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April 17, 2013

Ultra-efficient mechanical systems offer lots of ways to cut costs

  • Engineers used strategies such as water preheating, multiple-size chillers and dual-fuel condensing boilers.
    Affiliated Engineers NW


    Seattle Children’s new Building Hope is projected to be approximately 50 percent more energy-efficient than existing buildings on campus, thanks to energy- and cost-saving strategies incorporated into building design.

    The building was designed to meet the LEED gold criteria for new construction. Among these strategies, the design of energy-efficient mechanical systems contributed to a significant reduction in the facility’s overall energy use and costs.

    The hospital incorporates a lean approach to its operation and management in order to maximize its quality of care, patient safety, operating cost and other factors. The building’s mechanical design augments lean management goals by incorporating flexible systems that conserve energy and reduce costs, allowing resources to flow back into the larger patient care mission.

    Image courtesy of Affiliated Engineers NW [enlarge]
    Many of the patient rooms have HEPA recirculation units that allow staff to double hourly air changes, from six to 12, when needed. The option will allow the hospital to save 320,000 kilowatt hours of energy a year.

    As part of the lean process to determine the best strategies for Building Hope’s mechanical system design, the mechanical team developed a set of sustainable concepts and engaged Seattle Children’s health care and facilities personnel into the evaluation and decision-making process.

    In addition to sustainability, stakeholders analyzed the impact of mechanical design options on health care delivery, infection control, maintenance issues, equipment access, cleanliness and overall user satisfaction. Out of this process a set of options emerged for further evaluation.

    Working closely with Seattle Children’s, Seattle City Light and contractors, costs and opportunities for sustainable design options were strategically assessed.

    Innovative and sustainable mechanical design features that were incorporated into the final building design include HEPA recirculation units in 54 patient rooms, high-efficiency chillers, domestic water preheating, emergency generator heat recovery and dual-fuel condensing boilers.

    Air changes on demand

    The building has custom-designed high-efficiency particulate air (HEPA) filtration systems that enhance the efficient use of patient room space and bring a savings of 320,000 kilowatt-hours per year over traditional approaches, while providing more flexibility in the scheduling and operation of the rooms.

    The Washington State Department of Health requires 12 air changes per hour in “protective environment” patient rooms, while only six air changes per hour are required in a normal patient room. The additional six air changes are important for patient safety in certain situations. But when not needed, the extra air changes place an unnecessary burden on energy consumption and overall cost.

    After receiving Seattle Children’s approval for the HEPA recirculation systems concept, the mechanical team presented evidence to the state health department that demonstrates the ability to meet the requirement for 12 air changes per hour in protective environments. The recirculation units were also mocked up to ensure that a quiet patient room environment would be maintained during operation.

    HEPA recirculation units are mounted on ceilings in 54 patient rooms at Building Hope, allowing patient care personnel to easily modify air changes as needed. A switch located outside the patient room directs the number of air changes needed for either a normal environment or a protective environment.

    During the normal mode, the patient room receives the standard six air changes. When the switch is turned to protective mode, the HEPA unit is engaged and 12 air changes per hour are provided. This feature can also be used to provide additional room flushing during cleaning between patients, thus reducing the turnover time for patient rooms.

    Another benefit of the airflow reduction is that it minimizes the overall size of the air-handling units, which reduces the amount of space needed to house the mechanical systems and leaves more usable occupant space on each floor. The decrease in ventilation load also reduces the load on the boiler plant, chiller plant and condenser water system.

    High-efficiency chillers

    A mixed chiller plant was devised to contain large, water-cooled centrifugal units and small, modular 80-ton units equipped with high-efficiency, oil-free magnetic bearing compressors. The arrangement of these chillers contributes to greater flexibility for variable conditions.

    By engaging chillers with varied part-loading capacities and efficiencies, the plant has the ability to operate at peak efficiency across a wide range of operating conditions. During periods of low loading, the system can operate using the smaller units in a very efficient mode rather than the larger units, which are less efficient at low load.

    Preheated water

    Heat is rejected from both the modular and the 350-ton chillers into a condenser water loop. Before sending the hot condenser water to the building cooling tower, the water is passed through a double wall-plate heat exchanger, where it transfers heat to incoming domestic cold water, thus efficiently preheating cold water for domestic hot water use.

    Rejected heat can also be recaptured when produced by the emergency generator. During these occasions, the heat is funneled through the water loop on the building cooling tower, contributing to domestic hot water preheating and maximizing the energy benefit of the fuel consumed by the generator.

    Dual-fuel boilers

    Heat is provided by condensing boilers that have the capacity to operate at up to 95 percent efficiency. This is well in excess of standard water tube boiler efficiency of approximately 80 percent.

    In addition, these are among the first condensing boilers that can switch operation from gas to diesel oil in the event of a disruption to the gas supply.

    Additional measures, such as low-pressure drop duct and piping design, variable speed drives and others, when combined with the above mechanical system approaches, make the new Building Hope one of the most energy-efficient hospitals in the Pacific Northwest.

    Jim Sokol, a principal at Affiliated Engineers NW, specializes in health care facility design. He led the mechanical, plumbing and architectural lighting systems design for Seattle Children’s Building Hope.

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