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 ID fan sits
- Husk furnace
- Settling chamber
- Tube heat exchanger
- Cyclone
- ID fan
- Chimney
On an indirect generator the fan sits after a cyclone at the outlet of the heat exchanger. Most rice mill installations have only a settling chamber and a cyclone, so the fan sees a much heavier silica load than a boiler ID fan behind a bag filter. Suction covers the furnace, settling chamber, exchanger tubes and cyclone, typically 80 to 160 mmWC.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after the heat exchanger |
| Maximum or upset | Up to 350°C with ash-choked exchanger tubes |
| Dust at the fan | Fine angular silica and char after a cyclone, often 1 to 3 g/Nm³ |
| Moisture and dew point | Dry husk; water dew point well below the gas temperature |
| Fire risk | Glowing husk char in the gas and the cyclone |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 husk fired hot air generators
| Blade form | Radial tipped with replaceable liners, the robust choice behind a cyclone only |
| Material of construction | Wear-resistant plate impeller and liners; mild steel casing with lined scroll |
| Drive and arrangement | Belt driven or direct coupled arrangement 8, impeller speed kept low |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive or inlet damper on furnace draft |
| Also specified | Bolted scroll liners, inspection door and casing drain |
What this ID fan has to cope with
Silica erosion behind a cyclone
With only a cyclone in front, the fan receives grams of fine silica per cubic metre. Unlined mild steel blades can be worn through within a season. A radial tipped wheel in wear-resistant plate, bolted liners and a low tip speed keep the fan running through the paddy season. A multicyclone in place of a single cyclone also helps.
Ash choking the exchanger
Husk ash collects in the exchanger tubes, raising the resistance and the gas temperature at the fan. The fan loses suction and the furnace pressurises. Rapping or blowing the tubes regularly, and selecting the fan with pressure margin for partly fouled tubes, keep the furnace under suction.
Unbalance from erosion
Silica wears the blades unevenly, so vibration rises gradually through the season rather than suddenly. Vibration checks every few weeks on the bearing housings, and balancing at the end of each season along with liner replacement, prevent bearing failures in the middle of a crop when the dryer cannot be stopped for long.
Energy and control
Paddy drying is seasonal and batch-driven, and the generator runs hard when the dryer is full and at low fire while it loads and unloads. A variable speed ID fan following furnace draft saves energy through these swings. Keeping the exchanger tubes clean reduces both gas temperature and fan resistance, which lowers fan power and raises the heat delivered to the dryer air.
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 Forced Draft (FD) Fan for husk fired hot air generators.
Questions buyers ask
How long does the ID fan impeller last on a husk fired hot air generator?
It depends on the dust reaching the fan. Behind a single cyclone, unlined mild steel blades may last only one paddy season. A radial tipped wheel in wear-resistant plate with replaceable liners, at low tip speed, typically runs several seasons with liner changes between them, and a multicyclone in front extends this further.
Can a husk fired hot air generator be used directly for drying paddy?
Yes, with a settling chamber and cyclone to remove most of the ash, and dilution air to bring the gas to drying temperature. Many mills prefer an indirect exchanger so the paddy stays free of ash. The fan duty changes: direct units pull dilute gas through the dryer, indirect units pull flue gas only.
Why does the furnace of a husk hot air generator go positive after a few days?
Usually because ash has built up in the heat exchanger tubes and cyclone, raising the resistance until the ID fan cannot hold suction. Cleaning the tubes restores draft. If it happens too quickly, the fan may be undersized or worn, and its pressure should be checked against the clean and fouled system.
Information needed to quote an ID fan for husk fired hot air generators
- 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.
- Firing system. Step grate or cyclone furnace, and the husk firing rate.
- Collection. Settling chamber, cyclone or multicyclone in front of the fan, and present wear life.
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.