How wood log fired fixed grate boilers work
Wood log fired fixed grate boilers are small units, typically 0.5 to 6 TPH, used by textile dyeing and processing units, food plants and laundries. Split logs or firewood are loaded by hand through a firing door onto a fixed grate in a refractory furnace or a large furnace tube. Combustion is cyclic: each charge flares, burns down and smoulders until the next. Gas passes through the smoke tubes of a shell boiler, or a small water tube bank, and leaves at about 220 to 300°C, as most units have no economiser or air heater. Wood ash is low, but soot, tar and fine ash collect in the tubes. A cyclone, and sometimes a wet scrubber, precedes the ID fan.
Where the ID fan sits
- Fixed grate furnace
- Smoke tubes
- Smoke box
- Cyclone
- ID fan
- Chimney
The fan stands beside the chimney after a cyclone, with suction typically 60 to 120 mmWC. Each time the firing door opens, a large volume of cold air enters and the furnace draft collapses; the fan must recover draft quickly once the door closes. Between charges the logs smoulder at low air, producing soot and tarry vapour that can condense on cooler surfaces of the fan and duct.
Gas at the ID fan
| Gas temperature, normal | 220 to 280°C without economiser |
| Maximum or upset | Up to 320°C during a fresh charge flare |
| Dust at the fan | Fine ash, soot and some char after a cyclone |
| Deposits | Soot and tar from smouldering logs, especially at low load |
| Moisture and dew point | Moderate on seasoned logs; higher on green wood |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 wood log fired fixed grate boilers
| Blade form | Radial or radial tipped, self-cleaning against soot and tar |
| Material of construction | Mild steel casing and impeller; heat-resisting steel only if gas exceeds about 350°C |
| Drive and arrangement | Belt driven, arrangement 3 or 9 |
| Bearings and cooling | Plummer block bearings with cooling disc and heat shield |
| Flow control | Inlet damper or a small variable speed drive |
| Also specified | Access door for cleaning, casing drain, generous bearing span for hot gas |
What this ID fan has to cope with
Soot and tar build-up
Smouldering logs between charges release soot and tar vapour that condense on cooler fan surfaces, especially at low load and after shutdown. The deposits are uneven and cause vibration. Radial blades that shed deposits, a casing access door, and cleaning at each weekly stop keep the fan in balance.
Draft collapse at each charge
Opening the firing door floods the furnace with cold air. A fan with too little margin cannot hold draft, smoke spills out of the door into the boiler house. Sizing for the open-door condition, or adding a door interlock that briefly slows the FD fan, keeps smoke inside.
Heat at the bearings
At 250 to 300°C, small overhung fans conduct heat along the shaft into the bearings, and grease breaks down. A cooling disc, a heat shield between casing and bearing pedestal, and enough bearing span prevent the bearing failures that are common on undersized log-fired boiler fans, where bearings are often the first thing to fail.
Energy and control
Log-fired boilers usually run one or two shifts with an uneven firing cycle. Because the fan is small, energy savings come mainly from not oversizing it and from keeping the smoke tubes clean, which reduces resistance and exit temperature together. Where a variable speed drive is fitted, it should follow furnace draft so it eases off between charges.
ID fan and FD fan in wood log fired fixed grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclone, before chimney | Under the grate |
| What it handles | Hot gas with soot and fine ash | Ambient air |
| Temperature at the fan | 220 to 280°C | Ambient, 25 to 45°C |
| Main design concern | Heat, soot and draft recovery at charging | Interlocks and excess air |
| Usual drive | Belt driven | Motor mounted |
See also the Forced Draft (FD) Fan for wood log fired fixed grate boilers.
Questions buyers ask
Why does smoke come out of the firing door when we charge logs?
Opening the door lets in a large volume of cold air, and the ID fan cannot hold negative pressure against it. Either the fan has too little capacity for the open-door condition, or the FD fan keeps blowing air in. Checking both usually solves it.
What ID fan is used on a wood log fired boiler?
Usually a belt-driven radial ID fan designed for about 220 to 280°C, handling soot and fine ash after a cyclone. It needs a cooling disc, heat shield and access door for cleaning, and enough capacity to recover draft after the firing door has been opened. Belt drive lets the speed be set to the boiler on site.
How often should the ID fan be cleaned on a wood fired boiler?
It depends on how much the logs smoulder and whether the wood is seasoned or green. Many plants clean the fan and smoke tubes weekly. Rising vibration, or falling draft between cleanings, shows that deposits are building faster, and the interval should then be shortened or the firing practice changed.
Information needed to quote an ID fan for wood log fired fixed grate 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.
- Firing pattern. How often logs are charged and whether wood is seasoned or green.
- Collector. Cyclone, multiclone or wet scrubber after the boiler.
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.