How furnace oil fired smoke tube boilers work
A furnace oil fired smoke tube boiler is usually a three-pass wetback shell boiler of 1 to 20 TPH, serving dyeing houses, food plants and general process industry. Furnace oil is heated to about 100 to 120°C for atomisation by pressure jet, steam or air atomising, or a rotary cup. The flame burns in the corrugated furnace tube, the gas turns in the reversal chamber and passes two banks of fire tubes, then often an economiser, leaving at about 220 to 280°C without one and 160 to 200°C with one. The whole gas path runs under positive pressure from the burner fan. There is no ID fan in normal practice; the chimney only has to carry the gas away.
Where the FD fan sits
- Boiler house air
- FD fan
- Burner air register
- Furnace tube
- Fire tube passes
- Economiser
- Chimney
The fan is mounted on the burner or on a separate base at the boiler front, drawing air from the boiler house. On rotary cup burners a primary air fan on the burner shaft supplies atomising air while a secondary fan supplies most of the combustion air. The fan discharges into the burner air box and through the register into the pressurised furnace. Its pressure covers the register, the furnace and reversal chamber, the second and third pass tubes, any economiser and the chimney base.
Air at the FD fan
| Air temperature at the fan | Boiler house air, 30 to 50°C, hotter close to the boiler front |
| Pressure at the fan outlet | Typically 100 to 250 mmWC depending on boiler size and economiser |
| Furnace pressure | Positive, often 30 to 100 mmWC at full fire |
| Dust and contaminants | Oil mist and soot in the boiler house; fibre lint in textile plants |
| Turndown | About 3:1 to 4:1 on pressure jet, wider on rotary cup or steam atomising |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 furnace oil fired smoke tube boilers
| Blade form | Backward curved, for a stable rising pressure curve against variable tube fouling |
| Material of construction | Mild steel, or aluminium on smaller burner fans |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no separate cooling |
| Flow control | Air damper on linkage or servo; VFD with O₂ trim on larger burners |
| Also specified | Inlet filter where air carries lint or oil mist, intake silencer, pressure margin for fouled tubes |
What this FD fan has to cope with
Soot fouling raises back pressure
Heavy oil leaves soot in the fire tubes, especially at low fire and with poor atomisation. Furnace back pressure rises, the fan delivers less air, combustion worsens and more soot forms. A fan selected with pressure margin for the fouled condition, together with regular tube brushing, breaks this cycle before the boiler has to be derated.
Lint and oil on the impeller
In dyeing houses and textile processing, fibre lint and oil mist settle on the impeller and inlet cone, cutting air flow and causing unbalance. An inlet filter, or a ducted intake from a clean area, together with periodic cleaning, keeps the burner fan at its rated output and the air-fuel ratio where it was set.
Smoke at light-off and low fire
Furnace oil needs a stable air-fuel ratio at light-off and low fire. A fan that cannot hold a steady low flow, or a linkage with play in it, makes smoke and soot each time the burner starts or turns down. Servo positioning or VFD control with a calibrated low-fire air setting gives a cleaner start and cleaner tubes.
Energy and control
Smoke tube boilers serving batch processes such as dyeing see sharp steam swings and frequent burner cycling at low demand. Every cycle includes a pre-purge that cools the boiler and wastes fan power. A burner fan on VFD with wider turndown keeps the burner modulating instead of cycling, and O₂ trim brings excess air down from the 5 to 6% O₂ common on linkage burners to about 3 to 4%.
Questions buyers ask
Does a furnace oil smoke tube boiler need an ID fan?
Normally no. The burner fan pushes the gas through the furnace and tubes under pressure, and the chimney carries it away. An ID fan is added only in special cases, such as a long horizontal flue, a scrubber or heat recovery unit after the boiler, or a chimney that cannot discharge the gas on its own draft.
Why does my furnace oil boiler burner fan not give enough air?
The common causes are soot-choked fire tubes raising back pressure, a dirty impeller or inlet, a slipping linkage, or an economiser added without checking the fan. Measuring furnace pressure and fan outlet pressure at full fire shows whether the limit is the fan or the boiler side.
What is the difference between primary and secondary air fans on a rotary cup burner?
The primary air fan, often on the same shaft as the cup, supplies a small quantity of high-velocity air that shears the oil film leaving the cup into fine droplets. The secondary air fan supplies most of the combustion air. Both must be clean and in good condition for clean combustion of furnace oil.
Information needed to quote an FD fan for furnace oil fired smoke tube 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.
- Burner type. Pressure jet, steam or air atomising, or rotary cup, with the burner rating.
- Boiler back pressure. Furnace pressure at full fire with clean and with fouled tubes, if known.
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.