How coal fired hot air generators work
Coal fired hot air generators supply hot gas to ceramic spray dryers, mineral and sand dryers, and some kiln and foundry sand applications. Coal is burnt on a chain grate or reciprocating grate, Indian coal with 25 to 40% ash and about 3,500 to 4,500 kcal/kg, or imported coal with lower ash and higher calorific value. The furnace gas at 900 to 1,100°C is tempered with dilution air in a mixing chamber to 450 to 650°C for a tile body spray dryer, or lower for other dryers, and passes directly into the drying chamber. Exhaust from the dryer, humid and carrying fine ceramic or mineral dust with fly ash and SO₂, goes through cyclones and a wet scrubber or bag filter to the exhaust fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Grate zone dampers
- Chain or reciprocating grate
- Coal furnace
- Mixing chamber
The fan stands beside the furnace and blows ambient air into the compartments under the grate, each with its own damper. Dilution air for the mixing chamber is drawn in by the exhaust fan through a damper on many units, and blown by a separate fan on others. The FD fan works against the grate, the coal and ash bed and the ducts, typically 80 to 150 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 80 to 150 mmWC |
| Dust | Clean air; coal dust from the bunker area can be drawn in |
| Grate zones | Typically three to six compartments with separate dampers |
| Duty | Follows the dryer inlet temperature |
| 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 coal fired hot air generators
| Blade form | Backward curved, for a stable curve and efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive from dryer inlet temperature, with zone dampers |
| Also specified | Inlet screen, and a separate dilution air fan where fine temperature control is needed |
What this FD fan has to cope with
Unburnt carbon in the ash
High-ash coal needs enough air at the back of the grate to burn out before the ash drops. A fan without pressure margin starves the rear zones when the bed is thick. A stable rising curve and zone dampers set for the coal fired keep unburnt carbon in the ash down.
Air siftings and grate leaks
Worn grate links and side seals let air bypass the bed. The fan delivers more air than the fire receives, and the furnace runs with high excess air. Maintaining seals and measuring oxygen at the mixing chamber show whether the fan is supplying the bed or the leaks.
Dryer temperature hunting
On direct fired dryers the inlet temperature must be held within a few degrees for consistent granule moisture. A coal grate responds slowly, and an FD fan on damper control makes the temperature hunt. Speed control on the FD fan with fast dilution air control at the mixing chamber gives a stable inlet temperature.
Energy and control
A ceramic spray dryer generator runs for long continuous campaigns, and the FD fan follows the dryer inlet temperature through the day. A variable speed drive with zone dampers saves fan power and keeps excess air lower than full speed with throttled dampers. Lower excess air raises furnace temperature, so less coal is burnt for the same hot gas.
ID fan and FD fan in coal fired hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer cyclones | Under the coal grate |
| What it handles | Humid exhaust with ceramic dust and SO₂ | Clean ambient air |
| Temperature at the fan | 90 to 130°C, up to 400°C on feed loss | Ambient |
| Main design concern | Erosion and acid condensation | Zoned air for high-ash coal |
| Control | Dryer chamber pressure | Dryer inlet temperature |
See also the Induced Draft (ID) Fan for coal fired hot air generators.
Questions buyers ask
What pressure does the FD fan on a coal fired hot air generator need?
Usually 80 to 150 mmWC, covering the ducts, zone dampers, grate and the coal and ash bed. High-ash Indian coal on a thick bed needs the upper end, imported coal on a thinner bed the lower. The figure is confirmed from the grate supplier's data or from the plenum pressure measured at site with the grate at full load.
Is the dilution air on a coal hot air generator supplied by the FD fan?
Usually not. Dilution air enters at the mixing chamber, either drawn in by the exhaust fan through a damper or blown in by a separate dilution fan. The FD fan supplies combustion air under the grate. Keeping them separate gives better control of both combustion and the dryer inlet temperature.
Why is there so much unburnt coal in the ash of my hot air generator?
Often the rear grate zones are short of air, the grate speed is too high for the coal, or air bypasses the bed through worn seals. Checking zone pressures and damper settings, and confirming the FD fan has pressure margin with a thick bed, usually finds the cause.
Information needed to quote an FD fan for coal fired hot air generators
- 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.
- Coal. Type of coal, ash content and calorific value.
- Grate. Grate type, number of zones and the design plenum pressure.
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.