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 2.20 DIFFUSER CFM. It is frequently necessary to measure the airflow from
room-type diffusers such as that shown in Figure 2-34. While the velocity
can be measured, it is difficult to know the discharge area of the diffuser
in order to calculate cfm. For new construction, the air balancer will have
the specifications for each diffuser. This specification will include the
effective area (Ao) of that diffuser. Multiplying the velocity by the
effective area will give you the actual cfm. For the technician taking
measurements on an existing job, the diffuser specifications are sometimes
In this case, data from similar diffusers may be used.
unavailable.
FIGURE 2-34
Ceiling Diffuser
Another option is using the type of hood shown in Figure 2-35. The top of
the hood is placed against the ceiling so that all air from the diffuser
enters the funnel-shaped hood. The hood then tapers down to a l- by l-foot
area at the bottom. Velocity measurements taken at the bottom may then be
multiplied by one square foot in order to get cfm. In addition to the
obvious advantages of this hood, there is the added advantage of being able
to take all measurements in rooms with normal ceiling height from the floor,
without the need for a ladder.
2-32
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