"The project started out as an evaluation of CFC replacement options in two older chillers and evolved into a major mechanical system upgrade." That's the description by Paul Giles, project engineer with the U.S. Federal Government General Services Administration (GSA). The project culminated in the replacement of chillers and installation of thermal storage capability at the William S. Moorhead Federal Building in downtown Pittsburgh.
This 27-story structure was constructed in 1963 and is occupied by a wide range of federal government agencies. Standard occupancy hours are 8 a.m. to 5 p.m. with occasional off-hour usage on evenings and weekends. The building has two subbasement levels that include a parking garage and a mechanical area.
The original 30-year-old chiller plant consisted of two 990-ton centrifugal chillers equipped with open drives and reduction gears. By modern standards, the chillers were of comparatively low efficiency, about .90kW/ton, and had a history of CFC-12 leakage. The two original, constant-speed chilled water pumps provided 2,866 gpm with 125 feet of head using 125 hp. The two condenser water pumps had capacities of 2,847 gpm with 80 feet of head using 75 hp. One 1,980-ton roof-mounted cooling tower served the existing chiller plant.
Replacement Not PracticalIn 1992, GSA's mid-Atlantic region commissioned an engineering firm, H.F. Lenz Company (Johnstown, PA), to perform an in-depth survey and analysis of the building's cooling plant. The original focus of the survey was to evaluate the feasibility of converting the existing chillers with non-CFC refrigerant and possibly installing variable-speed drives (vsd's) to improve their efficiency.
The analysis determined that, while vsd's could improve efficiency, a much greater improvement could be achieved by complete replacement of the chillers with new, high-efficiency, non-CFC machines. At about this time, the electric utility Duquesne Light Company, encouraged the study of thermal storage as a means of reducing utility electric demand charges.
According to H.F. Lenz Company principal Robert Stano, it was at this point that the possibility of combining an ice storage system with new chillers came under consideration. "We could see some real possibilities for reducing operating costs. One of the many challenges was the physical constraints within the existing building." According to Giles, it wasn't just a question of physical space. "It was a budget issue as well. We needed to demonstrate that the project payback was real."
Ultimately, the consulting engineer, working with the GSA, came up with a plan that involved replacing the two original 990-ton chillers with two 600-ton, high-efficiency centrifugal chillers and an ice storage system. The amount of ice storage that could be used was constrained by physical space in the building as the storage tanks were installed in a subbasement space previously used for storage and shops. Despite the reasonably good condition of the original pneumatic control system, it was replaced with a new microelectronic system with advanced control capabilities.
As a U.S. government agency, the GSA was required to purchase chillers ranked within the top 25% for efficiency. One of several bidding options prepared by H.F. Lenz Company was two Trane (LaCrosse, WI) Model CVHE 600-ton chillers running on HCFC-123. For ice storage, the choice was 30 Calmac tanks rated at 190 ton-hours each for a total of 7,410 ton-hours. The new chillers have a full-load efficiency of .60kW/ton at ARI conditions. The efficiency of the chillers in the ice-making role is .75 kW/ton. The building automation system installed was a Trane Tracer Summit system.
Close Quarters in SubbasementThe mechanical contractor, James C. Eastley, Inc. faced several challenges when construction began. The first was finding a way to transport the replacement chillers and the ice tanks into the subbasement where they were to be located. During design, it was determined that the best way was to cut an access portal through a concrete truck ramp in the basement level of the building. The disassembled chillers and ice tanks were lowered from the basement parking area into the subbasement through the hatchway.
Another challenge was installation of the cooling towers. The new supporting grid and the new towers were airlifted into position by helicopter in January. "It was the coldest day of the year," said Eastley, "about 7° below zero and mighty windy on the rooftop. We lifted in the supporting grid sections first and bolted them together. Then the helicopter brought in the cooling towers. The whole operation took about three hours."
Following the installation of the chiller plant components, extensive piping, pump, and control revisions to the existing system were made to accommodate the ice storage addition and cooling system replacement. After installation, GSA conducted a detailed system commissioning procedure. "This took time," said Giles, but from my perspective, the start-up went very smoothly."
System Flexibility A PlusThe system offers wide flexibility in its use of the ice storage capability. In typical July-August operation, the ice storage capability is used to minimize electrical demand during the period from noon to 4 p.m., the electric utility's summer peak demand time. The chillers make ice from 6 p.m. until 6 a.m. The evaporator fluid is 25% ethylene glycol and the system uses a plate-frame heat exchanger to separate the glycol system serving the ice tanks from the building's chilled-water distribution system.
During the "shoulder" cooling months, the system is operated to produce a reduced amount of ice, generating only enough to meet anticipated cooling needs during the peak billable demand period. According to GSA's Giles, "Actually, on mild days during the cooling season, we don't run the chillers during the peak at all." Except by special arrangement, the building cooling system is not operated on weekends or holidays.
Robert Stano explained one of the system's added benefits for the existing high-rise building. "It can deliver low-temperature chilled water. This offers flexibility for the anticipated future replacement of the building's airside systems. The lower chilled water temperatures will simplify the installation of new ductwork within the tight physical constraints of the building, if low temperature supply air distribution is used."
In addition to the hvac system improvements, the GSA has completed numerous other efficiency improvements in the Moorhead building, including lighting upgrades, building enveloped improvements and power factor improvement with capacitors. The GSA's Giles is pleased with the results both from an efficiency and a comfort standpoint. "We had a fixed budget and limited space. This project gave us a good payback within those constraints." ES