Honeywell AUTOMATIC CONTROL Engineering Manual page 476

DAMPER SELECTION AND SIZING
Step
1
Calculate the approach velocity:
Approach velocity (fpm) =
2
Using the approach velocity from Step 1, calculate a correction factor:
Correction factor =
3
Calculate the pressure drop at 1000 fpm:
Pressure drop at 1000 fpm = Pressure drop at approach velocity x correction factor (Step 2)
4
Calculate free area ratio
For pressure drops (Step 3) ≥ 0.23:
Ratio = [1 + (21.3265 x pressure drop)] –0.3903
For pressure drops (Step 3) < 0.23:
Ratio = [1 + (79.7448 x pressure drop)] –0.2340
5
Calculate damper area (in
For parallel blade dampers:
Damper area (in
For opposed blade dampers:
Damper area (in
a
he free area of a damper is the open portion of the damper through which air flows. The free area ratio is the open area
T
in a damper divided by the total duct area.
Step
1
Approach velocity (fpm) =
2
3 Pressure drop at 1000 fpm = 0.6 in. wc x 0.64 = 0.038 in. wc
4
5
Damper area (parallel blades) =
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Table 4. Damper Sizing Procedure.
Airflow (cfm)
x
2
Duct Area (in )
6
10
[Approach velocity (fpm)]
a
:
2
):
( 2
Duct area (in ) x ratio
2
) =
0.37
( 2
Duct area (in ) x ratio
2
) =
0.3810
Table 5. Damper Sizing Example.
20,000 cfm
2304 in 2
10 6
Correction factor =
2
1250
Free area ratio = [1 + (79.7448 x 0.038)] –0.2340
= 4.03 –0.2340
= 0.722
2304 in 2 x 0.722
(
= 4496 0.9085
= 2083 in
Procedure
2
144 in
2
1 ft
2
0.9085
)
0.9217
)
Example
144 in 2
x = 1250 fpm
1 ft 2
= 0.64
) 0.9085
0.37
2
466