Engineered Systems - October 2008 - (Page 45)

THE TECHNOLOGY How does the active chilled beam work? Referring to Figure 1, primary air (1) is introduced into the active chilled beams through a series of nozzles; (2) room air is then induced; (3) up into the active chilled beams’ coil; and (4) the induced room air is cooled or heated by the water coil, as necessary, to maintain room temperature. The induced room air is then mixed with the primary air (5) and is discharged into the room. The water control valve is thermostatically controlled (6) to maintain setpoint. Occupants experience comfort through excellent and uniform air movement and temperature. The induced air is typically three to four times the primary air volume, and the flow is constant. Air temperatures are moderate, generally 60s for cooling and 80s for heating. The result is no cold air dumping and no drafts. IAQ and odor control are never an issue since full ventilation air requirements are delivered to each unit. This occurs continuously at all load conditions during the occupied cycles. IN ACTION At 250 South Wacker, the two primary AHUs operate continuously during the occupied cycle. Duct static pressure sensors maintain +0.6 in. w.c. of air pressure to the farthest chilled beam units. The supply air from the primary AHUs is 50°F drybulb. This removes large latent load from the primary air. The air is then tempered to 58°. Thus the primary air supplied to the chilled beam is dry and above dewpoint temperature. During the economizer cycle, at the primary AHUs, sensors check to ensure that the ambient dewpoint is 52° or lower. Three-way valves divert the building’s chilled-water return system to supply water to the chilled beams’ coil. Thus the spaces’ sensible loads are satisfied. The chilled water leaving the active chilled beam’s coil is returned to the 850 tons of central chillers. During the heating season, the primary AHUs are supplied hot water from the steam-to-water heat exchanger. The building’s hot water return is supplied to the chilled beam coils. Entering water is 140°. Leaving water can be as low as 105°. The BAS controls the HVAC systems. Dewpoint sensors are dispersed throughout the building, monitoring the temperatures. It is important to maintain a 3° to 5° spread between the systems’ fluid and air temperatures, as compared to ambient dewpoint temperatures. The 250 South Wacker building is equipped with 655 chilled beam units. The units are 2 x 4 foot modules, thus easily integrated into standard ceiling tiles. The units are placed within 18 in. of the perimeter building skin. The unit breakdowns are: the north building exposure is provided with 143 units, 209 units west side, 147 units east exposure, and 156 units south exposure; the total secondary heat is 3,238.5 MBtuh and 162.5 tons cooling. The existing building HVAC system was already equipped with both vertical water risers and duct air risers at perimeter columns, which made ceiling active chilled beams a natural system choice for the project. The fact that the vision glass extends from structural column to column (and from overhead girder to floor) simply reinforced the decision to keep the mechanical system off the occupied rentable floor space. The building is a shell-and-core design; therefore, the piping is arranged to allow individual control of each chilled beam, or depending upon office space planning, control groups of active chilled beam units. Primary air to chilled beams from the duct air risers are equipped with dampers for the balancing of airflows to the active chilled beams. In order to more closely match the diverse year-round heating and cooling loads and to assist in testing and balancing, VFDs were added to the existing primary AHUs. FIGURE 3. 250 South Wacker Drive is a shell-and-core design and piping is arranged to allow individual control of each chilled beam. FIGURE 4. The chilled beam system is integrated into standard ceiling tiles and are placed within 18 in. of the perimeter building skin. Two small natural gas-fired boilers (one is standby) with eight ductmounted hot water coils were added to the primary duct distribution system. These coils are intended to temper the primary air supplies to the chilled beams from 50° to 58°. The coils ensure that the primary air supplied to the chilled beams is always above dewpoint temperature. The outcome of this active chilled beam design is a green, ecofriendly building. It provides excellent thermal comfort, energy conservation, and efficient use of space. The system operation is trouble free, simple, and has minimum maintenance requirements. The system provides constant airflow mixed with balanced conditioned outside air to each chilled beam. Thus there will be no IAQ issues. The end result is a successful project and satisfied client. ES Posko is owner of Posko Associates, Inc. Consulting Engineers (Pewaukee, WI). Reach him by e-mail at tomposko@posko.com or by phone at 312-592-2660. w w w. esmag a zin e. c o m 45 http://www.esmagazine.com

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Engineered Systems - October 2008 Contents Editor’s Note Letters Back2Basics HVAC Challenge Case in Point Commissioning Building Automation HydroTech HVACR Designer Tips Special Insert: Engineer's Technical Library Fall 2008 Living Wild! Chilled Beams in Chicago Phoenix Convention Center: On The Rise Cold-Climate HVAC Challenges Humidification Strategies For Data Centers & Network Rooms Special Advertising Supplement: Tabb Talk Computers & Software Issues & Events Products Literature & Catalogs Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - October 2008

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