How quartz roasting furnaces work
Quartz lumps for high-purity silica, glass batch, ceramics and ferro-silicon are roasted at about 800 to 1,000°C and often quenched in water, so that the sudden temperature change cracks the quartz along grain boundaries and impurities. This makes crushing easier and exposes iron and mica inclusions for later removal. Rotary kilns, vertical shaft furnaces and batch car-bottom furnaces are used, fired on oil, gas or producer gas. Flue gas leaves the furnace at 400 to 700°C, often passes a recuperator that preheats combustion air, and then a cyclone or bag filter before the ID fan. Where the quench tank sits close to the furnace discharge, steam can enter the flue system. The FD fan supplies burner air.
Where the ID fan sits
- Kiln or shaft furnace
- Recuperator
- Dilution air damper
- Cyclone or bag filter
- ID fan
- Stack
The fan sits after the recuperator and dust collector, where the gas has cooled to a level mild steel can tolerate. If there is no recuperator, a dilution air damper ahead of the fan brings the gas down from 500°C or more. The fan holds the furnace slightly negative, typically against 100 to 250 mmWC of system loss. Quartz dust from the furnace is abrasive but usually light in quantity, since the lumps are large.
Gas at the ID fan
| Gas temperature, normal | 200 to 300°C after the recuperator |
| Maximum or upset | 400°C or more with recuperator bypass or dilution damper failure |
| Dust at the fan | Fine quartz from decrepitating lumps; abrasive but light in volume |
| Moisture and dew point | Low from combustion; higher where quench steam is drawn into the flue |
| Corrosive species | SO₂ when firing furnace oil or producer gas; acid dew point near 120 to 140°C |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 quartz roasting furnaces
| Blade form | Backward curved with wear protection at the leading edge |
| Material of construction | Mild steel for gas up to about 350°C; heat-resisting steel wheel where upsets are frequent |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and heat shield |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Temperature trip at the fan inlet, inlet damper for start-up and an insulated casing |
What this ID fan has to cope with
Overheating on recuperator bypass
When the recuperator is bypassed for cleaning or to protect it from overheating, hot gas goes straight to the fan. A dilution damper controlled on fan inlet temperature, a temperature trip and a wheel checked for creep at the upset temperature protect the fan. The fan casing and bearings are also checked at the upset temperature, not only the normal one.
Quartz fines and erosion
Quartz decrepitates when heated, and fine sharp particles are carried in the flue gas. Behind a cyclone, enough can reach the fan to wear the blade leading edges. A bag filter, or wear-resistant tips and reduced tip speed where only a cyclone is used, extends the impeller life considerably.
Steam from the quench
Where the quench is close to the discharge, steam is drawn into the flue by the fan's suction, especially on batch furnaces where the door is open during discharge. The gas then becomes humid and acidic deposits form at cold spots. Separating the quench vent from the furnace flue and insulating the duct prevents this.
Energy and control
Quartz roasting furnaces often operate in batches or with frequent changes in feed size, so the flue gas volume varies a lot. A variable speed drive controlled on furnace pressure follows these changes and saves much of the power a throttled damper wastes. Recovering heat in a recuperator reduces both fuel and the gas volume that the fan must move.
ID fan and FD fan in quartz roasting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator and dust collector | Before the recuperator |
| What it handles | Hot flue gas with quartz fines | Clean ambient air |
| Temperature at the fan | 200 to 300°C, up to 400°C upset | Ambient |
| Main design concern | Hot-gas rating and overheat protection | Burner pressure with preheated air |
| Control | Furnace pressure | Air main pressure and firing rate |
See also the Forced Draft (FD) Fan for quartz roasting furnaces.
Questions buyers ask
What temperature does a quartz roasting furnace ID fan handle?
Typically 200 to 300°C after a recuperator, with short upsets to 400°C or more when the recuperator is bypassed. Without a recuperator, dilution air is used to bring gas from 500 to 700°C down to a temperature the fan can handle, which increases the volume the fan must move.
Does quartz dust damage the ID fan?
Quartz is very hard, so even a small amount of fine dust erodes the blade edges over time. Behind a bag filter, wear is slow. Behind a cyclone alone, wear-resistant blade tips and a lower tip speed are recommended to extend the impeller life. Inspecting the leading edges at each furnace shutdown shows how quickly wear is progressing.
Why is steam a problem in the quartz roasting furnace flue?
Steam from the water quench raises the dew point of the flue gas. Combined with sulphur from the fuel, it forms acid condensate on cold surfaces, including the fan casing. Venting the quench separately, closing the discharge door promptly and insulating the flue avoid this, and keep the fan dry.
Information needed to quote an ID fan for quartz roasting 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.
- Furnace type. Rotary kiln, shaft or batch furnace, throughput and firing fuel.
- Heat recovery. Whether a recuperator is fitted and how the gas temperature at the fan is limited.
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.