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 FD fan sits
- Inlet filter
- Combustion air FD fan
- Air control valve
- Bottom burner
- Expansion furnace
The fan sits at the base of the furnace beside the burner and supplies air through a control valve at typically 300 to 700 mmWC. The flame fires upward into the furnace shaft, and the ore falls into it from a feed point above. The combustion air is a small share of the gas leaving the furnace, as the ID fan also draws in cooling air at the top.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Perlite dust in the plant; filtered intake needed |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Changes with feed rate and product grade |
| Fuel | Natural gas, LPG or light oil |
| 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 perlite expansion furnaces
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, burner-mounted or skid |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or modulating valve with ratio control |
| Also specified | Inlet filter, air pressure switch, flow measurement |
What this FD fan has to cope with
Flame stability with ore
Ore falling into the flame absorbs heat and can disturb it. If the combustion air pressure dips, the flame lifts or goes out. A fan with a steady curve and ratio control keeps the flame stable across feed rates.
Temperature for each grade
Light grades need a hotter furnace than dense grades. The fan and burner must cover the full firing range for the grades the plant makes, without being throttled into instability at the low end.
Perlite dust at the intake
Light perlite dust floats around the plant and is easily drawn into the fan, coating the impeller and burner. A filtered intake from a clean area is needed.
Energy and control
The combustion air fan is small but runs throughout each campaign. Ratio control on fuel and air saves fuel, which is the main operating cost of expansion, and speed control on the fan saves power at lower firing rates. Excess air adds to the gas the ID fan must handle and lowers furnace temperature.
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 Induced Draft (ID) Fan for perlite expansion furnaces.
Questions buyers ask
What is the FD fan on a perlite furnace?
On a perlite expansion furnace the forced draft fan is the combustion air fan for the bottom burner. Cooling air enters at the furnace top, drawn in by the ID fan.
What pressure does a perlite burner fan need?
Typically 300 to 700 mmWC, depending on the burner design and the furnace size. The fan must hold this steadily as the firing rate changes.
Why does flame stability matter in perlite expansion?
The ore falls into the flame and is expanded there. An unstable flame gives uneven expansion, unexpanded grains and density variation. Steady combustion air keeps the product consistent.
Information needed to quote an FD fan for perlite expansion 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 type, fuel and firing rate for each product grade.
- Furnace. Furnace diameter, height and feed rate.
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.