Structure and principle
- The damper has a double-layer blade structure, with a stainless steel sealing strip fitted around the perimeter of each blade. When the damper closes, sealing air is injected between the two blade layers so that the medium in front of and behind the damper is effectively isolated.
- Opening and closing are carried out by an electric actuator through a linkage. The linkage uses ball joints, and rotation is achieved without lost travel.
- An open/close positioning interlock device and external limit switches are fitted, which makes operation reliable and effectively prevents mis-operation.
The flue damper is driven by mechanical force transmission. The mechanical force is determined by the flow rate, pressure and temperature in the flue and air duct, and by the structure of the bearing and the linkage. When determining the driving force, the effect of the rotational torque must be considered: only when that torque is overcome can the damper blade turn freely during regulation.
Why the drive calculation matters
Under-sizing the actuator is a common cause of failure on hot ducts. The torque needed is not only the aerodynamic torque at design conditions, but also the friction in the packing glands, the weight of the blade off its centre of gravity, and the additional friction that appears once dust has baked onto the blade. A drive sized on aerodynamic torque alone will stall after a season of operation.
Where the gas is corrosive, the sealing strip and all wetted fasteners should be stainless, and the shaft should be in a grade that will not gall in the gland.