ASHRAE Journal- September 2008 - 51

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 air-handling unit sizes. Capacity and Energy Savings 100% Peak Power 37.5% Fan and Motors 57.5% Load From Lights Air Transport Load Pumps 18.8% Chiller 62.5% 9.3% 1.5% 7.5% 9.4% 1.9% 34.4% Other Loads 34.4% Conventional HVAC System Radiant Cooling and Chilled Beams Percentages Relative to Overall Peak Power for the Conventional System Figure 1: Typical fan and motor savings for ofﬁce building dedicated outdoor air systems (DOAS) vs. conventional HVAC systems.2 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 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 *For additional information on dedicated outdoor air systems, see http://doasradiant.psu.edu/. †Class I and II areas as deﬁned by CSA Standard Z317.2, Table 1, Ver. 01, 2001. 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 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 1,200 fpm to 2,400 fpm (6 m/s to 12 m/s), favoring lower duct velocities or less where possible to mitigate duct frictional losses. However, ASHRAE Journal 51 September 2008

ASHRAE Journal- September 2008

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