How sunflower husk fired boilers work
Sunflower husk is the hull separated from the seed before oil extraction, produced in quantity at oil mills in Karnataka, Maharashtra and Andhra Pradesh. It carries 8 to 12% moisture, 2 to 4% ash and a calorific value of about 4,000 to 4,400 kcal/kg, but its ash is rich in potassium and softens from about 850 to 1,000°C. Bulk density is low, around 100 to 150 kg/m³. The husk is blown or screw-fed onto a travelling or reciprocating grate, usually in the mill's own boiler, and a large part of it burns in suspension. The flue gas passes the boiler bank, economiser and air heater, then a multicyclone and bag filter, before the ID fan.
Where the ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Air heater
- Multicyclone
- Bag filter
- ID fan
- Chimney
The fan sits after the bag filter. Sunflower husk produces little ash, so dust at the fan is light and erosion is moderate. The difficulty is upstream: potassium-rich ash forms sticky deposits on the boiler tubes and economiser that grow steadily, and the draft loss across them can rise substantially within a few weeks. Suction at the fan is typically 150 to 250 mmWC when clean and higher before cleaning, and the fan must still hold furnace draft at the fouled condition.
Gas at the ID fan
| Gas temperature, normal | 150 to 170°C after a clean air heater |
| Maximum or upset | 200 to 240°C as the economiser and air heater foul |
| Dust at the fan | Low; fine potassium-rich ash after the bag filter |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Potassium chloride and sulphate deposit on heat surfaces; the fan sees little of them in normal running |
| 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 sunflower husk fired boilers
| Blade form | Backward curved, for efficiency on low dust gas |
| Material of construction | Mild steel, with light wear protection at the blade inlet |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc, rated for the fouled-condition temperature |
| Flow control | Variable speed drive on furnace draft, with pressure margin for the fouled boiler |
| Also specified | Inspection door and drain, and draft gauges across the economiser and air heater to track fouling |
What this ID fan has to cope with
Draft loss rising between cleanings
Fouling on the economiser and boiler bank raises resistance week by week. A fan sized on clean surfaces runs out of pressure before the planned cleaning and the boiler has to be derated. Selecting on the fouled resistance, with the drive supplying the extra pressure only when it is needed, avoids the derate without wasting power while the boiler is clean.
Hotter gas at the fan
Deposits cut heat transfer, so gas leaves the air heater hotter as fouling builds. Hotter gas is lighter and occupies more volume, so the fan has to move more cubic metres for the same mass. The impeller, shaft and bearings are designed for the fouled temperature, and the volume margin covers the expansion.
Ash bursts after sootblowing
When deposits are blown off, or fall during load changes, slabs and fine ash reach the collector in bursts. The bag filter takes most of this if it is sized for the peak rather than the average load. The fan should still have a drain and an access door, so material that does arrive can be removed at the next stop.
Energy and control
Oil mills crush for most of the year, but steam demand varies with seed throughput and the refinery. The ID fan draws most power just before cleaning, when the boiler is most fouled. A variable speed drive keeps power low after cleaning, and logging fan speed against steam output shows when cleaning is due, which saves fuel as well as fan power.
ID fan and FD fan in sunflower husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Before the air heater, feeding the grate |
| What it handles | Flue gas with low, potassium-rich ash | Ambient air |
| Temperature at the fan | 150 to 170°C, up to 240°C fouled | Ambient, 25 to 45°C |
| Main design concern | Rising draft loss from fouling | Primary air to prevent clinker |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Forced Draft (FD) Fan for sunflower husk fired boilers.
Questions buyers ask
Why does my sunflower husk boiler lose draft after a few weeks?
Sunflower husk ash is rich in potassium and forms sticky deposits on the boiler tubes and economiser. Resistance rises and the ID fan runs out of pressure. Regular cleaning, and an induced draft fan selected for the fouled condition with a variable speed drive, keep the boiler at full output between cleanings.
Is a wear-resistant ID fan needed for sunflower husk?
Usually not. Sunflower husk has low ash, typically 2 to 4%, and after a bag filter the dust at the fan is light. Modest protection at the blade inlet is enough. The real concern is fouling upstream and the higher gas temperature and pressure demand it causes at the fan.
What gas temperature should a sunflower husk ID fan be designed for?
The normal temperature is 150 to 170°C after a clean air heater, but it climbs as the surfaces foul. Designing the fan, its bearings and the motor for about 220 to 240°C covers the fouled case without derating, while the drive keeps power down when the gas is cooler.
Information needed to quote an ID fan for sunflower husk 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.
- Fouling record. Draft readings across the boiler and economiser after cleaning and just before cleaning.
- Cleaning interval. How often the boiler is cleaned or sootblown, and the method used.
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.