Technical
© ASHRAE , www . ashrae . org . ASHRAE Journal , September , 2008 . This article is re-printed with the necessary permission of ASHRAE . Placement of this article was sponsored by the ASHRAE South African Chapter . Use of this ASHRAE content does not state or imply ASHRAE endorsement . No additional distribution or reproduction of this content may be made whatsoever without direct permission from ASHRAE .
DESIGN CONSIDERATIONS
FOR ACTIVE CHILLED BEAMS
By Darren Alexander , P . Eng .; and Mike O ’ Rourke
Active chilled beams ( ACBs ) are becoming popular with design consultants as a means of managing large sensible cooling loads with excellent energy performance and low acoustic signatures .
This type of active chilled beam is generally installed within a T-bar ceiling grid and is often supplied in 1 ft or 2 ft ( 300 mm or 600 mm ) widths , and lengths ranging from 2 ft to 10 ft ( 0.6 m to 3.0 m ). Active chilled beams provide the advantage of a substantial sensible cooling effect , often by using as little as only the required ventilation air .
Also known as high induction diffusers , ACBs recirculate room air through a unit-mounted coil , driven by ventilation air ( primary air ) with mixing ratios as high as 6:1 ( discharge air to primary air ). The coil is provided chilled water , which is maintained above the dew point of the primary air to prevent condensation from forming on the coil . The driving force in dehumidifying a space is the mixing of moist room air with drier primary air .
By decoupling the ventilation load from the space load , perpetual fan energy savings are created when compared to VAV and other types of all-air systems . Although the low air volume advantage may be reduced when addressing higher latent loads , ACBs offer the benefit of warmer chilled water temperatures for cooling , and cooler hot water temperatures for heating ( low exergy systems ) to provide high levels of controlled comfort within the occupied zone . This allows for consideration of additional system options in the design process , and offers a new world of possible solutions to the total HVAC system design process .
A system advantage in terms of improving the chilled water temperature range is open to sites that use a dedicated outdoor air system ( DOAS )* evaporator to return water to service part of the secondary chilled water loop , minimizing the total pumping requirements . Fin-enhanced , closed-circuit fluid coolers allow for
ACB water-side economization , and mixed mode ventilation can be used to great effect to further reduce the carbon footprints of new and existing buildings .
In addition to their relatively shallow depth ( generally less than 12 in [ 300 mm ] clearance requirements ), ACBs offer an attractive solution to retrofit type applications as the mechanical service space requirements are exceedingly modest in comparison to systems such as variable air volume ( VAV ), constant air volume ( CAV ), or fan coil applications . In new construction , this reduction in clearance above the ceiling height could offer lower construction costs for buildings with lower floor-to-floor height . Essentially , active chilled beams are used to remove the sensible heat from the occupied zone via chilled water in lieu of air . Given the high specific heat of water , and the reduction in total fan energy , it is easy to understand why active chilled beams offer substantial water and fan energy savings over all-air type systems . Capital cost savings are often available due to a potential reduction in floor-to-floor height , ducting requirements , and airhandling unit sizes .
CAPACITY AND ENERGY SAVINGS Active chilled beams have the ability to reduce the total air handled by the ventilation system because they typically require as little as 99 cfm / ton to 250 cfm / ton ( 13 L / s per kW to 33.6 L / s per kW ) of sensible cooling as it relates to the volume of primary air
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