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 FD fan sits
- Inlet filter
- FD fan
- Recuperator
- Hot air main
- Burners
The fan sits at ground level before the recuperator and delivers ambient air into it. After the recuperator, preheated air at 250 to 450°C goes to the burners through an insulated main with a control valve at each burner or zone. The fan works against the recuperator, the hot air main, the valves and the burners, typically 300 to 600 mmWC depending on burner type.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 250 to 450°C, downstream of the fan |
| Dust | Quartz dust around the crushing and loading area |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| 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 quartz roasting furnaces
| Blade form | Backward curved, on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter, recuperator protection with a cold air bleed, and low-pressure interlock for burner safety |
What this FD fan has to cope with
Recuperator overheating at low air flow
The air passing through the recuperator cools its tubes. If combustion air falls too low on reduced firing, the tubes overheat. A minimum air flow, maintained by the fan and released through a hot air bleed if necessary, protects the recuperator. The fan selection includes this minimum flow at the lowest firing rate.
Hot air volume at the burners
Preheated air is much less dense, so the burner valves and the fan pressure must account for the larger volume and pressure loss at 300 to 450°C. A fan selected only for cold air will run short at full firing. The pressure loss is therefore calculated at the actual hot air temperature in each section of the main.
Quartz dust at the inlet
Crushing and screening areas produce sharp quartz dust. Drawn into the fan, it wears the impeller and bearings. A filtered inlet away from crushers is a simple cure, and the filter pressure drop, with an allowance for loading, is included in the fan duty so the burners do not run short as it fills.
Energy and control
Firing on a quartz furnace changes with batch stage or feed rate. A variable speed drive on air main pressure avoids the losses of a throttled fan. The larger saving is in fuel: a recuperator that preheats combustion air to 300°C or more reduces fuel use considerably, and keeping the fan supply matched to the fuel limits excess air.
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 Induced Draft (ID) Fan for quartz roasting furnaces.
Questions buyers ask
Does the combustion air fan on a quartz roasting furnace handle hot air?
No. It is placed before the recuperator and handles ambient air. The air is heated downstream in the recuperator, so the fan itself runs cool and is built in mild steel. The fan must, however, provide the pressure needed to push the hot, expanded air through the main and the burners.
What pressure does a quartz roasting furnace combustion air fan need?
Typically 300 to 600 mmWC, depending on burner type, the recuperator and the hot air main. High-velocity burners need the upper end. The fan is selected on the burner maker's data plus all losses at hot air conditions downstream of the recuperator, including the recuperator itself.
How do I protect the recuperator if the combustion air fan trips?
Burners trip on low air pressure, and a hot gas bypass around the recuperator or a dilution damper upstream of it opens to protect the tubes. Some plants also keep a standby fan or allow natural air flow through the recuperator. The aim is to avoid the tubes running hot with no air to cool them.
Information needed to quote an FD fan for quartz roasting 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.
- Burners. Number, type and firing capacity of the burners and the air pressure required at each.
- Recuperator. Air side pressure drop and the preheated air temperature.
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.