How rice husk fired thermic fluid heaters work
Rice husk fired thermic fluid heaters are common in rice mills, parboiling plants, solvent extraction units and food processors close to husk supply. Husk is fed by screw or pneumatically onto an inclined step grate, a reciprocating grate or into a small fluidised bed, and much of it burns in suspension. It has a calorific value of about 3,000 to 3,400 kcal/kg and 18 to 22% ash that is almost pure silica. The flue gas leaves the coil at 330 to 380°C, passes an air preheater, and goes through a cyclone or multicyclone, with a bag filter on newer units, to the ID fan and chimney. The FD fan supplies grate or bed air and a separate blower often handles pneumatic husk feeding.
Where the ID fan sits
- Husk feeder and grate
- Radiant and convective coils
- Air preheater
- Multicyclone
- Bag filter where fitted
- ID fan
- Chimney
On most heaters the fan follows a multicyclone, and many small units have no bag filter, so the fan handles hundreds of mg/Nm³ to a few g/Nm³ of fine silica. Gas reaches it at about 200 to 240°C with the air preheater working, and well above 300°C when the preheater is bypassed or fouled. Suction is typically 100 to 180 mmWC on heaters of 1 to 6 million kcal/h, made up of furnace draft, the coils, the air preheater and the multicyclone.
Gas at the ID fan
| Gas temperature, normal | 200 to 240°C after the air preheater |
| Maximum or upset | 300 to 360°C with preheater bypass or fouled coil |
| Dust at the fan | Fine, angular silica ash and some char; several hundred mg/Nm³ to a few g/Nm³ after a multicyclone |
| Moisture and dew point | Husk moisture of 8 to 12%; water dew point well below fan temperature |
| Corrosive species | Low sulphur; minor chlorides from some husk |
| 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 rice husk fired thermic fluid heaters
| Blade form | Radial tipped with replaceable liners after a multicyclone; backward curved with liners after a bag filter |
| Material of construction | Mild steel casing with scroll liners; impeller in wear-resistant plate suited to the operating temperature, or hard-faced mild steel |
| Drive and arrangement | Direct coupled or belt driven, at moderate speed to limit tip speed |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Bolted wear liners, inspection door, and backup power for the fan and oil pump |
What this ID fan has to cope with
Wear plate softening at heater temperatures
Quenched wear-resistant plates lose hardness if they run for long periods above about 250°C, and husk thermic fluid heaters can reach that when the air preheater is bypassed. Using a wear grade rated for the temperature, or hard-facing on a heat-tolerant base plate, keeps the liner working in the hot case.
Silica erosion after a multicyclone
A multicyclone removes coarse ash but passes much of the fine silica, which wears the blade leading edges and the scroll near the cut-off. Lower tip speed, radial tipped blades with replaceable liners, and a scheduled liner change keep the fan running through the season.
Glowing char and air preheater fires
Unburnt husk char can carry over glowing and lodge in the air preheater or fan. Deposits that smoulder can damage the casing and bag filter. Good overfire air, a spark arrestor and a cleanout door on the fan reduce the risk.
Power failure with hot oil
If power fails, the oil circulation pump stops while the furnace and refractory are still hot, and oil in the coil can overheat and crack. Husk heaters with a fluidised bed hold even more stored heat. The ID fan and pump on backup power let the furnace be cooled while the oil keeps moving.
Energy and control
Husk heaters in rice mills run with the milling season and follow parboiling or drying demand, often at part load. A variable speed drive on draft saves power and, by lowering speed, also reduces erosion. Keeping the air preheater clean holds the fan gas temperature down, cuts the gas volume and protects the wear materials.
ID fan and FD fan in rice husk fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone or bag filter, before the chimney | Before the air preheater, under the grate or bed |
| What it handles | Flue gas with abrasive silica ash | Ambient air |
| Temperature at the fan | 200 to 240°C, above 300°C upset | Ambient |
| Main design concern | Erosion at elevated temperature | Air split on a light husk bed |
| Control | Furnace draft | Oil outlet temperature |
See also the Forced Draft (FD) Fan for rice husk fired thermic fluid heaters.
Questions buyers ask
Why does the ID fan wear out so fast on a rice husk thermic fluid heater?
Husk ash is almost pure silica and is very abrasive. Most heaters use a multicyclone, which lets fine silica reach the fan, and the gas is hotter than on a husk boiler. Wear plate, liners, lower tip speed and a bag filter all extend fan life.
What temperature does a husk fired thermic fluid heater ID fan see?
About 200 to 240°C with the air preheater working, and above 300°C when the preheater is bypassed, fouled or leaking. That is noticeably hotter than on a husk fired steam boiler of similar size. Wear materials, shaft cooling and bearings are selected for the hotter case, not only the normal one.
Is a radial or backward curved ID fan better for a husk thermic fluid heater?
After a multicyclone, radial tipped blades with replaceable liners last longer and tolerate wear without large loss of performance. After a bag filter, a lined backward curved fan is more efficient. The choice follows the dust collector and the measured dust at the fan inlet.
Information needed to quote an ID fan for rice husk fired thermic fluid heaters
- 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.
- Dust collector. Multicyclone, bag filter or both, and the dust measured at the fan inlet.
- Wear history. Life of the present impeller and liners and photographs of worn parts.
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.