How bagasse travelling grate boilers work
Travelling grate bagasse boilers, typically 40 to 150 TPH, spread bagasse pneumatically across a slowly moving grate that carries ash continuously to the front of the furnace. Most of the fine bagasse burns in suspension; the coarser fibre lands on the grate and burns out as it travels. Continuous ash discharge gives steady furnace conditions without the cycles of a dumping grate. Bagasse at about 50% moisture produces a large, wet flue gas flow, which passes the superheater, boiler bank, economiser and a large air heater before an ESP. Two ID fans in parallel are common, with FD, secondary air and distributor air fans on the air side.
Where the ID fan sits
- Furnace and travelling grate
- Superheater
- Boiler bank
- Economiser
- Air heater
- ESP
- ID fans in parallel
- Chimney
The ID fans sit after the ESP, sharing a common suction duct, each typically sized for 55 to 60% of full load flow. Upstream resistance includes the furnace, superheater and bank, economiser, a large tubular air heater and the ESP, so suction is typically 200 to 300 mmWC. Because the travelling grate discharges ash continuously, gas conditions at the fan are steady and the ESP keeps dust low, which allows high efficiency blading.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the air heater |
| Maximum or upset | Up to 200°C with fouled air heater or ESP bypass |
| Dust at the fan | Low after ESP, typically 50 to 100 mg/Nm³; sand fraction if ESP fields trip |
| Moisture and dew point | About 20 to 25% water vapour by volume; dew point near 60 to 65°C |
| Arrangement | Two fans in parallel, each about 55 to 60% of full flow |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 travelling grate boilers
| Blade form | Backward curved or aerofoil, with bolted liners at the leading edge |
| Material of construction | Mild steel; leading edge liners against sand |
| Drive and arrangement | Direct coupled; double inlet double width on larger units |
| Bearings and cooling | Pedestal bearings with cooling disc; temperature and vibration monitoring |
| Flow control | Variable speed drives, with inlet dampers for isolation of one fan |
| Also specified | Outlet isolation dampers so one fan can be repaired while the other runs |
What this ID fan has to cope with
Parallel fan instability
Two fans on a common duct can share flow unevenly, and at low load one can be pushed into stall. Fans with matched curves, selected so that single-fan and two-fan operating points stay on the stable part of the curve, and speed control on both drives, prevent hunting.
ESP field trips
When an ESP field trips, sand and ash at the fan inlet can rise several times within minutes. Thin aerofoil blades are vulnerable to this, because the leading edge and hollow nose wear through quickly. Bolted leading edge liners, a vibration alarm and prompt restoration of the field protect the wheel through such periods.
Low temperature at start-up
At start-up and low load the gas leaving the air heater can fall close to the water dew point of bagasse flue gas. Condensation wets ash on the blades and casing. Warming the system before bagasse firing and a casing drain avoid deposits and corrosion.
Energy and control
Travelling grate bagasse boilers often run near rated output through the crushing season but drop when cane supply falls. Variable speed drives on both ID fans, controlled together from furnace draft, save power across this range and allow one fan to carry reduced loads. Efficient aerofoil or backward curved blades on clean gas after the ESP keep absorbed power low; on a large boiler the difference over a season is substantial.
ID fan and FD fan in bagasse travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP, two fans in parallel | Before the air heater and grate zones |
| What it handles | Large wet flue gas flow, low dust | Ambient air |
| Temperature at the fan | 140 to 170°C | Ambient, 25 to 45°C |
| Main design concern | Efficiency and parallel operation | Even air along a wide grate |
| Control | Furnace draft, VFDs on both fans | Steam pressure with oxygen trim |
See also the Forced Draft (FD) Fan for bagasse travelling grate boilers.
Questions buyers ask
Why do large bagasse boilers use two ID fans?
Two fans of about 60% each allow the boiler to keep running at reduced load if one fan is out of service, and spread the very large gas flow over two machines of practical size. They need matched curves and coordinated speed control to share load evenly.
Can an aerofoil ID fan be used on a bagasse boiler?
Yes, on a travelling grate bagasse boiler with a well-performing ESP the dust at the fan is low enough for aerofoil or backward curved blades, which are the most efficient choices for this large flow. Leading edge liners are added because field sand passes whenever an ESP field trips or rapping is disturbed.
What causes ID fan vibration on a bagasse travelling grate boiler?
Common causes are ash build-up on the blades after condensation at start-up, uneven wear from sand during ESP trips, and unstable flow sharing between parallel fans at low load. A vibration trend, an internal inspection at the next shutdown, and in-situ balancing usually identify and correct the problem without removing the rotor.
Information needed to quote an ID fan for bagasse travelling 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.
- Fan arrangement. Number of ID fans and the share of full flow each must carry alone.
- ESP performance. Design and actual ESP outlet dust and how often fields trip.
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.