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Engineering
G-25
Introduction
The size and quantity of actuators required depends on
several damper torque factors:
• Type of damper seals (Standard, low or very
low leakage)
• Quality of damper installation
• Number of damper sections
• Approach air velocity
• Static pressure
The following procedures can be used to determine the
damper torque, actuator size and quantity of actuators
required to operate a damper.
Determing Damper Torque
1. From the damper manufacturer get the Damper Torque
Rating (DTR) for the damper at the most severe
operating conditions.
If the damper torque rating is not available,
Table 1 can be used for estimating purposes only on
an interim basis. However, it is very important to get the
damper torque rating from the manufacturer as soon as
possible to assure accurate torque calculations.
2. Calculate the damper area (DA) in square feet from the
damper dimensions.
3. Calculate the Total Damper Torque (TDT) in lb-in using
the following formula:
TDT = DTR X DA
4. If the damper torque rating is not available, use a
torque wrench on the damper shaft while air is moving
through the duct to measure the TDT.
Table 1
Damper Type
Damper Leakage at 1"
H
2
O Static Pressure Drop
Damper Torque for
Approach Air Velocities of
1200 ft./min. or less
Standard leakage More than 10 CFM/ft.
2
2.5 lb.-in./ft.
2
Low leakage 5 to 10 CFM/ft.
2
5.0 lb.-in./ft.
2
Very low leakage Less than 5 CFM/ft.
2
7.0 lb.-in./ft.
2
Contact your local customer
service representative for additional
application assistance when specic
damper factors are known.
Actuator Size
1. From the actuator literature select the actuator type and
size whose actuator torque rating (ATR) in lb-in is most
appropriate for the application.
2. The ATR is normally based on 90° rotation of the
damper. For torque ratings of other than 90°f rotation,
use the following formula:
ATR @ X° rotation =
ATR @ 90° rotation x
3. If the actuator is rated in pounds of thrust, it can be
converted to torque using the following formula:
Torque = (*Crank arm length x 0.707) x Thrust
*The crank arm length is for 90° shaft rotation at
nominal actuator stroke.
Quantity of Actuators
1. Calculate the number of actuators required using the
following formula:
Number of actuators =
SF = Safety Factor: When calculating the number
of actuators required, a safety factor should be
included for unaccountable variables such as slight
misalignments, aging of the damper, etc. A suggested
factor is 0.8 or 80% of the rated torque.
2. If the number of actuators calculated is too large to be
practical, select a more powerful actuator or consider
using a positioning relay if it is a pneumatic actuator.
Crank Radius @ X°
Crank Radius @ 90°
( )
Total Damper Torque
SF x Actuator Torque Rating
Damper Actuators
Damper Actuators
Selection and Sizing