Air and gas dampers are the least glamorous yet most influential actuated devices in combustion adjustment. Two forum threads (19 and 18 replies) — one on air distribution of a 300MW boiler, one on Shanghai Boiler Works' secondary-air damper control logic — together give the full picture. For I&C engineers the keywords are: hot-state damper sticking and the reality of automatic mode.

1. Damper Layout of the Boiler

On the HG-1025/17.4-YM28 boiler, dampers are arranged by coal mills A~F and oil layers AB/CD/EF, with furnace-to-windbox differential pressure measurable (setpoint adjustable). When mills B~E run, the damper settings directly shape the flue-gas temperature deviation.

2. The Vendor's Control Logic (Shanghai Boiler Works)

• Pulverized-coal windbox dampers: control boundary (fuel) air, trim the ignition point of the primary-air stream — typically a function of the associated feeder speed
• Auxiliary windbox dampers: control the windbox-to-furnace differential pressure
• CCOFA/SOFA dampers: opening is a function of boiler load, mainly for NOx control
• All settings are finally determined by combustion tuning tests

3. Operating Practice: The Reality of Automatic Mode

• Some plants run automatic with a 20% minimum opening, adjusted by load; others have the logic (by unit load and mill output) but never use it — manual is the norm
• Few Shanghai-Boiler units run secondary-air dampers in automatic; on older 200MW, bin-storage units the dampers strongly affect combustion and have always been manual
• The top two layers mainly eliminate swirl and help cap furnace temperature
• The systematic rule (best reply): damper settings interact with coal quality, load, heat deviation, slagging, metal over-temperature and mill operation; without deviation or slagging keep the air distribution even and true to the tangential circle; trim the fireball center by metal temperatures; when slagging skews the heat pattern, bias the air to one side. The lowest layer lifts the pulverized coal; the top layer kills swirl, some are NOx trim — every layer has its own duty; ultimately follow the manual
• A plain rule of thumb: with mills B~E running, if main/reheat steam reaches rating, distribute air evenly with a bit more at the bottom layer

4. For I&C Engineers: The Actuator Layer

The recurring reality in the replies: many dampers cannot be operated when hot. Implications for damper actuators:

• Allow generous torque margin for hot-state sticking (see Step 4 of our sizing article)
• Exercise long-idle dampers periodically or via partial-stroke tests to prevent sticking and rust-seizure
• When windbox-furnace ΔP is the controlled variable, actuator dead band and accuracy set the control quality
• For dampers frequently switched between local and remote, verify the selector switch and command circuit in routine maintenance