The heating-/cooling-plant replacement at a Chicago office building features electric chillers and gas boilers
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.
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.
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Many other improvements were made to the building's mechanical systems, such as:
- Replacing the existing cooling-tower target nozzles with smaller orifice nozzles to help distribute the reduced condenser-water flow required by the new chiller banks.
- Replacing the existing HVAC control system's old direct-digital-control (DDC) panels and sensors with a state-of-the-art system that utilizes the interoperable open BACnet protocol.
- Replacing the domestic-water triplex house booster-pump system with a modern package that includes VFD controls for each pump.
- Replacing 12 aged primary hydronic pumps with updated pumps to suit new system requirements.
- Replacing the cooling-tower chemical-feed system and fine sidestream filters at the heating and cooling hydronic mains with new equipment that utilizes pipe-corrosion-monitoring hardware.
- Replacing an existing 3,000-amp building-lighting switchboard with old-style circuit breakers (which had not been removed during the 1980s renovation) with a new switchboard containing modern switch and fuse construction for better reliability.
Even producing the same amount of cooling, the new electric chillers use and reject one-third less thermal energy compared with the old absorption chillers. Therefore, the condenser flow was reduced sharply, and the cooling-tower fans did not have to work quite as hard.
At part load, a scroll modular electric chiller uses up to 25-percent less electrical energy than it does at full load. Therefore, multiple chiller compressors at each chiller bank can be controlled automatically in a lead-lag manner by a factory-furnished master control panel (one panel per bank) to help maximize chiller operating efficiency at part load.
Unlike an absorption chiller that must receive condenser water at a steady 85°F, the operating efficiency of a scroll modular chiller improves significantly at lower condenser-water-supply temperatures. Therefore, the building's cooling-tower-fan operation was programmed to reset condenser-water-supply temperature to within 8°F of the prevailing outdoor wet-bulb temperature.
The six new modular boilers are set for automatic lead-lag operation, injecting heat into the building's heating-water supply main as needed to maintain supply-main temperature. An inline pump and three-way control valve at each modular boiler ensure a minimum 140°F boiler entering-water temperature to prevent flue-gas condensation. The new boilers provide 88-percent thermal efficiency at full load vs. the 80-percent efficiency of the old absorption chiller-heaters.
Each cooling-and-heating-bank assembly includes energy meters to monitor operating efficiency (kilowatts per ton for the cooling bank and thermal efficiency for the heating bank). Parameters are displayed at the building-automation front-end computer system. A degrading performance over time signals alerts for critical maintenance needs, such as heat-exchanger cleaning or a gas-combustion tune-up. Back-flush valves also were added to each condenser (plate heat exchanger).
A year-by-year comparison of electric and gas utility bills before and after project construction shows the building's gas consumption was reduced by 29 percent and electricity consumption went up by only 1 percent. Keep in mind the old chiller-heater equipment operated mostly using natural gas. While the total of annual heating and cooling degree-days was nearly the same, the gross energy consumption on a British-thermal-unit basis went down by nearly 15 percent.
The building's design work began in early 2007, and project construction was completed by the middle of 2009. The building remained in full normal operation throughout the project. The construction work was completed smoothly and on time, with no significant glitches. The construction costs were about 25-percent below the State of Illinois Capital Development Board's original budget and included costs for correcting several previously undiscovered conditions.
The building can be kept comfortable with just one bank of boilers and no more than two banks of chillers. Therefore, standby equipment capacity is significant. The new replacement plant is easy to maintain and will continue to realize substantial energy savings in the future.
The authors wish to thank the following for their contributions to the success of the project: Jerry Adams, regional manager, Donald Barnes, energy manager, and John Teubert, assistant chief stationary engineer, Central Management Services, State of Illinois; Mohammed Haq, project manager, Capital Development Board, State of Illinois; Pat Liston and Larry McMahon, contractors, Anchor Mechanical; Stanley Lawrence, project manager, Control Engineering Corp.; Robert Fimbianti, contractor, Linear Electric; and Andrew Sebescak, commissioning services, Primera Engineers.
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President of Melvin Cohen and Associates Inc., a consulting-engineering firm, Ronald B. Cohen, PE, graduated with a bachelor's degree in electrical engineering from the University of Illinois, Champaign/Urbana. A senior mechanical engineer for Melvin Cohen and Associates, Om P. Gupta, PE, obtained a master's degree from the University of Toronto. Gupta also has an accreditation as a Leadership in Energy and Environmental Design (LEED) Green Associate.