ASHRAE Journal - April 2022 - 36

TECHNICAL FEATURE
FIGURE 1 Traverse locations.
Airflow
FIGURE 2 Louver and duct velocities.
350 to 400 fpm
700 to 800 fpm
600 to 1,500 fpm
500 fpm
8 Diameters Downstream
Airflow Monitor
Outside Air Louver
2 Diameters Upstream
Traverse Location
Airflow
Often, the mechanical engineer or designer does not
have enough space given to them by the architect or
architectural designer to fit all the ducts with adequate
straight duct diameters for optimal air measurement.
The design engineer and TAB professional should be
consulted to help locate the best places available.
If the outside air is delivered from a ducted louver,
the design is usually set up as shown in Figure
2.9 Michael Heilman, P.E., a mecchanical engineer,
states, " The typical louver is designed to keep entrance
velocity low to avoid moisture entrainment, at
approximately 350-400 feet per minute (fpm) [1.8-2
m/s] based on nominal louver sizing. Through the
louver the actual velocity is higher, typically roughly
700-800 fpm [3.6-4.1 m/s]. If ductwork out of [the]
louver remains the same size, this is an ideal location
for [the] filter giving approximately 350-400 fpm
[1.8 -2 m/s] face velocity. Where duct is continued
from [the] louver (and filter) to [the] unit, [the] standard
low-pressure duct friction rate design of 0.10 in.
w.g./100 ft (25 Pa at 30.5 m) of duct gives approximately
600-1,500 fpm [3- 7.6 m/s] (between 200 and 5,000
cfm [94.4 and 2360 L/s]). This is the generally acceptable
velocity for most airflow measuring stations.
The engineer will want to also ensure enough straight
duct lengths, sufficient to build consistent flow for
stable airflow readings. As the ductwork exits the airflow
monitor station (into the unit or duct casing) sizing
is optimized for cooling (or heating) coil pressure
drop efficiency and minimizing moisture carry-over at
approximately 500 fpm [2.5 m/s]. " 9
36
ASHRAE JOURNAL ashrae.o rg
A P R I L 2022
Commercial airflow monitors in the technology marketplace
offer a variety of location options in their installation
instructions. They can vary from 3 in. (76 mm)
to 6 duct diameters of straight ducts. The availability of
these required minimum tolerance distances depends
on how much mechanical room space the design engineer
is given to work with. The airflow monitor provider
needs to coordinate with the design engineer as
to what model airflow monitoring device is best suited
to install depending on the parameters available in the
installation.
Project at a Florida K - 12 Public School
One example project that was commissioned involved
a K-12 public school located in Florida with a hot and
humid ambient environment. It looked at outside airflow
meters that were reportedly calibrated within 10%
of design airflow when installed by the TAB professional.
These airflow meters use thermal technology and a hotfilm
anemometer to generate the airflow velocity signal
reporting to the building automation system (BAS).
It is best practice to require the TAB professional to be
involved in calibrating and documenting airflow monitors
before the time of building turnover. The design
specification should require this documentation to be
included in every TAB report.
Commissioning on this project was postponed until
after one year of service, so it uncovered a deficiency
that revealed itself over that year. The original design
and installation called for outside airflow monitors to
be provided in unfiltered outside airstreams. The building
automation system monitored the airflows using
airflow monitors. Table 1 shows the airflow readings and
outside air/return air damper positions that were read
by the BAS during the early phase of the commissioning
process. The readings and damper positions can be
Fan
Air-Handling Unit
Supply Air
Return Air
VFD
Cooling
Coil
Heating
Coil
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ASHRAE Journal - April 2022

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