How husk fired hot air generators work
Husk fired hot air generators are used mostly in rice mills for paddy drying after parboiling, and in some spice, maize and seed dryers. Rice husk is fed by screw or gravity hopper onto an inclined step grate, or blown tangentially into a cyclone furnace where it burns largely in suspension. Husk carries about 3,000 to 3,400 kcal/kg and 18 to 22% ash, almost all fine silica, and the furnace runs at 800 to 1,000°C. The hot gas either passes through a tube heat exchanger that heats clean air for the dryer, or is cleaned in a settling chamber and cyclone and diluted with ambient air for direct drying. The flue side leaves the exchanger at 180 to 250°C.
Where the FD fan sits
- Inlet screen
- FD (combustion air) fan
- Air ducts and dampers
- Step grate or tangential nozzles
- Husk furnace
The fan sits beside the furnace and blows ambient air under the step grate and through secondary air ports, or into the tangential nozzles of a cyclone furnace. On cyclone furnaces the husk itself is often blown in with part of this air. Pressure is typically 50 to 120 mmWC for a step grate and 150 to 300 mmWC for a cyclone furnace with pneumatic feed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | 50 to 120 mmWC step grate; 150 to 300 mmWC cyclone furnace |
| Dust | Clean air, but husk dust in mill air is heavy |
| Husk transport | Some cyclone furnaces convey husk in the combustion air |
| Duty | Follows the dryer air temperature |
| 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 husk fired hot air generators
| Blade form | Backward curved for combustion air; radial for any fan that carries husk |
| Material of construction | Mild steel; wear liners on a fan that carries husk |
| Drive and arrangement | Direct mounted or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive from the dryer air temperature |
| Also specified | Inlet screen or filter against husk dust |
What this FD fan has to cope with
Husk dust in the mill air
Rice mills are full of husk dust and bran, and an FD fan drawing mill air collects a fibrous layer on its blades and inlet screen. The wheel goes out of balance and the air flow falls. Taking the inlet from outside the mill building and fitting a cleanable screen keep the fan clean.
Husk carried through the fan
Where husk is conveyed pneumatically and passes through the fan itself, the silica wears the blades as it would in an ID fan. Feeding husk into the air duct after the fan, with an ejector or rotary valve, keeps the fan on clean air and reduces wear.
Cyclone furnace turndown
A cyclone furnace needs enough tangential velocity to keep husk in suspension. At low fire, cutting air too far drops husk to the floor, where it smoulders and clinkers. The fan control has a minimum speed, and turndown below it is taken by stopping and restarting the feed.
Energy and control
The FD fan on a husk generator runs through the drying season, following the batches. A speed drive saves energy against a damper and keeps excess air under control, which matters because husk furnaces often run with far more air than they need. Checking oxygen at the furnace outlet occasionally and adjusting the air settings saves both husk and fan power.
ID fan and FD fan in husk fired hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the heat exchanger and cyclone | Beside the husk furnace |
| What it handles | Flue gas with fine silica and char | Clean ambient air |
| Temperature at the fan | 180 to 250°C | Ambient |
| Main design concern | Silica erosion behind a cyclone | Grate or cyclone furnace air pressure |
| Control | Furnace draft | Dryer air temperature |
See also the Induced Draft (ID) Fan for husk fired hot air generators.
Questions buyers ask
What is the FD fan on a husk fired hot air generator?
It is the combustion air fan of the husk furnace. On a step grate furnace it blows air under the grate and above the bed. On a cyclone furnace it supplies tangential air that keeps the husk burning in suspension, sometimes carrying the husk with it. It handles clean air in both cases.
Should rice husk be blown through the FD fan into the furnace?
It is better not to. Husk passing through the fan wears the blades quickly. Feeding husk into the air duct after the fan, through a rotary valve or an ejector, keeps the fan on clean air while still conveying the husk pneumatically into the furnace.
Why does the combustion air fan on my husk furnace keep going out of balance?
Most often because husk dust and bran from the mill collect on the blades and the inlet screen. The build-up is uneven and grows until vibration rises and the bearings suffer. Taking the fan inlet from outside the building, fitting a cleanable screen and cleaning the impeller regularly solve this.
Information needed to quote an FD fan for husk fired hot air generators
- 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.
- Furnace type. Step grate or cyclone furnace, and how husk is fed.
- Air supply. Where the fan inlet draws from, and the required pressure at the furnace.
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.