ASHRAE Journal- September 2008

When zoning the ductwork, consider a duct ring around the glazed perimeter of buildings. In this way, the loads can be managed independently of one another. The perimeter ring may be equipped with a duct-mounted heating coil to address the skin losses that are not present within the core of the building. Increasing the primary air temperature to the exterior zone, via this offset during the heating season, allows the ACBs to yield the greatest heating output, without sacrificing internal cooling comfort capability. Although four-pipe ACBs are available, the aforementioned strategy typically requires less capital investment at the cost of individual room temperature control. Manufacturers’ data suggests that increasing the end-of-run operating static pressure from 0.4 in. w.c. to 1.0 in. w.c. (100 Pa to 250 Pa) will increase the active chilled beam capacity by more than 55%, at a fan brake horsepower penalty of only 14.1%.‡ Typical operating static pressures range from 0.3 in. w.c. to 1.2 in. w.c. (75 Pa to 300 Pa) at the pressurization plenum of the beam. This trend would hold true with all active chilled beam manufacturers. While an exhaustive study is not included here, it is reasonably easy to make a case for higher beam plenum pressures as this strategy ultimately requires fewer beams to satisfy the sensible cooling load, and little in terms of a fan energy penalty. A slight increase in the cross-sectional area of the primary air ductwork also helps to mitigate any additional fan power, and could be accomplished with little premium in terms of material or labor. However, the additional static pressure at the beam plenum generally adds more sound pressure to the space. The effect is typically considered inconsequential. Higher frequency acoustic content, which is typical of beams, generally dissipates quickly once the room attenuation effect is factored into the analysis. At 1 in. w.c. (250 Pa), the acoustic signature of a single active chilled beam in a typical room could be as low as 29.5 dBA,§ assuming no other sound sources within the space, and no room attenuation effect. As a result, a small increase in system working pressures would seem to remain essentially innocuous from a perspective of a total systems impact. Supercharging beam applications by considering working static pressures between 0.75 in. w.c. to 1 in. w.c. (185 Pa to 250 Pa) is fundamentally a sound approach from an optimization perspective in terms of first costs. As the sound source for an active chilled beam is the nozzle, and since these sound sources are added logarithmically, once the acoustic signature of a single beam is known, with dozens of nozzles per beam, the acoustic signature of the space is also known (as it relates to the beam’s contribution). Sealant materials and workmanship become important issues with pressurized ductwork. However, smaller ducting typically servicing ACB systems is easier to leak test, particularly when leaks in the duct network are easily identified by a whistling ‡Primary air volume = 4,000 cfm (1887 L/s) at 2.4 in. w.c. (600 Pa) increased to 3.0 in. w.c. (750 Pa). Total sensible capacity increase provided by active chilled beam = 50%. Increase in fan bhp = 14.1%. 15 in. SISW AFPF, 9-blade, Arr.3., CLII. §Acoustic sound pressure levels determined using selection program data from Twa Panel Systems, Inc. (MAC Beam software Ver. 11). www.info.hotims.com/19354-68 September 2008 ASHRAE Journal 53 http://www.carlyleCompressor.com http://www.carlyleCompressor.com http://www.info.hotims.com/19354-68

Table of Contents Feed for the Digital Edition of ASHRAE Journal- September 2008

ASHRAE Journal- September 2008 Section: Contents Contents Section: Commentary Options for Sustainability Section: Industry News The Silk Route for Energy Solar Thermal Is Unrealized Opportunity Industry Groups Sue City of Albuquerque Section: Letters Letters Section: Meetings and Shows Meetings and Shows Section: Feature Articles Article-Radiant Floor Cooling Systems Article-HVAC Design for Sustainable Lab Article-Mixed Mode Ventilation Article-Single- Design Considerations For Active Chilled Beams Article-Acoustic Design In Green Buildings Article-Teams, Contracts & BIM Section: Building Sciences Some Old Lessons Distilled Section: Washington Report Energy in Federal Buildings Section: Products HVAC&R Product Showplace Section: Emerging Technologies Toplighting & Lighting Controls For Commercial Buildings Section: Special Products Fans & Blowers Section: Classified Ads Classified Ads Section: Advertising Index Advertising Index

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