How Indian coal chain grate boilers work
Chain grate boilers firing Indian coal are common from about 2 to 20 TPH in textile processing, dyeing and food plants. Sized coal, typically 6 to 25 mm, flows from a hopper over a guillotine gate onto an endless chain of cast links, which carries it slowly through the furnace as a bed 100 to 150 mm deep. Indian coal is high in ash, often 30 to 45%, with a calorific value of about 3,500 to 4,500 kcal/kg, moderate volatiles and a high ash fusion temperature. The thick ash layer protects the links, and most ash falls off the end into the ash pit, but fines and light ash are carried into the gas. Flue gas passes the boiler, often an economiser, then a multiclone and bag filter or wet scrubber, the ID fan and chimney.
Where the ID fan sits
- Chain grate furnace
- Boiler tubes
- Economiser
- Multiclone
- Bag filter or wet scrubber
- ID fan
- Chimney
The fan sits after the final collector. Many dyeing and process units of 2 to 10 TPH have only a multiclone and a wet scrubber, with the fan placed between the two so it stays dry, and so it handles all the dust the multiclone passes. Newer installations put a bag filter ahead of the fan. Without an air heater, gas reaches the fan at 180 to 230°C. Inlet suction is modest, typically 120 to 200 mmWC on these smaller units.
Gas at the ID fan
| Gas temperature, normal | 180 to 230°C after the economiser |
| Maximum or upset | Up to 280°C with sooted tubes or no economiser |
| Dust at the fan | Hard, quartz-rich coal ash; 1 to 3 g/Nm³ after a multiclone, below 100 mg/Nm³ after a bag filter |
| Moisture and dew point | Water dew point 40 to 50°C; coal moisture usually 8 to 15% |
| Corrosive species | Sulphur in Indian coal is usually low, 0.3 to 0.8%; acid dew point rarely limits at these gas temperatures |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 Indian coal chain grate boilers
| Blade form | Radial tipped or backward curved with liners, depending on the collector ahead |
| Material of construction | Mild steel casing with scroll liners; impeller in wear-resistant plate where the fan follows a multiclone |
| Drive and arrangement | Belt driven arrangement 9 on small units, direct coupled arrangement 8 on larger |
| Bearings and cooling | Pedestal bearings with a cooling disc for gas above 200°C |
| Flow control | Variable speed drive on furnace draft, replacing manual dampers |
| Also specified | Inspection door, drain, and bolted liners at the cut-off |
What this ID fan has to cope with
Erosion behind a multiclone
On small chain grate boilers the fan often follows only a multiclone, which passes much of the fine ash below about 10 microns and a share of coarser grit. Indian coal ash is high in quartz and wears unlined mild steel blades within months. A radial tipped wheel in wear-resistant plate, or a lined backward curved wheel, at modest tip speed, extends life to years.
Belt drive maintenance and tip speed
Belt-driven fans on small boilers are often speeded up by changing pulleys when the boiler is pushed beyond rating. Higher speed raises erosion sharply, roughly with the cube of velocity, and overloads the belts and bearings. Setting the duty correctly and using a speed drive rather than pulley changes keeps wear and maintenance predictable.
Sooted boiler tubes raise gas temperature
Chain grate boilers firing variable Indian coal soot their tubes, and gas temperature at the fan rises between cleanings. Hotter gas is less dense, so the fan must move more volume, and the bearings run hotter. A fan selected at the sooted-tube temperature with a cooling disc handles this without tripping on bearing temperature.
Energy and control
Chain grate boilers in textile and process units often run at 50 to 80% load, and older ID fans are controlled by a manually set damper. A variable speed drive on furnace draft saves a large part of the fan power. Reducing air ingress at the grate seals, ash pit doors and the multiclone hopper also cuts the gas volume, and on many sites the fan has more margin than needed.
ID fan and FD fan in Indian coal chain grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone or bag filter, before the chimney | Beside the boiler, feeding the undergrate plenum |
| What it handles | Flue gas with hard high-ash coal fly ash | Ambient air |
| Temperature at the fan | 180 to 230°C | Ambient |
| Main design concern | Erosion behind a multiclone and hotter gas | Even air through a thick ash bed |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Forced Draft (FD) Fan for Indian coal chain grate boilers.
Questions buyers ask
Why does the ID fan wear out so fast on my chain grate coal boiler?
Indian coal ash is high in quartz, and on small chain grate boilers the fan often follows only a multiclone that lets fine ash through. Higher fan speed, often from pulley changes, multiplies the wear. A wear-resistant impeller, scroll liners and a bag filter ahead of the fan are the long-term fixes.
Should the ID fan be before or after the wet scrubber on a coal chain grate boiler?
Before the scrubber is usual, because the fan then handles dry gas and stays in mild steel. It must then tolerate the dust that passes the multiclone. Placing it after the scrubber keeps dust off but means wet, saturated gas, which needs stainless or lined construction and drains.
Can I convert my belt driven ID fan to direct drive on a small coal boiler?
Often yes. A direct coupled fan with a variable speed drive removes belt maintenance and lets the fan run at the speed it actually needs. The impeller may need redesign for motor speed, and the base frame must suit the new arrangement. We can assess this from the existing fan data and site layout.
Information needed to quote an ID fan for Indian coal chain grate 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.
- Collector arrangement. Multiclone, bag filter or wet scrubber, and where the fan sits relative to each.
- Present wear. Impeller life between repairs, present drive arrangement and running speed.
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.