.
The staff notes that this double filtration design allows the system to capture particles down to the small-micron sizes. This is part of GSA’s emphasis on maintaining good IAQ.
For the same reason, the facility management crew completely cleans the air handlers on an annual basis. The cleanup crew vacuums and washes every square inch of the air handler interior, including the coils.
According to the maintenance staff, now that the facility is up and running and the system is tuned, there are very few comfort complaints from building occupants. Faulkner suggests that the reason for the low complaint level is attention by the facility staff to detail in the design and selection of equipment, in operation, and in scheduled maintenance of the facility to maintain its high performance levels.
Publication date: 10/09/2000