How ceramic frit melting furnaces work
Ceramic frits for tile and sanitaryware glazes are made by melting a batch of quartz, feldspar, borax or boric acid, zinc oxide, zircon, limestone, dolomite and sometimes lead or fluoride compounds at about 1,400 to 1,550°C, then quenching the melt in water or between chilled rollers. Continuous frit furnaces are small refractory tanks, typically 1 to 3 tonnes per hour, fired by natural gas or LPG with air through a recuperator, or with oxygen. Batch rotary furnaces are still used for smaller runs. Waste gas leaves the furnace at about 1,300 to 1,450°C, carrying boron, zinc, alkali and fluorine compounds as fume. It is diluted with ambient air, passed through a recuperator where fitted, cooled further and filtered in a bag filter before the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Recuperator
- Hot air main
- Burners
- Frit tank
The fan sits near the recuperator and blows ambient air through its tubes to a short insulated hot air main and the burners, which usually fire through the end or side walls of the small tank. The air is preheated to about 400 to 600°C. The fan must overcome the recuperator air side, the hot main, the ratio valves and the burner, typically 300 to 500 mmWC. Dilution air on the gas side often comes from a separate small fan or a controlled opening.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 400 to 600°C, downstream of the fan |
| Dust | Raw material dust in the batch area; filtered intake |
| Pressure at the fan outlet | Typically 300 to 500 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 ceramic frit melting furnaces
| Blade form | Backward curved, medium pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive ratioed to fuel |
| Also specified | Inlet filter, standby provision and minimum flow for recuperator protection |
What this FD fan has to cope with
Recuperator overheating
A frit furnace recuperator sees gas at 900 to 1,000°C even after dilution. Without air flowing through its tubes, it overheats within minutes. At low fire, or if the fan trips, the tubes are exposed. A minimum air flow with a hot air bleed, a standby fan with automatic changeover and an interlock on recuperator temperature protect it.
Melt temperature and frit quality
Frit properties, such as melting range and transparency, depend on how completely the batch melts and fines. Unstable combustion air changes flame temperature in a small tank very quickly, leaving unmelted particles or seeds in the frit. A fan with stable flow under speed control, ratioed accurately to the fuel, keeps the melt temperature steady.
Fine dust in the intake
Frit plants handle fine borax, zinc oxide, zircon and quartz, and the air around the batch area is dusty. Drawn into the intake, this dust clogs filters, coats recuperator tubes on the air side and wears burner passages. A filtered intake from a clean location, with a filter pressure gauge, prevents it.
Energy and control
A frit furnace runs continuously, but output and firing rate change with each formulation. A variable speed drive on the combustion air fan follows these changes without throttling and keeps a minimum flow through the recuperator at low fire. Accurate air-fuel ratio matters more for cost than fan power: excess air carries heat out of a small, very hot tank and increases the volume the ID fan must handle.
ID fan and FD fan in ceramic frit melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Before the recuperator |
| What it handles | Diluted waste gas with frit fume | Ambient air |
| Temperature at the fan | 150 to 200°C | Ambient |
| Main design concern | Fume deposits and fluoride corrosion | Recuperator protection |
| Control | Furnace pressure | Air ratioed to fuel |
See also the Induced Draft (ID) Fan for ceramic frit melting furnaces.
Questions buyers ask
Does a frit melting furnace have a combustion air fan?
An air-gas frit furnace does: the FD fan supplies all the combustion air through the recuperator to the burners. Oxy-fuel frit furnaces burn gas with oxygen and have no combustion air fan, although they still need a fan for dilution or quench air and an ID fan for the waste gas.
What pressure does a frit furnace combustion air fan need?
Typically 300 to 500 mmWC at the fan outlet, enough to overcome the recuperator air side, the hot air main, the ratio valves and the burners. The recuperator maker's pressure drop at maximum air flow and the burner manufacturer's requirement set the final figure, with a margin for tube fouling.
Why is a standby combustion air fan used on frit furnaces?
If the duty fan fails, air stops flowing through the recuperator tubes, which then overheat quickly in gas at 900°C or more. The furnace also loses firing and the melt cools. A standby fan with automatic changeover protects the recuperator and keeps production going while the duty fan is repaired.
Information needed to quote an FD fan for ceramic frit melting 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.
- Firing type. Air-gas recuperative or oxy-fuel, burner type and number of burners.
- Recuperator data. Air side pressure drop, preheat temperature and the minimum air flow for tube protection.
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.