A sample application compares outlet dampers and variable-speed drives under varying flow conditions. Results show the latter offers significant cost advantages.

Outlet-damper power requirements
Cubic feet per minute (cfm)
Duty cycle (%)
Horsepower (hp)
Weighted horsepower
100
10
35
3.5
80
40
35
14.0
60
40

31

12.4
40
10
27
2.7
Total
32.6

Variable-speed-drive power requirements
Cubic feet per minute (cfm)
Duty cycle (%)
Horsepower (hp)
Weighted horsepower
100
10
35
3.5
80
40
18
7.2
60
40
7.56
3.024
40
10
2.24
0.224
Total
13.948

Comparing costs
 
Outlet damper
Variable speed
Weighted horsepower
32.6
13.9
X kW/hp
0.746
0.746
X hr/month
730
730
= kW-hr/month
17,753
7,596
X electricity cost
$0.07
$0.07
Total cost
$1,242.73
$531.07