You have selected your damper by size and functional requirements, but now the question, “How much actuator do I need to obtain maximum close-off and to withstand the many cycles of operation?” This is a good question, and yet little information is published on this subject. Hopefully the following guidelines will aid you in your selection process.
Damper torque loading, inch pounds per square foot, used in selecting the correct size actuator should be provided by the damper manufacturer; however, if this information is not available, the following general guidelines can be used.
Damper Type Torque Loading Factor
Opposed blade, without seals, for non-tight close-off applications – 3 in-lb/sq.ft.
Parallel blade, without seals, for non-tight close-off applications – 4 in-lb/sq. ft.
Opposed blade, with seals, for tight close-off applications – 5 in-lb/sq. ft.
Parallel blade, with seals, for tight close-off applications – 7 in-lb/sq. ft.
Where direct mount (rotary motion) actuators are used, the above torque loading will work with most applications under 2” WC static pressure or 1000 FPM face velocity. For higher applications up to 3” WC or 2500 FPM, the torque loading should be increased by a multiplier of 1.5. If your application calls for even higher criteria up to 4” WC or 3000 FPM, use a multiplier of 2.0. This is a good general set of rules to follow. We have found it better to have more torque available because dirt build up and corrosion of the damper over time will put an extra burden on the actuator.
With these things taken into consideration, along with your initial static and velocity parameters, you may find it better to oversize the actuator by using the next larger unit available. This suggestion may offer you some insurance of tight close-off and longer life of the actuator.
Example 1: The damper is an opposed blade type with seals for tight shut off and has an area of 14 sq. ft. The design velocity through the damper is 2200 FPM. From the selection chart, you find the torque loading factor to be 5 in. lb. per sq. ft. The required torque would be calculated as follows:
Since the velocity is 2200 FPM, the torque loading will be increased by a multiplier of 1.5.
5 in.-lb. per sq. ft. x 1.5 = 7.5 in.-lb. per sq. ft. = Actual Torque
14 sq. ft. x 7.5 in.-lb. per sq. ft. = 105 in.-lb. = Required Actuator Torque
Select an actuator that meets or exceeds 105 in.-lb. of torque.
i.e.: Belimo AFB series spring return actuator and AMx series non-spring return are rated at 180 in.-lb.
The above method holds true when selecting direct mount (rotary motion) actuators. However, in choosing an electric (gear train or hydraulic) or pneumatic actuator where linking arms are used, you may find the manufacturer has rated their actuator by the square foot area it can effectively control. This is done, in part, because of the change in torque produced throughout the stroke of the actuator when using crank arms and linking rods. These ratings are typically best case scenarios based on nominal conditions and dampers without seals. To correctly apply these actuators, you must know the conditions under which the actuators will be used. If your application involves pressures or velocities higher than the conditions the actuators were rated for, or involves dampers with seals, you should derate the manufacturer’s square foot area by at least 33% (0.67 multiplier). If your application involves pressure or velocities higher than the conditions the actuators were rated for, and the dampers also have seals, you should derate the manufacturer’s square foot area by at least 50% (0.50 multiplier).
Example 2: An actuator is rated for 24 sq. ft. at 1” W.C. and 2000 FPM. The next larger actuator is rated for 40 sq. ft. under the same conditions. The application is for a 14 sq. ft. low leakage damper with seals, and a velocity of 2200 FPM. Which of these actuators should be used?
Since the application involves both seals and a velocity higher than the actuator was rated for, the actuator square foot rating must be reduced by 50%. Therefore, the 24 sq. ft. actuator should be applied to a damper no larger than 12 sq. ft. (24 sq. ft. X 0.50 = 12 sq. ft.). The actuator rated for 40 sq. ft. would be required in this application.
This method may not be as precise as selecting the direct mount actuators, but is a good rule of thumb to use as a general guideline to select an appropriate actuator. If you are still unsure of your selection, Kele’s sales department will be glad to assist you in making a suitable selection.