How grate kiln pellet plants work
A grate kiln pellet plant splits induration across three machines. A travelling grate dries and preheats green pellets until they are strong enough to tumble; a rotary kiln, fired by a coal, gas or oil burner at the discharge end, hardens them at 1,250 to 1,320°C; and an annular cooler cools them. Hot air from the first cooler section enters the kiln as secondary air, and the kiln off-gas at about 1,000 to 1,100°C flows to the preheat zone of the grate. Gas drawn down through the preheat bed is returned by a windbox fan to the drying zones, and the final gas from the drying windboxes passes an ESP to the main waste gas fan. Most Indian grate kiln plants fire coal, which adds ash and sulphur to the gas.
Where the FD fan sits
- Inlet filter
- FD (cooler) fan
- Cooler section air box
- Pellet bed
- Cooler hood
- Kiln or grate
The annular cooler has typically three to five sections, each with its own fan beneath the pallets. Pellets fall from the kiln at about 1,200°C onto a bed 700 to 800 mm deep. Air from the first section goes to the kiln as secondary air, the second to the grate preheat zone, and later sections to the drying zone or atmosphere. Each fan works against its own bed and air box, typically 400 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the bed | About 1,000°C from the first section, falling to 150 to 300°C in the last |
| Dust | Pellet fines and ore dust in plant air |
| Section pressures | Different for each section; highest under the freshly charged bed |
| Pressure at the fan outlet | Typically 400 to 700 mmWC |
| 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 grate kiln pellet plants
| Blade form | Backward curved, for efficiency and a stable pressure curve |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Arrangement 8, one fan per section |
| Bearings and cooling | Pedestal bearings with condition monitoring |
| Flow control | Variable speed drive on each fan, by section air flow and hood pressure |
| Also specified | Inlet filters and silencers, and isolation dampers |
What this FD fan has to cope with
Secondary air to the kiln
The first section fan provides the kiln secondary air. If its flow drops, kiln combustion suffers and the burner flame lengthens. This fan is sized with the most margin and controlled with kiln hood pressure. Hood pressure at the kiln discharge guides control.
Uneven bed from the kiln
Pellets leave the kiln unevenly, with fines and clusters on one side. Air takes the easiest path, leaving hot spots. Fans with steep characteristics hold flow better through uneven beds. Spreading pellets evenly onto the cooler helps as well. Fans are checked against measured section pressures.
Seal leakage
The cooler's moving seals leak air, especially with wear. The fans move more air for the same cooling, and secondary air temperature falls. Seal maintenance is part of fan performance. Secondary air temperature trending reveals seal wear. Seal maintenance belongs in the fan care plan, and replacing worn seals restores both air flow and kiln heat recovery.
Energy and control
Each cooler fan can be controlled to its section's duty, and a variable speed drive on each allows close matching to production. The first section fan sets heat recovery to the kiln, so its flow should not be cut to save power; later section fans give the main saving opportunity at reduced output.
ID fan and FD fan in grate kiln pellet plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP at the grate feed end | Below each annular cooler section |
| What it handles | Moist gas with coal ash and ore dust | Ambient cooling air |
| Temperature at the fan | 110 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Erosion and acid near dew point | Secondary air and section balance |
| Control | Kiln inlet draught | Section air flow and hood pressure |
See also the Induced Draft (ID) Fan for grate kiln pellet plants.
Questions buyers ask
How many cooler fans does a grate kiln pellet plant have?
Typically three to five, one per section of the annular cooler. Each is selected for its own section pressure and air flow, and for where its heated air is used. The first section fan is usually the most critical, because it feeds the kiln. Their sizes differ accordingly.
What does the annular cooler fan supply to the kiln?
The first section fan's air is heated by the hot pellets to about 1,000°C and enters the kiln as secondary air for the burner. That recovery greatly reduces fuel consumption. If that fan falls short, the burner must burn more fuel for the same pellet hardness. Hood pressure helps confirm its flow.
What pressure do grate kiln cooler fans need?
Typically 400 to 700 mmWC, depending on the bed depth, pellet size and section. The first section often needs the highest pressure because its bed is freshly charged and densest. We confirm duties from cooler data before selecting each fan. Pellet size and fines content also matter.
Information needed to quote an FD fan for grate kiln pellet plants
- 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.
- Cooler sections. Number of sections, area and air flow for each.
- Air use. Where the air from each section is used and the temperatures required.
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.