Engineered Systems - August 2008 - (Page 36)

Looking Radiant MAIN EXHIBIT FLOOR SYSTEM The first floor exhibit space (38,000 sq ft) was a challenge to ventilate and condition. The goal to maintain monolithic, sheer surfaces within this great volume, coupled with the desire to display the geometry of the green roof above, prevented the design team from using traditional overhead air systems. Instead, the team’s approach was to take advantage of San Francisco’s mild climate, using natural ventilation with supplemental heating and cooling via a radiant floor slab. Solar gains are minimized by the roof overhang and by motorized sun shades protecting some of the glass walls and canopies. Complex analyses were necessary to hone the system design and prove that comfort conditions would be maintained throughout operating hours. In early design stages, Arup used its own thermal analysis software (ROOM) to determine exhibit area surface temperatures, which were then used as input to STAR-CD computational fluid dynamics software. This CFD study looked at a calm day and a windy day to determine indoor air temperatures and velocities and occupant comfort levels. They were also used to determine how much additional cooling was required to augment the natural ventilation scheme. This analysis demonstrated that air movement in the occupied zone would be within acceptable limits, and that good air mixing produces a relatively even air temperature distribution throughout the large volume spaces. As the building design was refined, Arup used EnergyPlus energy simulation software to evaluate thermal and airflow behavior of the exhibit hall at design conditions. The resultant data allowed the team to determine the most appropriate configuration and control of the radiant floor and natural ventilation systems. The EnergyPlus model results were also used as input for detailed comfort and ventilation effectiveness CFD studies. The CFD studies proved that at the design summer day (79°F), with the floor slab at 68°, the occupied zone’s operative temperature is expected to be in the mid-70s and the average air velocity is expected to be 35 fps. Likewise, the CFD studies proved the exhibit area would be maintained at an average temperature of 69° and air velocity of 60 fpm in the winter. For both design conditions, these parameters fall well within comfort requirements. SYSTEM DETAILS Natural ventilation does the majority of the space cooling, while the radiant slab provides all space heating requirements and supplementary cooling. The massive concrete surfaces exposed to the exhibit area serve as a thermal capacitor, thereby reducing peak loads and ensuring space FIGURE 1. A natural ventilation system meets the California Academy of Sciences’ space cooling needs. 240-ton McQuay centrifugal chillers and circulated by a constant flow primary / variable flow secondary pumping system. This system serves both the building cooling system and the exhibit life support system (LSS) cooling demands. Condenser water is cooled via three Baltimore Aircoil closedcircuit, indirect evaporative cooling towers. While most of the cooled tower water feeds directly into the chiller condensers, some is routed through the building to serve AHU coils and to absorb heat rejected from refrigerators and freezers. Six 2,000 MBtuh condensing boilers from HydroTherm generate heating hot water which is then distributed via a variable flow, primary only pumping system. Sixteen custom Governair AHUs have been strategically located within internal mechanical rooms — some spanning multiple floors — so as to avoid interrupting the green roof. These units, in conjunction with an array of variable and constant volume air delivery systems, provide ventilation and comfort conditioning to most of the spaces. Since the Academy’s life cycle is expected to be at least 50 years, it was relatively easy to justify the added first cost of energy saving components such as condenser water cooling coils, flat plate heat exchangers and enthalpy wheels, carbon dioxide sensors, and temperature- and enthalpy-based economizers. The research and administration areas at the south facade are conditioned and ventilated via a mixed-mode system involving natural ventilation and finned-tube heating at the perimeter. 36 En gi neer ed S y stem s August 2008 FIGURE 2. More than 100,000 linear ft of tubing was used in the radiant system underneath the main exhibit floor. The system meets the building’s space heating and supplemental cooling requirements.

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Engineered Systems - August 2008 Contents Editor's Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Efficiency Incentives HVACR Designer Tips Glass Box to Grand Casino Looking Radiant In Green Mechanical Products Sourcebook Q&A: Dehumidification In Schools Building Oversight Management: M&V And More Products Classifieds Advertiser Index Tomorrow's Environment

Engineered Systems - August 2008

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