Heartbeat of the Vision: BNA's Central Utility Plant
September 22, 2021
by Wesley King, P.E., Sean Ashburn, P.E., & Ken Parrish, P.E.
With an average of over 25,000 passengers a day, Nashville International Airport (BNA) is among the fastest growing airports in the U.S. In order to "accommodate Middle Tennessee's booming population and the airport's record-breaking passenger increases" (https://bnavisionnashville.com/), BNA enacted a growth and expansion plan known as "BNA Vision." This "Vision" has so far included Concourse D, Terminal Wings Expansion, Central Utility Plant, Terminal Garage & Administration Building, International Arrivals Facility, and the Airport Hotel which broke ground recently.
Quite literally central to BNA Vision, a new Central Utility Plant (CUP) was constructed as the foundation for BNA's major expansion. The CUP was designed for robust and energy-efficient operation to reach sustainability objectives and serve the future strategic vision for the airport. The new 11,000 SF CUP contains a new 8,400-ton chilled water system and 56,000 MBH hot water system. Below is a detailed description of the CUP.
Electrical
The electrical service for the CUP included two 4000 Amp, 480-Volt fully redundant Main-Tie-Main Switchboards with automatic transfer capabilities. Each Switchboard was fed from two redundant Utility (NES) transformers. The Switchboard lineups also included a 4,000 Amp auxiliary circuit breaker that fed to exterior Generator Quick Connect Switchboards so that rental generators could provide power to the entire CUP in the event of a Utility power outage. The 4,000 Amp auxiliary breakers were key-interlocked with their respective main breakers to prevent normal and emergency power sources to be energized together on the Switchboards. The CUP was also designed with Variable Frequency Drives (VFDs) for all of the chiller system's pumps and fans, and for the Boiler system pumps.
Chilled Water System
Chillers
The new CUP consists of a total of seven 1200-ton centrifugal chillers, and one of these chillers was relocated from the existing CUP underneath the existing terminal. The chillers are setup so that three of the chillers are fed by an existing geothermal loop, using an existing nearby water-filled quarry as the heat sink. The existing chiller uses R-123 refrigerant, and the new chillers use R-134a refrigerant. In addition to the geothermal chillers, there are three new chillers that are designed to use the new cooling tower. One additional chiller sits in between the geothermal chillers and cooling tower chillers as a standby chiller for either system. In the event that either a geothermal or cooling tower chiller fails or is under maintenance, manual control valves can be opened to the standby chiller to restore the system to full capacity. Additionally, 12” supply and return pipe connections are provided for a temporary air-cooled chiller to be connected to the CUP in the event of an emergency. The chilled water system is designed to provide 42˚F supply water and handle up 54˚F return water.
Chilled Water Pumps
Four chilled water split case double suction pumps are designed for variable primary flow and are each provided with a VFD. There is an additional pump provided so that the system can operate at full capacity if one of the pumps is down or in maintenance.
Condenser Water System
Quarry Geothermal Lines
The new CUP utilizes the existing geothermal lines from the quarry for three of the chillers in the CUP. The existing geothermal line pumps and air separator were reused since the equipment had only been in service for 3 years. An additional new pump was provided to the geothermal system to allow for redundancy in the system, so there will be no capacity lost if a pump is down or under maintenance.
Cooling Tower
In addition to the geothermal system, a single cooling tower with three 1500-ton cross flow cells is utilized. The cooling tower cells are provided with a VFD for each fan. Three split case double suction pumps located outside are provided for the cooling tower. Each VFD-controlled pump is sized at 50% of the total capacity of the cooling tower, which allows for redundancy in the cooling tower. The cooling tower utilizes a shared basin with a basin sump pump and a single 24-kW basin heater per cell to prevent freezing. The cooling tower loop is provided with a plate-and-frame heat exchanger in series with the chillers to allow the CUP to provide a water side economizer to BNA.
Hot Water System
Boilers
The new CUP consists of eight 6,000 MBH natural gas condensing boilers that are 93% efficient. Each boiler is provided with a stainless-steel double wall exhaust flue and a condensate neutralization kit to prevent corrosive condensate from entering the sanitary system. An inline recirculation pump is provided at each boiler to maintain minimum flow through the boilers during normal operations. The boilers are designed to have two redundant boilers in the system. The hot water system is designed to provide 140˚F hot water to BNA.
Heat Pump Chiller
Two heat pump chillers are provided in the new CUP to generate free pre-cooling to the chiller water system. Each heat pump chiller provides 4,000 MBH of heat, for a total of 8,000 MBH of heat, to the hot water system. The heat pump chillers are the primary source for heating to take advantage of the free cooling that they can provide since BNA will require both heating and cooling for the entire year. The heat pump chillers are provided with inline pumps for both the chilled water side and the hot water side. The heat pump chillers use refrigerant R-134a refrigerant.
Hot Water Pumps
Four hot water split case double suction pumps are designed for variable primary flow and are each provided with a VFD. There is an additional pump provided so that the system can operate at full capacity if one of the pumps is down or in maintenance.
Controls
The CUP is provided with an advanced demand flow control program to optimize the plant efficiency. The control system is designed so that if the front-end controller is unavailable, the equipment will default to standard operation so that system operation would not be lost due to controller failure.
Miscellaneous
The CUP includes two (2) offices, a break room, a comm room, and a main electrical room. Each of these spaces are conditioned by a single 4-pipe fan coil unit, and the electrical room is provided with an additional ventilation system to be used during cooler months. The CUP’s main mechanical area is provided with ventilation by wall-mounted propeller fans and intake louvers with security bars. The CUP is also equipped with a refrigerant detection system to activate emergency ventilation and provide an alarm for R-134a and R-123 in the event of a refrigerant leak.