Blending In

Space constraints, restricted funding and a short time schedule all came together to make designing a new central chilled-water plant for the Fox Chase Cancer Center in Philadelphia a challenging problem. Aging chillers, as well as plans for growth and a need for redundant systems, made it clear that a new chilled-water plant was required.

12/01/2000


Space constraints, restricted funding and a short time schedule all came together to make designing a new central chilled-water plant for the Fox Chase Cancer Center in Philadelphia a challenging problem. Aging chillers, as well as plans for growth and a need for redundant systems, made it clear that a new chilled-water plant was required. Philadelphia-based Ewing Cole Cherry Brott recommended that the client focus on the long-term benefits.

"Our goal was to help them understand that this was an investment for the future," says Keith R. Cockerham, P.E., the mechanical engineer on the project. "Even though we were going to be adding extra cooling-with newer, more efficient units-we were going to use less energy."

Choosing a location for the plant was the first major challenge for the design team, because most of the land on campus had been used for new research facilities. The best location was near what is known as the "west building," where three 450-ton chillers less than 10 years old were located in the basement. Electrical systems in the west building also were the only ones on campus optimized for future growth.

The only land available in the area, however, was a landscaped courtyard. The decision was made to bury the plant under the courtyard, level with the mechanical room in the west building basement. A waterproof membrane and insulation would be added to the roof, and the courtyard restored.

Keeping it small

Because of the tight space requirements in the underground bunker, both vertical and horizontal, designing a system with the smallest possible footprint was critical. Electrical centrifugal chillers were chosen because they provided the greatest efficiency and reliability with the smallest size

The next challenge was deciding how to divide the 3,800 tons of capacity needed. After looking at multiple options, the team decided to use two 1,900-ton dual-compressor chillers with 480-volt operation, which provided redundancy and offered excellent turndown capacity.

The manufacturer suggested using a single-pass chiller with a lower evaporator pressure drop as a way to save on initial cost. At first it was thought that efficiency would be sacrificed slightly: from the 0.55 kilowatts (kW) per ton that was specified to 0.61 kW per ton for the units now suggested. However, through analysis of part-load performance and the reduction in chiller-pump horsepower (hp), the design team determined that the single-pass chiller would be more efficient.

Plumbing considerations

As if the design team didn't have enough problems with space constraints, they also had to deal with a 12-inch main sewer line that ran through the courtyard and into the projected bunker. Space was also needed for a sewer ejector.

"I personally enjoyed this because normally plumbing [engineers] accommodate everyone else, and in this case everyone else had to consider the plumbing requirements first," says Irina Apeltsin, plumbing engineer on the project.

A major challenge was coordinating electrical and plumbing designs, because codes did not allow electrical equipment under any plumbing or sewer pipe. According to Apeltsin, it took four tries before a successful design was achieved.

Addressing harmonics

Another significant test for the design team was how to control harmonics caused by the use of variable-frequency drives (VFDs) on secondary chilled-water pumps. Potential voltage and current distortions had to be analyzed, and in the end, 5-percent line reactors were installed on each of the three VFDs serving the 150-hp, secondary chilled-water pumps. According to Alan Mayzenberg, P.E., electrical engineer on the project, determining an effective mitigation solution required close coordination not only among mechanical and electrical team members, but also with equipment manufacturers.

"This was a challenging project that required compromise," Mayzenberg says. "Everybody had to work together."





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