Chilled Beam Design Guide - TROX

Airside Design Considerations
Space temperature control in passive beam systems is
accomplished by varying the amount of sensible heat
removed by the chilled water. The chilled water supply
to several beams within a single zone is generally
controlled by a single chilled water valve. Although the
zone may consist of multiple spaces, a certain degree
of temperature compensation for each space will be
affected by the passive beam itself. As the cooling
requirement of the space is reduced, the temperature of
the air entering the beam will also be reduced. This will
result in less heat transfer to the water circuit and a
lower return water temperature.
Passive chilled beams cannot be used for heating as its
airflow would be reversed. They are typically applied
with some type of separate heating system such as low
level finned tube heaters. Radiant (ceiling or wall
mounted) heating panels can also be used depending
on the façade heat losses expected.
Thermal comfort considerations with active beams
While the primary (conditioned) airflow rate for active
chilled beams can be greatly reduced, their induction
ratios (2 to 6 CFM of room air per CFM primary air)
result in discharge airflow rates that are slightly higher
than those of conventional all-air systems. As such,
attention should be exercised in the beam placement to
avoid drafty conditions and maximize occupant thermal
comfort. Figure 19 predicts maximum occupied zone
velocities for various combinations of primary airflow
rates and active beam spacing. This nomograph
suggests local velocities which will maintain acceptable
levels of occupant comfort per ASHRAE.
Active beams can be for heat in moderate climates. Hot
water can either be delivered to each perimeter area
beam or to a hot water heating coil in the duct supplying
a number of beams within the same thermal control
zone. The use of a zone hot water heating coil feeing
multiple chilled beams is a generally more economic
option than piping each chilled beam for heating as it
may save considerable labor and piping material costs.
If active chilled beams are used for heating, the following
recommendations should be observed:
• Chilled beam discharge temperatures should
be maintained within 15˚F of the room
temperature.
• Velocities at the mid-level of outside walls and
windows should be maintained within the
region indicated in figure 19.
Unoccupied periods demanding heating via the chilled
beams or primary air system will require that the AHU
remain operational.
Variable air volume operation using active beams
Although normally operated as constant air volume
delivery devices, active chilled beams can also be used
as variable air volume (VAV) devices. VAV operation
may be advantageous when space occupancy and/or
ventilation demands vary widely. Recommendations for
the control of chilled beams in VAV applications can be
found in the control section of this document.
As the room air distribution provided by active
beams is identical to that provided by ceiling slot
diffusers, their selection for (total) discharge airflow
rates greater than 40 CFM per linear foot of slot is
not recommended when high levels of occupant
thermal comfort are required!
The v L velocities shown in figure 19 are those predicted
within 2 inches of the window or wall surface during
cooling operation. It is recommended that beams which
are configured for both heating and cooling of perimeter
spaces be selected such that v L (selected for cooling
operation) is between 120 and 150 FPM in order to
assure that the warm air is adequately projected down
the perimeter surface. Velocities taken 6 inches away
from the surface can be expected to be about half those
values.
Heating in chilled beam applications
Ceiling or high sidewall mounted passive chilled beams
exert no motive force on their discharge airflow, and
cannot be used for overhead heating. Heating must be
provided by a separate source, either the primary air
supply or a separate heating system (finned tube,
radiant panel, etc.).
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