How the calculation works
Actuator torque is sized from the aerodynamic load on the damper blades, not from the duct size alone. Manufacturers publish a torque loading in in-lb per ft² of damper face area, which varies with blade arrangement, seal type and the pressure the damper closes against. Multiply the face area by the torque loading and you have the base torque:
Torque [in-lb] = Damper area [ft²] × Torque loading [in-lb/ft²]
A sizing factor is then applied for linkage loss, small misalignment, dirt and ageing, giving the design torque the actuator must guarantee. Divide in-lb by 8.85 to get Nm.
Torque loading by damper construction
Torque loadings in in-lb per ft² of damper face area, as published in damper-actuator selection guides (for example Siemens document A6V12077765). They describe standard multi-blade rectangular dampers.
A round damper uses a single blade on a central shaft, so the aerodynamic moment goes as Δp × 4r³/3 — for a 400 mm damper at 500 Pa that is already about 5 Nm, an order of magnitude more than the per-area loadings above would suggest. Mapping round dampers onto this table would under-size the actuator, so we do not do it. Send us the damper schedule and we will size those from the damper maker's data.
Before you order an actuator, also check
- Fail position — spring return for outside-air and life-safety dampers, fail-in-place for most modulating duties.
- Shaft and mounting — shaft diameter, square or round, and whether a mounting kit is needed.
- Control signal — on/off, 3-point floating, 0–10 V or 4–20 mA, and the supply voltage available.
- Multi-section dampers — one actuator per section on its own shaft, not one actuator driving a coupled bank.
- Air velocity — above roughly 12 m/s the aerodynamic load rises sharply and the loading class should be reviewed with the damper maker.