ASHRAE Journal - September 2013 - 79

Column HVAC AppliCAtions

by electronic devices that have a number of features,
including the ability to record and average readings.
These newer measuring devices, typically hot wire
or hot film anemometers, while much more accurate,
convenient, and portable, are highly dependent on measurement location, sensor tip geometry, and temperature sensitivity. More importantly, there is no accepted
“industry standard.” The air leaving a diffuser or grille is
typically a very thin and highly variable jet. The mechanical 2000 and 6000 Series vane anemometers, with the
appropriate sensing tips, essentially averaged the air
velocity in a repeatable manner, allowing for standardized presentation of area factors. The variety of hot wire
geometries in the newer devices, usually protected by a
shield to protect the sensitive film or wire, greatly affects
the measurement when in close proximity to a surface.
This means that while an area factor could be determined for one type of anemometer, it would not be valid
for another.
As mentioned earlier, measurements of discharge
velocities are, of course, greatly affected by inlet effects.
The most common installation involves a 90-degree
bend of the flexible duct that connects the diffuser to the
HVAC system. This was discussed at length in an ASHRAE
research project conducted at UNLV and was reported
in the Journal in April 2012. Averaging the measured
discharge velocity on all four sides of a square ceiling diffuser is a minimum requirement, but is probably not a
perfect measurement technique.
A better and more common measurement technique
is to use a flow hood to determine airflows into and out
of air outlets/inlets. Unfortunately, these are not absolute measurement instruments. With a high induction
device, which probably includes most ceilings and linear
slot diffusers, there can be significant errors between
reported and actual air quantities. Differences greater
than 20% are not uncommon. They are difficult or
impossible to use on duct-mounted grilles or installations with no ceilings.
The most accurate technique for determining airflow
quantity is a pitot traverse in a straight run of duct. This
is seldom practical, as straight runs of duct are not often
found when needed, and typically have low velocities,
making accurate measurements difficult. With VAV systems using pressure independent VAV boxes with multipoint averaging inlet sensors, proportional balancing
can be a very effective technique. Most VAV box sensors

are accurate to within 5% of the full-scale reading of
the values presented on most manufacturers’ products,
given a relatively straight inlet to the terminal. (Note:
It is highly unlikely that any VAV box sensor, from any
manufacturer, is accurate to 5% of reading at minimum
flow!) My advice, which I have presented at a number of
NEBB sessions in the past couple of years, is to compare
the VAV box sensor reading to the carefully aligned anemometer sensor readings from all the diffusers attached
to that VAV terminal and then determine an “effective”
Ak for that sensor and that type diffuser. (Note: One cannot assume that all the diffusers shown on the plans are
actually connected to that VAV terminal, or even connected to anything, without actually checking). I also
recommend adjusting the adjustable outlets as shown in
the plans, such as slot diffusers. As mentioned in a prior
article, adjusting a linear slot diffuser so that it no longer
blows down on occupants will invalidate any prior system balance.
In any case, the reported or determined area factor cannot be used to determine the actual free area of an air outlet/inlet. The best way to compare actual free area between
devices is to compare static pressure or sound levels, as
they should be related. Blade configurations, angle, and
the relationship to a specified probe location all affect the
measured Ak and may have little relationship to the actual
free area. This is especially true for return grilles.
So, when asked for these factors, manufacturers have
to ask for what purpose the Ak value is to be used. In most
cases, they find that the balancer is attempting to determine the air quantities and has a hot wire anemometer of
some sort. When he is using one of the old Alnor devices,
the manufacturer may have data for many products.
Newer electronic devices, however, seem to be the rule.
Most manufacturers recommend that the users determine
their own area factors by carefully determining the airflow
for a single device and sensor orientation and use this on
others on the project. If the user wants to use the values
for some diagnostic purpose, such as the actual discharge
air velocity, that value is not reflected in the traditional Ak
values. A better result can be found using software from
the manufacturer and calculated throw data.
Until the time when a single anemometer and anemometer geometry again become the defacto industry
standard, manufacturers will likely not be able to report
area factors that can be used with any of the newer
instrumentation types available.
Septem ber 2013

ashrae.org

ASHRAE JouRnAl

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ASHRAE Journal - September 2013

Table of Contents for the Digital Edition of ASHRAE Journal - September 2013

Contents
ASHRAE Journal - September 2013 - Cover1
ASHRAE Journal - September 2013 - Cover2
ASHRAE Journal - September 2013 - 1
ASHRAE Journal - September 2013 - 2
ASHRAE Journal - September 2013 - Contents
ASHRAE Journal - September 2013 - 4
ASHRAE Journal - September 2013 - 5
ASHRAE Journal - September 2013 - 6
ASHRAE Journal - September 2013 - 7
ASHRAE Journal - September 2013 - 8
ASHRAE Journal - September 2013 - 9
ASHRAE Journal - September 2013 - 10
ASHRAE Journal - September 2013 - 11
ASHRAE Journal - September 2013 - 12
ASHRAE Journal - September 2013 - 13
ASHRAE Journal - September 2013 - 14
ASHRAE Journal - September 2013 - 15
ASHRAE Journal - September 2013 - 16
ASHRAE Journal - September 2013 - 17
ASHRAE Journal - September 2013 - 18
ASHRAE Journal - September 2013 - 19
ASHRAE Journal - September 2013 - 20
ASHRAE Journal - September 2013 - 21
ASHRAE Journal - September 2013 - 22
ASHRAE Journal - September 2013 - 23
ASHRAE Journal - September 2013 - 24
ASHRAE Journal - September 2013 - 25
ASHRAE Journal - September 2013 - 26
ASHRAE Journal - September 2013 - 27
ASHRAE Journal - September 2013 - 28
ASHRAE Journal - September 2013 - 29
ASHRAE Journal - September 2013 - 30
ASHRAE Journal - September 2013 - 31
ASHRAE Journal - September 2013 - 32
ASHRAE Journal - September 2013 - 33
ASHRAE Journal - September 2013 - 34
ASHRAE Journal - September 2013 - 35
ASHRAE Journal - September 2013 - 36
ASHRAE Journal - September 2013 - 37
ASHRAE Journal - September 2013 - 38
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ASHRAE Journal - September 2013 - 40
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ASHRAE Journal - September 2013 - Cover4
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