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Are Chilled Beams Viable in Humid Climates?
As the effort to improve energy efficiency in buildings increases, many new technologies are under consideration by designers, contractors and owners. The use of chilled beams, a technology that has been successfully employed in European buildings for decades, is gaining popularity in the U.S.
But can this technology be used effectively in the hot, humid environments that much of the U.S. experiences every summer? That was a significant concern for the U.S. Army Corps of Engineers (USACE) and the National Geospatial-Intelligence Agency (NGA) in 2007 and 2008 when the new NGA Campus East (NCE) facility in Springfield, VA, was being designed.
The outcome of that project provides significant evidence that chilled beams are a viable option for today’s modern buildings.
The Facility and Campus
NGA began a phased move-in of some 9,000 personnel that was completed in September 2011. NCE is a 2.4 million-square-foot campus whose primary structure is a 2.1 million-square-foot Main Office Building (MOB) that consists of offices, conference facilities, dining, a fitness center and other ancillary spaces. The design team was charged with ensuring the entire campus would be certified LEED Silver. The majority of construction on NCE was completed in late 2010.
A large data center is also on the campus. Achieving significant energy improvements over the baseline in data centers is a notoriously difficult challenge. In order to meet the energy efficiency goals for the entire campus, the design firm’s approach included finding a way to make the MOB especially energy-efficient. After evaluating concepts, the firm concluded that the best solution was incorporating chilled beams.
Summers in Virginia are humid, so USACE was concerned that chilled beams would create a significant risk of condensation on the beams and associated chilled water piping above the ceiling. However, the designer was able to convince the government that chilled beams were safe to install, so design and construction moved forward.
At completion, the MOB had 4,166 chilled beams installed. During the four years that NGA has occupied the facility, the beams have been remarkably reliable and incident-free. The only regular maintenance required is an annual vacuuming of the coils in the beams, which is a one-minute operation per beam. There have been no incidences of condensation on the chilled beams since NGA moved into the facility.
Chilled Beam Basics
Chilled beams can be used for both heating and cooling, but their primary application in office environments is cooling. The beams consist of a water-cooled coil similar to a finned-tube radiator.
There are two general types of beams, passive and active. Passive chilled beams do not utilize forced air, instead relying solely on convection for circulation of air across the beam. Air cooled by the beam falls toward the occupied zone, and warm room air that rises toward the ceiling is induced by the falling cool air into the beam where the warm air is then cooled in turn. Passive chilled beams also have a radiant cooling component, similar in nature to standing next to a wood-burning stove, but radiating cold instead of heat.
While passive beams and similar structures like chilled “sails” have their place in the built environment, the real bang for the energy buck comes with active chilled beams. At NCE, roughly 3,900 of the 4,000-plus beams are active.
A cross-section of an 8-foot by 2-foot active chilled beam is shown on page 35. The bottom of the unit sits flush with the adjacent ceiling so all that is visible is a perforated panel that covers the coil and the two slots on either side that discharge cool air to the space. The cooling capacity of this beam is a little over 6,000 BTUs per hour (half a ton), about one-third of which is provided by the primary air at 52 degrees F., with the remaining two-thirds provided by chilled water.
A relatively small amount of air is provided from an air handling unit (AHU) – ideally, an amount equal to the ventilation requirement for the space to minimize airflow, fan size, duct size, etc. This primary air is forced through a row of nozzles along each side of the beam. The air shooting through these nozzles uses the Venturi effect to pull warm room air up through the coil, cooling the room air, mixing it with the primary air, and providing a tempered stream of air into the space.
To provide an equal amount of cooling to the space, a typical VAV system would need almost three times as much air from the AHU, or about 250 cfm of air at 52 degrees F. The chilled beam uses 90 cfm at 52 degrees F., which is then blended with ambient air to provide about 500 cfm of 65 degrees F. air to the space. The 8-foot-long beam provides a lower velocity stream of air that is not nearly as cold, thereby eliminating cold drafts, one of the more common VAV system complaints.
The real benefit of the chilled beam is reducing the size of the AHU, ductwork, fans, and more by about two-thirds. The energy savings realized from pumping cool water throughout the building rather than blowing cool air is significant.
What about the danger of condensation on the cooling coil in the chilled beam? Just as a can of cold soda will cause condensation to occur, people fear that the same thing will happen with a cold metal object hanging over their desk. However, proper system design avoids this risk.
Typically, HVAC systems are designed to maintain interior space conditions at 75 degrees F. and 50% relative humidity (RH). That equates to a dewpoint temperature of about 55 degrees F. That cold soda can has a surface temperature of 34-40 degrees F., which is far below the dewpoint temperature of the surrounding air, hence the immediate condensation.
In contrast, the entering water temperature for the chilled beam is at 58 degrees F. – above the dewpoint. This keeps the beam from ever producing condensation. A dewpoint sensor in the space can be incorporated into the controls scheme to reset the water temperature higher if needed or shut the pump down entirely if control of the space dewpoint is completely lost. Dehumidification for the space is provided at the AHU, which has a lower water temperature (typically 44 degrees F.). After periods of HVAC shutdown, such as weekends, the AHU may have to run for a period of time at start-up to properly dehumidify the space before the cool water is introduced into the chilled beams.
One note of caution: If your design incorporates operable windows, the use of chilled beams or any other radiant cooling device should be considered carefully. It may be necessary to incorporate some kind of sensor and interlock that shuts down the system when the window is open and the dewpoint exceeds the chilled water temperature. Of course, it’s generally not a good idea to run the air conditioning system and leave the windows open at the same time.
In a properly designed and installed chilled beam system, condensation should never be an issue. The long-term savings in energy is substantial. The savings in ductwork, fan, and AHU size typically come close to offsetting the added cost in pumps, piping, and chilled beams. In addition, reducing duct size can reduce slab-to-slab heights, which can provide a significant savings in building envelope costs, particularly in taller buildings.
Chilled beams have been used successfully for decades in Europe, where they are now one of the most common HVAC system choices. While their introduction in the U.S. is more recent, there have been enough successful installations to allay any concerns owners or engineers might have about condensation.
Daniel Kailey, P.E., LEED AP, CEM, and QCxP can be reached at Daniel.P.Kailey@nga.mil.