How rice husk dumping grate boilers work
A dumping grate is a stationary grate whose sections are tilted at intervals, by hand lever or pneumatic cylinder, to drop accumulated ash into the hopper below. Rice husk dumping grate boilers are typically water tube or hybrid units of about 4 to 20 TPH. Husk is fed by pneumatic spreaders or chutes and burns partly on the grate, partly in suspension. Between dumps, every one to four hours depending on load, the ash layer grows and grate resistance rises. During a dump, air to that section is shut off and the furnace sees a brief surge of ash and a change in draft. The gas then passes the boiler bank, economiser, air heater and a multiclone with ESP or bag filter before the ID fan.
Where the ID fan sits
- Dumping grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan follows the final collector and holds the furnace a few millimetres negative. Total suction at the fan inlet is typically 150 to 250 mmWC. When a grate section tilts, the ash bed breaks up and fine silica and char are lifted into the gas stream for a short time, and sometimes a flare of unburnt husk follows. The collectors take most of this surge, but the fan sees a brief rise in dust and temperature at every dump.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C after the air heater |
| Maximum or upset | Brief rises of 20 to 40°C after each dump |
| Dust at the fan | Silica ash, with short surges after dumps; 50 to 200 mg/Nm³ after a bag filter or ESP |
| Draft variation | Short swings of several mmWC at each dump |
| Corrosive species | Low; husk is low in sulphur |
| Density at the design temperature | About 0.79 kg/m³ at 175°C, 66% 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 rice husk dumping grate boilers
| Blade form | Backward curved with wear liners, chosen for a stable pressure curve across the draft swings |
| Material of construction | Mild steel casing; wear-resistant plate or lined impeller blades |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft, tuned for fast response, with damper for isolation |
| Also specified | Liners at the scroll, access door, casing drain, temperature margin for post-dump peaks |
What this ID fan has to cope with
Draft swings at each dump
With a slow damper control, the furnace goes positive for a few seconds at each dump and hot gas escapes at the doors. A variable speed drive with a correctly tuned draft loop, and a fan whose pressure curve is stable over the operating range, keeps the furnace negative.
Ash surge to the collector and fan
Dumping lifts fine ash and char that would otherwise stay on the grate, and a short flare can carry glowing particles forward. A bag filter with spark arrestors, or an ESP, takes most of it. Liners at the impeller leading edge and the scroll cut-off protect the fan from the share that passes during the surge.
Rising resistance between dumps
As ash builds on the grate between dumps, operators often raise air to keep the husk burning, so excess air and gas volume drift upward through the cycle. A fan sized on the average condition runs short of capacity just before each dump. We size it on the highest gas volume in the cycle, not the average.
Energy and control
Dumping grate husk boilers usually serve parboiling and rice mill loads that vary through the day. A variable speed drive on the ID fan saves energy at part load and does the extra job of holding draft through each dump. Dumping more often at high load keeps the grate resistance and excess air lower, which reduces gas volume and fan power together.
ID fan and FD fan in rice husk dumping grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After ESP or bag filter | Before the air heater and section dampers |
| What it handles | Flue gas with silica ash, surges at dumps | Ambient undergrate air |
| Temperature at the fan | 160 to 190°C | Ambient, 25 to 45°C |
| Main design concern | Holding draft through each dump | Steady pressure as sections close |
| Control | VFD on furnace draft | VFD on plenum pressure |
See also the Forced Draft (FD) Fan for rice husk dumping grate boilers.
Questions buyers ask
Why does furnace pressure go positive when we dump the grate?
Tilting a section disturbs the ash bed and changes the air flow through the grate, and unburnt husk can flare as it falls. If the ID fan control is slow, the furnace briefly loses draft. A variable speed drive with a fast, well-tuned draft loop, and a fan with a stable curve, keep the furnace negative through the dump.
What ID fan is used on a rice husk dumping grate boiler?
Typically a backward curved ID fan with wear liners placed after an ESP or bag filter, designed for about 160 to 190°C and short temperature peaks after each dump, with a variable speed drive on furnace draft. Where the boiler has only a multiclone, a radial tipped wheel with liners is the safer choice.
Does grate dumping increase ID fan wear?
Somewhat. Each dump lifts a short burst of silica ash and char into the gas stream. Most is caught by the collector, but over a year these bursts add measurably to erosion at the blade inlet edges and the scroll cut-off. Liners at those points and a check of blade thickness at each shutdown handle it.
Information needed to quote an ID fan for rice husk 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.
- Dumping cycle. Dumping interval, method (manual or pneumatic) and number of grate sections.
- Draft behaviour. Furnace pressure trend through a dump, if recorded.
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.