Case study: Patient-bed tower addition

A hospital required paralleled generators to ensure adequate power distribution and prepare for future growth.


A hospital campus added a 144,000-sq-ft patient tower to the existing campus. This new tower includes approximately 80 patient beds and is mechanically independent from the existing campus central plant. The project was designed using the 2012 edition of NFPA 101: Life Safety Code, the 2010 edition of NFPA 110: Standard for Emergency and Standby Power Systems, and the 2012 edition of NFPA 99: Health Care Facilities Code.

Figure 3: The existing glass storefront (top) was replaced with dampers. Courtesy: IMEG Corp.

Figure 3: The existing glass storefront (top) was replaced with dampers. Courtesy: IMEG Corp.

The total connected building loads were calculated to be 1,200 kW, with 550 kW on automatic emergency power and another 350 kW on manual transfer. Generator backup was required by NFPA 99, NFPA 101, and NFPA 70: National Electrical Code for lighting, branch power, and mechanical infrastructure. All of the loads were located within the building, with the exception of the standby generator systems. More than 1,000 ft away was an empty building that had sufficient space and structure for the proposed generators. This location would keep noise and fumes away from the new patient tower.

The existing campus has three 400-kW standby diesel generator sets operating at 480 V adjacent to this building, and they were already loaded to 70% capacity. The design team considered revising/upgrading the existing system but decided that a new system with room for future growth would provide more flexibility. Eventually, the campus will expand to the new emergency power supply (EPS) and emergency power supply systems (EPSS) and transition the existing campus distribution to the new systems.

Figure 4: An existing exterior façade (top) was replaced with a discharge air plenum. Courtesy: IMEG Corp.

Figure 4: An existing exterior façade (top) was replaced with a discharge air plenum. Courtesy: IMEG Corp.

Due to the length of electrical feeders between the generators and the new building, the generator voltage was chosen at 4,160 V. The underground duct bank included a spare conduit for future growth and a dedicated conduit for transfer switch control wiring. The design team evaluated several generator options including paralleled and a single large generator. To provide redundancy, temporary connection capability, and room for future growth, the paralleling gear selected had four generator-input sections.

The room is sized for three 1,000-kW generators. Two 600-kW generators were provided with space allocated for a third, and an exterior connection provided for a fourth. The exterior connection also provides a temporary generator connection in the event of maintenance, which complies with 2016 NFPA 110, Article 7.13. Screen walls were added to the existing exterior walls to allow for adequate airflow. A dedicated room was created to house the paralleling gear separate from the generators to allow for a clean, conditioned place to operate electrical gear. The generator room is heated and the EPSS room has heating and cooling with a dedicated fan-coil unit.

Figure 5: The layout of the emergency power supply systems (EPSS) room shows a generator that is remotely located from switchgear. Courtesy: IMEG Corp.The air intakes were located on the opposite wall from the unit-mounted radiators to provide ideal airflow and minimize pressure drop (see Figure 3). Recirculation dampers alleviate pressure in the discharge plenum, as plenum pressurization can prevent the discharge louvers from opening. There are several options for discharge louvers, such as gravity, center-pivot motorized, and end-pivot motorized. Each style offers different pressure-drop specifications, and each is affected differently by plenum pressurization.

Because motorized dampers are typically held closed by a motor, a spring in the louver system will force the dampers open when voltage is removed from the motor. It can take 10 to 20 seconds for a motorized louver to change state. A generator fan will start pressurizing the discharge plenum in a few seconds. The recirculation damper provides pressurization relief to prevent the radiator fan's pressure from holding a motorized damper closed. While gravity dampers do not have a delay to open, they do result in additional pressure drop due to the airflow forcing the damper open.

Center-pivot motorized louvers were chosen due to low-pressure-drop specifications. Acoustical louvers were used at the generator discharge, with consideration given to the calculated property-line noise levels (see Figure 4). Additional considerations included an in-floor trench for fuel-oil piping and a fuel polisher for the existing long-term fuel-storage system.

The EPSS room has a dedicated heating and cooling system, which will prolong the life of the equipment in the room. The location also has a lower ambient noise level, which makes operation safer by allowing for effective concentration and communication. The 5-kV circuit breaker switchgear and generator control panels were housed in separate enclosures on an opposite wall in the EPSS room (see Figure 5). Having controllers in separate enclosures enhances safety by preventing personnel from standing in front of the switchgear when controlling 5-kV-rated breakers.

A redundant master-control screen is located in the generator room as well. The paralleling-gear busing is rated 1,200 amp, which allows for 5 MW of generator capacity. Because the new generator building required electricity from the new EPS and EPSS, a unit substation, equipment branch, and life safety branch transfer switches were provided. The patient tower's EPSS gear includes a single-ended unit substation with space allocated to provide a double-ended unit substation for future feeder redundancy. The draw-out switchgear has several spare breaker cubicles to facilitate future growth and allow the new EPSS distribution to refeed existing portions of the hospital campus.

Richard Vedvik is a senior electrical engineer and acoustics engineer at IMEG Corp. He is a member of the Consulting-Specifying Engineer editorial advisory board.


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