How subcritical 200 MW class pulverised coal units work
The 200 MW class drum boiler, in its 195 to 250 MW variants, is the most common legacy coal unit in Indian utilities. Bowl mills grind coal of 35 to 45% ash and blow it with hot primary air to tangential corner burners. Gas leaves the economiser at about 330 to 360°C, crosses two rotary air preheaters, and passes a multi-field ESP, often enlarged since commissioning to meet tighter emission limits. Two or three ID fans then discharge to the chimney, or to an FGD absorber where one has been retrofitted. Two FD fans supply secondary air and two PA fans serve the mills. Many units are 30 to 40 years old and running under renovation and modernisation programmes.
Where the ID fan sits
- Furnace
- Economiser
- Rotary air preheaters
- ESP
- ID fans
- Chimney or FGD
The ID fans stand at ground level between the ESP outlet and the chimney, on many units two working fans with or without a standby. Many original fans are double inlet centrifugal machines driven through a hydraulic coupling, while others use inlet guide vanes. Upstream they carry the full draft loss of the boiler, air preheaters and ESP; where an absorber is added downstream they also take part or all of its resistance. Their position after the ESP limits the ash they see, provided the ESP stays healthy.
Gas at the ID fan
| Gas temperature, normal | 130 to 150°C after the air preheaters |
| Maximum or upset | 170 to 200°C with a preheater out or heavily fouled |
| Dust at the fan | Fly ash, about 50 to 150 mg/Nm³ behind an upgraded ESP; much higher on units with weak ESP fields |
| Moisture and dew point | Water vapour about 7 to 10%; acid dew point near 110 to 125°C with low-sulphur Indian coal |
| Air ingress | Leakage at the preheaters, ESP and ducts commonly adds 10 to 25% over theoretical gas volume on older units |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% 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 subcritical 200 MW class pulverised coal units
| Blade form | Backward curved or radial on double inlet double width wheels, reusing the existing casing where the duty allows |
| Material of construction | Mild steel casing; impeller in high-strength low-alloy steel with wear liners at the blade inlet |
| Drive and arrangement | Medium-voltage motor through hydraulic coupling on original fans; VFD on many retrofits |
| Bearings and cooling | Sleeve or antifriction bearings with oil circulation, cooling water where originally fitted |
| Flow control | Scoop tube or VFD on furnace pressure, with inlet dampers for isolation |
| Also specified | Replacement impeller designed to fit the existing shaft line, bearings and foundation |
What this ID fan has to cope with
Fans short of head after upgrades
ESP field additions, extended ductwork and higher air ingress push the system curve above the original fan curve, and the fans run at full scoop with no margin left. A site test of flow and pressure at full load, followed by a new impeller designed for the measured system on the existing casing, usually restores margin without civil work.
Hydraulic coupling losses
A hydraulic coupling reduces speed by slipping, and the slip energy is lost as heat in the coupling oil. At the part loads these units now run, the loss is large. Replacing the coupling with a VFD on the existing motor, or a new motor if the insulation is weak, is a common retrofit with clear payback.
Erosion from weak ESP fields
Older ESPs with failed fields or overflowing hoppers pass heavy ash to the fans, and Indian fly ash is highly abrasive. Erosion of the blade inlet and back plate weakens the wheel over a few years. Thickness surveys at each overhaul, liners on the worn zones and attention to ESP health keep this under control.
Ageing rotors and fatigue
Impellers that have run three or four decades, with repeated weld repairs, carry fatigue risk at the blade roots and hub welds. Non-destructive testing at each overhaul catches cracks early, and once cracking appears, a new rotor balanced to ISO 21940-11 is safer than another round of repair.
Energy and control
These units now cycle down to 55 to 60% load at night and through hours of high renewable output, so the ID fans spend long periods well below design flow. A hydraulic coupling or inlet damper wastes a large share of the power at those loads. Converting to VFD, sealing air ingress at the air preheaters and ESP, and reducing excess air together cut ID fan power far more than any one measure on its own.
ID fan and FD fan in subcritical 200 MW class pulverised coal units
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before chimney or FGD | Ahead of the rotary air preheaters |
| What it handles | Flue gas with fly ash and air ingress | Clean ambient air |
| Temperature at the fan | 130 to 150°C | Ambient, 25 to 45°C |
| Usual drive | Hydraulic coupling or VFD | Direct motor with blade pitch |
| Main retrofit | Head for FGD, coupling to VFD | Pitch mechanism overhaul |
| Control | Furnace pressure | Total air and excess oxygen |
See also the Forced Draft (FD) Fan for subcritical 200 MW class pulverised coal units.
Questions buyers ask
Can the existing ID fans of a 210 MW unit handle the FGD retrofit?
Usually not without changes. The absorber, ducts and dampers add substantial resistance, and most original ID fans have little head margin left after earlier ESP upgrades. A site performance test followed by a system study shows whether a new impeller and motor on the existing casing, a booster fan or new fans is the practical route.
Is it worth replacing the hydraulic coupling with a VFD on a 210 MW ID fan?
On units that spend long hours at part load, usually yes. The VFD avoids the slip losses of the coupling and gives finer furnace pressure control. The decision depends on the load profile, the condition of the motor, and space for the drive panels and transformer near the fan.
What is the life of an ID fan impeller in a 210 MW coal power plant?
With a healthy ESP and periodic liner replacement, impellers often run many years between major repairs. With poor ESP performance, erosion can force repair at every overhaul. Thickness measurements, vibration trends and NDT findings decide when a new rotor is due, rather than any fixed number of years.
Information needed to quote an ID fan for subcritical 200 MW class pulverised coal units
- 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.
- Performance test. Measured flow, pressure, temperature and motor current at the present full load.
- Existing fan details. Casing drawings or dimensions, shaft and bearing details, and the drive type.
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.