The Velocity Traverse

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2.10 VELOCITY AND CFM.  When we talk about airflow quantities, we are
talking about cubic feet per minute (cfm).  When we talk about velocity, we
are describing how fast a quantity of air is moving, in feet per minute
(fpm).  The general relationship between velocity and air quantity is:
=
AxV
Q
where:
= flow in cfm
Q
V
= velocity in fpm
= cross-section area, in ft 2
A
In almost all cases, airflow is not measured directly with a meter that
reads out in cfm.  Rather, we usually use a meter that measures velocity,
and we use the above formula to calculate the actual air quantity.
2.11 THE VELOCITY TRAVERSE.  The velocity of air flowing inside a duct is
rarely uniform across the entire cross section.  In order to arrive at an
accurate average velocity through the duct, it is necessary to measure the
velocity at a number of different points in the duct.  This is called taking
a velocity traverse.
For air flowing through a duct, the velocity distribution might look
something like Figure 2-18.  If the length of the arrows represent the
actual air velocity, you can see that the air velocity is highest in the
center of the duct, and drops off very quickly near the sides of the duct.
In order to arrive at an average, the technician mentally divides the duct
cross section into equal areas, and measures the velocity at the center of
each of the equal areas (see Figure 2-19).  In a straight duct, some service
technicians get rough estimates of the average velocity by measuring just
the maximum velocity in the center of the duct, and multiplying that value
by 0.85.
FIGURE 2-18
Velocity Distribution of Air
Flowing Inside a Duct
When selecting a location to take a duct traverse, a location preceded
by a long straight duct is ideal.  If measurements are made close to a fan
or an elbow, a very odd and nonuniform set of readings will result (see
Figure 2-20).  When measuring velocities from a supply air or return air
grille, the face area is mentally divided and individual velocity readings
are taken in the center of each area and averaged.
2-18