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 FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate air duct
- Grate zones
- Fuel bed
The fan sits beside the heater at ground level and pushes ambient air through the air preheater into the undergrate plenum, which is divided into zones along the chain grate. On a thermic fluid heater the air preheater does a large share of the heat recovery, so its air side is often larger than on a steam boiler of the same rating and adds noticeably to the fan pressure. Fan outlet pressure is typically 80 to 150 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air preheater | 150 to 230°C, downstream of the fan |
| Pressure at the fan outlet | Typically 80 to 150 mmWC |
| Dust | Clean air; coal dust near the bunker |
| 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 chain grate thermic fluid heaters
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft or direct coupled |
| Bearings and cooling | Motor or pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on oil outlet temperature, trimmed by flue gas oxygen |
| Also specified | Inlet screen, zone dampers, and an interlock to stop with the ID fan |
What this FD fan has to cope with
Hot air backflow on fan stop
When the FD fan stops with the heater hot, hot air and gas can flow backwards through the air preheater into the fan. A non-return or isolating damper at the fan outlet, and a check that the fan casing and motor are clear of this path, protect the fan during trips and planned stops.
Air preheater leakage
Tubular air preheaters on thermic fluid heaters see high gas temperatures at their hot end and can crack or leak. Leaking air passes directly to the gas side, so the FD fan delivers more than the grate receives and the ID fan carries the extra. Oxygen readings before and after the air preheater show the leakage.
Grate zone balance
A chain grate needs more air in the middle zones, where the coal burns most actively, and less at the ends. With a single fan and manual zone dampers, the balance drifts as the coal changes. Setting the dampers on zone pressure and checking ash for unburnt carbon keep the distribution right.
Energy and control
The FD fan on a thermic fluid heater follows oil outlet temperature, and many plants run it at constant speed with a damper. A variable speed drive saves power across the typical batch load pattern and holds excess air steadier. Since a large share of the heater's efficiency comes from the air preheater, keeping its air side clean also reduces the pressure the fan must develop.
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 Induced Draft (ID) Fan for coal fired chain grate thermic fluid heaters.
Questions buyers ask
Does the FD fan on a thermic fluid heater handle hot air?
Not normally. The FD fan takes ambient air and pushes it through the air preheater, where it is heated to 150 to 230°C before the grate. The fan is selected at ambient density. Only if hot air recirculation is added to its inlet would the selection change.
What controls the forced draft fan on a coal fired thermic fluid heater?
Usually the oil outlet temperature controller sets the firing rate, which changes grate speed and FD fan air together. Flue gas oxygen trims the air. A variable speed drive allows this smoothly, and the ID fan follows on furnace draft. Grate speed and air should move together, not in separate loops.
Why do we need an interlock between the FD fan and ID fan?
If the ID fan stops while the FD fan keeps running, air continues to feed the fire and the furnace goes strongly positive, pushing hot gas and ash out of openings. The interlock stops the FD fan and grate as soon as the ID fan trips.
Information needed to quote an FD fan for coal fired chain grate thermic fluid heaters
- 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.
- Air preheater. Air side pressure drop and the hot air temperature at full load.
- Control. Present firing control, whether modulating or on-off, and the signals available.
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.