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 ID fan sits
- Burner and furnace shaft
- Product take-off
- Product cyclone
- Dilution air inlet
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, well clear of the product stream, because a fan wheel that sees the exfoliated granules breaks them and spoils the grade. Upstream are the furnace, the take-off duct, the product cyclone and the filter, and a cold-air bleed that brings gas from 300 to 450°C at the cyclone down to a level the filter bags accept. Suction at the fan is typically 150 to 300 mmWC, and the operator trims it to change how high the light grains are lifted.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after dilution and the filter |
| Maximum or upset | 250°C or more if the dilution damper sticks or feed stops with the burner on |
| Dust at the fan | Light mica flakes and some quartz or feldspar fines; low after a bag filter, heavy if a bag fails |
| Moisture and dew point | Steam from the ore raises the dew point to about 45 to 60°C; condensation forms at shutdown and on cold duct walls |
| Gas composition | Combustion gas with a large share of tempering air; sulphur is low on gas or LPG firing |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 vermiculite exfoliation furnaces
| Blade form | Backward curved, with a steep enough curve to hold suction as the bag filter loads |
| Material of construction | Mild steel casing and impeller; abrasion-resistant plate on the blade tips where gangue content is high |
| Drive and arrangement | Belt driven or direct coupled, arrangement 9 or 8 on smaller furnaces |
| Bearings and cooling | Pedestal bearings outside the gas stream; shaft cooler only if the inlet can exceed 200°C |
| Flow control | Variable speed drive on furnace or cyclone suction, set per product grade |
| Also specified | Inlet damper for start-up, casing drain and an access door for clearing flake deposits |
What this ID fan has to cope with
Grade drift with filter loading
As the bag filter cake builds between cleaning cycles, the fan moves back along its curve and suction at the furnace falls. Lighter grains then drop out early and the product becomes coarser and heavier. A fan with a steep characteristic and a speed loop on cyclone suction keeps the take-off velocity steady between pulses.
Hot excursions at the filter and fan
If ore feed stops while the burner stays on, the furnace exit temperature climbs quickly. Without enough dilution air, filter bags are scorched and the fan sees gas well above its rating. Interlocks on filter inlet temperature that open the bleed damper and trip the burner protect both.
Flake build-up on blades
Vermiculite fines are flat and slightly hygroscopic. Where the gas cools close to its dew point, they paste onto the pressure face of the blades and fall off in pieces, causing sudden vibration. Insulating the filter and duct and keeping the fan inlet above about 110°C prevents this.
Energy and control
Exfoliation lines often change grade several times a week, and each grade needs a different furnace suction and gas velocity. Running the ID fan on a damper means it spends most hours throttled. A variable speed drive with a stored speed setting for each grade saves power and makes changeovers repeatable. Reducing the dilution air, by fixing leaks and controlling the bleed on filter inlet temperature, cuts the gas volume the fan must move.
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 Forced Draft (FD) Fan for vermiculite exfoliation furnaces.
Questions buyers ask
Why does my vermiculite come out heavier when the ID fan is slowed down?
In furnaces where the gas stream carries the expanded grains to the cyclone, lower suction means lower velocity at the take-off. Only the lightest particles are lifted, while heavier, partly expanded grains fall out, and the furnace residence time also changes. Product density is therefore partly set by the fan speed.
Should the ID fan on an exfoliation furnace handle the product directly?
It is better not to. Passing expanded vermiculite through a fan wheel crushes the soft granules and produces fines. Placing the fan after the product cyclone and bag filter keeps the granules whole and leaves the fan handling only gas and a small amount of dust.
What temperature should an exfoliation furnace exhaust fan be rated for?
Rate it for the normal filter outlet temperature, usually 120 to 180°C, and for a short upset of about 250°C when the dilution control fails. The casing, shaft and bearings are checked at the upset value, and a temperature trip on the filter inlet protects against anything higher.
Information needed to quote an ID fan for vermiculite exfoliation furnaces
- 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.
- Product transport. Whether the expanded grains are carried to the cyclone by the gas stream or discharged at the furnace base, and the grades produced.
- Dilution. How the filter inlet temperature is controlled and the volume of bleed air at full feed.
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.