How coal fired chain grate thermic fluid heaters work
A coal fired thermic fluid heater burns sized coal on a chain grate in a furnace lined with helical coils, through which thermal oil is pumped at outlet temperatures of typically 250 to 300°C. Because the oil is hot, the flue gas cannot be cooled below about 330 to 400°C in the coil bank itself. Most units therefore add an air preheater, an economiser or a small waste heat boiler to bring the gas down to 180 to 230°C before a cyclone or multiclone and, increasingly, a bag filter. Common ratings are about 0.5 to 6 million kcal/h. The FD fan supplies undergrate air, often through the air preheater; the ID fan draws gas through the coils, heat recovery unit and collector to the chimney.
Where the ID fan sits
- Chain grate furnace
- Thermal oil coils
- Air preheater or WHR unit
- Cyclone or multiclone
- Bag filter
- ID fan
- Chimney
The fan sits after the collector, usually at ground level beside the chimney. Upstream are the radiant and convective coil banks, the air preheater or waste heat unit and one or two stages of dust collection, giving inlet suction of about 120 to 220 mmWC. Where there is no heat recovery after the coils, or it is bypassed, gas at 330°C or more reaches the fan, so many plants specify the fan for that condition as an upset case.
Gas at the ID fan
| Gas temperature, normal | 180 to 230°C after the air preheater |
| Maximum or upset | 330 to 400°C with heat recovery bypassed or absent |
| Dust at the fan | Fine coal ash; 30 to 100 mg/Nm³ after a bag filter, more after a cyclone alone |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Corrosive species | Low; coal sulphur gives an acid dew point well below the normal gas temperature |
| Density at the design temperature | About 0.73 kg/m³ at 210°C, 61% 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 chain grate thermic fluid heaters
| Blade form | Backward curved, with leading edge liners where only a cyclone precedes the fan |
| Material of construction | Mild steel casing and impeller, rated for the bypass temperature; heat-resisting steel only if the fan regularly runs above about 350°C |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 3, with bearings well outside the casing |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc sized for the upset temperature |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Interlocks with the circulation pump, FD fan and grate drive; inspection door and drain |
What this ID fan has to cope with
Fan trip with a live coal fire
A chain grate holds a large amount of burning coal. If the ID fan trips, the furnace goes positive and the fire continues, while the oil in the coils keeps absorbing heat. The ID fan trip must stop the FD fan and grate and confirm oil circulation, so that the coils are not overheated. A reliable fan and motor are part of process safety here.
High inlet temperature during bypass
When the air preheater or economiser is bypassed for cleaning or repair, the gas reaches the fan at 330 to 400°C. Gas density falls, volume rises and bearings run hot. Selecting the fan, its cooling disc and its motor for this condition avoids the heater being restricted during maintenance.
Coil and preheater fouling
Fly ash and soot settle in the convective coil bank and air preheater tubes. Resistance rises and gas temperature at the fan climbs. A fan with pressure margin, on a variable speed drive, keeps the heater on load until the planned cleaning, and its speed trend shows when cleaning is due.
Energy and control
Thermic fluid heaters in chemical, pharmaceutical and textile plants follow process demand, often batch by batch. A variable speed ID fan on furnace draft saves energy at partial load and holds draft steadily as the grate speed changes. Keeping the air preheater in service, rather than bypassed, lowers gas volume at the fan and recovers fuel heat at the same time, which is the larger saving of the two.
ID fan and FD fan in coal fired chain grate thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the collector, before the chimney | Ahead of the air preheater and grate |
| What it handles | Hot flue gas with fine coal ash | Clean ambient air |
| Temperature at the fan | 180 to 230°C, up to 400°C with bypass | Ambient, 25 to 45°C |
| Main design concern | High inlet temperature and trip safety | Grate zone balance and air preheater |
| Control | Furnace draft | Oil outlet temperature and excess oxygen |
See also the Forced Draft (FD) Fan for coal fired chain grate thermic fluid heaters.
Questions buyers ask
Why is the flue gas so hot on a thermic fluid heater?
The thermal oil leaves the coils at 250 to 300°C, so the gas can only be cooled to some margin above that, typically 330 to 400°C, within the heater. An air preheater or economiser after the coils brings it down further. Without one, the ID fan and the chimney see the higher temperature.
What happens if the ID fan trips on a coal fired thermic fluid heater?
The furnace loses draft and goes positive, but the coal on the grate keeps burning. The interlocks must stop the FD fan and grate, and the oil must keep circulating to carry heat away from the coils. A fan trip is therefore a safety event, not just a production loss.
Can a mild steel induced draft fan run at 350°C?
Mild steel is generally acceptable up to about 350 to 400°C, with reduced strength at the top of that range. The impeller stress and tip speed must be checked at the actual temperature. For regular operation above about 350°C, heat-resisting steel is usually specified. Bearing cooling also needs checking.
Information needed to quote an ID fan for coal fired chain grate thermic fluid heaters
- 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.
- Heat recovery. Whether an air preheater, economiser or WHR unit is fitted, and the gas temperature at the fan with it bypassed.
- Interlocks. Present trip logic between the ID fan, FD fan, grate drive and oil circulation pumps.
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.