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 FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Grate plenum or bed distributor
- Husk bed
- Furnace
The fan pushes ambient air through the air preheater, where it heats to 150 to 220°C, then into the grate plenum or, on fluidised bed heaters, through the air distributor into the sand bed. A separate secondary air fan supplies air above the grate, and a pneumatic feeding blower may also carry husk into the furnace. On grate heaters the FD fan works at about 80 to 150 mmWC; on fluidised bed heaters the duty rises to 700 to 1,000 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 220°C, downstream of the fan |
| Dust | Husk dust in the plant air; inlet screen needed |
| Pressure at the fan outlet | 80 to 150 mmWC on grates; 700 to 1,000 mmWC on fluidised beds |
| Air split | A large share of air is secondary, because husk burns in suspension |
| 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 rice husk fired thermic fluid heaters
| Blade form | Backward curved; high pressure backward curved for fluidised beds |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on oil outlet temperature |
| Also specified | Inlet screen against husk, inlet silencer |
What this FD fan has to cope with
Blowing husk off the grate
Husk is very light, and too much undergrate air lifts it off the grate before it burns, raising unburnt char in the ash and the dust load on the ID fan. A fan with fine control and the right air split keeps the bed in place.
Husk dust on the impeller
Rice mills and parboiling plants are dusty, and husk fibre collects on unscreened impellers and in the air preheater. The fan goes out of balance and the preheater loses performance. A screen, and an intake placed away from the husk store and the feeding area, keep the fan in balance.
Clinker on fluidised beds
On fluidised bed heaters, too little air or uneven distribution lets the sand bed sinter with husk ash, and the heater must stop for bed replacement. The primary air fan must hold bed pressure steadily across the load range, with margin for distributor nozzle fouling.
Energy and control
A variable speed drive on the FD fan, linked to oil outlet temperature, follows process demand through the milling season and keeps excess air under control. Getting the split between undergrate and secondary air right reduces char in the ash, saves husk and cuts the dust load on the ID fan, as well as saving fan power.
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 Induced Draft (ID) Fan for rice husk fired thermic fluid heaters.
Questions buyers ask
How much air does the FD fan supply on a husk fired thermic fluid heater?
Only part of the total. Husk burns largely in suspension, so a secondary air fan supplies much of the air above the grate, and a pneumatic feed blower may add some more. The FD fan supplies the undergrate or bed air, and its share depends on the grate design.
Does the husk thermic fluid heater FD fan handle hot air?
No. It pushes ambient air into the air preheater, and the air is heated downstream before it reaches the grate or bed. The fan is therefore selected at ambient density in mild steel, which keeps it smaller and simpler than the ID fan on the same heater.
What FD fan is used on a fluidised bed husk thermic fluid heater?
A high pressure primary air fan, typically 700 to 1,000 mmWC, to pass the distributor and fluidise the sand bed, with a separate secondary air fan above the bed. On a fluidised bed husk thermic fluid heater the primary air fan is often the largest motor on the unit.
Information needed to quote an FD fan for rice husk fired thermic fluid heaters
- 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.
- Firing system. Step grate, reciprocating grate or fluidised bed, and husk firing rate.
- Air split. Share of undergrate or bed air and secondary air.
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.