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 ID fan sits
- Dumping grate furnace
- Boiler bank
- Air heater
- Mechanical collector
- Wet scrubber
- ID fan
- Chimney
On a mill with a wet scrubber, the ID fan sits after the scrubber and its mist eliminator, handling gas saturated at about 60 to 75°C. On a mill with an ESP, it handles dry gas at about 150 to 180°C. Suction is typically 200 to 300 mmWC, with the scrubber adding a significant share. Each dump briefly increases ash and sand in the gas and changes furnace pressure.
Gas at the ID fan
| Gas temperature, normal | 60 to 75°C after a wet scrubber, or 150 to 180°C dry |
| Maximum or upset | Up to 200°C if scrubber water fails |
| Dust at the fan | Wet sand and ash slurry droplets after a scrubber; surges after each dump |
| Moisture and dew point | Saturated gas with entrained droplets after the scrubber |
| Corrosive species | Scrubber water with dissolved ash; mildly acidic to alkaline depending on make-up water |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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 bagasse dumping grate boilers
| Blade form | Radial or backward curved, self-cleaning shape against wet deposits |
| Material of construction | Stainless steel or rubber-lined mild steel after a wet scrubber; mild steel with liners on dry ESP duty |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings; shaft seal against wet gas |
| Flow control | Variable speed drive on furnace draft with fast response |
| Also specified | Casing drains, wash connections and a design check at the dry scrubber-failure temperature |
What this ID fan has to cope with
Mud build-up on blades
Droplets carrying ash and sand stick to the blades and harden. Build-up is rarely even, so the impeller goes out of balance within weeks. A good mist eliminator, blade shapes that shed deposits, water wash connections and a drain keep the fan running between stops.
Corrosion and erosion together
Wet sand slurry both corrodes and erodes mild steel faster than either would alone, because erosion keeps removing the protective rust layer. An unprotected mild steel impeller after a scrubber may last only part of a season. Rubber lining, or stainless steel with hard liners at the leading edge, gives the fan a full season of life.
Scrubber water failure
If scrubber water stops, the fan suddenly sees dry gas at 150 to 200°C. Rubber linings, shaft seals and bearings must survive this until the boiler trip acts. We check materials, lining grade and shaft cooling at the dry temperature, and recommend a low scrubber water flow interlock to the boiler trip.
Draft swings at dumping
Tilting a section loaded with bagasse ash and field sand disturbs furnace pressure, and fresh bagasse spread on the dumped section can flare. A slow damper lets the furnace go positive and flame escapes at the doors. A variable speed drive with fast draft response keeps the furnace negative through the dump.
Energy and control
Dumping grate boilers with wet scrubbers have high system resistance, so the ID fan is often the largest auxiliary motor. Replacing damper control with a variable speed drive, keeping the scrubber pressure drop at its design value, and cleaning the mist eliminator regularly bring the largest savings. Mills converting from scrubber to ESP should re-check the fan, since the new duty is hotter, drier and lower in resistance.
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 Forced Draft (FD) Fan for bagasse dumping grate boilers.
Questions buyers ask
What ID fan material is used after a wet scrubber on a bagasse boiler?
Stainless steel or rubber-lined mild steel is used, because the gas is saturated and carries wet ash and sand that corrode and erode plain mild steel quickly. The fan also needs casing drains, water wash connections and a design check at the higher dry temperature in case scrubber water fails during operation.
Why does the ID fan vibrate after a few weeks on a bagasse dumping grate boiler?
Wet ash and sand from the scrubber build up unevenly on the blades and harden. A better mist eliminator, regular water washing through fixed nozzles and blade shapes that shed deposits slow the build-up. Balancing restores the fan for a time, but it does not remove the cause, so the deposits return.
Can we change the ID fan when converting from wet scrubber to ESP?
Yes, and it is usually necessary. Gas at an ESP outlet is hotter and drier, at lower density, and the system resistance changes once the scrubber is removed. The existing fan may be the wrong size, speed and material for the new duty. We recalculate the duty for the new system before reusing or replacing the fan.
Information needed to quote an ID fan for bagasse dumping grate boilers
- 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.
- Scrubber details. Type of scrubber, pressure drop, and whether a mist eliminator is fitted.
- Dumping cycle. Frequency of dumping and number of grate sections.
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.