How rice husk fixed grate smoke tube boilers work
Fixed grate smoke tube boilers are the workhorse of small rice mills and parboiling units, usually 1 to 6 TPH at 10 to 17.5 bar. Husk is fed by screw, by pneumatic blower or by hand onto a stationary grate in an external furnace or a large furnace tube. The hot gas then makes two or three passes through the smoke tubes inside the shell. Many units have no economiser or air heater, so gas leaves the boiler at 220 to 280°C. Ash is raked off the grate periodically, and fly ash collects in the tubes and the smoke box. A cyclone or multiclone, sometimes followed by a wet scrubber, cleans the gas before the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Damper
- Ash pit under grate
- Fixed grate and husk bed
- Furnace
The fan is usually mounted at the side or front of the boiler and blows into the closed ash pit under the fixed grate. Air passes up through the grate bars into the husk bed. Some units add a small secondary air supply through ports above the bed from the same fan or a separate one. Pressure is low, typically 40 to 100 mmWC, because the bed is thin and there is no air heater on most smoke tube units.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | About 40 to 100 mmWC |
| Preheat | Usually none; a few units have a small air heater after the boiler |
| Dust | Husk dust from the feeding area is common at the inlet |
| Duty variation | Bed depth changes with feeding and raking |
| 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 fixed grate smoke tube boilers
| Blade form | Backward curved or radial, compact, often motor-mounted |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft, arrangement 4 |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Outlet or inlet damper, or a small variable speed drive interlocked with the ID fan |
| Also specified | Inlet screen; interlock so the FD fan stops if the ID fan trips |
What this FD fan has to cope with
Positive furnace when the ID fan trips
If the FD fan keeps running after the ID fan stops, the small furnace pressurises within seconds and flame and ash blow back through the firing door and sight holes. An electrical interlock between the two fans, so the FD fan trips with the ID fan and cannot start before it, is essential on these boilers.
Too much air on a thin bed
After raking, the bed is thin and the same air flow lifts husk off the grate, raises excess air and increases carry-over into the smoke tubes and the ID fan. Reducing FD air during and after raking, by damper or drive, keeps combustion steady and reduces the ash reaching the tubes.
Husk drawn into the inlet
Fans placed near the husk hopper or feeding screw ingest fibre, which clogs the inlet and builds up unevenly on the wheel, causing vibration and bearing wear on a small motor-mounted fan. An inlet screen that is easy to clean, or a short duct to a clean area away from the husk handling, prevents it.
Energy and control
The FD fan on a small smoke tube husk boiler is a small motor, but it runs continuously and is often left at full air regardless of load. A simple variable speed drive, or a damper set by steam pressure, lowers excess air, cuts the gas volume the ID fan must move and improves boiler efficiency. Measuring oxygen at the boiler outlet occasionally shows whether air is being wasted.
ID fan and FD fan in rice husk fixed grate smoke tube boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclone, before chimney | Under the grate, ambient intake |
| What it handles | Flue gas with silica ash | Ambient air |
| Temperature at the fan | 220 to 260°C | Ambient, 25 to 45°C |
| Main design concern | Heat, dust and tube fouling margin | Matching air to bed depth, interlock |
| Usual drive | Belt driven | Motor mounted |
| Control | Damper or VFD on draft | Damper or VFD on steam pressure |
See also the Induced Draft (ID) Fan for rice husk fixed grate smoke tube boilers.
Questions buyers ask
Does a small rice husk smoke tube boiler need an FD fan?
Most do. A fixed grate with a husk bed needs forced undergrate air to burn the fuel at the rating, and natural draft alone is rarely enough. The FD fan is small, typically 40 to 100 mmWC, and blows into the ash pit under the grate.
Why must the FD and ID fans be interlocked?
If the FD fan runs without the ID fan, the furnace becomes positive and hot gas and ash blow out of the firing door and sight glasses, which is a direct hazard to the fireman. The interlock stops the FD fan whenever the ID fan trips and prevents the FD fan starting until the ID fan is running.
Should the FD fan on a smoke tube boiler be direct driven?
Usually yes. It handles ambient air at low pressure, so a compact fan mounted directly on the motor shaft is simple, quiet and reliable, with no belts to adjust. Speed control, where used, is by a small variable speed drive, which also helps match air to the husk feed rate on single-shift operation.
Information needed to quote an FD fan for rice husk fixed grate smoke tube boilers
- 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.
- Feeding method. Whether husk is fed by screw, pneumatic blower or by hand, and the raking interval.
- Secondary air. Whether overfire air ports are fed from the FD fan or a separate fan.
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.