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 ID fan sits
- Coal furnace
- Mixing chamber
- Spray dryer or rotary dryer
- Cyclones
- ID (exhaust) fan
- Wet scrubber
- Stack
The fan most often sits after the cyclone battery and before the wet scrubber, so it handles dry exhaust at 90 to 130°C and the scrubber runs on its pressure side. Some plants place it after the scrubber, where the gas is saturated at 50 to 65°C and the fan must be corrosion resistant. Suction includes the furnace draft, the dryer chamber, ducts and cyclones, typically 150 to 300 mmWC.
Gas at the ID fan
| Gas temperature, normal | 90 to 130°C after the cyclones |
| Maximum or upset | 250 to 400°C if slip feed to the dryer stops |
| Dust at the fan | Fine clay, quartz and feldspar with coal fly ash; 0.5 to 3 g/Nm³ after cyclones |
| Moisture and dew point | Humid exhaust with a dew point of 60 to 70°C |
| Corrosive species | SO₂ and SO₃ from coal; acid condensate wherever the gas cools below its dew point |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 coal fired hot air generators
| Blade form | Radial tipped or backward curved with wear liners, chosen on the dust at the fan |
| Material of construction | Wear-resistant plate impeller; mild steel casing with liners before the scrubber, SS 316 or rubber lined after it |
| Drive and arrangement | Direct coupled arrangement 8, at moderate speed to limit erosion |
| Bearings and cooling | Pedestal bearings with a cooling disc; vibration monitoring |
| Flow control | Variable speed drive on dryer chamber pressure |
| Also specified | Bolted liners at the scroll cut-off, casing drain and large inspection door |
What this ID fan has to cope with
Erosion by ceramic fines
Spray dryer fines that pass the cyclones contain quartz, harder than mild steel. They erode the blade tips, the backplate near the blade root and the scroll cut-off. Wear-resistant plate, liners designed for site replacement and a tip speed kept moderate let the fan run between annual shutdowns.
Acid and mud deposits
If the exhaust cools below its dew point at low load or at shutdown, sulphur from the coal forms acid and the ceramic dust turns to mud on the blades. It dries hard and unevenly, causing unbalance. Keeping the exhaust temperature up, insulating the casing and washing the wheel at stops control this.
Slip feed interruption
If the slip pump trips, the dryer stops evaporating and the exhaust temperature climbs within a minute. A fan built for 120°C humid gas then sees 300°C or more. A high exhaust temperature trip that cuts coal feed and opens dilution air protects the fan and bag filter where fitted.
Coal quality changes
A change from imported to Indian coal can double the ash and change the SO₂. Fly ash adds to the dust load at the fan, and the gas volume per unit heat rises. The fan is sized on the coal that will actually be fired, with margin for the poorer grade.
Energy and control
Ceramic spray dryers run continuously for long campaigns at a throughput set by the press shop. The exhaust fan is one of the largest motors in the body preparation section, and a variable speed drive holding chamber pressure saves a large share of its power against damper control. Raising slip solids and reducing exhaust temperature to just above dew point cut the gas volume, so they reduce fan power as well as coal.
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 Forced Draft (FD) Fan for coal fired hot air generators.
Questions buyers ask
Why does the exhaust fan on a ceramic spray dryer wear so fast?
The fines passing the cyclones contain quartz and feldspar, which are harder than mild steel, and on coal firing fly ash is added. Erosion concentrates on blade tips and the scroll cut-off. Wear-resistant plate, replaceable liners, efficient cyclones and lower tip speed extend impeller life considerably.
Should the ID fan on a coal fired hot air generator be before or after the wet scrubber?
Before the scrubber the fan handles dry, dusty gas and can be built in lined mild steel. After it, the gas is cleaner but saturated and acidic, so the fan must be stainless or rubber lined. Both work; the choice depends on cyclone efficiency, scrubber pressure drop and layout.
What temperature is the exhaust fan on a coal fired spray dryer designed for?
Normal operation is usually 90 to 130°C after the cyclones. The fan should be rated for the excursion when slip feed stops, often 250 to 400°C for a short period, with a trip on high exhaust temperature that cuts coal feed and protects the fan bearings and liners.
Information needed to quote an ID fan for coal fired hot air generators
- 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.
- Dryer and product. Spray dryer or rotary dryer, the product and its dust at the fan inlet.
- Scrubber. Whether the fan is before or after the wet scrubber, and the scrubber pressure drop.
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.