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 ID fan sits
- Pinhole grate furnace
- Superheater and bank
- Economiser
- Air heater
- Mechanical collector
- ESP or wet scrubber
- ID fan
- Chimney
The ID fan follows the ESP or scrubber, with suction typically 200 to 300 mmWC. When the operators lance or rake the grate, the ash layer is broken up and much of it, with sand, is lifted into the gas stream rather than falling into a hopper. The mechanical collector and ESP take most of this, but outlet dust rises for the duration of de-ashing, and the furnace draft fluctuates as the bed is disturbed.
Gas at the ID fan
| Gas temperature, normal | 145 to 175°C after the air heater |
| Maximum or upset | Up to 210°C with fouled air heater |
| Dust at the fan | Low in normal running after ESP; rises during grate lancing |
| Moisture and dew point | About 20 to 25% water vapour; dew point near 60 to 65°C |
| After a wet scrubber | Saturated at 60 to 75°C with wet sand droplets |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 pinhole grate boilers
| Blade form | Backward curved with liners after an ESP, radial or self-cleaning after a scrubber |
| Material of construction | Mild steel with wear liners on dry duty; stainless or rubber lining after a scrubber |
| Drive and arrangement | Direct coupled, single or double inlet by size |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Liners at the scroll cut-off, inspection door, casing drain |
What this ID fan has to cope with
Ash and sand bursts during lancing
Steam or air lancing throws ash and sand from the grate into the furnace, and the fine fraction travels to the collectors. If an ESP field is weak, the fan receives a short, heavy dose of sand. Leading edge liners and a moderate tip speed protect the impeller against this repeated exposure.
Draft fluctuation during de-ashing
Breaking up the ash layer suddenly changes the resistance to undergrate air, and steam lancing adds its own volume to the gas. A slow damper loop lets the furnace swing positive. A variable speed drive tuned for prompt response holds the furnace negative, avoiding flame and ash puffing out at the doors where operators are working.
Ash deposits at low gas temperature
If the air heater is oversized or load is low, gas temperature at the fan approaches the water dew point and moist fine ash sticks to the blades and casing. A temperature margin in the air heater design, and a drain and inspection door on the fan, prevent gradual build-up and unbalance.
Energy and control
Pinhole grate boilers in sugar mills run continuously through the crushing season. A variable speed drive on the ID fan saves power at the many hours of part load caused by uneven cane supply, and handles de-ashing disturbances better than a damper. Keeping the air heater free of sand deposits holds both draft loss and fan power at their design values.
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 Forced Draft (FD) Fan for bagasse pinhole grate boilers.
Questions buyers ask
Why does ID fan current rise when we clean the pinhole grate?
Lancing or raking the grate adds steam and ash to the gas and changes air flow through the grate, so the fan moves more gas briefly to hold draft. A short rise in current is normal. A large or lasting rise suggests the fan lacks margin or the air heater is fouled.
What type of ID fan suits a bagasse pinhole grate boiler?
After an ESP, a backward curved ID fan with wear liners at the leading edge and scroll cut-off, designed for about 145 to 175°C and the wet flue gas of bagasse. After a wet scrubber, a corrosion-resistant fan with a self-cleaning blade form. A variable speed drive is recommended in both cases.
How is a pinhole grate boiler different for the ID fan from a dumping grate?
On a dumping grate, ash falls into a hopper when a section tilts. On a pinhole grate, de-ashing by lancing or raking puts more of the ash into the gas stream, so the collectors and the ID fan see more sand during cleaning. Normal running conditions are otherwise similar.
Information needed to quote an ID fan for bagasse pinhole 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.
- De-ashing method. Whether the grate is cleaned by steam lancing, air lancing or raking, and how often.
- Collector. Mechanical collector with ESP, or wet scrubber, and measured outlet dust.
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.