Actuator and linkage selection
Torque, duty cycle, fail position and lost motion: the four numbers that decide whether a control damper holds a loop.
Most control damper problems are actuator or linkage problems. The blade is usually fine. What fails is the interface between the motor and the blade, or the motor itself when it has been sized for torque and not for how often it has to move.
Torque
Calculate the torque from the blade area, the design differential and the shaft geometry, then add a margin for dust build-up and for seat friction. A damper that opens freely on the bench can need twice the calculated torque after a year in a flue duct. We publish the assumed margin on the data sheet so the client can see what was allowed for.
Duty cycle
A modulating damper may move several times a minute. An actuator rated for 30 % duty will fail within months on that duty even though its torque is correct. Specify the cycle rate and let the actuator be chosen against it. Where the cycle rate is high, a pneumatic actuator with a positioner is often more durable than an electric one.
Fail position
Decide what the damper should do when air or power is lost, and confirm it in writing before ordering. It cannot be changed afterwards without a new actuator. On a draft control damper the usual answer is open; on a combustion air damper it may be closed; on a bypass it is almost always open.
Lost motion
Use a ball-jointed linkage with reamed pins and a shaft coupling that is keyed and clamped rather than pinned with a grub screw. Grub screws slip, and a slipped coupling looks exactly like a failed actuator from the control room.
Setting in the works
Limit switches, mechanical stops and the position transmitter are set and marked before despatch. Commissioning should only need power, a signal and a confirmation that the blade reaches both ends without binding.
Talk to us about this duty
Send the duct section, design temperature, pressure difference and the duty point. We reply with the gate type, the material selection, the drive and a general arrangement drawing for approval.