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SKILLS WORKSHOP

HVAC&R SKILLS WORKSHOP MODULE 37 PULLOUT

The essentials of
chilled beams – Part 1
This month’s skills workshop explains the functionality of an active chilled beam.

What is an active chilled beam? The induction process

An active chilled beam (ACB) is an air terminal device used to provide
localised cooling by inducing room air over a chilled water heat exchanger
located in the space to be conditioned.
An ACB relies on the use of primary air as the motive force for inducing air
over the secondary heat exchanger.
Primary Air
This primary air is conditioned at a conventional air handler and distributed Stream
to the ACB’s through conventional air ductwork.
ACB’s can be configured to suit a variety of applications or designs.
PULLOUT

For the purpose of visualising the following explanations, a typical

two-way discharge ceiling mounted unit will be used.
Supply Air Return Air Inlet Supply Air

Primary Air Plenum

Figure 2: Primary air
Primary
Air Nozzles
Primary air is delivered into the primary air plenum and out through the
nozzles into the mixing chamber on its way to the conditioned space.
The motive force in an ACB is the induction process created by the induction
Air Mixing Chamber
nozzle and primary air stream delivered through the induction nozzle.
Secondary
Heat
Exchanger The primary air can be any temperature but it is beneficial to use lower
Supply Air Outlet
temperature primary air to ensure adequate latent capacity is delivered
in the primary air and to permit the primary air to deliver a proportion
of the required sensible cooling to the space.

Secondary Air Inlet

Figure 1: Typical construction

The ACB is configured of an insulated well-sealed primary air plenum

Low Pressure
containing a primary air connection spigot and an array of primary air Kernel
nozzles through which the primary air is delivered.
The lower part of the unit is comprised of a sheet metal air mixing
chamber which houses the secondary cooling coil and supply air outlet
chambers. The supply air chambers are formed by smooth extrusion
shapes on either side of the unit and are open to the conditioned space.
Supply Air Return Air Inlet Supply Air
The secondary air cooling coil is centrally located in the mixing chamber of
the unit and is accessible through a perforated metal return air grille panel. Figure 3: Low pressure kernel
The coil and mixing chamber are separated from the primary air plenum
by the nozzle plate to ensure the mixing chamber is exposed only to room In the primary air stream, at the outlet of the nozzle, is a low pressure
air from the space to be conditioned. kernel which is created as the pressure at the nozzle is degraded
as air flows through the nozzle.
The unit is fitted with mounting brackets to facilitate installation and to
permit the unit height and fore and aft attitudes to be adjusted during All pressure degradation occurs at the primary air nozzle,
installation. resulting in a low pressure area immediately at the nozzle outlet.
ACB’s can include return air lint screens although this is not an essential This low pressure area is only slightly lower in pressure than
component. the surrounding room air.

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What really happens?

30 L/s Primary
Air
Low Pressure
Kernel

Supply Air Return Supply Air

Air Inlet
Secondary Heat
Exchanger
Air from Conditioned Space

Supply Air to Space Supply Air to Space

87 L/s
Secondary
Figure 4: Induced secondary air

The low pressure kernel, or area, attracts the surrounding air which is at a Figure 5: Operating details
slightly higher pressure than that of the low pressure kernel.
Air from the room (conditioned space) known as secondary air is drawn or Typically, an active chilled beam can induce secondary air of up to 3 times
induced into the low pressure kernel. the primary air quantity. Secondary air is routed through a chilled water
A secondary heat exchanger coil is imposed in the path of the room heat exchanger, producing cooling to be delivered to the space for no fan
secondary air, forcing the induced air to pass through the heat exchanger energy requirement.
on its way to join the low pressure kernel. As the primary air and secondary air are mixed in the ACB prior to delivery
The efficiency of the induction process is dependent on the number, size to the space, the total supply air quantity delivered by the ACB is the sum
and efficiency of the induction nozzles plus the density and geometry of of the primary and secondary air quantities. Primary air sensible cooling
the secondary cooling coil. capacity, usually around 25-35% of the ACB sensible capacity, is a function
of the primary air quantity and temperature difference between the
This secondary air from the local room space to be conditioned is induced, primary air and room air temperatures. The secondary air cooling is ideally
or moved, across the heat exchanger (cooling coil) for no fan energy a strictly sensible cooling process and usually represents 65-75% of the ACB
requirement whatsoever. This is a significant contributor to the energy sensible cooling capacity, not because of its higher air quantity but the
reduction potential of active chilled beams.

