Answer
Incinerator Fan Type and Why
Marine Incinerators primarily use an Induced Draft (ID) Fan.
Why Induced Draft?
- Positive Safety / Vacuum Containment: The main purpose is safety. The ID fan creates a continuous negative pressure (vacuum) inside the combustion chamber. If any leakages occur (e.g., faulty gaskets or during sludge loading), air leaks into the furnace, preventing hazardous, high-temperature gases, soot, or sparks from leaking out into the engine room.
- Optimal Combustion: By controlling the vacuum, the ID fan ensures a precise amount of combustion air is drawn into the burner and the tertiary air intake for efficient mixing and complete combustion.
- Controlled Flue Gas Temperature: The ID fan controls the flow rate of the hot flue gases to the exhaust stack, preventing overheating of the furnace structure and managing the exhaust gas temperature profile.
(Note: Some large incinerators may use a small Forced Draft (FD) fan for the burner itself, but the overall combustion air and exhaust flow is dominated by the Induced Draft Fan.)
Why no ID fan For Boiler?
- FD fan provides sufficient pressure
- The FD fan forces combustion air through the burner register into the furnace.
- This pressure is sufficient to overcome the resistance of the furnace and boiler gas passages.
- Hot gases naturally rise
- Combustion gases are very hot and less dense.
- The uptake/funnel creates a natural chimney (stack) effect, assisting gas flow to atmosphere.
- Gas-side resistance is relatively low
- A typical marine auxiliary boiler is compact and designed so that the combination of FD pressure + natural draft can handle the required gas flow.
- ID fan would add unnecessary equipment
- Extra electrical consumption
- Additional maintenance and failure point
- More complicated draft-control system
- Not justified when the FD fan alone can provide the required combustion-gas flow.
Normally a marine auxiliary boiler does not require an ID fan because the FD fan provides sufficient pressure to overcome the boiler flow resistance, while the hot flue gases are assisted by natural chimney draft through the uptake. Providing an ID fan would add unnecessary power consumption and complexity. However, large boilers may use both FD and ID fans as a balanced-draft system.