How rice husk dumping grate boilers work
A dumping grate is a stationary grate whose sections are tilted at intervals, by hand lever or pneumatic cylinder, to drop accumulated ash into the hopper below. Rice husk dumping grate boilers are typically water tube or hybrid units of about 4 to 20 TPH. Husk is fed by pneumatic spreaders or chutes and burns partly on the grate, partly in suspension. Between dumps, every one to four hours depending on load, the ash layer grows and grate resistance rises. During a dump, air to that section is shut off and the furnace sees a brief surge of ash and a change in draft. The gas then passes the boiler bank, economiser, air heater and a multiclone with ESP or bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Main air duct
- Section dampers
- Dumping grate sections
The FD fan sits near the air heater at ground level and feeds a common undergrate duct with a damper to each grate section. When a section dumps, its damper closes so that ash is not blown up into the furnace. The fan then sees a sudden fall in flow and a rise in pressure, followed by a recovery when the damper reopens. Secondary air for the suspended husk usually comes from a separate fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | About 100 to 180 mmWC |
| Air after the air heater | 120 to 180°C, downstream of the fan |
| Flow variation | Sudden drops of a quarter to a third when a section dumps |
| Dust | Clean air; husk dust near the boiler house |
| 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 rice husk dumping grate boilers
| Blade form | Backward curved with a steadily rising pressure curve toward lower flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, pedestal or motor-mounted |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive on plenum pressure, with section dampers for distribution |
| Also specified | Inlet screen; damper actuators sized for frequent operation |
What this FD fan has to cope with
Pressure rise when a section closes
Closing one section's damper pushes the fan back on its curve. A fan with a flat or drooping curve can surge or overfeed the remaining sections. Selecting a fan with a rising curve and controlling to plenum pressure keeps air to the burning sections steady.
Air heater bypass during dumping
Some operators open bypasses or change air heater dampers to cool the grate before dumping, which changes the system resistance the fan works against. If the fan is checked only at the design point it may overload or run unstable in the bypass condition. We check the selection at both the normal and the bypass conditions.
Damper wear from frequent cycling
Section dampers operate many times a day, often in a hot, dusty location below the boiler. Robust dampers with good shaft seals and actuators sized for frequent cycling, rather than the fan itself, are often the weak point of the air system. We specify the dampers along with the fan so the whole system holds its setting.
Energy and control
Because one section is periodically shut, the FD fan on a dumping grate boiler works over a wider range than on a continuous grate. A variable speed drive controlling plenum pressure saves the power otherwise lost across a throttled main damper, and smooths the transitions. Keeping excess air in check after each dump also reduces the gas the ID fan must handle.
ID fan and FD fan in rice husk dumping grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP or bag filter | Before the air heater and section dampers |
| What it handles | Flue gas with silica ash, surges at dumps | Ambient undergrate air |
| Temperature at the fan | 160 to 190°C | Ambient, 25 to 45°C |
| Main design concern | Holding draft through each dump | Steady pressure as sections close |
| Control | VFD on furnace draft | VFD on plenum pressure |
See also the Induced Draft (ID) Fan for rice husk dumping grate boilers.
Questions buyers ask
Why is air shut off before dumping a grate section?
So that the ash dropping into the hopper is not blown back up into the furnace, and so the remaining husk on that section does not flare. The section damper closes, the grate tilts and returns, fresh husk spreads onto it, and air is restored. The FD fan control must then follow the change in total flow.
Does the FD fan need a VFD on a dumping grate boiler?
It is strongly recommended. The drive holds plenum pressure steady as sections close and reopen, and saves energy compared with throttling a main damper all day. A fixed speed fan with damper control works, but air to the burning sections rises each time one section is shut, so air distribution is rougher.
What pressure FD fan is used for a rice husk dumping grate boiler?
Typically about 100 to 180 mmWC at the fan outlet, covering the air heater, ducts, section dampers, grate and the ash layer just before a dump. The exact figure depends on boiler size, grate design and how heavy the ash layer is allowed to become, so we size on the condition just before dumping.
Information needed to quote an FD fan for rice husk dumping grate boilers
- 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.
- Grate sections. Number of sections and whether they share one fan or have individual air supplies.
- Secondary air. Whether overfire air comes from this fan or a separate one.
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.