How straight grate pellet induration machines work
A straight grate induration machine carries green pellets on one travelling grate, over a hearth layer of fired pellets that protects the pallets, through updraught drying, downdraught drying, preheat, firing, after-firing and two cooling zones. Burners along the hood, firing oil, natural gas or coke oven gas, bring the gas to about 1,300°C. Air blown up through the first cooling zone reaches the hood at 900 to 1,000°C and serves as combustion air; air from the second cooling zone goes to updraught drying. A recuperation fan draws hot windbox gas from the firing zones to the downdraught drying hood, and the waste gas fan pulls the cooler windbox gas through an ESP to the stack. Four or more large fans run as one balanced system.
Where the ID fan sits
- Downdraught drying hood
- Green pellet bed
- Drying and preheat windboxes
- ESP
- Waste gas (ID) fan
- Stack
The waste gas fan sits after the ESP at the end of the windbox system. The recuperation fan works alongside it on the firing zone windboxes, handling gas at 350 to 450°C, so the two fans divide the windboxes between them and their suctions interact. The waste gas fan pulls through the bed, hearth layer, pallets and windboxes, typically 450 to 700 mmWC. Because the gas has just dried wet pellets, it reaches the fan cool and laden with water vapour.
Gas at the ID fan
| Gas temperature, normal | 90 to 140°C after the ESP |
| Maximum or upset | Up to 250°C when the grate stops with the firing hood hot |
| Dust at the fan | Fine iron oxide and bentonite, typically 30 to 80 mg/Nm³ |
| Moisture and dew point | High water vapour from 8 to 10% green pellet moisture; water dew point 55 to 65°C |
| Corrosive species | SO₂ from fuel oil or sulphur in the ore; acid dew point near 110 to 130°C |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 straight grate pellet induration machines
| Blade form | Backward curved with wear liners, double inlet on large machines |
| Material of construction | Mild steel casing with internal insulation; wear-resistant plate on blades; SS 316 for the inlet box where gas runs close to dew point |
| Drive and arrangement | Arrangement 3 or 8 with a heavy rotor sized for the large flow |
| Bearings and cooling | Oil-lubricated sleeve or roller bearings with vibration monitoring |
| Flow control | Variable speed drive on drying zone windbox suction, coordinated with the recuperation fan |
| Also specified | External insulation, casing drains, large cleaning doors and water wash nozzles for deposit removal |
What this ID fan has to cope with
Mud on the impeller
Water vapour from drying and fine bentonite dust that passes the ESP form a mud on the blades when the gas approaches its dew point, especially at start-up and low output. Insulation, keeping the gas above about 90°C and periodic washing with drying afterwards prevent heavy vibration.
Interaction with the recuperation fan
The waste gas fan and the recuperation fan both draw on windboxes connected along the grate. A change in one shifts the suction under the other zone and moves the burn-through point. Coordinated speed control of the two fans keeps zone pressures stable.
Over-wetting of the bed
If the drying gas cools too far, water condenses in the lower layers of green pellets, which soften and collapse, reducing bed permeability. The fan then sees falling flow at rising suction. Proper updraught drying and gas temperature control prevent it.
Hot stops
When the grate stops with the hood hot, hot gas can reach the waste gas fan. Its impeller stress is checked at the upset temperature, and dilution or bypass dampers limit the rise. The fan keeps running on such stops to protect the bed and pallets.
Energy and control
A straight grate machine runs a set of very large fans continuously, and the waste gas fan is one of the largest. Variable speed control on windbox suction saves power when production or pellet size changes. Sealing at pallet side walls and windbox seals reduces the leakage air the fan carries, and good updraught drying improves bed permeability, which lowers the suction needed.
ID fan and FD fan in straight grate pellet induration machines
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP at the windbox end | Below the cooling zones |
| What it handles | Wet drying gas with ore dust | Ambient cooling air |
| Temperature at the fan | 90 to 140°C | Ambient, 25 to 45°C |
| Main design concern | Condensation and mud build-up | Hood balance and deep bed pressure |
| Control | Windbox suction | Hood pressure and cooling air flow |
See also the Forced Draft (FD) Fan for straight grate pellet induration machines.
Questions buyers ask
What is the waste gas fan on a straight grate pellet machine?
It is the ID fan of the straight grate. It draws gas down through the green pellet bed in the downdraught drying and preheat zones, through the windboxes and ESP, and discharges it to the stack. It sets the suction across the wet bed. Its duty interacts with the recuperation fan.
Why does the waste gas fan of a pellet induration machine vibrate?
Most often from mud build-up: moist drying gas and fine bentonite-laden dust stick to the blades when the gas nears its dew point. Keeping the gas warm, insulation and periodic washing are the remedies, followed by balancing. Inspection doors make cleaning practical. Vibration trending gives early warning.
How many fans does a straight grate induration machine have?
Typically four or more process fans: the cooling air fan, the hood exhaust or updraught drying fan, the windbox recuperation fan and the waste gas fan, plus combustion air fans. They work as one balanced system. A change in any one fan affects the zone pressures of the others.
Information needed to quote an ID fan for straight grate pellet induration machines
- 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.
- Machine data. Grate area and width, bed and hearth layer depth, production rate and zone suctions.
- Fan system. Duties of the recuperation and waste gas fans, and how windboxes are divided between them.
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.