How mineral flash calciners work
Flash or gas suspension calciners treat fine minerals in a hot gas stream for a few seconds instead of hours in a kiln. Kaolin is converted to metakaolin at about 700 to 850°C, alumina hydrate to alumina at 1,000 to 1,100°C, and gypsum to plaster at 150 to 180°C, while magnesite and phosphates have their own windows. Feed passes up through cyclone preheaters, enters the calciner vessel where a burner fires oil or gas, and is separated in a hot cyclone. Product then cools in cyclone coolers or a fluid bed, which preheat the combustion air. The whole system typically runs under suction from a single ID fan after the ESP or bag filter, with a smaller FD fan supplying burner air.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner primary air line
- Calciner burner
The fan sits near the calciner burner and delivers ambient air to it as primary air. The main combustion air enters through the product cooling cyclones under the suction of the ID fan, picking up heat from the mineral before it reaches the calciner. Some designs also use a separate fan to fluidise a fluid bed cooler. The primary air fan works against the burner and its piping, often 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Mineral dust in the plant; a filtered inlet is essential |
| Pressure at the fan outlet | Typically 300 to 700 mmWC, set by the burner |
| Share of total air | Primary air is usually a small fraction of the total combustion air |
| 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 mineral flash calciners
| Blade form | High-pressure backward curved or radial, to suit the burner |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive or damper, linked to burner firing |
| Also specified | Inlet filter, silencer and low-pressure interlock |
What this FD fan has to cope with
Flame instability
The burner relies on primary air for flame shape and mixing. If the primary air pressure fluctuates, the flame can become unstable or impinge on the calciner wall, causing localised overheating. A fan with a steep pressure curve and good control at the burner keeps the flame steady.
Balance between primary and cooler air
Too much primary air reduces the air drawn through the coolers, so the product leaves hotter and less heat is recovered. The primary air flow is kept to the minimum the burner needs, and its control is coordinated with the ID fan draft so the total air and the share drawn through the coolers both stay on target.
Mineral dust at the fan inlet
Calcining plants handle fine powder, and an unfiltered inlet draws it into the fan and the burner, wearing both. A filtered inlet from a clean area is part of a good specification and costs little. Fine powder that reaches the burner can also block small air passages and upset the flame.
Energy and control
The primary air fan is small compared with the ID fan, but excess primary air costs fuel because it is cold air that bypasses the cooler heat recovery. Keeping primary air at the burner maker's minimum and using a variable speed drive saves both fan power and fuel. Checking the primary air setting after any burner change is a simple, low-cost step.
ID fan and FD fan in mineral flash calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | At the calciner burner |
| What it handles | System gas with fine mineral dust | Clean ambient air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Steady draft through many cyclone stages | Flame stability |
| Control | Calciner pressure or temperature | Burner firing rate |
See also the Induced Draft (ID) Fan for mineral flash calciners.
Questions buyers ask
Does a flash calciner have an FD fan?
It usually has a burner primary air fan, which acts as the FD fan. Most of the combustion air is drawn through the product coolers by the ID fan, where it is preheated. Some designs also have a fluidising air fan for a fluid bed cooler.
Why is the primary air fan pressure so high on a calciner burner?
Burner primary air is used to shape the flame and mix fuel with the hot secondary air. High-momentum burners need pressures of 300 to 700 mmWC or more. The fan is selected on the burner design to keep the flame stable throughout the firing range.
How is combustion air controlled in a flash calciner?
The ID fan draft controls the main air flow through the coolers and calciner, and the burner primary air fan supplies a controlled, small amount of air to the burner. Oxygen at the calciner outlet is used to trim the overall air-fuel ratio. The two fans are tuned together.
Information needed to quote an FD fan for mineral flash calciners
- 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.
- Burner. Burner type, firing capacity and primary air pressure required.
- Cooler air. Whether cooler air is drawn by the ID fan or blown by a separate fan.
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.