ASHRAE Journal: Designing Chilled Beams for Thermal ... - TROX

or equal to 6 ft (1.8 m) can be sized such that their throw to a
terminal velocity of 150 fpm (0.75 m/s) does not exceed half
the distance to a beam with an opposing discharge.
Figure 3 can be used to determine the maximum collision
velocity that limits velocities within the occupied zone to 50
fpm (0.25 m/s) or less for any given distance H1 using the following
equation:
V C = 50 / (V H1 / V C ) (4)
where V H1 / V C is the value from Figure 3 that corresponds to
the distance H1.
For example, if H1 is equal to 4 ft,
V C = 50 / 0.43 = 116 fpm
Room Humidity Design Considerations
The sensible cooling contribution with chilled beams affords
the designer an opportunity to significantly reduce the
primary airflow rate compared to all air systems. As 50% to
75% of the sensible heat gains are typically removed by the
chilled water coil, proportional reductions in the primary
airflow rates within the system may be achievable. However,
this should be done with caution as the beam must also deliver
sufficient ventilation air and maintain acceptable space
humidity levels. The primary airflow rate to the room must
be the greater of that required to (1) ventilate the space in
conformance to ASHRAE Standard 62.1-2007 6 (or other
applicable ventilation codes); (2) offset space latent gains
to control the room humidity level within ASHRAE Standard
55-2004 recommendations; and (3) provide sufficient
sensible cooling to complement the sensible heat removed
by the chilled water coil.
In most common interior space applications, the primary
airflow rate required to offset space latent gains will exceed
both the ventilation airflow rate and the airflow rate required
to complement the coil’s sensible cooling. The space airflow
rate will be determined by the latent gains and the design
room humidity ratio (W ROOM ). In perimeter spaces, the
primary airflow rate to the beams will be driven by the sensible
load (laboratories may be exceptions due to their high
mandated ventilation rates). The use of beams whose water
side cooling capacity contributes to more than about 65%
of the total space load may be impractical due to architectural
constraints that limit the installed beam quantities and
lengths in these spaces. The goal of the design should be to
reduce the primary airflow rate to as close to the required
ventilation rate as possible.
Designing chilled beam systems to maintain room air humidity
levels lower than necessary can result in considerably
higher primary air requirements. Figure 4 is presented in
Standard 55-2004 7 and prescribes acceptable ranges of room
temperatures and humidity ratios. Assuming a clothing level
of 1.0 clo (0.15 m 2 · k/W), this diagram defines the thermal
comfort window for a dry-bulb temperature of 75°F (24°C)
to include room dew-point temperatures as high as 62°F
Distance H1 (ft)
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
0.3 0.4 0.5 0.6 0.7
V H1 /V C
Figure 3: Velocities entering the occupied zone.
Figure 4: ASHRAE Summer and Winter Comfort Zones. Acceptable
ranges of operative temperature and humidity with air speed ≤ 40
fpm (≤0.20 m/s) for people wearing 1.0 and 0.5 clo (0.15 m 2 · k/W
and 0.08 m 2 · k/W) clothing during primarily sedentary activity
(≤1.1 met [≤63.9 W/m 2 ]). 8
(16.7°C). Where chilled beams are applied, the room dewpoint
temperature must not exceed the chilled water supply
temperature, so design for dew points above about 57°F (14°C)
is not recommended.
Most conventional HVAC systems condition delivered air to
about a 52°F (11°C) dew-point temperature, which coincides
with a humidity ratio (W PRIMARY ) of 58 grains (3.8 g). The
airflow rate (Q PRIMARY ) to offset the space latent heat gains
(q LATENT ) can be determined by Equation 5.
Q PRIMARY = q LATENT / [0.68 × (W ROOM – W PRIMARY )] (5)
Using this equation and assuming the humidity ratio of the
primary air is 58 grains (3.8 g), the primary airflow requirement
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