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19
Canopy Hood
®
255
(4663.44 m/h)
248
(4535.42 m/h)
256
(4681.73 m/h)
240
(4389.12 m/h)
250
(4572 m/h)
Measure the velocity of each location.
A digital 2.75 in. (70 mm) rotating
vane anemometer or equivalent
is suggested. The center of the
anemometer should be held 2 in.
(50 mm) from the face of the filters as
shown in Fig. 30. It is helpful to make
a bracket to keep the anemometer
at the 2 in. (50 mm) distance and
parallel to the filter. Both squareness
and distance are very important for
accuracy.
Calculate the average velocity for the filter.
3. Determine the filter’s conversion factor from the table.
4. Calculate the filter’s volume in CFM (m
3
/hr) by multiplying the average velocity by the
conversion factor.
5. Calculate the hood’s volume by repeating the process for the remaining filters and summing the
individual filter volumes.
x
x
x
x
x
2 in.
H/4
H/2
H/4
H
Airflow
Rotating Vane Anemometer
Fig. 30
Nominal Filter Size (H x L)
Imperial
Conversion Factor
Metric
Conversion Factor
Inches Millimeters
16 x 16 400 x 400 1.63 .157
16 x 20 500 x 400 2.13 .198
20 x 16 400 x 500 1.90 .177
20 x 20 500 x 500 2.48 .230
Total hood volume = (Filter 1 Volume) + (Filter 2 Volume) + (Filter 3 Volume)
= 474.6 + 455.4 + 470.1 =
1400.1 cfm
= 809 + 880 + 799 =
2488 m
3
/hr
For a nominal filter size of 20 x 16, the conversion factor is 1.90 Imperial (.177 Metric)
Volume for one filter = Conversion Factor x Average Velocity
= 1.90 x 249.8 fpm =
474.6 cfm
= .177 x 4568 m/hr =
809 m
3
/hr
Example: Exhaust Only Hood with three 20 x 16 Filters
Measured velocities in fpm for one 20 x 16 Filter
Average Velocity =
Sum of Velocity Readings
Number of Readings
(Imperial)
=
255 + 250 + 256 + 248 + 240
5
=
249.8 fpm
(Metric)
=
4663 + 4572 + 4681 + 4535 + 4389
5
=
4568 m/hr