Located in the Chicago Loop, the Michael A. Bilandic Building is a 21-story office tower serving State of Illinois agencies, including the Illinois Supreme Court. In 1989, the existing building structure was gutted and renovated. At that time, a new top floor was added to house major mechanical equipment, including three gas-fired, two-stage absorption chiller-heaters. Each unit had an output of 600 tons of cooling and 7,200 mbh of heating. Dedicated dual-cell cooling towers with 20-hp high-/low-speed fans were installed on the roof.

Since then, the chiller-heaters have been replaced with equivalent-capacity modular electric chillers and sets of duplex hydronic gas boilers. The update resulted in enhanced equipment reliability, easier maintenance, and, during the first year of operation, a 15-percent savings in measured thermal-energy use.

Out With the Old
The existing hydronic system clearly needed an update. With pumps mounted on the 21st floor in the vicinity of the chiller-heaters, four-pipe primary heating and cooling mains served multiple variable-air-volume (VAV) air-handling units and perimeter radiant ceiling coils throughout the building. Hydronic return mains fed into four 30-hp primary heating pumps and four 40-hp chilled-water pumps. Three of each type pumped into the respective chiller-heaters, while the fourth heating and chilled-water pumps provided manual standby duty. Likewise, four 100-hp condenser-water pumps (which since have been replaced with 40-hp pumps) took in water from the cooling-tower supply header, pumped cool water into an absorption chiller, and returned hot condenser water to a dedicated cooling tower.

Because of year-round use, one chiller-heater broke down after only about 16 years of useful life and became unserviceable. The building continued to sustain itself comfortably on the remaining two functional chiller-heaters but no longer benefited from emergency standby capacity. The building's facility-management staff urgently replaced the old absorption chiller-heaters with new units, one at a time. Melvin Cohen and Associates Inc. was chosen by the State of Illinois Capital Development Board to prepare design documents for bid and construction. Primera Engineers in Chicago was chosen to provide commissioning.

Challenges Ahead
Absorption chiller-heaters are heavy, bulky machines that weigh up to 40,000 lb each. Replacing the chiller-heaters on the building's 21st floor required disassembling the units into smaller pieces for rigging by helicopter, creating a large opening in the roof, and procuring special permits to close downtown streets around the building on several occasions during the construction period. Further, the new absorption units were unavailable locally and required a minimum of six months of lead time before they could be shipped from Japan.

Given the urgency of the task at hand, these scenarios were daunting. Additionally, absorption chiller-heaters are not well-known for thermal efficiency. Therefore, a new approach was needed. One option was to use modular electric chillers and modular gas boilers that could be transported via the building's freight elevator. Although convenient, obtaining the vast amount of new electric power needed on the top floor as well as guaranteeing a proper fit for all of the required modular equipment within the space vacated by the old absorption units, would have been difficult. However, building surveys indicated that existing 480-v electric switchboards in the basement had the sufficient physical and electrical capacity available to serve new power needs. It also was feasible to run new power risers from the basement switchgear room up to the 21st floor, and sufficient storage space was available to expand the existing chiller-heater room if necessary. This strategy generally was acceptable to the project team.

Final Design and Construction
The three old gas-absorption chiller-heaters each were replaced (in three distinct contiguous phases) with a bank assembly of eight modular electric chillers served by existing spare 800-amp bolted pressure switches in the basement's existing switchgear. Power feeders were extended from each bolted pressure switch to a modular chiller bank on the 21st floor.

At 28 in. by 49 in. by 69 in. and 2,200 lb, the chillers feature two 35-ton, R-410a scroll compressors (with independent refrigerant circuits) and stainless-steel plate heat exchangers. Each chiller-bank assembly was accompanied by two duplex gas-fired boilers for replacement heating. At 39 in. by 41 in. by 85 in. and 2,400 lb, the boilers feature 2,640-mbh output and a variable-frequency-drive (VFD) forced-draft burner.

The three modular-chiller/duplex-boiler assemblies are located in the same area previously occupied by the old gas-absorption chiller-heaters. No additional space was required, and all of the equipment was able to fit properly without undue congestion. Most of the equipment was purchased within North America.