Basic Documentation

Damper Airflow Noise
C = 15.9 x 10
6
DP S
2
÷ Q
2
Where:
DP = the pressure drop across the damper from the manufacturer’s data.
S = the duct area at the damper in square feet
Q = the CFM.
In lieu of S
2
/Q
2
, the term 1/V
2
may be substituted, where V is the air velocity in feet/minute.
Calculate Damper Blockage Factor BF
To calculate damper blockage factor BF for a multi-blade damper, use the following formula:
If C ≠ 1, BF = (C
½
-1) ÷ (C - 1)
If C = 1, BF = 0.5
To calculate damper blockage factor BF for a single-blade damper, use the following formula:
If C < 4, BF = C
0.5
- 1) ÷ (C - 1)
If C > 4, BF = (0.68 x C
-0.15
) - 0.22
Calculate the Velocity Factor U
To calculate the velocity factor U, use the following formula:
U = (Q ÷ S) ÷ BF
In lieu of Q/S, the term V may be substituted, where V is the air velocity in feet/minute.
The graph in Figure 9 provides Velocity Factor values for dampers within common airflow
ranges and pressure drops. Using the graph eliminates the need to go through the foregoing
three equations to calculate U.
Siemens Building Technologies, Inc. 31