How perlite expansion furnaces work
Perlite is a volcanic glass holding 2 to 5% combined water. Crushed and sized ore is fed into a vertical expansion furnace fired at the bottom by a gas or oil burner. The grains heat to about 850 to 1,100°C in a second or two; the glass softens and the water flashes to steam, expanding each grain 4 to 20 times into a light, porous particle. The expanded perlite is carried up by the hot gas, cooled by ambient air drawn in at the furnace top, and separated in a cyclone, with a bag filter collecting the fines. Product density, typically 30 to 150 kg/m³, is set by the furnace temperature and the gas velocity. The ID fan draws the gas and product through the system; the burner has its own combustion air fan.
Where the ID fan sits
- Burner and furnace
- Cooling air inlet
- Product cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan sits after the bag filter, drawing through the furnace, the transport duct, the product cyclone and the filter, typically 250 to 450 mmWC. Cooling air drawn in at the furnace top lowers the gas temperature before the cyclone and filter. On some smaller plants the fan sits between the cyclone and the filter, where it handles more fine perlite. The fan speed sets the gas velocity in the furnace, which decides how quickly the expanded grains are carried away from the flame.
Gas at the ID fan
| Gas temperature, normal | 100 to 180°C after cooling air and the filter |
| Maximum or upset | Up to 250°C if the cooling air inlet is closed or feed stops |
| Dust at the fan | Low after the filter; light, glassy fines if the fan is before the filter |
| Moisture and dew point | Moderate; steam from the ore and combustion water |
| Dust character | Very light but abrasive volcanic glass |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 perlite expansion furnaces
| Blade form | Backward curved after a filter; radial-tipped with wear allowance if before the filter |
| Material of construction | Mild steel; wear-resistant plate on blades and scroll where fines reach the fan |
| Drive and arrangement | Belt driven or direct coupled with variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive for furnace gas velocity and product density |
| Also specified | Inspection door, drain and fine speed adjustment |
What this ID fan has to cope with
Density control by gas velocity
If the gas velocity is too high, grains leave before they fully expand and the product is too dense; too low and they over-expand, break or fall back into the burner. Fine speed control on the ID fan lets the operator hold the density specification as ore and feed rate change.
Abrasive glassy fines
Expanded perlite is light but its fragments are sharp volcanic glass. Where the fan handles fines before the filter, blade edges and the scroll wear steadily. Wear-resistant plate, or moving the fan after the filter, extends life.
Fallback and furnace upsets
If draft is lost, unexpanded and partly expanded grains fall into the burner, disturbing the flame and fouling the furnace bottom. A low-draft interlock that stops the feed protects the furnace.
Energy and control
Perlite furnaces run in campaigns, often switching between density grades. A variable speed drive on the ID fan is the main tool for grade changes and avoids wasteful dampers. Drawing just enough cooling air to protect the filter, rather than excess, keeps the gas volume and fan power down. A fan matched to the normal grade, not the extreme one, runs closer to its best efficiency.
ID fan and FD fan in perlite expansion furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | At the furnace base, feeding the burner |
| What it handles | Cooled gas, steam and perlite fines | Ambient air |
| Temperature at the fan | 100 to 180°C | Ambient |
| Main design concern | Gas velocity for product density | Flame stability with falling ore |
| Control | Fan speed for density | Air-fuel ratio |
See also the Forced Draft (FD) Fan for perlite expansion furnaces.
Questions buyers ask
How does the ID fan affect perlite density?
On a perlite expansion furnace, the fan sets the gas velocity in the furnace, which controls how long grains stay in the hot zone. Higher velocity gives denser product; lower velocity lets grains expand more. Speed control on the fan is the usual way to adjust density.
What temperature does a perlite furnace exhaust fan handle?
Typically 100 to 180°C after cooling air and the bag filter. The furnace gas is far hotter, but ambient air drawn in at the top cools it before the cyclone.
Should the perlite fan be before or after the bag filter?
After the filter is better for fan life, as the fan then handles clean gas. Some small plants place it between the cyclone and filter, where it handles light but abrasive fines and needs wear protection.
Information needed to quote an ID fan for perlite expansion 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. Product density range, feed rate and ore grading.
- Layout. Fan position relative to the cyclone and filter, and the cooling air arrangement.
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.