How float glass furnaces work
Float glass furnaces are the largest glass melters, cross-fired regenerative tanks of about 500 to 1,000 tonnes per day with six to eight port pairs along each side. Batch of sand, soda ash, dolomite, limestone, salt cake and in-house cullet is melted and refined at up to about 1,580°C, and the glass flows into a tin bath where it spreads into a ribbon. Firing is natural gas or heavy fuel oil, reversing between sides every 20 to 30 minutes. Waste gas leaves the regenerators at 450 to 600°C and passes a waste heat boiler where fitted, then a dry or semi-dry scrubber, ESP and often SCR for NOx before the ID fan and a tall chimney. A campaign lasts 15 to 18 years without a cold stop.
Where the ID fan sits
- Melter and refiner
- Cross-fired regenerators
- Main flue
- Waste heat boiler
- Scrubber and ESP
- ID fan
- Chimney
The fans, usually two in parallel or one duty and one standby, sit after the ESP and SCR at the base of the chimney. The main flue from both regenerator banks runs to the gas cleaning plant, with a bypass to the chimney. Gas reaches the fans at about 180 to 250°C after a waste heat boiler, or 300 to 400°C without one. Total suction is typically 250 to 400 mmWC because of the long flues, boiler, reactor, ESP and catalyst.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after a waste heat boiler; 300 to 400°C without one |
| Maximum or upset | Up to 500°C when the boiler is bypassed |
| Dust at the fan | Low after the ESP; fine sodium sulphate and lime |
| Moisture and dew point | About 10 to 15% water with gas firing; dew point 50 to 55°C |
| Corrosive species | SO₂ and SO₃, high with fuel oil firing; ammonium bisulphate risk after SCR |
| Flow | Large; among the highest gas volumes of any glass furnace |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 float glass furnaces
| Blade form | Backward curved aerofoil or backward inclined, for efficiency at large flow |
| Material of construction | Boiler quality plate; heat-resisting steel where the boiler bypass case exceeds about 400°C |
| Drive and arrangement | Double inlet, arrangement 3 or 7, or single inlet arrangement 8, on variable speed drives |
| Bearings and cooling | Sleeve or heavy duty rolling bearings with temperature and vibration monitoring; cooling disc |
| Flow control | Variable speed drive on crown pressure, with inlet dampers for isolation and parallel operation |
| Also specified | Duty and standby or 2 × 60 to 100% fans, tight isolation dampers, turning gear and emergency power |
What this ID fan has to cope with
Single point of failure
A float furnace that loses draft cannot be stopped and restarted. Flame blows out of the ports, the regenerators overheat and the tin bath atmosphere is disturbed. Two fans, each able to carry the furnace at reduced pull, with tight isolation dampers, automatic bypass to the chimney and supply from emergency power, are the normal arrangement on these furnaces.
Ammonium bisulphate after SCR
Where SCR is used for NOx, ammonia slip reacts with SO₃ to form ammonium bisulphate, a sticky deposit that condenses between about 150 and 250°C. It fouls air heaters and, if the gas cools further, the fan inlet. Keeping the gas temperature at the fan above the deposit range and controlling ammonia slip protect the fan.
Parallel fan instability
Two large fans in parallel can share load unevenly, and if one is pushed to the left of its pressure peak it may stall, hunt and set up duct pulsation. Fans with a stable, steadily rising curve, matched speed control on both drives and correct sequencing of the inlet dampers when a fan is brought in or out keep them sharing evenly.
Fuel oil sulphur
Float plants firing heavy fuel oil see far more SO₃ than gas fired furnaces, which lifts the acid dew point towards 170°C or more. Uninsulated casings, inlet boxes, flanges and expansion joints become local cold spots where sulphuric acid condenses. Complete insulation and cladding, with attention to these details, prevents the pitting and thinning that otherwise start there.
Energy and control
A float glass ID fan system can absorb several hundred kilowatts and runs continuously for the full campaign. Variable speed drives on crown pressure, rather than throttled dampers, follow changes in pull and gradual fouling of regenerators and the boiler. Fan efficiency at the normal operating point is worth evaluating carefully, since even a few points of efficiency represent a large energy saving over 15 years.
ID fan and FD fan in float glass furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP and SCR, at the chimney base | Before the reversing valves and regenerators |
| What it handles | Large waste gas flow with SO₃ and fine dust | Large ambient air flow |
| Temperature at the fan | 180 to 400°C | Ambient |
| Main design concern | Redundancy, acid and deposit control | Port air balance and reliability |
| Control | Crown pressure | Air flow ratioed to fuel |
See also the Forced Draft (FD) Fan for float glass furnaces.
Questions buyers ask
Why do float glass furnaces have two ID fans?
A float furnace runs 15 years or more without stopping, and losing draft would damage the regenerators and upset the tin bath. Two fans, each sized to keep the furnace running alone, allow maintenance and protect against a trip. A chimney bypass provides a further fallback while the gas cleaning plant is out of service.
What is the gas temperature at a float glass ID fan?
It depends on the heat recovery fitted. With a waste heat boiler the fans see about 180 to 250°C after the ESP. Without a boiler, gas leaves the regenerators at 450 to 600°C and is cooled for the scrubber, giving 300 to 400°C at the fan. The bypass case sets the maximum.
How does SCR affect the ID fan on a float furnace?
SCR adds resistance, so the fans need more pressure. It also introduces the risk of ammonium bisulphate deposits if ammonia slip and SO₃ combine in cooler parts of the system. The fans must be selected for the added pressure and kept above the deposit temperature range.
Information needed to quote an ID fan for float glass 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.
- Redundancy philosophy. Duty and standby or parallel fans, and the pull each fan must carry alone.
- Gas cleaning chain. Waste heat boiler, scrubber, ESP and SCR, with the pressure drop and temperature at each.
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.