ASHRAE Journal - November 2014 - 52

typically 200 to 500 fpm (1 to 2.5
FIGURE 3A Return fan with airflow tracking control.
m/s).
The economizer outdoor air
Return
damper (D-3) should be selected for
Relief Damper
Airflow
Fan
D-1
Sensor
low pressure drop to minimize supReturn/Relief
Relief Air
Return Air
Air Plenum
ply fan energy. Damper face velociPL-1
M
ties from 400 to 1,500 fpm (2 to 8
Speed
Control
m/s) at design flow are typical; the
[A]
[B]
V-1
range is a function of space availAtmosphere
Building
MIN T-2
T-1
(Outdoors)
ability and cost. From a functionalReturn Air
Supply
V-2
M Damper D-2
ity standpoint, damper size does
Fan
M2
M
Mixed Air
Supply Air
Airflow
not matter. Often, because of space
Outdoor Air
Plenum
Sensor
limitations, this damper is the same
Economizer OA PL-2
Filter Cooling Airflow
Speed
Damper D-3
Coil Sensor Control
size as the outdoor intake louver and
thus has very low velocity to avoid
P-1
rain entrainment.
Similarly, the return air damper
(D-2) pressure drop should genFIGURE 3B Return fan with direct pressure control.
erally be low to minimize supply
fan energy. Damper face velocities
from 800 to 1,500 fpm (4 to 8 m/s)
Return
Fan
at design flow are typical. In cold
M
Return/Relief
Relief Air
climates where coil freezing is posReturn Air
Air Plenum
PL-1
Relief Damper
sible and no other mixing device
Speed
D-1
(e.g., air blender) is provided, the
Control
P-3
[A]
[B]
design velocity should be on the
Atmosphere
Building
MIN T-2
T-1
high end of this range to encour(Outdoors)
Return Air
Supply
V-1
age mixing (see ASHRAE Research
M Damper D-2
Fan
M2
M
Mixed Air
Project 10457).
Supply Air
Airflow
Outdoor Air
Plenum
Sensor
Where outdoor air and return air
Economizer OA PL-2
Filter Cooling
Speed
dampers are controlled so that one
Damper D-3
Coil
Control
opens when the other closes (Figure
P-1
4), both outdoor air and return air
dampers should be PBDs to minimize
pressure drop and reduce fan energy.
Figure 8 shows why: the pressure drop across the mixing
improvement in mixing efficiency vs. dampers that
plenum is almost constant for PBDs and rises significantly were rotated 90°, but their tests were limited, and earwhen dampers are 50% open for OBDs. The reason is
lier research8 found angled parallel blade dampers did
improve mixing.
apparent from Figure 7: at 50% open, PBDs have a much
larger free area than OBDs.
Relief Fan System
Where outdoor air and return air dampers are
Economizer outdoor air and return air damper selecsequenced (Figure 5) as recommended, either PBDs or
OBDs may be used. Many designers (including the author) tions for VAV systems using Relief Fans (Figure 2) are the
same as those with Relief Dampers described earlier.
still prefer parallel blade dampers with the outdoor air
The one difference is the relief damper (D-1). This
and return air damper blades oriented to direct airflow
damper is typically a two-position damper interlocked
into each other to encourage mixing. RP 1045 found this
to the relief fan. As such, it should be selected for low
provided only marginal and statistically insignificant
52

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