Ashrae Journal - December 2008 - 29

determined (by minimum ACH rate or Dual Energy Reduction in Size of makeup for exhaust) all require that this Traditional Design Recovery With Air-Handling System air be reheated to prevent overcooling. Chilled Beams Even in the load-driven rooms, the actual 2 0.9 cfm/ft2 1.8 cfm/ft NJEDA 50% load at any time could be less than the Advance Care 12 A/C 6 A/C peak amount used to design the system. 2.6 cfm/ft2 1.9 cfm/ft2 For example, when some heat-producing Project X 27% 17.3 A/C 12.7 A/C equipment is turned off, the cooling requirement is substantially lower. In some 3.0 cfm/ft2 2.0 cfm/ft2 Project Y 33% cases a safety factor is also built into the 20 A/C 13.3 A/C maximum design load for future growth. 2.0 cfm/ft2 1.5 cfm/ft2 Project Z Under these conditions, energy is wasted 25% (8 A/C Minimum) 12.9 A/C 9.7 A/C on reheating the supply air to prevent 2.0 cfm/ft2 1.3 cfm/ft2 overcooling. A variable air volume (VAV) Project Z 35% (6 A/C Minimum) system can reduce this reheating effect, 12.9 A/C 8.4 A/C but minimum ACH rates or minimum air Project X: 70,000 ft2 of laboratory space in New England. Project Y: 60,000 ft2 of laboratory space in Midrequired for making up exhaust limit the Atlantic Region. Project Z: 45,000 ft2 of laboratory space in North Atlantic Region. amount of airflow reduction with the vari- Table 1: Reduction in air-handling system. able air volume system, still resulting in a penalty on energy used to reheat. Energy consumed for cooling range from 25% to 35%. The table also illustrates that the overis particularly high, using 100% outside air-handling units dur- all air change rates for all four buildings ranges from 6 ACH ing the summer months, since the air needs to be dehumidified to a little more than 13 ACH using the neutral air supply with chilled beams design, as compared to a range from 12 ACH to as well as cooled to 55°F (13°C). In recent years, there has been a different approach to the 20 ACH using a traditional design. design of air-handling systems for laboratories. The approach is based on a room-by-room calculation for minimum ACH Chilled Beam Characteristics rate and exhaust makeup requirements, but not cooling reFigure 1 is a diagram of an active chilled beam. The neutral quirements, using dual energy recovery to produce “neutral air at 68°F (20°C) is introduced to each beam as the primary air.”3 Neutral air refers to air that is slightly lower than room airflow. As this primary air expresses through the beam, it intemperature, say 68°F (20°C), that has been dehumidified to duces additional air to pass through the two chilled water coils maintain the relative humidity level in the building. Cool- inside the beam, mixing with the primary air. For this reason, ing for individual rooms is accomplished with local cooling active chilled beams are sometimes referred to as ceiling inducdevices. For the purpose of this article, the local cooling tion units, and use convection to cool the space. The resulting devices are considered to be active chilled beams. By taking mixture of air is supplied to the room through a linear diffuser the load-driven criteria out of the equation to determine the slot along each side of the beam, at a velocity between 40 to size of the “100% outside air” air-handling system, the total 60 fpm (0.20 to 0.30 m/s) at a distance of 2 ft (0.61 m) from amount of outside air introduced into the building can be the beam. The chilled water supplied to the coils in the beam reduced significantly by 25% to 50% of that required by a can typically be controlled to 59°F or 60°F (15°C or 16°C), traditional system. and is about 3°F or 4°F (1.7°C or 2.2°C) above the dew-point Table 1 illustrates the calculated potential reduction in size temperature for the room. The result is that the beam acts as a of the air-handling system for the NJEDA Tech IV Building sensible cooling device without condensation. Passive chilled and three other laboratory buildings. As seen in this table, the beams, by comparison, typically have much less cooling capacNJEDA Tech IV facility benefits from a 50% reduction in the ity since they rely on natural convection only, without primary size of the air-handling system. This reduction has been pos- air and induction. Since chilled beams do not have filters, fans, sible because the building does not contain many fume hoods, or other moving parts, they inherently require less maintenance so the laboratory spaces are “load driven.” With only a single than do fan coil units. fume hood present in each lab, the design ventilation rate was Based on the active chilled beam selections run with manuselected as 6 ACH at the low end of the acceptable range, with facturer’s software, the beams planned for the NJEDA project chilled beams to provide cooling. The other three laboratory each provide about 600 W (2,100 Btu/h) of sensible cooling buildings of similar design listed in the table each contain labs capacity, based on 6 ft (2 m) beam length, with approximately 30 with a higher concentration of fume hoods, resulting in many cfm (14 L/s) flow of primary neutral air into each beam, a static spaces that are exhaust driven. These spaces result in higher pressure drop of about 0.25 in. w.g., (62 Pa) water flow between required ACH rates and less reduction in total airflow at the 0.8 and 1.3 gpm (0.05 L/s and 0.08 L/s) per beam at 59°F (15°C) air-handling unit with a design using chilled beams. In these supply water temperature, and less than 5 ft (1.5 m) of water-side other buildings the reductions in size of the air-handling system pressure drop. Based on a two-pipe installation with dual cooling December 2008 ASHRAE Journal 29

Ashrae Journal - December 2008

Table of Contents for the Digital Edition of Ashrae Journal - December 2008