ASHRAE Journal - November 2014 - 58

COLUMN HVAC APPLICATIONS
Dan Int-Hout

Chilled Beams Selection
BY DAN INT-HOUT, FELLOW ASHRAE; AND LILLI WILBAR

Chilled beams have been prevalent in Europe for many years, and have become one
of the latest HVAC products to be adopted by engineers in the U.S. In fact, millions of
square feet (square meters) are heated and cooled with these devices every day. Simply
put, they are air-to-air induction diffusers with a coil. Much like any other piece of
building equipment, there is a need to balance both first cost and effective performance. However, the complexity of the product is such that if improperly selected, may
lead to unsatisfactory room temperatures, drafty conditions, or wasted energy.
History of Chilled Beams

Chilled Beam Selection Considerations

Chilled beam technology is not new by any means; in
fact, it was originally conceived and patented by Willis
Carrier in 1939, with the first installation occurring
sometime after World War II. Back then, they were typically located under a window and used relatively high
pressure (3 in. w.c. [750 Pa]) to induce room air near
the floor, which was passed through a coil, then out the
top of the unit. By keeping the water in the coils above
dew point, they were able to prevent condensation,
which eliminated the need for a drain pan. By the mid
1990s, European engineers had fabricated a linear slot
diffuser with induction nozzles and a coil. Their design
reduced the required inlet pressure to 0.75 in. w.c. (190
Pa), which was mainly due to a different design for the
induction nozzles and having reduced the fins per inch
on the coil to open it up. Similar to today's designs, these
products were installed in, or hung from, the ceiling
where they projected air along that plane.

Aside from operational savings, there is also a desire to
minimize first costs. We often see engineers accomplish
this by both minimizing the active length and maximizing
capacity of the units. While well intentioned, this action
is likely to result in a non-uniform load capacity, creating pockets of hot and cold air as well as drafts. However,
design errors like this can be avoided once there is a better
understanding of the issues regarding proper selection.

Why Chilled Beams?
Given their success in Europe along with their potential energy savings, it's no surprise that building owners
are often the driving force behind the decision to use
chilled beams. A few of the features and benefits include
reduced building fan airflow rates (where only outside/
ventilation air is provided by a dedicated outdoor air
unit or DOAS), small ducts, low unit heights, and the use
of water, which when slightly above the building's dew
point, can allow for water-side economizer mode. By the
nature of the system, they can also eliminate the need for
a building's central air handler, freeing up interior space.
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Principle of Induction
To begin, let's discuss the principle of induction. For
chilled beams, there are typically two induction sites.
One is inside the unit, where primary air is introduced
through nozzles, which create high velocity jets that in
turn generate low pressure and induce room air at the
bottom of the unit through a coil. The second location is
just outside the unit where the jet of leaving air (through
the slot) induces room air at the ceiling. By using the
potential energy stored in the form of pressure in the primary airstream, room air is drawn through the coil and
delivered to the space with low building fan energy use.

Induction Rate
At a constant supply pressure, the induction rate is a
function the airflow rate and external pressure. If the airflow varies, the induction rate will vary by the square or
cube of the airflow rate, depending on the geometry of the
device. Most chilled beams are used as constant volume
Dan Int-Hout is a chief engineer and Lilli Wilbar is communications manager at Krueger
in Richardson, Texas. Int-Hout is a member of SSPC 55, SPC 129 and consultant to
SSPC 62.1.

ASHRAE Journal - November 2014

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