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 ID fan sits
- Frit melting tank
- Dilution air inlet
- Recuperator
- Gas cooler
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. Dilution air at the furnace exit brings the waste gas down to about 900 to 1,000°C for the recuperator, and further air bleed or a gas cooler brings it to 150 to 200°C for the filter bags. The dilution air means the fan handles a much larger volume than the furnace produces. Suction is typically 150 to 300 mmWC, mostly the filter and recuperator.
Gas at the ID fan
| Gas temperature, normal | 150 to 200°C after the bag filter |
| Maximum or upset | 300 to 400°C if the gas cooler or dilution fails |
| Dust at the fan | Low after the filter; fine borates, zinc oxide and alkali salts |
| Moisture and dew point | Moderate; higher where water quench steam is drawn in |
| Corrosive species | HF and boron compounds from fluoride and borate raw materials; corrosive in condensate |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 ceramic frit melting furnaces
| Blade form | Backward curved, on gas cleaned by the bag filter |
| Material of construction | Mild steel where gas is dry and hot; stainless steel where fluorides are significant or condensation occurs |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Cooling disc and heat shield |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Casing drain, inspection door and insulation |
What this ID fan has to cope with
Fume deposits
Boron, zinc and alkali compounds evaporate from the frit melt and condense as fine fume that coats ducts and anything downstream of the filter if bags leak. Deposits on the impeller cause imbalance. Keeping the fan after an efficient bag filter, with bag leak detection, keeps it clean.
Fluoride corrosion
Frits made with fluorspar or cryolite, and some opaque and matt frits, release HF that condenses into strongly corrosive acid wherever gas cools below its dew point, such as at weekend stops or in uninsulated duct sections. Stainless steel for the gas-wetted parts, full insulation and a drain at the casing low point protect the fan in these plants.
Frequent product changes
Frit plants change formulation often, between transparent, opaque, matt and coloured frits, and each change alters the firing rate, gas volume and fume composition. A fan sized for one formulation runs poorly on others. A selection with good efficiency over the whole operating range, on variable speed control, copes better than one optimised for a single point.
Energy and control
Frit furnaces run continuously but change formulation and output often. The dilution air is a large share of the fan's volume, so controlling dilution to the minimum that protects the recuperator and filter saves fan power. A variable speed ID fan on furnace pressure follows the changes in load and formulation.
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 Forced Draft (FD) Fan for ceramic frit melting furnaces.
Questions buyers ask
Why does a frit furnace need dilution air?
Waste gas leaves the frit furnace at about 1,300 to 1,450°C, too hot for the recuperator and the bag filter. Ambient air is mixed in to cool it. The ID fan must handle this extra volume, so dilution air control has a large effect on fan size and power.
What material should a frit furnace ID fan be?
Mild steel is used where the gas is dry and well above the dew point after an efficient bag filter. Where fluoride frits are made, or the fan sees condensation at shutdowns or from quench steam, stainless steel is used for the gas-wetted parts to resist attack by HF and boron compounds.
What dust reaches the frit furnace ID fan?
After a working bag filter, very little. The fume from frit melting is sub-micron boron, zinc and alkali compounds that become sticky when they condense. If bags leak or tear, this fume deposits on the impeller and unbalances it, so bag leak detection and regular inspection of the fan are the practical protection.
Information needed to quote an ID fan for ceramic frit melting 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.
- Frit types. Frit formulations produced, especially those containing fluorides, lead or high boron.
- Dilution and cooling. Dilution air quantity and the gas temperature at the bag filter.
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.