How solid fuel fired coil steam generators work
A solid fuel coil steam generator is a compact, once-through unit: feedwater is pumped through a helical tube coil wrapped around or above a small furnace, and steam leaves the coil without a large drum. The furnace is usually a fixed or reciprocating grate burning firewood, wood chips, briquettes or coal, with outputs of about 0.5 to 3 TPH. Because the water content is small, the unit raises steam in minutes rather than hours, which suits food, dairy and laundry users with intermittent demand. The flue gas leaves the coil at about 220 to 300°C, often passes an economiser coil, then a cyclone or multiclone, the ID fan and a short chimney. An FD fan supplies air under the grate.
Where the FD fan sits
- Inlet guard
- FD fan
- Air damper
- Undergrate plenum
- Grate and fuel bed
- Overfire nozzles
The fan is mounted on the unit skid or on the floor beside the furnace and blows ambient air into the plenum under the grate. A branch with its own damper may feed overfire nozzles above the bed. On these small units there is rarely an air heater, so the fan works only against the duct, damper, grate and fuel bed, typically 40 to 120 mmWC, with the higher figures for a thick coal or briquette bed with clinker.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air under the grate | Ambient; no air preheating on most units of this size |
| Dust | Clean air, but wood dust and sawdust around the fuel yard can be drawn in without a guard |
| Bed resistance | Rises with clinker and ash between cleanings; higher for coal and briquettes than for wood logs |
| Pressure at the fan outlet | Typically 40 to 120 mmWC |
| 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 solid fuel fired coil steam generators
| Blade form | Backward curved, for a stable pressure curve and quiet running on clean air |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 4, on the unit skid |
| Bearings and cooling | Motor bearings only on arrangement 4; no cooling needed |
| Flow control | Variable speed drive or damper from steam pressure, interlocked with the ID fan |
| Also specified | Inlet guard or mesh, and a split to overfire air with its own damper where fitted |
What this FD fan has to cope with
Clinker and uneven air
As clinker forms on a fixed grate, air takes the easy path through thin areas, leaving dead zones and raising unburnt carbon. A fan with a steep enough pressure curve keeps flow steady as resistance rises, and regular de-ashing restores an even bed.
Positive furnace on load swings
On a fast-responding coil unit the FD fan can increase air faster than the ID fan clears the gas, pushing the furnace positive. Interlocking the FD fan to follow the ID fan, and limiting its ramp rate, prevents puffing at the fuel door.
Excess air at low load
A fixed-speed FD fan with a part-closed damper still delivers more air than the fuel needs at low firing, which cools the coil, lowers steam output per kilogram of wood and raises stack loss. Speed control from steam pressure, checked against flue gas oxygen at the economiser outlet, keeps excess air within a sensible band across the load range.
Energy and control
A coil steam generator follows demand closely, often swinging from full output to idle within minutes. The FD fan on a variable speed drive tracks firing rate and drops to minimum speed when the unit idles, saving power and avoiding the fuel loss of cold excess air through the coil. On manual firing, a simple two-speed scheme can give much of the same benefit.
ID fan and FD fan in solid fuel fired coil steam generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | Beside the furnace, under the grate |
| What it handles | Flue gas with soot and fly ash | Clean ambient air |
| Temperature at the fan | 200 to 260°C, up to 350°C fouled | Ambient, 25 to 45°C |
| Main design concern | Coil fouling and start-stop cycling | Bed resistance and fast load changes |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for solid fuel fired coil steam generators.
Questions buyers ask
What size FD fan is needed for a 1 TPH wood fired steam generator?
The air quantity follows from the fuel burnt and the excess air, typically 40 to 60% on a fixed grate. The pressure depends on the grate and bed. A typical duty is a few thousand m³/h at under 100 mmWC, but we confirm it from the fuel and grate data.
Should the FD fan on a coil steam generator have a VFD?
It is worthwhile where the unit modulates or idles for long periods, which is common on coil steam generators serving batch processes. The drive reduces power, gives better air-fuel control at low fire and allows a soft start for frequent starts. On very small, steadily loaded units a simple damper may be enough.
Why does the forced draft fan start after the ID fan?
Starting the ID fan first puts the furnace under suction and purges it, so fuel gases are drawn away from the fuel door. Starting the FD fan first would pressurise a furnace that may contain smouldering fuel and push smoke and flame into the boiler house.
Information needed to quote an FD fan for solid fuel fired coil steam generators
- 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.
- Grate type. Fixed, reciprocating or chain grate, with grate area if known.
- Overfire air. Whether the FD fan also supplies overfire air, and the split.
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.