Consulting-Specifying Engineer - May 2008 - (Page 36) Constitution Center chills out By using chilled beam technology, this Washington, D.C., building takes on new life with a different HVAC system. BY GEORGE KARIDIS, PE, LEED AP, Vice President, SmithGroup, Detroit s Washington, D.C.-based David Nassif Assocs. rejuvenates its namesake building, the full-block marble clad structure is shedding its aging skin and MEP systems. It will re-emerge in late 2009 as Constitution Center, the first large-scale building in the United States to use chilled-beam technology, and one of the largest USGBC LEED Core and Shell Development (LEED CS) projects to date. The building’s spacious 30- x 30-ft bay and 10-ft 4-in. floorto-floor waffle plate structure demands a compact ceiling sandwich to maximize the room height and abundant daylighting streaming from a glazed perimeter that is never more than 45 ft from any interior point. This being the case, conventional HVAC systems would have required excessive cavity depth and multiple “fixed monument” fan-rooms in the midst of the wide open floor plate—a major detriment to tenant planning flexibility. Instead, the owner’s representatives and engineers from SmithGroup’s Washington, D.C., and Detroit offices chose active chilled beams. This tamed the duct distribution issue and provided for increased comfort and IAQ for tenants, with substantial energy savings, especially compared to the conventional variable air volume (VAV) systems prevalent in the Washington, D.C., office market. Operational prototype To verify that choice, the team developed a fully monitored, operational prototype installation. Engineers selected chilled beam technology from Trox USA Inc., Alpharetta, Ga., following a review of test results, preliminary construction pricing, and the company’s willingness to consider modifications to its base design to better suit Constitution Center’s actual conditions. Prototype testing also honed the strategies for ensuring chilled beam condensation control, including active building pressurization control, air dew point monitoring, water temperature monitoring, condensation sensors, and a sequence for restarting using only primary air after a system interruption in humid weather. A Other integrated design fundamentals to avoid condensation include a low-air-leakage building envelope and the architectural isolation of program areas with high latent load. Active chilled beams The active, or ventilated, chilled beams handle the majority of cooling and all heating with overhead coils and piping systems, which are both space- and energy-efficient. The temperature difference of the constant primary air flow merely supplements the water-based cooling effect. During occupied schedules, primary air flows at a constant volume to each 2-ft x 6-ft x 9-in. unit. Interior zone chilled beams receive 50 cfm of 65 F air. This supply temperature eliminates the need for reheat, because internal heat from lighting alone offsets the primary air cooling capacity. The chilled water is supplied to the beams at 60 F, which is above the space dewpoint. The perimeter chilled beams, which can heat or cool as needed for each zone, typically receive 75 cfm of primary air. In the summer, this air is distributed at 57 F, which helps augment cooling capacity, though this may be reset upward in the winter. On a call for heating, 110 F heating hot water feeds a portion of the coil tubes in these perimeter beams. Indoor environmental quality The primary air flow through the beams exceeds all required ventilation. It also induces several times the flow of recirculated room air through the beam’s coil, and then that mixture of air diffuses into the space via 12 ft of linear air diffuser. The result is a moderate temperature discharge offering remarkably draft-free comfort and a consistent, quiet acoustical environment. Furthermore, since the local recirculation is essentially limited to the area served by each chilled beam, the beams offer greater isolation from the vitiated air of other office areas. (Continued on page 40) 36 Consulting-Specifying Engineer • MAY 2008
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