ACTIVE SERVICE A
Widely used in the UK and Europe, chilled beams have only recently taken off in the US. The lessons and experience from America can usefully be applied elsewhere, including optimising the primary airfl ow and maximising the use of water coils in the beam. ASHRAE members Andrew Livchak and Chris Lowell report
ctive chilled beams have been used for more than 20 years but are only now gaining popularity in the US where, as both cooling
and heating solutions, they are often called ‘active beams’ or simply ‘beams’. This article refl ects American experience
where beams are being designed with higher airfl ows to match increasing space loads. The article is derived, and converted into SI units, from one originally published in the ASHRAE Journal in April 2012. Beam designs with primary airfl ows
This article was published in ASHRAE Journal, April 2012. Copyright 2012 ASHRAE. Reprinted here by permission from ASHRAE. This article may not be copied nor distributed in either paper or digital form by other parties without ASHRAE’s permission. For more information about the society visit
www.ashrae.org
46 CIBSE Journal November 2012
signifi cantly exceeding space latent load and minimum ventilation requirements, are also driven by engineers attempting to reduce system fi rst costs and total number of beams. Unfortunately, this approach compromises the system’s energy performance and diminishes advantages of active beam systems over all-air systems. This often leads to active beams being
used as expensive diffusers. Primary air in active beams (Figure 1) is supplied into a mixing chamber through rows of nozzles. Negative pressure that is created in the mixing chamber induces room air through the cooling coil and the cooled air mixes with the primary air. This mixture of recirculated cooled air and primary air is supplied to the space. In an optimum design, primary airfl ow is intended to satisfy space outside air requirements and dehumidifi cation to avoid any condensation on beam’s surfaces. The cooling coil is used to meet only the space sensible load. Primary air is cooled and dehumidifi ed before it enters a beam.
Designing chilled beam systems When fi rst introduced in northern Europe, the design objective for active beam systems was to separate ventilation load from space sensible load and handle space cooling and dehumidifi cation with minimum airfl ow. Water is a more effective medium than air to transport energy, due to its higher density and specifi c heat. One unit volume of water can carry about 3,500 times more energy than air. Already high space loads in the US are often
further overestimated by design programs not accounting for transient heat transfer, as well as the tendency of engineers to put
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