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 FD fan sits
- Air intake
- FD fan
- Steam coil air preheater
- Rotary air preheaters
- Hot secondary air duct
- Corner windboxes
Two FD fans, axial on many of these units, sit near the air preheaters and feed a cross-connected cold air duct so either fan can serve both preheaters. The air passes the steam coil air preheater where fitted, picks up heat in the rotary preheaters and enters the four corner windboxes, where auxiliary and fuel air dampers set the tangential firing pattern. The fan works against the preheaters, ducts and windbox dampers, plus whatever air leaks across to the gas side.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Hot secondary air | About 290 to 330°C after the preheaters, downstream of the fan |
| Pressure at the fan outlet | Typically 250 to 400 mmWC at full load |
| Leakage allowance | Preheater leakage often 10 to 15% on worn seals |
| Dust | Clean air; coal and ash dust near the intake on some layouts |
| 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 subcritical 200 MW class pulverised coal units
| Blade form | Variable pitch axial on many units; backward curved centrifugal with inlet guide vanes on others |
| Material of construction | Mild steel casing and hub; replacement blades matched to the original material |
| Drive and arrangement | Direct coupled medium-voltage motor |
| Bearings and cooling | Oil-lubricated bearings with lube unit and vibration pick-ups |
| Flow control | Blade pitch or inlet vanes on total air flow |
| Also specified | Tight cross-connection dampers, intake silencer, recalibrated air flow measurement |
What this FD fan has to cope with
Lost capacity from worn pitch mechanisms
After decades of service the hydraulic pitch actuator and blade bearings of an axial FD fan wear, and the blades no longer reach full pitch or move together. The fan falls short at full load and hunts at low load. Overhaul of the hub and actuator, or replacement of the rotor assembly, restores both capacity and control.
Drifting air flow measurement
Old aerofoil or venturi air flow elements foul and drift, and the combustion control then drives the FD fans to the wrong air flow. Before concluding the fan is short of capacity, the flow element should be checked against a pitot traverse at site, since many apparent fan shortages turn out to be measurement errors.
Leaking cross-connection dampers
With both fans feeding a common duct, a leaking isolation damper lets air recirculate through the stopped fan, spinning it backwards and wasting part of the running fan's output. Tight-shut isolation dampers and an anti-reverse-rotation provision on each fan prevent this during single-fan operation and make restarting the idle fan safe.
Energy and control
Two-shift and part-load running means the FD fans spend long hours below 70% flow. Axial fans with healthy pitch control hold efficiency; centrifugal FD fans on inlet vanes gain clearly from VFD conversion. Lowering excess air through calibrated oxygen measurement and sealing windbox and preheater leaks often saves as much as a fan upgrade, at a fraction of the cost.
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 Induced Draft (ID) Fan for subcritical 200 MW class pulverised coal units.
Questions buyers ask
Why is my 210 MW FD fan not reaching full air flow?
The common causes are worn pitch mechanisms, a fouled steam coil air preheater, high preheater leakage or an air flow measurement that reads low. A site test of fan pressure, flow by pitot traverse, and the actual blade angle against the indicated pitch separates a fan problem from a system or instrument problem.
Can an axial FD fan be replaced with a centrifugal fan on a 200 MW unit?
It can, but a centrifugal fan for the same duty is larger and needs a new foundation and duct changes. It is usually considered only when the axial fan is beyond economic repair or spares are unavailable. Refurbishing the hub, blades and actuator of the existing fan is normally the cheaper route.
Does the FD fan in a 210 MW plant see hot air?
No. The FD fan takes ambient air, and the air is heated after the fan in the steam coil and rotary air preheaters. Selection is at ambient density, which is why FD fans are smaller than the ID fans on the same unit. Hot air reaches the fan only if a damper fault lets it flow back from the preheater.
Information needed to quote an FD fan for subcritical 200 MW class pulverised coal units
- 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.
- Pitch mechanism condition. Last overhaul of the hub and actuator, and any pitch tracking problems.
- Air flow test. Pitot traverse results or the basis of the present air flow reading.
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.