ACB
Primary
Primary Air 12°C Air AHU
Air On
Humidity, g/kg

Room Load Outside Air Load

AHU Air

Dry bulb, °C

Figure 6: Primary air

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HVAC&R SKILLS WORKSHOP MODULE 37 PULLOUT

Room Condition = 23°C, 50%rh

Secondary Water Secondary Water Temp = 13.5°C
Temperature 13.5°C
Secondary Water Flow = 0.06 L/s

Primary Air Quantity = 30 L/s

Air Mix Condition 14.8°C
Primary Air + Secondary Air
Secondary Coil Cooling = Sensible Cooling
Determined by Secondary Air Quantity &
Room Dew Point Water Flow & Temperature
Temperature 23.0°C /

Humidity, g/kg
12.1°C 50% rh
4
4
5
5 3
3 ∆w

AHU Air Off 1

1 22 Absolute Humidity
Approx 11°C Differential
Room Cooling (Mixed Supply Air to Room Air)
Primary Air Determined by Total Supply Air Quantity
Condition 12°C
PULLOUT

Primary Air Cooling = 417W Secondary Coil Cooling = 773W Room Sensible Cooling = 1190W

Dry bulb, °C

Figure 7: Process lines

result of having passed this relatively small secondary air quantity over a quantity. In the example above, the mixed air condition is 14.8ºC.
localised cooling coil operating with 13-14°C chilled water.
Mixed supply air (point 5) is delivered to the conditioned space (point 3)
Primary air is processed at the AHU to be delivered to the ACB at the representing the total cooling capacity delivered to the conditioned space
design conditions specific to the project. In these examples, 12ºC and by the ACB. The mixed supply air condition (point 5) will also determine
89%rh primary air will be used. the latent cooling delivered by the ACB based on the absolute humidity
differential (Δw) between points 3 and 5 on the psychrometric chart.
For determining primary air sensible cooling capacity, only the difference
Generally, since the secondary cooling coil process is a sensible cooling
between the primary air and room air temperatures are used. Any loads
process, the absolute humidity condition at point 5 will be determined by
higher than room loads are to be included in the AHU loads since the
the absolute humidity of the primary air at point 2.
ACB is a room terminal device and should only be related to room cooling
loads. (See figure 6 on previous page). Using too warm or too moist a primary air condition will result in
insufficient latent cooling delivered resulting in difficulty maintaining the
design room condition at point 3.
Process Lines
In the above example, primary air cooling (417W) represents 35% of the
The following describes the psychrometric process lines associated to an sensible cooling delivered by the ACB, meaning the ACB is capable of
active chilled beam or induction unit cooling process at the operating being turned down to 35% part-load cooling by simply isolating the
parameters listed. (See figure 7 above). secondary water flow.
In the psychrometric process lines above, primary air condition of 12ºC The amount of sensible cooling delivered by the secondary heat
and 89%rh (point 2) is achieved from an assumed AHU air off condition exchanger can also be varied by modulating secondary water flow
of 11ºC (point 1). The primary air is delivered through the ACB to the if required.  n
conditioned space. It is important that the primary air temperature be
expressed as the terminal temperature, not AHU air leaving temperature.
Room air of 23ºC and 50%rh (point 3) is induced through the secondary
cooling coil and sensibly cooled to 15.8ºC (point 4) by the 13.5ºC secondary
water circulating in the cooling coil. The capacity of the secondary coil will
be a function of the secondary air quantity, secondary water temperature
and secondary water flow rate.
The primary air and secondary air streams are mixed in the ACB (point 5) at
the ratio determined by the amount of secondary air to primary air known
This month’s Skills Workshop is reproduced
as the entrainment ratio. Entrainment ratio is determined by fixed unit
with the kind permission of its authors, Dadanco.
selection parameters and cannot be site adjusted for a given primary air

Next month’s workshop — Chilled beams – Part 2 17