How bagasse pinhole grate boilers work
A pinhole grate is a stationary, water-cooled grate made of membrane tubes with flat fins drilled with many small holes, through which undergrate air passes. Because it is cooled by boiler water, it tolerates the high air preheat bagasse needs. Bagasse is spread pneumatically, burns largely in suspension, and the remainder burns on a thin bed. Ash and sand settle on the grate and are removed periodically, by steam or air lancing or by raking, rather than by tilting or moving the grate. Pinhole grate boilers are typically 30 to 100 TPH and pass the gas through superheater, bank, economiser and air heater to a mechanical collector and ESP or wet scrubber.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Pinhole grate fins
- Furnace
The fan takes ambient air at ground level and delivers it through the tubular air heater, where it is heated to about 200 to 260°C, into a plenum under the grate. The air leaves the plenum through the pinholes. The water-cooled grate tolerates this hot air without burning. Overfire air and distributor air for the pneumatic spreaders come from separate fans.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 200 to 260°C, downstream of the fan |
| Pressure at the fan outlet | About 180 to 280 mmWC including the pinhole grate |
| Grate resistance | Rises as pinholes block with sand and fused ash |
| 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 pinhole grate boilers
| Blade form | Backward curved, selected with pressure margin for blocked pinholes |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with pedestal bearings |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on plenum pressure and oxygen |
| Also specified | Inlet screen, plenum pressure measurement, outlet isolation damper |
What this FD fan has to cope with
Blocked pinholes
Sand and molten ash can plug the small holes, especially where bagasse piles on the grate. Resistance rises and air shifts to the open areas, leaving dead spots that clinker. A fan with enough pressure margin, and regular grate cleaning, keeps air flowing through the whole grate.
Grate cooling depends on air distribution
Although the grate is cooled by boiler water, areas with poor air flow sit under thick, hot ash and the fins between tubes run hotter than designed. Steady plenum pressure from the FD fan, rather than large swings from damper hunting, keeps air moving through every part of the grate and protects the fins from local overheating and cracking around the holes.
High air preheat and air heater leaks
Pinhole grates run with high air preheat, which makes the air heater work hard and, with sand on the gas side, leads to tube leaks. Leaks pass FD air to the flue gas, increasing FD duty and ID gas volume together. The fan is sized with a leakage allowance and checked at the end-of-season condition, not only when new.
Energy and control
The pinhole grate itself absorbs pressure, so the FD fan on these boilers works harder than on a dumping grate of the same size. Keeping the holes clear is the main energy measure: blocked holes raise fan power while also worsening combustion. A variable speed drive on plenum pressure with oxygen trim keeps the fan close to the real requirement through the season.
ID fan and FD fan in bagasse pinhole grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP or scrubber | Before the air heater and pinhole grate |
| What it handles | Wet flue gas with sand, bursts when lancing | Ambient air |
| Temperature at the fan | 145 to 175°C | Ambient, 25 to 45°C |
| Main design concern | Erosion and draft during de-ashing | Pressure margin for blocked holes |
| Control | Furnace draft | Plenum pressure and oxygen |
See also the Induced Draft (ID) Fan for bagasse pinhole grate boilers.
Questions buyers ask
Why does a pinhole grate need a higher pressure FD fan?
Air must pass through many small drilled holes in the grate fins, which create a deliberate pressure drop to spread air evenly under a thin bagasse bed. That drop, plus the air heater and ducts, usually puts the FD fan at about 180 to 280 mmWC, higher than an open dumping grate.
What happens if the FD fan is too small on a pinhole grate boiler?
As pinholes block during the season, an undersized fan cannot maintain plenum pressure. Air goes through the clearest areas only, ash fuses on the starved areas, and the boiler loses output and efficiency until the next cleaning. Fan selections therefore include pressure margin for partly blocked holes, typically checked against measured plenum pressure late in the season.
Can the FD fan on a pinhole grate boiler be controlled by VFD?
Yes. A variable speed drive controlled from plenum pressure, with oxygen trim, gives steady air through the grate as load and hole condition change, and saves power at part load compared with throttling. An inlet or outlet damper remains for isolation and start-up, and the drive is interlocked with the ID fan.
Information needed to quote an FD fan for bagasse pinhole 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 data. Grate area, pinhole size and pitch, and design plenum pressure.
- Air preheat. Air heater outlet temperature and design 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.