Engineered Systems - March 2008 - (Page 39)

• Demand-based ventilation control sequences in intermittently occupied spaces such as the gymnasium and the flex room/cafeteria; • Heating system capacity assumes a portion of building heat will be provided from internal gains (i.e., lighting, computers, and occupants). WEATHER AND BUILDING BEHAVIOR: PUSHING THE ENVELOPE The first two prototype schools are equipped with air cooled packaged chillers, and the electrical rates justified an ice storage system at one of the schools. With Rice, PSD pushed the design team to find an alternative to a traditional chilled water system. The cooling system design began with an evaluation of a local non-potable irrigation distribution system for possible use as a heat sink for building cooling. For a number of reasons, it was determined the system was not a feasible alternative. Our design team then turned its attention to a means of evaporative cooling. The District had no desire to apply direct evaporative cooling, so the team began looking into different types of indirect evaporative cooling. A centralized system was chosen in an effort to minimize maintenance, minimize air handling fan energy, and centralize water treatment. The design team utilized the semi-arid climate along the Colorado Front Range to its fullest advantage. The high elevation and the relatively dry air results in a daily temperature range of approximately 24°, which aids in maintaining the building mass at a temperature that assists with indoor temperature stability during the cooling season. For the typical school day occupancy, and from the months of September through May, there are approximately 350 hrs that the building requires cooling. Of those hours, the coincident wetbulb temperature is below 55° for 300 of those hours (86%), which forms the basis of design for the indirect evaporative cooling. During more extreme cooling demand conditions, indoor temperatures will rise, but this occurs only during the the last hour or so of full building occupancy. After 3 p.m., the building cooling system is deactivated, and the building uses its own thermal inertia to “float” until the next day. The indirect evaporative cooling system design uses the 95% design wetbulb temperature of 56° for the months of September through May, allowing the cooling tower to be right-sized for the traditional school year. The cooling tower is sized for a water temperature drop of approximately 5° to deliver the coldest practical evaporative-cooled water to the building. Due to the nature of Colorado’s climate, the performance of the building, and the innovation of the design team, we were able to completely eliminate a chiller from the design equation, which met one of the school district’s primary goals. SYSTEM DESCRIPTIONS A significant design team goal, and one that leads to lower first cost and maintenance costs, is to maintain a level of simplicity in the design of all building systems. For the hydronic systems, several simple design ideas were added to gain efficiency. The heating system consists of two high-efficiency, gas-fired boilers with staged capacity control; two variable-flow, base-mounted distribution pumps; heating water distribution piping arranged in a reverse-return configuration; secondary pumped coils at each AHU; terminal reheat; and local The cooled water system at Rice Elementary includes an open cooling tower with a variable-speed fan. building energy performance, and allowed the HVAC systems to be significantly downsized. Some of the highlights of the architectural features of the 63,000-gross-sq-ft facility includes the following. • Commitment to a high-performance building envelope; • Attention to building orientation and minimizing the building footprint; • Integrating day-lighting design with exterior shading and lighting controls; • Understanding the contribution of building mass in the energy equation. In concert with the design team, PSD has been progressive in its approach to energy conservation. The district understands that as energy efficiency goals become increasingly lofty, building systems require more attention during commissioning, and may not be able to carry the building through extreme weather conditions as seamlessly as conventionally designed, oversized systems. This approach has allowed the design team to push the envelope to a degree that may be considered too marginal by many building owners, but achieves superior results in energy consumption. “These improvements are helping us redirect money that would normally go to utilities back into the classroom,” said Stu Reeve, energy manager for PSD. Mechanical measures taken to reduce the system capacity include the following. • No cooling in the gymnasium — heating and ventilation only; • A cooling space setpoint of 75°F; w w w. esmag a zin e. c o m 39 http://www.esmagazine.com

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Engineered Systems - March 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips Application Checklist Exemplary Design = Elementary Success The Modern Unit Ventilator On A Mission VAV Systems And Green Design – Part II Issues & Events Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - March 2008

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