Ashrae Journal - December 2008 - (Page 12) Designing Active Chilled Beams September’s ASHRAE Journal article “Design Considerations for Active Chilled Beams” by Darren Alexander, P.Eng., and Mike O’Rourke provided a good overview of chilled beam technology and its energy-efficiency benefits. However, it primarily describes the design methodology for a high cooling approach, i.e., high chamber pressure of 1 in. w.c. (250 Pa) to maximize the sensible cooling capacity of these units, reduce the number of units, and thereby optimize first costs of the system. What was not taken into account is the lower comfort in the occupied zone due to increased risk of local drafts and the detrimental effects on the building for occupants and owner. A chilled beam is a high induction unit (induction ratio of 1:3 to 8)— the higher the chamber pressure, the higher the amount of induced room air per linear unit. Furthermore, the induction ratio is constant during all operating conditions, and users do not have influence on it. In fan coil systems, users can control the fan speed and in a VAV system the air volume is reduced when cooling is not required. High air volumes from shorter beams will create high local air velocities in the occupied zone. In a sustainable design approach, several aspects need to be considered, not just the investment cost of the system and whether the cooling demand is fulfilled. As in the LEED building rating system, both energy efficiency and indoor environmental quality are taken into account when evaluating the design. High enough supply air temperature 60°F (16°C) and low velocities below 50 fpm (0.25 m/s) in the occupied zone during all operating conditions give good overall thermal comfort and prevent localized drafts. Primary air volume should be selected based on occupancy level with 20 to 40 cfm (10 to 20 L/s) per person to ensure high indoor air quality and removal of internal latent loads. Outdoor air humidity is controlled at the cooling coil of the air-handling unit. The energy efficiency of the system is further increased when higher inlet water temperatures (60°F [16°C]) and low chamber pressures (0.3 to 0.4 in. w.c. [75 to 100 Pa]) are used. 12 ASHRAE Journal Higher inlet water temperature reduces the risk of condensation, increases the COP of the chiller, enables use of ground water heat pumps, or gives a greater duration of free cooling. Chilled beam technology enables both design approaches: sustainability approach and high cooling approach. A typical European design today follows the sustainability approach with approximately 260 to 420 Btu/h of sensible cooling per linear foot of chilled beam (250 to 400 W/m). This results in slightly longer terminal units but ensures excellent indoor conditions and the highest possible energy efficiency. Maija Virta, M. Sc., Vice-president of REHVA, Vantaa, Finland Risto Kosonen, Dr. Sc., Member ASHRAE, Vantaa, Finland The Authors Respond While it is true that the design methodology describes an approach focused on first cost optimization, the risk to comfort may have regional subjectivity and may not account for the ability to reset both the primary air dry bulb and chilled water temperatures in the off-peak operating condition. Unlike fan coils and VAV systems, active chilled beams offer the benef it of vigorous and thorough mixing of the room air and primary airstreams within the active chilled beam body. The typical discharge air temperature of an active chilled beam (in cooling) is around 63°F – 67°F (17.2°C – 19.4°C). This is signif icantly warmer than conventional discharge air temperatures observed by variable air volume (VAV) and fan coil type systems. Consequently, in comparison to conventional systems, the occupant comfort is enhanced due to a decrease in the approach temperature delivered by the active chilled beams. The risk of draft can be further mitigated by using a reset schedule for both the primary air dry bulb and chilled water temperatures, the building automation system, and a double-wheel dedicated outdoor air system (DOAS) unit. It is also important to note that VAV can be easily applied to active chilled beams with an appropriashrae.org December 2008 http://www.ashrae.org
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.