ASHRAE Journal - May 2013 - 19

to a design/build approach, and Firm
A’s design became the bridging documents.
One of the design/build teams bidding on the project proposed a VAVR
system (designed by Firm B, the authors’ ﬁrm) and also carried the design through about the DD level in its
bid. Another bidder proposed a hybrid
chilled beam + VAV reheat system, designed by Firm C. The three designs
included equipment schedules, detailed
equipment layouts and zoning plans.
The team with the hybrid design was
awarded the job, but the project has
been placed on hold since.
After UC Davis selected a design/build
team, the ASHRAE Golden Gate chapter
decided to use this building for a competition between chilled beams and VAV reheat
since it had already been designed with
both systems and with a hybrid combination of the two. All three ﬁrms were eager to
participate. The results of the competition
were presented at a seminar sponsored by
the ASHRAE Golden Gate Chapter at the
Paciﬁc Gas & Electric Energy Center in
San Francisco on Oct. 17, 2012.

Figure 1: Rendering of graduate studies building.

HW
Coil

VFD
CHW
Supply
Coil with
Fan
Bypass
Array

VFD
Exhaust

CH
Beam

CH
Beam

T

Active Chilled Beam Design
The ACB+DOAS design consists of
a 100% outside air, constant volume air
handler serving two supply risers. The
air handler has a chilled water coil with a Figure 2: Active chilled beam design.
bypass damper and a hot water coil. The
air handler has a variable speed drive primarily for reducing cludes one set of chilled water pipes supplying 45°F (7°C)
fan speed when the bypass damper is open. The air handler chilled water to the air handler and a separate chilled water
provides the primary air to the active chilled beams. The aver- heat exchanger and chilled water pump in the building to
age DOAS primary airﬂow rate is about 0.6 cfm/ft2 (0.28 L/ maintain the chilled water supply temperature to the chilled
[s·m2]), which is considerably higher than the minimum ven- beams at 57°F (14°C). The design engineer felt that a heat
tilation in low occupant density spaces, like ofﬁces, but close exchanger was needed, rather than just a blending valve, as an
to the minimum ventilation in higher density spaces such as added layer of protection against condensation.
conference rooms and classrooms. A DOAS ﬂow rate higher
Each ACB has both heating and cooling coils. The primary
than the minimum ventilation in low density spaces is often air is maintained at 63°F (17°C), and the ACB heating and
needed to meet the space loads as the capacity of the chilled cooling coils are controlled by the thermostat to maintain
beams is a function of the primary airﬂow rates. In densely space conditions.
occupied spaces, it also ensures space dew point does not rise
The ACB design also includes a partially ducted exhaust
above the surface temperature of the chilled beams, possibly system—exhaust ducts extend from the air handler down the
causing condensation. It also improves the indoor air quality shafts and into the ceiling return plenum about two thirds of the
compared to code minimum ventilation.
way from the exhaust shafts to the building skin. This was in
Hot water and chilled water are provided by an existing response to perceived owner preference for ducted return. Howcampus central plant supplying 45°F (7°C) chilled water to ever, neither of the other two designs in the design/build compethe buildings on campus. However, 45°F (7°C) typically can- tition included ducted return and, therefore, the return ductwork
not be supplied to the chilled beams because of the likelihood was not included for any design in the cost model or energy
of condensation and dripping. Therefore, the ACB design in- model for the Golden Gate ASHRAE Chapter competition.
May 2013

ASHRAE Journal

19
ASHRAE Journal - May 2013

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