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 ID fan sits
- Furnace
- Boiler bank
- Economiser
- Air heater
- ID fan
- Stack
The ID fan stands after the air heater, at ground level beside the stack. There is normally no collector; on some plants a small cyclone traps cenospheres ahead of the fan. Suction covers furnace draft, the boiler bank, the economiser and the air heater, typically 100 to 180 mmWC on a 20 to 40 TPH unit. Placing the fan at the cold end keeps it on the smallest gas volume, but it also puts the fan where any unburnt oily carbon and acid condensate finally settle.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | 220 to 260°C with fouled surfaces or the air heater bypassed |
| Dust at the fan | Light; cenospheres and oily carbon after poor atomisation or cold starts |
| Moisture and dew point | Acid dew point typically 110 to 130°C, lower than on furnace oil |
| Corrosive species | Some SO₃ and vanadium, both less than with furnace oil |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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. |
ID Fan selection for LSHS fired boilers
| Blade form | Backward curved, with smooth blade faces that resist carbon build-up |
| Material of construction | Mild steel casing and impeller; SS 304 or 316 impeller where the plant runs a low stack temperature |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | VFD on furnace draft |
| Also specified | Casing drain, large access door for washing, insulation to keep the casing warm |
What this ID fan has to cope with
Oily carbon from cold fuel
If LSHS reaches the burner below its atomising temperature, through failed tracing or a heater fault, the spray is coarse and unburnt waxy carbon travels to the fan. It sticks unevenly to the blades and vibration climbs within days. An alarm on fuel temperature at the burner and a fan designed for easy washing limit the damage.
Running too close to the dew point
Lower sulphur tempts plants to push stack temperature down with a larger economiser. At low load the gas at the fan can then fall near the acid dew point, and corrosion starts on the casing and impeller back plate. A minimum gas temperature limit in the control, and corrosion-resistant steel where margins are thin, protect the fan.
Cenosphere accumulation
Hollow carbon spheres from heavy fuel droplets that did not burn out are light and travel with the gas. They cause little erosion but collect in the casing bottom and inlet box, where they absorb condensate and form an acidic sludge. A drain point, an access door and removal at each shutdown stop the build-up.
Energy and control
LSHS boilers in chemical and pharmaceutical plants usually run continuously but at varying load as batches start and finish. A VFD on the ID fan, linked to furnace draft and coordinated with the FD fan, follows these swings without throttling losses. Keeping air heater leakage low matters as well, since every unit of air that leaks across adds to ID fan flow without doing any work in the furnace.
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 Forced Draft (FD) Fan for LSHS fired boilers.
Questions buyers ask
What is LSHS and how is it different from furnace oil for a boiler?
LSHS, low sulphur heavy stock, is a residual fuel with lower sulphur than furnace oil but a higher pour point, so it must be kept heated in storage and lines. For the boiler fans this means a lower acid dew point and less corrosion, but a higher risk of waxy carbon deposits whenever fuel heating fails.
Can I lower the stack temperature on an LSHS fired boiler?
Within limits. The lower sulphur allows a cooler exit than furnace oil, often 150 to 170°C, but the ID fan and stack must stay above the acid dew point at all loads, including low load. Checking the actual fuel sulphur and measuring the dew point at site gives a safe basis for any economiser upgrade.
How often should the ID fan be cleaned on an LSHS boiler?
It depends on burner condition and fuel heating. With good atomisation, inspection at each boiler shutdown is usually enough. If vibration rises between shutdowns, deposits are the likely cause, and an access door with washing provision lets the plant clean the impeller without removing the rotor.
Information needed to quote an ID fan for LSHS fired boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Fuel heating. Fuel temperature at the burner and how it is maintained.
- Stack temperature target. Planned gas temperature at the fan after any economiser upgrade.
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.