Damper Actuator Torque and Sizing

SEALS ARE THE WHOLE STORY IN ACTUATOR SIZING AND THEY ARE SPECIFIED BY SOMEONE ELSE. A low-leakage damper is chosen for its leakage class -- an outside air intake that must shut tight in freezing weather, an isolation damper holding a pressure boundary -- and that specification travels to the DAMPER schedule and not necessarily to the ACTUATOR schedule. Blade and jamb seals then roughly double the torque required, because a sealed damper must COMPRESS its seals to close and that is torque the unsealed one never needs. An actuator sized from the ordinary factor stalls short of closed, and both selections are computed here side by side so the disconnect is visible on one line rather than across two documents that never meet. THE FAILURE IS CHARACTERISTIC AND EASY TO MISDIAGNOSE. The damper strokes almost fully, the actuator sits at its stall torque against the last few degrees of seal compression, and the control system reports the COMMANDED position rather than the achieved one. Leakage continues, the freeze stat trips or the space will not hold pressure, and what gets investigated is the freeze stat. The actuator is eventually found warm and buzzing at ninety-eight percent closed, and it has been reporting closed the whole time. CLOSE-OFF IS A SEPARATE RATING FROM RUNNING TORQUE and it is the one that matters for isolation service. Moving a damper through still air is a modest demand; holding it shut against a significant pressure difference, with fan pressure trying to push it open, is another, and manufacturers publish the two separately. An actuator adequate to STROKE a damper can be inadequate to KEEP IT CLOSED once the fan starts, and that figure is not computed here -- it is a separate line on the same datasheet and it has to be read. AND A LARGE DAMPER EXCEEDS ANY SINGLE ACTUATOR, at which point the choice is multiple actuators or a jackshaft distributing one actuator's torque across sections. A multi-section damper is normally sized and driven section by section rather than as one area, which changes the arithmetic entirely and is the manufacturer's arrangement to specify. This is a sizing screen. Torque factors are entered and are properties of the specific damper: blade style, bearing type, linkage arrangement, frame size, air velocity and pressure across it, and seal type all move the figure, and the damper manufacturer publishes the required torque for their product rather than leaving it to a table. It does not address the spring-return requirement where a fail-safe position is needed -- a spring-return actuator delivers less torque than the same frame size without one -- the actuator's power supply, the control signal type, stroke time and whether it suits the sequence, or the mounting, linkage and shaft coupling, which is where field installations actually fail. It does not address jackshaft sizing and deflection, or the leakage class the damper was chosen for and how it is verified. The damper and actuator manufacturers' published torque and close-off data, the applicable leakage and smoke damper standards where they govern, and the controls engineer govern.

area = width x height / 144; required torque = area x the torque factor in in-lb per sq ft; design torque = required x the safety factor; the selection is the smallest standard actuator at or above it, from the 35 / 70 / 140 / 180 in-lb ladder, and beyond the largest it is multiple actuators or a jackshaft.

The damper torque factor convention. Factors are ENTERED: roughly 3 to 5 in-lb per sq ft for an ordinary low-pressure damper, 5 to 7 at higher velocity and pressure, and 7 to 10 or more with blade and jamb seals.

An area calculation and a ladder lookup.

Estimate. AHJ and licensed professional govern.

Field names used by the API: damper_width_in, damper_height_in, torque_factor_in_lb_ft2, sealed_torque_factor_in_lb_ft2, safety_factor, damper_area_ft2, required_torque_in_lb, design_torque_in_lb, selected_actuator_in_lb, sealed_required_torque_in_lb, sealed_design_torque_in_lb, sealed_selected_actuator_in_lb

• Torque factors are entered blade style, bearings, linkage, frame, velocity and seals all move themthe damper manufacturer's published torque
• Close-off is NOT computed holding shut against fan pressure is a separate published ratingthe actuator datasheet
• Single-section damper a multi-section damper is normally sized and driven section by sectionthe manufacturer's arrangement