Intelligence agency investigates
innovative HVAC in Virginia facility
As the effort to improve energy efficiency in buildings increases, many new technologies are under consideration by designers, contrac- tors 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.
BY DANIEL KAILEY, PE
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.
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