ASHRAE Journal - October 2014 - 55

COLUMN ENGINEER'S NOTEBOOK

column), and because their pressure
FIGURE 1 Airflow tracking control (adapted from ASHRAE Guideline 16).
drop is small relative to the overall
return system pressure drop, they
will have only a small impact on the
Return
Airflow
Relief Damper
Fan
Sensor
fan speed vs. airflow relationship.
D-1
Return/Relief
Relief Air
Return Air
Air Plenum
Again, this is not precise but preciPL-1
M
sion is not needed.
Speed
Control
c) Airflow Measuring Station
[A]
[B]
V-1
(AFMS). Return airflow can be meaAtmosphere
Building
T-1
MIN T-2
(Outdoors)
sured at the air handler by a number
Return Air
Supply
V-2
M Damper D-2
of different devices:
Fan
M2
M
Mixed Air
Supply Air
* Sensor array, as shown scheAirflow
Outdoor Air
Plenum
Sensor
PL-2
matically in Figure 1. This may be a
Economizer OA
Filter Cooling Airflow
Damper D-3
Coil Sensor Speed
pitot array or thermal anemometer
Control
array. The latter is preferred if low
P-1
return airflow rates can be expected
because they have much lower minimum velocity limits. One major disadvantage of this
return fan airflow setpoint can be adjusted accordoption is the space required both upstream and downingly so that acceptable pressurization is maintained.
stream of the AFMS for accurate readings.
For example, if the air handler provides makeup air to
* Inlet bell sensors. Airflow for centrifugal fans
an intermittently operated kitchen, the return fan setcan be measured using pitot or thermal anemometer
point can be adjusted by the design kitchen exhaust fan
sensors mounted in the inlet bell. This provides an
airflow rate when the fan status (e.g., current switch)
accurate airflow reading but can generate noise and
indicates the fan is on. Airflow tracking would not work
adds to pressure drop. Several manufacturers offer
well if intermittent exhaust rates were significant and
differential pressure sensors mounted across the inlet not known by the control system since the building may
bell with differential pressure factory correlated to
be drawn negative.
airflow. This design is the best option for fan airflow
Direct Pressure Control
sensing, if available, because it has lower costs and
Figure 2 shows a VAV system with Direct Pressure
zero pressure drop. For multi-fan arrays, airflow senControl, which is the control logic recommended by
sors may be installed in a subset of fans (typically no
fewer than two for redundancy) to reduce costs while ASHRAE Guideline 167 and ASHRAE Research Project
RP-14558 for systems with return fans.
still maintaining reasonable accuracy. But control
The relief damper is controlled to maintain building
logic needs to be adjusted if the fan(s) with the airflow
pressure directly using a pressure sensor located in a
sensor fail(s).
representative space inside the building. The pressure
Supply airflow can be measured using options similar
setpoint is typically +0.05 in. w.c. (12 Pa). The range of
to a) or c) above. Option b) should not be used for supthe sensor may be as low as ±0.1 in. w.c. (±25 Pa) but
ply air fans because it is not sufficiently accurate; VAV
spikes due to wind and other factors may cause a higher
box dampers significantly vary system geometry so fan
range of pressures to occur so a ±0.25 in. w.c. (±60 Pa)
speed and airflow do not closely track. Option a) is generally preferred for supply airflow measurement since it range is recommended.
Locating the DP sensor port pickup inside the building
always is available for VAV systems with digital controls
can be a challenge. While one of the main purposes of
at no added cost (other than programming) and thus is
building pressure control is to prevent high pressures
the least expensive option and requires no added space
at entry doors, the pickup should never be located in
in the air handler.
If there are significant variable or intermittent exhaust entry lobbies. This is because the reading will fluctuate dramatically as entry and elevators doors open and
fans that are monitored by the control system, the
O CT O B E R 2 0 1 4

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ASHRAE Journal - October 2014

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