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 ID fan sits
- Fixed grate furnace
- Smoke tube passes
- Smoke box
- Cyclone or multiclone
- ID fan
- Wet scrubber or chimney
The fan sits after the cyclone or multiclone, close to the boiler's rear smoke box. It must overcome the furnace, the smoke tube passes and the collector, typically 60 to 120 mmWC. When the firing door is opened to add coal or clean the fire, cold air rushes in, and the fan must hold suction so that flame and smoke do not blow out at the operator.
Gas at the ID fan
| Gas temperature, normal | 220 to 280°C after the smoke tubes |
| Maximum or upset | Up to 330°C with sooted tubes |
| Dust at the fan | Fly ash and soot; 0.5 to 2 g/Nm³ after a cyclone |
| Moisture and dew point | Water dew point 40 to 50°C |
| Corrosive species | Low sulphur with most coals; soot can be acidic if wetted |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 fixed grate smoke tube boilers
| Blade form | Radial tipped or backward curved, simple and robust |
| Material of construction | Mild steel; boiler quality plate for the impeller at the higher temperatures |
| Drive and arrangement | Belt driven arrangement 9 or direct coupled arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Inlet damper or variable speed drive |
| Also specified | Inspection door for soot removal, casing drain |
What this ID fan has to cope with
Soot from hand firing
Hand firing gives a burst of smoke and soot each time fresh coal is added, before the volatiles catch fire. Soot builds up in the smoke tubes and reaches the fan, coating the blades. Regular tube brushing and an inspection door on the fan casing for cleaning prevent unbalance.
Draft when the firing door opens
Opening the firing door lets a large volume of cold air into the furnace. A fan with too little margin loses suction and the fire blows back at the operator. A fan with steady suction and a fast-responding damper or speed drive holds the furnace negative.
High gas temperature
Without an economiser, gas reaches the fan at 220 to 280°C, and more once the smoke tubes soot up. The impeller plate, shaft and bearings must be selected for this, with a shaft cooling disc to protect the bearings. Adding an economiser, where space allows, lowers the gas temperature and volume and improves boiler efficiency at the same time.
Energy and control
Small fixed grate boilers often lose more heat up the chimney than any other way. An economiser lowers gas temperature, which reduces the gas volume the ID fan handles. A variable speed drive on furnace draft gives smoother control than a damper and saves power when the boiler idles between batches.
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 Forced Draft (FD) Fan for coal fired fixed grate smoke tube boilers.
Questions buyers ask
Does a coal fired smoke tube boiler need an ID fan?
Most do. A short chimney cannot pull the gas through the furnace, two or three smoke tube passes and a dust collector by natural draft alone, particularly once the tubes are sooted. The ID fan provides steady draft, keeps the furnace under suction when the firing door is opened, and allows a smaller chimney.
Why does smoke come out of the firing door on my coal boiler?
The furnace has lost suction. Usually the ID fan cannot overcome sooted smoke tubes or a choked cyclone, the fan damper is set too far closed, or belts are slipping on a belt-driven fan. Cleaning the tubes and collector and checking fan speed normally cures it. If not, the fan may simply be too small for the boiler.
Can I add an FD fan to a fixed grate smoke tube boiler?
Yes. A small undergrate blower helps the coal burn more evenly and raises the boiler output. It must be balanced against the ID fan so that the furnace stays under suction, and interlocked so the blower stops if the ID fan trips. In some cases the ID fan then needs more capacity to handle the extra gas.
Information needed to quote an ID fan for coal fired fixed grate 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.
- Boiler data. Boiler rating, number of smoke tube passes, and whether an economiser is fitted.
- Draft problems. Whether smoke blows back at the firing door, and the present fan speed and damper setting.
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.