The installation of duplex chillers has helped increase cooling capacity and maximize the efficiency of the existing plant at the Johns Hopkins Medical Campus on the east side of downtown Baltimore, MD.

The campus includes a world-renowned hospital, one of the nation’s premiere medical schools, a school of nursing, and a large number of medical research facilities.

Chilled water needed year round

The campus has a year-round requirement for chilled water. Johns Hopkins commissioned a comprehensive central plant lifecycle evaluation.

One of the immediate drivers for the study was increased campus cooling requirements resulting from the construction of three major new buildings: a new School of Nursing Building, a Comprehensive Cancer Center, and a Cancer Research Building, totaling 600,000 sq ft of new space.

All of this space was to come on-line in 1998 and 1999.

The campus’s heating-cooling requirements were provided by a central plant on the east side of the campus. In 1997, the plant had 25,000 tons of chilled water cooling capacity. This was supplied by 12 chillers, with about three-to-one between electric- and steam-driven centrifugal chillers, all on a common chilled water header.

RMF Engineering, Baltimore, conducted a study that included an evaluation of chiller energy consumption, pumping energy costs, and system maintenance costs. According to RMF’s Steve McAdams, the goal was to maximize the efficiency of the existing plant and position it for future growth.

“The owner needed to know what changes were necessary to prepare for the future,” said McAdams.

Engineered solutions

Part of the solution recommended by RMF was revamping the chilled water distribution system. By optimizing the system, the usable capacity of the plant would be increased.

This required reworking hundreds of feet of chilled water pipe throughout the campus. In addition, the work had to be done without cutting off the supply of chilled water to the building.

The study also identified a near-term requirement for an additional 5,000 tons of chiller capacity.

As part of the study, RMF determined the optimum size and type of chillers. A number of considerations went into the evaluation, including the need for optimum efficiency. The new chillers were principally intended for base-load service, so efficiency was important.

Another issue was reliability. In working with RMF on the study and subsequently on the project, Johns Hopkins project manager Paul Matuska emphasized the importance of being able to keep cooling capacity on-line.

“Lifecycle cost is important,” said Matuska, “but most important is that we have cooling capacity when we need it.”

As a result of the equipment evaluation, the engineer recommended and Johns Hopkins chose two 2,400-ton duplex CenTraVac® chillers (manufactured by The Trane Company).

According to power plant manager David Chalmers, the units were chosen because of their operating efficiency and system redundancy.

“In effect, even if you lose one compressor, you still have half of that machine’s capacity available,” Chalmers noted.

Chalmers went to Wisconsin to observe the chillers’ performance tests at the manufacturer’s plant. “It was our objective to get a baseline for capacity and efficiency that we could use in our installation.”

Complications for the contractor

In early 1998, the units were shipped to Baltimore and installed on the site by Green Contracting Co.

A big part of the challenge of this installation was the limited existing floor space, but especially the need to keep the plant on-line — through the chilled water piping augmentation, the cooling tower modifications, and the chiller installation.

David Jacobson, project manager with Green Contracting, noted that a complicating factor was the need to add steel structure to the existing mechanical room floors to support the additional equipment.

He remarked that installation was a straightforward rigging job, although it required using an extremely tight staging area. To fit the duplex units into their spaces, it was necessary to temporarily remove the economizers and water boxes. (Full disassembly of the units was not needed.)

Despite the challenges, the two new units were up and running by May 31, 1998, thanks to cooperation among the team (see related sidebar article).

Finally, RMF determined that this additional chiller capacity would require additional cooling tower capacity. As with the chillers, space limitations were a challenge for additional tower capacity.

For this reason, two existing Marley cooling towers were extended upward 6 ft and larger diameter fans were installed. In addition, the existing wood fill in the towers was replaced with plastic.

Emergency power for chillers, too

An interesting aspect of this project is that the chillers can be powered directly by the hospital’s backup electric generation equipment.

The hospital has two 2-kW Caterpillar diesel generator sets for emergency power supply to the central power plant. These generators also have been connected so they can alternatively supply power to the duplex chillers, to reduce system demand during the billable peak-demand period.

In case emergency power generation is needed elsewhere, the chiller load can be quickly dropped.

Because the duplex units invariably would be operating at times of summer peak demand, it was logical to power them directly. This arrangement is attractive to Johns Hopkins, because it permits lower summer utility demand charges while having the generators available for emergency service.