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 ID fan sits
- Fixed grate furnace
- Smoke tube passes
- Smoke box
- Cyclone or multiclone
- ID fan
- Chimney
The fan usually stands at ground level beside the chimney, after the cyclone or multiclone. Upstream are the furnace, two or three tube passes and the collector, with a total suction of about 80 to 150 mmWC when clean. As husk ash builds up in the smoke tubes this can rise by a third or more before the weekly or daily tube cleaning. With only a cyclone ahead of it, the fan handles a substantial silica load.
Gas at the ID fan
| Gas temperature, normal | 220 to 260°C without economiser |
| Maximum or upset | Up to 300°C with choked tubes or overfiring |
| Dust at the fan | Silica ash and char after a cyclone, typically 0.5 to 2 g/Nm³ |
| With a wet scrubber | Saturated gas at 60 to 70°C with water droplets and wet silica slurry |
| Draft variation | Swings when the firing door is opened or the grate is raked |
| Density at the design temperature | About 0.69 kg/m³ at 240°C, 58% 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 fixed grate smoke tube boilers
| Blade form | Radial or radial tipped, open or shrouded, tolerant of silica dust |
| Material of construction | Mild steel casing; impeller blades in wear-resistant plate or mild steel with hard-facing; stainless or rubber lining after a wet scrubber |
| Drive and arrangement | Belt driven, arrangement 3 or 9, so speed can be trimmed by changing pulleys |
| Bearings and cooling | Plummer block bearings outside the casing with a cooling disc for 240°C gas |
| Flow control | Inlet damper, or a variable speed drive on larger units |
| Also specified | Generous pressure margin for tube fouling; casing drain; heat shield between casing and bearings |
What this ID fan has to cope with
Smoke tubes choking with husk ash
Fine silica and char deposit in the tubes, raising resistance and exit temperature together. A fan selected exactly on the clean duty runs out of draft before cleaning is due, and the furnace turns positive. We add pressure margin for the fouled condition and check the motor against the higher hot-gas flow.
High gas temperature on a small fan
At 250°C and above, an overhung belt-driven fan needs a cooling disc, adequate bearing spacing and a heat shield between casing and bearings. Bearing failures on these small units usually trace to heat soaking along the shaft, not to load. We design the shaft, bearing span and cooling for the actual gas temperature, including the choked-tube peak.
Scrubber carry-over
Where a wet scrubber is added for emissions, water droplets and silica slurry reach the fan, which then corrodes and builds up mud on the blades until it vibrates. A mist eliminator after the scrubber, a casing drain at the lowest point, and stainless steel or rubber-lined construction suit this wet, gritty duty.
Cyclone-only dust
With no bag filter or ESP, the fan handles a steady load of silica and wears continuously rather than occasionally. Radial blades with replaceable liners, a moderate tip speed and a wheel that can be relined at site give a predictable service life, and a spare impeller kept in stock avoids long stoppages in the milling season.
Energy and control
Small rice mills often run the boiler on a single shift or to the parboiling batch cycle. A fan oversized for safety and throttled by a damper wastes power for most of the day. Matching the pulley ratio to the real duty, keeping the tubes clean on a schedule, and fitting a small variable speed drive on units above about 3 TPH give practical savings without major changes.
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 Forced Draft (FD) Fan for rice husk fixed grate smoke tube boilers.
Questions buyers ask
Which ID fan is suitable for a 2 TPH rice husk smoke tube boiler?
Usually a radial or radial tipped ID fan, belt driven, designed for about 220 to 260°C and silica dust after a cyclone. The exact flow and pressure depend on excess air, tube passes and the collector. Send the boiler data or present fan nameplate and we will calculate it.
Why does the furnace go positive on our husk smoke tube boiler before tube cleaning?
Husk ash choking the smoke tubes raises resistance until the ID fan can no longer hold draft, and gas escapes at the firing door. Either the fan has too little pressure margin, or the cleaning interval is too long for the husk quality. Measuring suction at the smoke box before and after tube cleaning shows which of the two it is.
Can a smoke tube boiler ID fan be direct driven?
Yes, but most small units use belt drive because it allows the speed to be matched to the boiler after commissioning and keeps the motor away from the hot casing. Direct drive suits larger fans, or installations where a variable speed drive is used, since speed then no longer needs to be set by pulleys.
Information needed to quote an ID fan for rice husk fixed grate smoke tube boilers
- 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.
- Tube cleaning interval. How often the smoke tubes are cleaned, and furnace draft just before cleaning.
- Wet scrubber. Whether a wet scrubber is fitted or planned after the cyclone.
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.