Figure 13-13.Measuring fan rpm.
Figure 13-14.Fan static pressure measurement.
test may not be necessary in the field; however, if
it is, the results can be compared with the
manufacturers fan curve and system specifica-
tions to determine fan performance.
You can quickly locate problems caused by
blockages in duct systems by performing static
pressure readings. The total air volume in cubic
feet per minute (cfm) for a fan can be determined
by the following procedures:
1. Downstream of the air handler, establish a
point along the duct that has the longest
straight run and drill test holes into the
duct. Holes should be far enough
downstream from any elbows or from the
fan discharge to minimize the effect of
turbulence. The holes must be closed and
sealed after the test is completed.
2. Take velocity pressure readings using a
pitot probe and manometer or velometer.
For rectangular ducts, velocity readings
are taken at the center of equally divided
areas. On round ducts, readings are taken
across each of two diameters on lines at
right angles to each other. (See fig. 13-15.)
3. Calculate the cubic feet of air per minute
by multiplying the average velocity
pressure in feet per minute found in the
above reading by the cross-sectional area
of the duct in square feet. Total airflow in
cfm = Average velocity in fpm x duct cross-
sectional area in square feet.
The results are compared with design condi-
tions to determine performance. Measured cfm
should be approximately equal to design cfm plus
10 percent to allow for leakage.
In the event that fan performance is not
consistent with design conditions, the necessary
adjustments or repairs should be made at this
point in the balancing procedure. For example,
Figure 13-15.Velocity pressure measurement.