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 FD fan sits
- Inlet filter
- FD (cooling) fan
- Cooling zone windboxes
- Pellet and hearth layer
- Hood
- Firing and drying zones
The cooling air fan delivers into the windboxes beneath the first and second cooling zones. Air passes up through the hearth layer and pellet bed, together typically 400 to 550 mm deep. Air from the first cooling zone reaches 900 to 1,000°C and flows along the hood to the firing zones; air from the second cooling zone, at 300 to 500°C, goes to updraught drying. The fan works against windboxes, pallets and the bed, typically 500 to 800 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air leaving the bed | 900 to 1,000°C from the first cooling zone, downstream of the fan |
| Dust | Fine ore and pellet dust in the plant air |
| Bed | Hearth layer and fired pellets, 400 to 550 mm deep |
| Pressure at the fan outlet | Typically 500 to 800 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 straight grate pellet induration machines
| Blade form | Backward curved aerofoil, high efficiency on ambient air at high pressure |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Arrangement 8 or 3, double inlet on large machines |
| Bearings and cooling | Oil-lubricated pedestal bearings with vibration and temperature monitoring |
| Flow control | Variable speed drive or inlet guide vanes on cooling air flow and hood pressure |
| Also specified | Inlet filter and silencer, and an outlet damper for each cooling zone |
What this FD fan has to cope with
Hood pressure balance
Air from the cooling fan becomes firing hood air. If cooling air exceeds what the hood exhaust and windbox fans take away, the hood goes positive and hot gas escapes at seals; if it is short, cold air leaks in and firing suffers. Fan control is tied to hood pressure.
Hot pellets at discharge
If cooling air falls short, pellets leave the machine above about 150°C and damage conveyor belts. The fan has flow margin for maximum production and the coarsest pellet size distribution. Discharge temperature is the clearest sign that cooling air is short.
Ore dust at the intake
Plant air around a pellet machine carries fine, hard ore dust that erodes blades and gets into bearings. A filtered intake away from the discharge end keeps the fan clean. Bearings sealed against fine dust last longer, and inlet filters with pressure alarms show when to clean them.
Energy and control
The cooling air fan sets the heat brought back to the firing hood, so its flow is chosen for both pellet cooling and fuel economy. A variable speed drive on hood pressure and cooling air flow follows production without throttling. Reducing seal leakage at the cooling zone also saves fan power and fuel together.
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 Induced Draft (ID) Fan for straight grate pellet induration machines.
Questions buyers ask
What does the cooling air fan do on a straight grate pellet machine?
It cools the fired pellets by blowing air up through them, and that heated air then supplies the firing hood as combustion air and the updraught drying zone as drying air. It is the forced draft fan of the straight grate. Its flow therefore affects fuel rate.
What pressure does a straight grate cooling fan need?
Typically 500 to 800 mmWC, to push air through the windboxes, pallets, hearth layer and pellet bed. It depends on bed depth, pellet size and machine width, which we confirm before selection. Larger and wider machines often use double inlet fans to handle the flow.
Why does hood pressure matter for the cooling fan?
The cooling fan supplies the air that flows along the hood to the firing zones. Its flow must match what the exhaust fans remove, or the hood either blows hot gas out at the seals or draws cold air in. Hood pressure is the main control signal.
Information needed to quote an FD fan for straight grate pellet induration machines
- 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.
- Cooling zones. Area of each cooling zone, bed depth and cooling air flow.
- Hood balance. Hood pressure set point and the fans that control it.
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.