Technical
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delivered to the beam plenum for dehumidification , creating the induction effect within the ACB .
Potentially , for an office type building , this could translate into a reduction in the amount of total air processed at the air-handling unit to between 25 % to 50 % of that which is required by an all-air system . Consequently , by using the fan laws , one can quickly calculate the potential fan energy reduction compared to all-air systems when using DOAS equipment .
Figure 1 is an approximate representation of the possible energy savings associated with DOAS ventilation systems . Although the primary means of heat transfer is radically different , radiant chilled ceilings and ACB projects using DOAS offer the potential fan energy savings of this type of air-distribution system for many conventional building types including offices , schools , etc .
Lab spaces may require additional airflow to provide prescribed air change rates , or for fume hood makeup , but have used ACBs to manage the sensible loads within the space with great success . 1
The energy savings associated with the load from lights may not be available to ACB installations , as this graphical representation assumes that the sensible heat gain from the lighting load would be transported from the building envelope via the plenum , and that this heat gain would not be added to the space load , as it would in the conventional HVAC system and ACB installations .
Enthalpy wheels and heat pipes can be added inexpensively to makeup air-handling units as the total air volumes managed by DOAS equipment is much smaller than that required for VAV or CAV systems . There is an additional cost that must be considered for ducted return systems , which is unnecessary if there is no heat reclaim option included with the DOAS unit . However , an energy model will quickly and clearly identify if the added expense is justifiable .
SUITABILITY FOR VARIOUS SPACES Active chilled beams should not be considered a silver bullet in terms of addressing high sensible cooling requirements in all spaces . Certain spaces are well suited to chilled beam use , and others are not appropriate for this technology . Areas that may not be suitable for active chilled beams may include , but are not limited to :
• Large vestibules / atriums : latent load is difficult to control and could be addressed via other design strategies ;
• High latent cooling requirements : kitchens , pools , locker rooms , spas , gymnasiums , etc .;
• Hospitals : Class I and Class II areas where recirculated room air is not permitted ;† and
Figure 1 : Typical fan and motor savings for office building dedicated outdoor air systems ( DOAS ) vs . conventional HVAC systems . 2
• High ceilings : room air movements should be for ceilings in excess of 14 ft ( 4.3 m ).
Many spaces lend themselves well to the use of active chilled beams . These may include , but are not limited to , offices , schools , labs computer rooms ( i . e ., desktop farms as opposed to rack rooms ), and low-ceiling height building retrofits ( i . e ., < 10 ft [< 3 m ] and other space that provides little clearance for mechanical services ).
DUCT DESIGN AND WORKING STATIC PRESSURES Active chilled beams are dependent on pressure for driving the induction engine . One must consider reviewing the duct design to ensure that the pressure is efficiently delivered to the beam plenum . Although not a requirement , it is generally preferable to consider a low-velocity downstream ducting strategy . The main ducts may be sized as they would normally for offices at 1200 fpm to 2400 fpm ( 6 m / s to 12 m / s ), favoring lower duct velocities or less where possible to mitigate duct frictional losses .
However , when considering the branch ducts to the beam plenums , a slight oversizing of the ductwork helps avoid duct pressure losses , which can be used more effectively at the beam plenum to enhance the ACB capacity . In the strategy of avoiding downstream pressure losses , one can ensure more pressure is available to operate the beam , and improve its capacity , resulting in a reduction in the number of beams required on a project , and consequently , lowering the initial capital costs for this type of system .
One must be aware that higher operating static pressures increases the acoustic signature of the ACBs . A balance between acoustic requirements and capital cost must be evaluated . Active chilled beam projects often are installed using round
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