How glass fibre melting furnaces work
Glass fibre is made in two main forms. Continuous filament or textile fibre, usually E-glass for reinforcement, is melted in furnaces of about 50 to 200 tonnes per day, often oxy-fuel or air-gas recuperative with electric boost, at up to about 1,600°C, and the glass flows through forehearths to platinum bushings. E-glass batch contains colemanite or other boron sources and sometimes fluorspar, so the waste gas carries boron compounds and HF along with particulates. Glass wool for insulation is melted in electric or gas-fired furnaces and fiberised by spinners. Air-gas furnaces cool waste gas through a recuperator and dilution air; all types use a dry or wet scrubber and filter before the ID fan. The FD fan supplies the recuperator and burners.
Where the ID fan sits
- Melter
- Recuperator or quench
- Gas cooler
- Dry or wet scrubber
- Bag filter
- ID fan
- Chimney
The fan sits after the scrubber and filter. With a dry scrubber and bag filter, gas reaches it at about 150 to 200°C. With a wet scrubber, the gas is saturated at 60 to 80°C. Suction is typically 200 to 400 mmWC because scrubbers for boron and fluorine removal have significant resistance. Furnace pressure is held slightly positive to protect the batch and forehearth atmosphere.
Gas at the ID fan
| Gas temperature, normal | 150 to 200°C after a dry scrubber and bag filter; 60 to 80°C after a wet scrubber |
| Maximum or upset | 350 to 450°C if gas cooling fails |
| Dust at the fan | Low after the filter; fine borates, fluorides and sulphates |
| Moisture and dew point | Saturated after wet scrubbing; dew point 50 to 70°C otherwise, higher with oxy-fuel |
| Corrosive species | HF, boric acid and SO₂; very 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 glass fibre melting furnaces
| Blade form | Backward curved, for clean gas; radial tip if deposits are expected |
| Material of construction | Stainless steel (SS 316 or similar) for wet or fluoride-bearing gas; FRP or rubber-lined for wet scrubber duty |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Cooling disc on hot dry duties; sealed bearings on wet duties |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Casing drain, inspection door, standby provision and chimney bypass |
What this ID fan has to cope with
Borate deposits
Boron compounds evaporate from the E-glass melt and condense in the waste gas system as glassy, hard deposits. They build in ducts, the recuperator and any fan that sees unfiltered gas, and are difficult to remove. Keeping the fan after an efficient filter, and keeping duct temperatures controlled to avoid condensation in the wrong places, limit the build-up.
Fluoride corrosion
Where fluorspar is used in the batch, HF in the gas forms highly corrosive acid wherever it condenses. Even after a scrubber, residual HF attacks mild steel. Stainless steel, or FRP or rubber-lined construction after a wet scrubber, is normal for the gas-wetted parts of the fan, together with a casing drain at the low point.
Wet scrubber carry-over
After a wet scrubber, mist and droplets carrying dissolved borates and fluorides reach the fan and leave hard salt deposits on the blades as they evaporate. The deposit builds unevenly and causes vibration. A mist eliminator before the fan, a casing drain and periodic washing through an inspection door keep the impeller clean and balanced.
Energy and control
Glass fibre furnaces run continuously, often at steady pull because the bushings require constant glass flow. The ID fan's load changes mainly with scrubber resistance and filter cleaning cycles. Speed control on furnace pressure follows these changes. Selecting the fan for efficiency at the normal operating point rather than at the absolute maximum avoids continuous losses over the campaign.
ID fan and FD fan in glass fibre melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber and filter | Before the recuperator |
| What it handles | Waste gas with borates and HF | Ambient filtered air |
| Temperature at the fan | 60 to 200°C depending on scrubber | Ambient |
| Main design concern | Corrosion and deposits | Steady flow and recuperator protection |
| Control | Furnace pressure | Air main pressure ratioed to fuel |
See also the Forced Draft (FD) Fan for glass fibre melting furnaces.
Questions buyers ask
Why does an E-glass furnace need special gas cleaning?
E-glass batch contains boron and often fluorine compounds, which evaporate during melting. Boron compounds and HF are regulated emissions and are corrosive. A dry or wet scrubber and filter remove them before the ID fan and chimney, which adds pressure drop to the fan duty.
What material is used for an ID fan on a glass fibre furnace?
Stainless steel, such as SS 316, is usual for gas-wetted parts because of HF and boron compounds. After a wet scrubber, FRP or rubber-lined fans are also used. Mild steel is used only where the gas is dry, hot and clean after an efficient filter.
Does a glass fibre furnace use oxy-fuel?
Many continuous filament furnaces use oxy-fuel firing, which reduces gas volume and NOx. Others are air-gas recuperative with electric boost. The ID fan duty differs between them: oxy-fuel gives less volume but wetter, more concentrated gas, while air-gas furnaces have more volume because of the combustion nitrogen and the dilution air.
Information needed to quote an ID fan for glass fibre 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.
- Batch chemistry. Boron and fluorine content of the batch and the type of glass fibre.
- Scrubber type. Dry or wet scrubber, with its pressure drop and outlet 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.