How coal fired fixed grate smoke tube boilers work
Coal fired fixed grate smoke tube boilers are small shell boilers, usually 0.5 to 5 TPH, still used in dyeing, food processing and small process units. Coal is fed by hand or by a simple ram onto fixed cast iron grate bars inside the furnace tube or an external refractory furnace. Ash is raked off manually through the firing door, and the fire is cleaned every few hours. The flue gas passes through two or three passes of smoke tubes inside the shell, then usually leaves at 220 to 280°C, often with no economiser. A cyclone or multiclone, and sometimes a wet scrubber, follows. Many units rely on an ID fan for draft, with a small FD fan blowing undergrate air.
Where the FD fan sits
- Inlet screen
- FD fan (undergrate blower)
- Ash pit duct
- Fixed grate bars
- Coal bed
The blower sits beside the boiler front and delivers ambient air into the ash pit below the grate, through a duct and damper. Pressure is low, typically 30 to 80 mmWC. The ash pit door must be closed for the air to pass through the grate rather than escape. The blower is interlocked with the ID fan so that the furnace stays under suction.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Ambient; no air heater |
| Pressure at the fan outlet | Typically 30 to 80 mmWC |
| Dust | Coal dust and ash near the firing area |
| Duty | Small, intermittent with firing cycles |
| 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 fixed grate smoke tube boilers
| Blade form | Backward curved, small direct-mounted blower |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings only |
| Flow control | Damper or variable speed drive |
| Also specified | Inlet screen, ash pit door seals and interlock with the ID fan |
What this FD fan has to cope with
Imbalance with the ID fan
If the FD blower delivers more air than the ID fan can remove, the furnace goes positive and smoke escapes at the firing door. The blower must be sized and controlled in proportion to the ID fan, with an interlock that stops it when the ID fan trips.
Uneven air through the grate
Fixed grates with a thick or clinkered bed in places pass air unevenly. Air flows through thin, burnt-out patches while thick areas smoulder and make smoke. The blower has only modest pressure, so regular cleaning of the fire, raking of clinker and an even bed thickness are what let its air reach the whole grate.
Ash pit leakage
Leaky ash pit doors and gaps around the grate frame let air escape around the bed instead of through it. That wastes blower output, adds cold air to the furnace and increases the gas the ID fan must move. Good door seals and a closed ash pit make even a small undergrate blower effective.
Energy and control
The undergrate blower is a small motor, but its effect on combustion is large. Matching air to the firing rate, with a damper or speed drive, avoids high excess air that wastes fuel and overloads the ID fan. Turning the blower down during idle periods saves both power and fuel.
ID fan and FD fan in coal fired fixed grate smoke tube boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, at the boiler rear | At the boiler front, feeding the ash pit |
| What it handles | Hot flue gas with soot and fly ash | Ambient air |
| Temperature at the fan | 220 to 280°C | Ambient |
| Main design concern | Steady draft, soot and temperature | Balance with the ID fan |
| Control | Furnace draft | Firing rate |
See also the Induced Draft (ID) Fan for coal fired fixed grate smoke tube boilers.
Questions buyers ask
Is an FD fan necessary on a coal fixed grate boiler?
Not always. Many small fixed grate smoke tube boilers run on natural undergrate air drawn through the ash pit by ID fan suction. An undergrate blower improves combustion, makes the fire easier to control and allows higher output, especially with thick beds or lower grade coal, so it is worth fitting on most units.
What size FD blower does a small coal smoke tube boiler need?
Usually a small blower delivering 30 to 80 mmWC, with an air flow matched to the coal firing rate and the grate area. It is almost always a direct-mounted unit on the motor shaft. We size it from the boiler rating, the grate area and the coal used, and check it against the ID fan capacity.
Why must the FD fan be interlocked with the ID fan?
If the ID fan stops while the FD blower keeps running, the furnace goes positive, and hot gas and smoke blow out at the firing door where the operator stands. The interlock stops the blower whenever the ID fan trips, and prevents it starting until the ID fan is running and the furnace is under suction.
Information needed to quote an FD fan for coal fired fixed grate 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.
- Grate area. Grate area and typical coal firing rate.
- Existing draft. Whether an ID fan is fitted and how the ash pit is closed.
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.