Active Chilled Beams: Saving Energy and Space
Chilled beams are ceiling-mounted fixtures that use chilled water flowing through finned heat-exchanger coils to supply cooling, and sometimes heating, in commercial buildings. They are available in a variety of styles, typically in one- or two-foot widths (0.3 or 0.6 m) and in lengths up to ten feet (3 m).
Passive chilled beams operate through simple convection: as warm room air rises, it passes through the water-cooled heat exchanger fins inside the chilled beam, where it cools and settles back into the room. This pattern of rising and settling air circulates the cooling energy. Passive beams can provide an energy-efficient HVAC solution, especially in retrofits or modular office layouts where duct space is a problem, but they require the use of added ventilation—usually underfloor—to provide fresh air and humidity control, and they do not provide heating.
Active chilled beams (ACBs) account for most chilled beam sales and are the focus of this article. They use the same heat-exchanger technology as passive beams but can supply both cooling and heating, as well as ventilation air. ACBs contain an added compartment (plenum) that is connected to the ventilation air supply (see diagram). This primary ventilation air (dehumidified and filtered outdoor air) enters the beam’s plenum under pressure where it is forced through nozzles that direct the flow of the air along the outside of a second chamber and into the room. This action pulls secondary air (room air) into the unit from underneath via induction past the heat exchanger coils. The now-cooled/heated secondary air mixes with the primary air in this second chamber and is blown back into the room. Unlike passive beams, active beams do not rely solely on convection for air circulation, so they can force warm air down into the room, supplying heating as well as cooling. (Multi-service beams are custom ACBs that include lighting, sprinklers, security, sensors, or other features). There are no electrical connections or moving parts in the beams, minimizing maintenance and creating a quiet HVAC system.
ACBs use water’s ability to transport energy more efficiently than air. In general, moving a single Btu in air consumes 20 times more energy than moving that same Btu in water, according to John Nodson, application engineer at Trox USA. Because of these efficiencies, higher water temperatures can be used for cooling (55°F–61°F/12.7°C–16.1°C) and lower temperatures for heating (85°F–115°F/29.4°C–46.1°C), leading to improvements in chiller performance or the possibility of reducing chiller size, as well as improved boiler efficiencies, especially for condensing boilers. Other cooling and heating sources, such as ground-source heat pumps, could further reduce the demands on chillers and boilers, and in some cases could replace them altogether.
HVAC fans are one of the biggest energy consumers in commercial buildings. With ACBs, fans only need to move ventilation air—and less of it—so a secondary fan for heated air is unnecessary. Twa Panel Systems claims that its MAC Beams can produce one ton of cooling by moving as little as 99 ft3 (2.8 m3) per minute of air. And some ACBs can provide sensible cooling (which handles “dry” heat from lights, equipment, and people) of up to 1,100 Btu/hr per linear foot, according to Trox. A 2009 study by the American Council for an Energy-Efficient Economy showed that a 100,000 ft2 (9,290 m2) building saved 20% (1,600,000 kWh/year versus 2,000,000 kWh/year) on its energy consumption using an ACB system with dedicated outdoor air supply compared with a standard variable air volume (VAV) system with chiller.
Because ACBs demand less airflow to supply the primary ventilation air, ducts can be sized significantly smaller than those for all-air systems; ACBs use four- or five-inch ducts compared to 14-inch ducts or greater for a comparable VAV system. And as little as eight inches of ceiling clearance is necessary for the ACBs, their pipes, and ductwork, which means ceilings can be taller or overall room height adjusted: five floors could potentially fit into the same space typically dedicated to four floors, and according to some proponents, equipment rooms on those floors could be eliminated, creating usable space. Fewer ducts and a plug-and-play design simplify HVAC installation and lower first costs. “With multi-service beams [which can include lighting, security, etc.], the benefit for the contractor is that everything is factory-mounted, which cuts down on installation time for each of the contractors,” said Nodson. “The Empress State Building in London used multi-service chilled beams, and the contractors saved four weeks off the commissioning time.”
Chilled beams are used in laboratories, office buildings, hospitality, schools, and retrofits with limited duct space, but there are applications where chilled beams are not appropriate, such as hospital areas that do not allow recirculated room air. And ACBs may not be worth the investment in buildings smaller than 20,000 ft2 (1,900 m2) if installation requires the addition of a chiller, according to Nodson. (An alternative cooling source could make it competitive in these smaller commercial buildings.) In general, ACBs are well suited for rooms with ceiling heights of less than 14–16 feet (4.3–4.9 m) because of air circulation.
But controlling humidity is the primary factor in whether an ACB is appropriate for a building. ACBs require that chilled water be kept 2°F–8°F above the dew point of the primary air to prevent condensation (condensation sensors should be used on the incoming pipes to detect changes in humidity and adjust the ventilation and water temperature accordingly). In some buildings, controlling the humidity is difficult, and ACBs may not be the best HVAC option. Some examples include buildings with high humidity (latent cooling loads), such as gyms, pools, and kitchens, and buildings with operable windows where cooling/heating zones cannot be controlled or where the building envelope is not well sealed.