ASHRAE Journal - November 2014 - 51

COLUMN ENGINEER'S NOTEBOOK

* The relief damper modulates as
the inverse of the return damper.
For Return Fan systems using
Direct Pressure control, the
sequence is the same except the
relief damper is controlled independently off of building pressure.
It may be counterintuitive, but
economizer outdoor air dampers
on air handlers with return fans
have no impact on the outdoor airflow rate into the mixing plenum.
Instead the return fan and return
damper controls dictate outdoor
airflow. From ASHRAE Guideline
16:4 "The flow of outdoor air ... is
always the difference between the
supply airflow and the return airflow through path PL-1 to PL-2, and,
therefore, the [outdoor air] damper
size and type have no effect on the
amount of outdoor air." Fully opening the outdoor air damper rather
than tracking the other dampers
reduces pressure drop and fan
energy, and it ensures there is no
backflow through the relief damper
for systems using Airflow Tracking
control (see Reference 3 for more
details).

Damper Selection

FIGURE 1 Relief damper.

P-2
M

Return/Relief
Air Plenum
PL-1

Relief Air
Relief Damper
D-1
[A]
Atmosphere
(Outdoors)
P-3

Outdoor Air
Outdoor Air
Minimum OA
Damper D-4

M
Mixed Air
Plenum
Economizer OA PL-2
Damper D-3
M

Return Air

MIN T-2
Return Air
Damper D-2
M

T-1

[B]
Building

Supply
Fan
Supply Air

Filter Cooling
Coil

M2

Speed
Control
P-1

FIGURE 2 Relief fan.

P-2
Speed
M-2 Control

Return/Relief
Air Plenum
PL-1

Relief Air
Relief
Damper Relief
D-1 Fan

Return Air

[A]
Atmosphere
(Outdoors)
V-1

Outdoor Air
Outdoor Air

M
Mixed Air
Plenum
Economizer OA PL-2
Damper D-3
Airflow Sensor

Minimum OA M
Table 1 is a summary of recomDamper D-4
mendations for damper action
and sizing, adapted from ASHRAE
Guideline 16. Recommended
damper action is referred to as either parallel blade
damper (PBD) or opposed blade damper (OBD) (Figure 7).
One important point in selecting economizer
outdoor air and return air dampers is that the concept of "damper authority"5,6 does not apply. The
damper authority concept applies to dampers that
throttle airflow but economizer dampers do not;
they are mixing dampers and are not intended to
vary overall airflow. This fact, and other factors, are
considered in Guideline 16 and Table 1 damper sizing
recommendations.

MIN T-2
Return Air
Damper D-2
M

M

Filter Cooling
Coil

T-1

[B]
Building

Supply
Fan
Supply Air
Speed
Control
P-1

Relief Damper System
For VAV systems using Relief Dampers (Figure 1), the
most important damper from a sizing perspective is the
relief damper (D-1). The relief system relies on building pressurization to cause excess outdoor air to relieve
to atmosphere. The relief damper and entire relief
path (Path B to A in Figure 1) must be sized large enough
to prevent overpressurization of the building, i.e., no
more than about 0.05 to 0.10 in. w.c. (12 to 25 Pa), when
the VAV system is supplying maximum outdoor air.
The resulting face velocity will therefore be very low,
N OVEM BER 2014

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ASHRAE Journal - November 2014

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