Relationship between flue damper opening and negative pressure

Generally, the capacity of the flue gas exhaust fan in boilers is several times greater than that of the blower. When the opening degree of the flue dampers is high, the amount of flue gas exhausted is large, resulting in a negative pressure condition. When the opening of the flue gas dampers is small and the draft volume decreases, with the blower volume remaining unchanged, a positive pressure condition will inevitably occur in the furnace, resulting in flames escaping outward.

Excessive positive or negative pressure occurs in the furnace; excessive negative pressure is caused by an overly large opening of the chimney damper. Excessive negative pressure causes the burner to deviate from its designed operating conditions, resulting in unstable combustion and a high risk of flame drift and extinguishment. The chimney damper should be adjusted to maintain a negative pressure in the furnace chamber at -10 to -40 Pa. A positive pressure condition occurs when the chimney dampers are opened too little or the system is operated beyond its capacity, which can lead to flash explosions and damage to the furnace door and furnace walls; it is necessary to increase the opening of the chimney dampers promptly to bring the negative pressure value back to standard. Sometimes, due to the lack of regular cleaning of the convection furnace tubes, severe ash accumulation and scaling occur, increasing the smoke flow resistance. It can also cause positive pressure in the furnace, so soot blowing measures should be taken promptly to reduce the resistance in the convection section.

The boiler flue exhaust fan is equipped with a larger capacity than the blower; when the flue dampers are opened wide, the amount of exhaust air is high, resulting in a negative pressure condition. When the opening of the flue gas dampers is small and the exhaust air volume decreases, with the blower volume remaining unchanged, the furnace will inevitably experience a positive or slightly positive pressure condition, resulting in flames escaping outward.