ASHRAE Journal - November 2014 - 62

COLUMN HVAC APPLICATIONS

(1.2 m) gap between the ends, it would provide 240 ft2 (22
m2) coverage or 8 Btu/h·ft2 (25 W/m2) and is probably the
right design for a typical open plan office (Figure 2, Page 59).
For instances where the load might be higher, it is
likely that drafts will be created if the units are placed
closer together, so to combat this, the end spacing
should be reduced. Giving a 2 ft (0.6 m) gap between the
ends will allow coverage for 180 ft2 (17 m2) or 11 Btu/h·ft2
(35 W/m2). (Figure 3)
Some would say an effective strategy to avoid collisions
would be to offset opposing units. This can work, but
only if the end spacing is equal to the active length of
the unit. Done correctly, the units could be placed closer
together, which would then increase the per square foot
(square meter) load capability, delivering 16 Btu/h·ft2
(50 W/m2) (in our example). There has been some push
back from architects with regard to this type of configuration, but often times, locating the lights in the gaps
seems to be an acceptable solution (Figure 4).
Some manufacturers offer a "high capacity" chilled
beam, which is around 1,000 Btu/h·ft (961 W/m). For
units this large, the airflow might be near 100 cfm/ft (155

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L/s·m) (at 10°F ΔT) and provide throws upwards of 28 ft
(8.5 m). To avoid jet collisions, the units would need to
be spaced about 50 ft apart. (Figure 5)

Turndown
Reducing the supply airflow rate in response to lower
loads tends to complicate the situation. As the primary
flow is reduced, the induction nozzle velocity drops,
which in turn lowers the induction effect and pressure
across the coil. While one might think that the supply airflow and throw may lower in proportion to each
other, this is not always the case. To make matters worse,
if the flow rate to one chilled beam is varied, it will affect
the flow to all the others in the system, so for those particular cases, a pressure independent supply damper
would be required to effectively employ this strategy. If
you recall, the ASHRAE Journal article published in April
2012 titled "Don't Turn Active Beams Into Expensive
Diffusers," explains this issue very well.
In the recently completed ASHRAE Research Project
1515, it was found that interior loads at the Yahoo facility
in California were as low as 6 Btu/h·ft2 (19 W/m2). This is
a far lower number than many buildings are designed
around today, which on average is 23 Btu/h·ft2 (73 W/
m2) or more. So, if a typical design load is used, the
turndown of the chilled beam is an important consideration. As mentioned earlier, at constant primary flow,
the unit's response to water flow rate will determine the
reduction in discharge temperature.
Finally, there are limitations that are inherent to the
design of chilled beams that may make the ability to
handle high perimeter loads problematic, whether
it is heating or cooling. Reference the article from
the ASHRAE Journal published in August 2014 titled
"Variable Volume DOAS Fan-Powered Terminal Unit,"
which discusses the use of a "chilled box" that employs
many similar system features, but differs in that it uses
an integral ECM fan to help overcome the performance
that results from using induction from a central fan.
While not a new technology, chilled beams are not
without complexities or challenges. To ensure that
designs using these devices are comfortable, energy
efficient, and able to realize expected loads (both imagined and actual), it is important that engineers not only
become more familiar with the product, but also take
into account the many selection considerations, including first cost, that can make or break its success.

ASHRAE Journal - November 2014

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