How vermiculite exfoliation furnaces work
Crude vermiculite concentrate is dropped through a vertical or slightly inclined furnace fired on gas, LPG or light oil, where the flame zone runs at about 900 to 1,100°C. The water held between the mica layers flashes to steam in a second or two and the grains expand eight to twelve times into a very light product, often only 60 to 130 kg/m³. The expanded granules fall to a discharge or are carried upward by the gas stream into a product cyclone. Gas leaving the cyclone carries fine vermiculite flakes and gangue, and passes through dilution air, a bag filter and the ID fan to the stack. A small combustion air fan supplies the burner at the base of the furnace.
Where the FD fan sits
- Inlet filter
- FD fan
- Air valve
- Burner
- Furnace shaft
The fan stands beside the furnace at ground level and delivers ambient air to a nozzle-mix or packaged burner through a ratio valve. Combustion air is only part of the gas in the furnace; much more enters as induced air through the ore feed and inspection openings, drawn in by the ID fan. The FD fan therefore works against the burner and air train resistance, usually 150 to 400 mmWC depending on burner type, rather than against the whole system.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Ore dust from the feed hopper and bagging area can reach an unprotected inlet |
| Pressure at the fan outlet | Typically 150 to 400 mmWC, set by the burner |
| Turndown | Wide, as firing rate follows ore feed and grade |
| 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 vermiculite exfoliation furnaces
| Blade form | Backward curved or high-pressure radial, chosen to suit the burner pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft for small burners; direct coupled for larger |
| Bearings and cooling | Motor bearings or standard pedestal bearings; no cooling needed |
| Flow control | Ratio valve linked to fuel; variable speed drive on larger furnaces |
| Also specified | Inlet filter and silencer, and a pressure switch for burner safety interlock |
What this FD fan has to cope with
Ore dust at the fan inlet
Vermiculite plants are dusty at the feed and bagging ends, and fine mica drawn into the combustion air fan coats the blades and plugs the burner air passages. A filtered inlet taken from outside the building avoids drift in the air-fuel ratio, and the filter is easy to check at each shift because its pressure drop is shown on a simple gauge.
Unstable flame at low feed
When the plant runs a light grade at low throughput, the burner is turned down hard. If the fan curve is flat near shut-off, air pressure wanders and the flame becomes unsteady. A fan selected with its normal point well right of the peak and a stable rising curve holds pressure at low flow.
Noise near operators
The combustion air fan often sits near the feed platform where operators work. High-pressure burner fans are noisy, and an inlet silencer and acoustic lagging on the outlet duct are part of a sensible specification. Mounting the fan on anti-vibration pads also stops noise travelling into the furnace platform.
Energy and control
The combustion air fan on an exfoliation furnace is small, and its energy is minor compared with the ID fan. Savings come mainly through the burner: an accurate ratio between air and fuel keeps excess air down, which reduces fuel use and the gas volume the ID fan must handle. Where a fixed-speed fan runs against a nearly closed valve for long periods, a variable speed drive is worth fitting.
ID fan and FD fan in vermiculite exfoliation furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the product cyclone and bag filter | At the burner, base of the furnace |
| What it handles | Combustion gas, steam and mica fines | Clean ambient air |
| Temperature at the fan | 120 to 180°C, up to 250°C upset | Ambient |
| Main design concern | Suction that sets product lift and density | Stable air-fuel ratio |
| Control | Cyclone or furnace suction per grade | Burner firing rate |
See also the Induced Draft (ID) Fan for vermiculite exfoliation furnaces.
Questions buyers ask
Does a vermiculite exfoliation furnace need an FD fan?
Most do, for the burner combustion air, although packaged burners carry their own fan. The bulk of the gas in the furnace, however, is induced air drawn in by the ID fan, so the FD fan is much smaller than the exhaust fan. It still matters, because a steady air supply keeps the flame and the expansion of each grade repeatable.
What pressure does the combustion air fan need for a gas burner on an exfoliation furnace?
It depends on the burner design. Nozzle-mix burners commonly need 150 to 300 mmWC at the burner, and high-velocity designs more. The fan is selected on the burner maker's figure plus the losses in the filter, ratio valve, safety shut-off valves and piping, with a modest margin. Too much margin leaves the fan throttled at the valve for most of its life.
Can one combustion air fan serve two exfoliation furnaces?
It can, if each furnace has its own ratio valve and the fan holds a steady header pressure. Separate fans are usually simpler, because the two furnaces often run different grades and one can stop without disturbing the other. A shared fan also leaves both furnaces stopped if it fails.
Information needed to quote an FD fan for vermiculite exfoliation furnaces
- 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 make and type, firing rate and the required air pressure at the burner inlet.
- Furnace count. Number of furnaces served and whether they run independently.
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.