How imported coal chain grate boilers work
Small chain grate boilers firing imported coal, from about 2 to 15 TPH, are common in textile dyeing and process units. Sized Indonesian or South African coal flows from a hopper over a gate onto a chain of cast links, burning as a bed 80 to 120 mm deep as it moves to the ash discharge. Indonesian coal is high in moisture and volatiles and low in ash, so the ash layer protecting the links is thin and the coal ignites quickly at the front. Many of these units have no air heater, only an economiser. The gas passes a multiclone, then a bag filter or a wet scrubber, with the ID fan placed before the scrubber or after the bag filter.
Where the ID fan sits
- Chain grate furnace
- Boiler tubes
- Economiser
- Multiclone
- ID fan
- Wet scrubber or bag filter
- Chimney
On smaller units the fan usually sits after the multiclone and before a wet scrubber, so that it handles dry gas at 170 to 210°C. Where a bag filter is fitted instead, the fan follows it. Suction at the fan inlet is modest, typically 100 to 180 mmWC. A few plants place the fan after the scrubber; it then handles saturated gas and must be built for it.
Gas at the ID fan
| Gas temperature, normal | 170 to 210°C after the economiser |
| Maximum or upset | Up to 260°C with sooted tubes |
| Dust at the fan | Light ash with soot; 0.3 to 1 g/Nm³ after a multiclone, far less after a bag filter |
| Moisture and dew point | Water dew point 50 to 60°C with Indonesian coal |
| Corrosive species | Low sulphur; wet scrubber carry-over is the main corrosion risk if the fan follows the scrubber |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 imported coal chain grate boilers
| Blade form | Backward curved, with light liners if after a multiclone only |
| Material of construction | Mild steel when dry; SS 304 or SS 316 if after a wet scrubber |
| Drive and arrangement | Belt driven arrangement 9 on small units, direct coupled on larger |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain, inspection door, and an outlet transition matched to the scrubber |
What this ID fan has to cope with
Hot gas without an air heater
Small imported coal chain grate boilers often have no air heater, and with sooted tubes the gas at the fan rises above 220°C. Lower density means more volume and less draft for the same speed. Bearings run hotter. Selection at the sooted-tube temperature, with a shaft cooling disc, prevents the fan falling short or tripping on bearing temperature.
Wet scrubber carry-over
When the fan follows a wet scrubber, droplets and slurry carry over into the impeller. Ash particles in the water deposit on blades and corrode mild steel. Stainless construction, a good mist eliminator, drains and periodic washing are needed in that position. Placing the fan before the scrubber avoids most of this.
Soot deposits and unbalance
Imported coal releases volatiles quickly, and small chain grate boilers often lack enough overfire air. Soot forms, coats the tubes and builds on the fan blades unevenly, causing vibration. Better overfire air, soot blowing and a smooth backward curved impeller keep the fan in balance.
Energy and control
Small chain grate boilers in dyeing units run on production cycles with frequent load changes. A variable speed ID fan saves considerably compared with a damper. Keeping tubes clean holds gas temperature down, which reduces gas volume and fan power, and an air heater or larger economiser, where space allows, reduces both.
ID fan and FD fan in imported coal chain grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multiclone, before the scrubber or after the bag filter | Beside the boiler, feeding the undergrate plenum |
| What it handles | Hot, humid gas with light ash and soot | Ambient air |
| Temperature at the fan | 170 to 210°C | Ambient |
| Main design concern | Hot gas and scrubber position | Link cooling with a thin bed |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Forced Draft (FD) Fan for imported coal chain grate boilers.
Questions buyers ask
Should the ID fan be before or after the wet scrubber on a small imported coal boiler?
Before the scrubber is usual on small imported coal boilers. The fan then handles dry gas at 170 to 210°C and can be built in mild steel. After the scrubber, it handles saturated gas carrying droplets and fine slurry, and needs stainless construction, drains and a mist eliminator upstream. The trade-off is that a fan before the scrubber sees the dust the multiclone passes.
Why is the ID fan gas temperature so high on my chain grate boiler?
Many small chain grate boilers have only an economiser and no air heater, so gas leaves at 170 to 210°C even when the tubes are clean. Imported coal soots the tubes quickly when overfire air is short, which raises the temperature further. Regular tube cleaning, good overfire air and, where space allows, added heat recovery keep it down.
Can I fit a variable speed drive to the ID fan on a small chain grate boiler?
Yes, on almost any fan with a suitable motor. A variable speed drive controlled from furnace draft replaces the manually set damper, holds suction steadily through load changes and saves power at part load. Belt-driven fans can be converted to direct drive at the same time, which removes belt maintenance and slip.
Information needed to quote an ID fan for imported 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.
- Scrubber position. Whether a wet scrubber is fitted and whether the fan is before or after it.
- Heat recovery. Whether the boiler has an economiser, air heater, or both, and the gas temperature 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.