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 ID fan sits
- Grate furnace
- Steam coil
- Economiser coil
- Cyclone or multiclone
- ID fan
- Chimney
The fan usually stands on the floor beside the cyclone, close-coupled to the unit, with a short duct to the chimney. Upstream it sees the furnace draft of a few mmWC, the resistance of the closely spaced coil turns, the economiser and the cyclone, typically 80 to 150 mmWC in total. Because the coil passages are narrow, soot build-up raises this resistance week by week, so the fan is selected with margin for a fouled coil rather than for a clean one.
Gas at the ID fan
| Gas temperature, normal | 200 to 260°C after the economiser coil |
| Maximum or upset | 300 to 350°C with a sooted coil or no economiser |
| Dust at the fan | Fly ash and soot after a cyclone, typically 0.3 to 1 g/Nm³; coarser char with wet wood |
| Moisture and dew point | Wood at 25 to 40% moisture raises water vapour in the gas; water dew point is reached only if the fan runs on cold gas at start-up |
| Corrosive species | Low with wood; coal firing adds SO₂, which condenses on a cold casing during long idle periods |
| 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 solid fuel fired coil steam generators
| Blade form | Radial tipped or backward curved with thick blades, chosen for soot and fly ash after a cyclone |
| Material of construction | Mild steel throughout, with a wear-resistant plate on the blade leading edges for coal or husk briquettes |
| Drive and arrangement | Direct coupled on a common base, arrangement 4 on small units and arrangement 8 above about 15 kW |
| Bearings and cooling | Bearings outside the gas path with a cooling disc; no water cooling needed at these temperatures |
| Flow control | Variable speed drive tied to furnace draft, or an inlet damper on the smallest units |
| Also specified | Cleaning door in the casing, casing drain, and an interlock so that the ID fan starts before the FD fan |
What this ID fan has to cope with
Soot build-up in the coil
Smoky combustion during start-up and low firing coats the coil and economiser with soot. Draft at the furnace falls and the operator opens the damper fully, so the fan runs at the end of its curve. Selecting the fan for the fouled resistance and providing regular coil cleaning keeps the furnace under suction.
Thermal cycling from start-stop duty
The fan may be stopped and restarted several times a day as steam demand comes and goes. Repeated heating and cooling loosens welds and hub fits. A shrink-fitted and keyed hub, stress-relieved impeller and a fan kept running at low speed during short stops reduce this.
Smoke from the fuel door
When the fan is undersized or the coil is choked, the furnace goes positive each time the door opens for manual firing. The fan needs enough reserve to hold suction with the door open for a few seconds, which is a larger flow than the steady duty.
Energy and control
Coil steam generators spend much of the day modulating or idling between demands. An ID fan on a variable speed drive, following furnace draft, falls to low speed during idle periods instead of running at full speed against a closed damper. On units of this size the absolute saving is small, but the drive also gives a soft start, which suits the frequent starts typical of this plant.
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 Forced Draft (FD) Fan for solid fuel fired coil steam generators.
Questions buyers ask
Does a solid fuel coil steam generator need an ID fan?
Yes, in nearly all cases. The narrow coil passages, an economiser and a cyclone add more resistance than a short chimney can overcome, and the furnace must stay under suction for safe manual or automatic firing. The ID fan provides that suction. It also lets the unit respond quickly when steam demand returns after an idle period.
Why does the ID fan on my coil type boiler draw less current when the coil is dirty?
A sooted coil adds resistance, so the fan moves less gas at a higher pressure. On most fan curves power falls as flow falls, so current drops while draft is lost. Low current with poor draft is a sign the coil or cyclone needs cleaning.
What temperature should an ID fan be designed for on a wood fired steam generator?
Typically for the normal gas temperature after the economiser, around 200 to 260°C, with the impeller stress checked at the upset temperature with a sooted coil, often 300 to 350°C. Mild steel is suitable for this range. Bearings outside the gas path with a cooling disc need no water cooling.
Information needed to quote an ID fan for solid fuel fired coil steam generators
- 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.
- Fuel and firing. Fuel type and moisture, and whether firing is manual or by an automatic feeder.
- Operating cycle. Number of starts per day and typical idle periods between steam demands.
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.