Technical
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spiral downstream ducting . A slight increase in the internal diameter of the duct network would result in little impact on the capital cost , and the associated labor to hang the larger ducting ( i . e ., 6 in . [ 150 mm ] diameter round duct versus 4 in . [ 100 mm ] diameter ).
A good rule of thumb would be to restrict the branch ducting to less than 600 fpm ( 3 m / s ). An extended plenum design is the ultimate goal of the downstream duct design for ACB applications . The cross-sectional area required to achieve this design is not always available or possible due to restrictions in height and / or costing restraints . The duct optimisation will remain a design issue tailored to each project .
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 labour .
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
Figure 2 : Schematic operation of an active chilled beam .
pressures would seem to remain essentially innocuous from the
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 noise generated at any breach in the seal . The goal in the downstream duct design is to efficiently deliver the pressure to the beam plenums through the use of low velocity ducting .
BEAM PLACEMENT AND ROOM AIR DISTRIBUTION A high induction ratio provides discharge air normalization as it relates to discharge temperatures . As a result , a lower approach exists between the beam ’ s discharge air temperature and the room air temperature . Consequently , the occupants can tolerate higher average room air velocities ( 30 fpm [ 0.15m / s ]), and remain in compliance with ANSI / ASHRAE Standard 55 , Thermal Environmental Conditions for Human Occupancy , for the design setpoints of 75 ° F ( 24 ° C ) and higher , which is common criteria for ACB projects .
Typical discharge temperatures from ACBs range between 64 ° F to 66 ° F ( 18 ° C to 19 ° C ), and ultimately yield lower overall
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