How coal fired hybrid water and smoke tube boilers work
A hybrid boiler combines a water tube furnace, where the coal burns and radiant heat is absorbed in membrane or tube walls, with a smoke tube shell in which the gas makes one or two convective passes before leaving. Typical sizes are 1 to 10 TPH, fired on a fixed, chain or reciprocating grate with Indian or imported coal. Many have no air heater, so the gas leaves the shell or a small economiser at about 180 to 240°C. Dust collection is usually a cyclone or multiclone, increasingly followed by a bag filter, and some older units still use a wet scrubber. The FD fan blows ambient air under the grate and the ID fan draws gas through the smoke tubes and collector to the chimney.
Where the ID fan sits
- Water tube furnace
- Smoke tube passes
- Economiser (if fitted)
- Cyclone or multiclone
- Bag filter or scrubber
- ID fan
- Chimney
The fan is usually mounted on a small foundation next to the chimney, after the cyclone and any bag filter or scrubber. The suction it must develop includes the furnace, the smoke tube passes, the economiser and the collectors, typically 100 to 180 mmWC on a clean boiler and noticeably more when the tubes are choked. On packaged units with limited floor space, the fan is often belt driven and placed close to the collector outlet.
Gas at the ID fan
| Gas temperature, normal | 180 to 240°C, often with no air heater |
| Maximum or upset | Up to 280°C with heavily fouled smoke tubes |
| Dust at the fan | Fine coal ash; 30 to 100 mg/Nm³ after a bag filter, 300 mg/Nm³ or more after a cyclone alone |
| Moisture and dew point | Dew point about 45 to 55°C dry; saturated at 55 to 65°C after a wet scrubber |
| Corrosive species | Dilute sulphurous acid in the gas after a wet scrubber |
| Density at the design temperature | About 0.73 kg/m³ at 210°C, 61% 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 coal fired hybrid water and smoke tube boilers
| Blade form | Backward curved behind a bag filter; radial or radial tipped after a cyclone or wet scrubber |
| Material of construction | Mild steel for dry gas; SS 316 or rubber-lined impeller and casing after a wet scrubber |
| Drive and arrangement | Belt driven, arrangement 1 or 3, on small units; direct coupled above about 30 kW |
| Bearings and cooling | Pedestal bearings outside the casing with a cooling disc |
| Flow control | Variable speed drive on furnace draft; damper on the smallest units |
| Also specified | Casing drain, inspection door, and access for washing where a scrubber is upstream |
What this ID fan has to cope with
Smoke tube choking
Fly ash and soot settle in the smoke tubes, and resistance rises steadily between cleanings. Operators then open the ID damper fully and still lose draft. A fan selected with 20 to 30% pressure margin over the clean condition, on a variable speed drive, keeps the boiler on load until the next scheduled tube cleaning.
Corrosion after a wet scrubber
Where a wet scrubber is used for dust, the gas leaves saturated and slightly acidic. Mild steel impellers corrode and collect wet ash, and unbalance follows. An SS 316 or rubber-lined fan with a casing drain, or replacing the scrubber with a bag filter, removes the problem.
High fan inlet temperature without an air heater
With no air heater, the gas reaches the fan at 200°C or more, so it is less dense and the fan must move more volume for the same mass flow. Bearings also run hotter. The fan is selected at the actual inlet temperature with a shaft cooling disc.
Energy and control
Hybrid boilers in dyeing and food units usually follow batch demand, with steam draw rising and falling through the day. A variable speed ID fan on furnace draft saves energy at part load and compensates gradually as smoke tubes foul. Adding an economiser, where the gas leaves above about 220°C, lowers the gas volume at the fan and recovers fuel heat at the same time.
ID fan and FD fan in coal fired hybrid water and smoke tube boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, bag filter or scrubber | Beside the grate, feeding the plenum |
| What it handles | Flue gas with fine coal ash, sometimes wet | Clean ambient air |
| Temperature at the fan | 180 to 240°C, often no air heater | Ambient, 25 to 45°C |
| Main design concern | Margin for smoke tube fouling | Stable air at modest turndown |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for coal fired hybrid water and smoke tube boilers.
Questions buyers ask
Why does my hybrid boiler lose draft after a few weeks?
The smoke tubes collect soot and fly ash, and their resistance rises until the ID fan cannot hold furnace draft. Regular tube cleaning restores it. If the boiler loses draft quickly, check the coal ash, the cyclone performance and whether the fan has pressure margin over the clean boiler condition.
Can an ID fan run after a wet scrubber on a coal fired hybrid boiler?
Yes, but the gas is saturated and corrosive, so the fan should be in stainless steel or rubber lined, with a drain at the lowest point of the casing. A mild steel induced draft fan in this duty usually corrodes and goes out of balance within a year or two.
What ID fan size suits a 4 TPH hybrid boiler?
It depends on the coal, excess air, gas temperature and collector. Typical duties are a few thousand cubic metres per hour at 120 to 180 mmWC, but the right selection needs the boiler data. Send us the boiler details or the existing fan nameplate and we will calculate it.
Information needed to quote an ID fan for coal fired hybrid water and 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. How often the smoke tubes are cleaned and the draft observed before and after cleaning.
- Collector. Cyclone, bag filter or wet scrubber, and whether the gas is wet at the 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.