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 FD fan sits
- Inlet screen
- FD fan
- Air duct
- Grate plenum
- Fuel bed
The fan is usually mounted on the boiler skid or on the floor beside the grate, discharging directly into the undergrate plenum. With no air heater, the path is short and the pressure needed is mostly the grate and bed, typically 50 to 100 mmWC. Some designs split the air between a forward and rear zone with manual dampers, and a few add a small overfire air fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the grate | Ambient; no air heater on most units |
| Dust | Clean air; coal dust near the bunker |
| Pressure at the fan outlet | Typically 50 to 100 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 coal fired hybrid water and smoke tube boilers
| Blade form | Backward curved, compact single inlet |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft, arrangement 4 |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper, linked to the steam pressure switch or controller |
| Also specified | Inlet screen and a guard; low noise selection where the boiler house is near work areas |
What this FD fan has to cope with
On-off firing and cold air
Small hybrid boilers often run the FD fan on-off with steam pressure. Every stop lets the bed cool and every restart pushes cold air through, causing smoke and clinkers. A variable speed drive that modulates the air keeps the bed alive and reduces cycling of the coal feed.
Over-sized fans from rule-of-thumb selection
Packaged boilers are sometimes supplied with FD fans larger than needed, and the damper is kept nearly closed. The fan runs inefficiently and noisily. Selection on calculated air at the actual excess oxygen gives a smaller, quieter fan. Where an existing fan is oversized, a smaller impeller or a speed change often corrects it without replacing the casing.
Coal dust at the inlet
In compact boiler houses the FD fan inlet is close to the coal stack. Fine coal dust builds on the blades and causes unbalance. Taking the inlet from a clean area, or fitting a screen, prevents it. A cleaning check at each boiler inspection keeps the fan smooth.
Energy and control
Hybrid boilers rarely run at steady full load; dyeing and food plants draw steam in batches. A variable speed FD fan matched to steam pressure saves power and, more importantly, keeps excess air under control so the coal burns efficiently. Many small boilers run at 10 to 12% oxygen because the air is never adjusted; bringing this down to about 7 to 9% improves fuel efficiency noticeably.
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 Induced Draft (ID) Fan for coal fired hybrid water and smoke tube boilers.
Questions buyers ask
Does a small hybrid boiler need a variable speed FD fan?
It is not essential, but it helps. A variable speed forced draft fan lets the combustion air follow steam demand smoothly instead of switching on and off, which keeps the bed stable and reduces excess air. On boilers with frequent load swings the fuel saving is usually larger than the power saving.
What pressure does the FD fan need on a coal fired hybrid boiler?
Typically 50 to 100 mmWC, depending on grate type and bed depth. With no air heater in the path, the fan sees only the duct, dampers, plenum and fuel bed. The grate supplier's plenum pressure is the starting point for the selection. Margin is added for a deeper bed and for wear in the grate seals.
Can the FD fan be mounted on the boiler skid?
Yes, on smaller units this is common. The fan should be on its own anti-vibration mounts with a flexible connection to the plenum, so that fan vibration does not pass into the boiler structure and the grate seals. The motor terminal box and cable must be clear of hot surfaces and ash handling.
Information needed to quote an FD fan for coal fired hybrid water and smoke tube boilers
- 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.
- Firing control. Whether the boiler runs on-off or modulating firing, and the controller used.
- Grate. Fixed, chain or reciprocating grate and the plenum pressure at full load.
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.