How bagasse dumping grate boilers work
Dumping grate bagasse boilers, typically 20 to 80 TPH, are common in Indian sugar mills. Bagasse is spread pneumatically over a stationary grate made of tilting sections. Fines burn in suspension; the rest burns on the grate, where ash and sand collect until a section is tilted to dump it, usually every few hours depending on cane cleanliness. Many older mills fitted wet scrubbers after mechanical collectors to meet emission limits, while newer units use ESPs. Air comes from an FD fan through an air heater to each section, with secondary and distributor air from separate fans.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate duct
- Section dampers
- Dumping grate
The FD fan pushes ambient air through a tubular air heater, raising it to about 200 to 250°C, then into a duct with a damper for each grate section. When a section is dumped, its damper closes and the fan flow falls by a share proportional to that section. Distributor air for the bagasse spreaders and secondary air are from separate fans. Pressure at the fan is typically 120 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 250°C, downstream of the fan |
| Pressure at the fan outlet | About 120 to 200 mmWC |
| Flow variation | Drops when each section is dumped |
| Dust | Bagasse pith around the intake |
| 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 bagasse dumping grate boilers
| Blade form | Backward curved with a rising curve toward low flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on plenum pressure |
| Also specified | Inlet screen; section dampers with good seals |
What this FD fan has to cope with
Sand-heavy ash raises dumping frequency
When cane carries much soil, sand ash builds quickly on the grate and sections must be dumped more often, sometimes every hour or two. Each dump changes FD flow. A variable speed drive controlling to plenum pressure handles frequent dumping without upsetting air to the other sections, where a fixed damper setting cannot.
Hot air heater exit
High air preheat helps dry wet bagasse, but it raises tube metal temperatures and, together with sand erosion on the gas side, leads to leaking air heater tubes. Leakage adds to FD flow and to the gas volume through the ID fan, so it is included in the fan selection and checked at each annual overhaul.
Section damper leakage
Worn section dampers let air blow through a section while it is being dumped, lifting ash and sand into the furnace and raising the dust load toward the collector and ID fan. They also waste air through idle sections. Damper seals and actuators deserve the same attention as the fan itself at each overhaul.
Energy and control
The FD fan runs the whole crushing season. A variable speed drive on plenum pressure saves power compared with throttling and handles dumping cycles smoothly. Lower excess air through better bagasse distribution also cuts the wet gas volume the ID fan must handle, which is usually where the larger saving lies.
ID fan and FD fan in bagasse dumping grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After wet scrubber or ESP | Before the air heater |
| What it handles | Saturated gas with sand slurry, or dry gas | Ambient air |
| Temperature at the fan | 60 to 75°C wet, 150 to 180°C dry | Ambient, 25 to 45°C |
| Main design concern | Corrosion, deposits, draft at dumping | Pressure as sections close |
| Control | Furnace draft, fast response | Plenum pressure |
See also the Induced Draft (ID) Fan for bagasse dumping grate boilers.
Questions buyers ask
Why does our bagasse boiler lose steam after each grate dump?
During the dump one section has no air and no burning bed, and the fresh bagasse spread on it takes time to dry and ignite. A well-controlled FD fan holds air to the other sections so the drop is short. Hot undergrate air speeds ignition on the dumped section.
What size FD fan is needed for a 40 TPH bagasse dumping grate boiler?
It depends on bagasse moisture, excess air, the share of undergrate air and the air heater resistance. Typically the fan delivers about 120 to 200 mmWC at its outlet. Send us the boiler design data or the existing fan nameplate and, if possible, measured plenum pressure, and we will calculate the duty.
Is a separate fan used for bagasse distributor air?
Usually yes. Distributor or spreader air needs higher pressure than undergrate air to throw bagasse evenly across the grate, so it comes from its own fan. On some smaller boilers it is taken from the FD fan discharge through a booster fan. Either way, it is sized separately from the undergrate air duty.
Information needed to quote an FD fan for bagasse 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.
- Section layout. Number of grate sections and size of each.
- Air heater. Air heater outlet temperature and 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.