How marine auxiliary boilers work
Marine auxiliary boilers supply steam for fuel oil heating, accommodation, tank heating and, on tankers, cargo pump turbines and cargo heating. Most are oil-fired vertical or D-type units burning marine gas oil or residual fuel through a pressure-jet or rotary cup burner, often combined with an exhaust gas economiser on the main engine uptake. The furnace is pressurised: the FD fan draws air from the engine room or a dedicated air trunk and pushes it through the burner, furnace and convection bank to the funnel. There is no ID fan. Ship motion, vibration, salt-laden air and the requirements of the classification society set the conditions the fan works under.
Where the FD fan sits
- Engine room air or air trunk
- FD fan
- Air register
- Burner
- Pressurised furnace
- Convection bank
- Funnel
The fan is mounted on the burner or on a bracket beside the boiler, usually in the engine room casing. It draws air from the engine room, which the ventilation fans keep supplied, or from a separate trunk. It delivers air against the furnace pressure, the convection bank and the uptake to the funnel, often sharing the funnel with the engine exhaust. On tankers with cargo boilers, a larger separate FD fan on its own foundation is common.
Air at the FD fan
| Air temperature at the fan | Engine room air, typically 35 to 50°C |
| Pressure at the fan outlet | Set by burner, furnace and uptake resistance |
| Ship motion | Must run at the static and dynamic inclinations required by class rules |
| Vibration | Hull and engine vibration transmitted through the seating |
| Atmosphere | Warm, humid, salt-laden air with oil mist |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for marine auxiliary boilers
| Blade form | Backward curved, compact and rigid |
| Material of construction | Mild steel with a marine coating system; stainless steel fasteners |
| Drive and arrangement | Direct mounted or direct coupled on a common bedplate with the motor |
| Bearings and cooling | Bearings and lubrication suited to sustained inclination; no cooling needed |
| Flow control | Air damper linked to the burner modulation, or variable speed drive on larger cargo boilers |
| Also specified | Marine-duty motor for the ship supply, often 440 V 60 Hz; anti-vibration mounts; documentation to the class society's requirements |
What this FD fan has to cope with
Running at sea under inclination
A fan on a rolling ship must keep its bearings lubricated and its rotor stable at inclinations a shore fan never sees. Bearing type, lubrication and mounting are chosen to suit the inclinations in the class rules, and the rotor is balanced closely to limit loads on the seating.
High engine room temperature
Engine room air at 45 to 50°C is less dense, so a fan sized on 20°C air delivers less mass flow and the burner runs short of air at full fire. The fan is selected at the hottest engine room condition expected in the vessel's trading area.
Salt and oil mist corrosion
Humid, salt-laden air with oil mist corrodes unprotected steel and loads the impeller with sticky deposits. A marine coating system, stainless fasteners and an inlet guard that can be cleaned at sea keep the fan in balance between dry dock visits. Moisture separators on the air trunk help on open-deck intakes.
Energy and control
Auxiliary boilers run hardest in port, during cargo discharge and in cold weather, and idle or shut down at sea when the exhaust gas economiser covers the steam demand. The fan therefore sees long periods of low or no load and short periods at full duty. Damper modulation is usual on small boilers; on large cargo boilers a variable speed drive saves power during the long hours at low fire alongside.
Questions buyers ask
Does a marine auxiliary boiler have an ID fan?
No. Marine auxiliary boilers have pressurised furnaces, and the FD fan on the burner pushes the flue gas through the boiler and up the funnel. The inert gas blowers on tankers handle scrubbed flue gas taken from the boiler uptake, but they are a separate system, not a boiler draft fan.
Can a replacement FD fan be made for a marine boiler?
Yes. We can measure the existing fan or work from the boiler and burner data, and build a fan to match the duty, mounting and motor. The ship owner should state the classification society and the documentation it needs so the fan is supplied accordingly.
Why does the boiler smoke in hot weather?
Hot engine room air is less dense, so the same fan delivers less oxygen to the burner. If the fan was sized for cooler air, combustion becomes rich at full fire. Checking the fan duty at the actual engine room temperature, and the damper linkage, usually explains it.
Information needed to quote an FD fan for marine auxiliary boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Vessel and class. Type of vessel, classification society and the inclinations the fan must run at.
- Ship supply. Voltage, frequency and the motor enclosure required in the engine room.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.