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 FD fan sits
- Inlet filter and silencer
- FD fan
- Air main
- Reversing valves
- Regenerators
- Ports and melter
The fans stand at one end of the furnace building and feed an air main running along the regenerators. Reversing valves direct the air into the regenerator on the air side, where it is preheated to 1,200 to 1,300°C before entering the ports. Port air may be balanced by dampers. The pressure required is moderate, about 150 to 350 mmWC, but the flow is very large. Duty and standby fans on emergency power are normal.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the regenerators | 1,200 to 1,300°C, downstream of the fan |
| Dust | Batch dust in the furnace building; filtered intake |
| Pressure at the fan outlet | Typically 150 to 350 mmWC |
| Duty pattern | Continuous, reversing sides every 20 to 30 minutes |
| 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 float glass furnaces
| Blade form | Backward curved aerofoil, for efficiency at large flow |
| Material of construction | Mild steel |
| Drive and arrangement | Double inlet or single inlet direct coupled, with standby |
| Bearings and cooling | Heavy duty bearings with condition monitoring |
| Flow control | Variable speed drive on total air flow, with port dampers for distribution |
| Also specified | Inlet silencer, filter, automatic changeover and emergency power |
What this FD fan has to cope with
Port air balance
The temperature profile along a float furnace is set port by port, and each port must receive the air its fuel needs. If the main pressure varies, the ports nearest the fan get more air and the far ports less. A correctly sized air main and a fan holding steady pressure, with port dampers for fine adjustment, give an even profile.
Noise from a large fan
A float furnace combustion air fan moves a very large air flow and can be a dominant noise source in the furnace building and at the site boundary. Inlet silencers, acoustic lagging and selecting the fan near its peak efficiency, where it runs quietest, keep noise down.
Changeover disturbance
Switching from the duty fan to the standby can cause a short dip in air if the dampers and drives are not sequenced. Starting the standby fan, bringing it to speed and then transferring the dampers in a controlled sequence avoids a pressure dip at the ports.
Energy and control
The combustion air fan runs for the whole campaign, so efficiency and speed control matter. A variable speed drive on air flow follows the pull rate and avoids throttling losses. Excess air through the regenerators wastes fuel and raises NOx, so accurate air flow control saves far more fuel than the fan saves in power.
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 Induced Draft (ID) Fan for float glass furnaces.
Questions buyers ask
How large is the combustion air fan on a float glass furnace?
It handles the full combustion air for a furnace burning fuel for 500 to 1,000 tonnes of glass a day, so the flow is very large while the pressure is moderate, about 150 to 350 mmWC. Two fans of this size, one standby, are normally installed and both are on emergency power.
How is combustion air distributed to the ports of a float furnace?
The FD fan feeds an air main along the regenerators. The reversing valves send air into the regenerator on the air side, and port dampers or valves balance the air to each port in ratio to its fuel. Steady main pressure from the fan is what makes the port balance work.
Does the float furnace FD fan handle hot air?
No. The fan blows cold ambient air into the regenerators, where the checkers preheat it to 1,200°C or more before it reaches the ports. Handling the preheated air would need a fan many times larger, built from special heat-resisting materials, so this arrangement is never used on a regenerative float furnace.
Information needed to quote an FD fan for float glass 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.
- Port arrangement. Number of port pairs and the air flow split along the furnace.
- Redundancy and power. Standby fan arrangement and emergency power supply.
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.