How LSHS fired boilers work
LSHS, low sulphur heavy stock, is a waxy residual fuel with a pour point typically 40 to 60°C, a calorific value near 10,000 kcal/kg and sulphur usually below 1%. It must be kept heated in tanks and lines and raised to about 120 to 140°C for atomisation. It is fired in shell boilers and in water tube boilers of 10 to 60 TPH in chemical, pharmaceutical and fertiliser plants. On the larger units the gas passes the boiler bank, economiser and often a tubular air heater before an ID fan and the stack, while the FD fan feeds the burners through the air heater. Lower sulphur than furnace oil allows a cooler stack and more heat recovery.
Where the FD fan sits
- Air intake
- FD fan
- Steam coil air preheater
- Tubular air heater
- Windbox
- Burners
The FD fan stands near the air heater and pushes ambient air through a steam coil air preheater, used at start-up and low load, then through the tubular air heater to the windbox. Preheating the air helps LSHS burn out at low fire and keeps the air heater cold end above dew point. The fan works against the preheaters, ducts, windbox and burner registers, typically 150 to 250 mmWC on balanced draft units of this size.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 150 to 250 mmWC |
| Dust | Clean air |
| Excess air | About 15 to 20% at full load, higher at low fire |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 LSHS fired boilers
| Blade form | Backward curved, giving a stable curve at low flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Pedestal bearings; no cooling needed |
| Flow control | VFD with O₂ trim on total air |
| Also specified | Inlet silencer, air flow element in the cold air duct |
What this FD fan has to cope with
Poor burnout at low fire
At low fire the air velocity through the register falls, mixing weakens and LSHS leaves unburnt carbon. A fan that cannot control steadily at low flow makes this worse. Sizing the fan for the real duty rather than throttling a large margin, and setting a minimum air flow in the control, keep low-fire combustion clean.
Steam coil preheater out of service
When the steam coil preheater fails or is bypassed, cold air reaches the tubular air heater, its cold end corrodes and starts to leak air into the gas side. The FD fan then carries more air for the same combustion. Keeping the steam coil in service at start-up and low load protects both heaters and the fan duty.
Oversized fan after a fuel change
Boilers converted to LSHS from furnace oil or coal often keep their original FD fan, sized for a different air requirement and resistance. The fan runs throttled, wastes power and gives poor control at low fire. A new impeller or a VFD sized for the LSHS duty corrects both the power and the control.
Energy and control
Most LSHS boilers run continuously at 50 to 90% load. A VFD on the FD fan with O₂ trim holds excess air at about 3 to 4% O₂ across this range and saves more energy than damper control. Because LSHS is a costly fuel, the larger saving comes through the fuel bill: every point of excess air removed reduces both fan power and stack loss.
ID fan and FD fan in LSHS fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the air heater, before the stack | Ahead of the steam coil and tubular air heater |
| What it handles | Flue gas with occasional oily carbon | Ambient air |
| Temperature at the fan | 150 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Deposits and acid dew point | Steady low-fire air for burnout |
| Control | Furnace draft | Firing rate and oxygen |
See also the Induced Draft (ID) Fan for LSHS fired boilers.
Questions buyers ask
Why does an LSHS boiler need a steam coil air preheater?
LSHS burns better with warm combustion air, and the tubular air heater corrodes if it receives cold air at start-up. The steam coil preheater warms the air from the FD fan before it reaches the main air heater, which protects the cold end and helps clean light-off. It is usually switched out once the boiler is at steady load.
What controls the FD fan on an LSHS fired boiler?
The fan speed or damper is set by the combustion control from the firing rate and trimmed by flue gas oxygen. On larger boilers the air flow is measured and kept in ratio with fuel flow, with cross-limits so that air leads fuel on a load increase and lags it on a decrease.
Can the same FD fan be used if the boiler changes from LSHS to natural gas?
Often yes, but the duty changes. Natural gas runs at lower excess air, and a low NOx gas burner may need more pressure at the register than the oil burner did. The fan curve should be checked against the new burner's air flow and pressure before the conversion is finalised.
Information needed to quote an FD fan for LSHS fired 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.
- Air preheating. Whether a steam coil and tubular air heater are fitted, and their air-side resistance.
- Burner data. Number of burners and register pressure at full fire.
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.