How glass melting furnaces work
Glass melting furnaces melt a batch of sand, soda ash, limestone, dolomite and cullet at about 1,450 to 1,600°C in a refractory tank. The main types differ in how they recover heat. Regenerative furnaces, end-port or cross-fired, alternate combustion air and waste gas through checker chambers every 20 to 30 minutes, preheating air to 1,100 to 1,300°C. Recuperative unit melters preheat air to 600 to 800°C in a metallic recuperator. Oxy-fuel furnaces burn with oxygen and need no air preheat. Electric melters and day tanks fill out the range. Fuel is natural gas, LPG, fuel oil or producer gas, often with electric boost. Waste gas leaves the heat recovery at 400 to 800°C and goes to the chimney, or through a waste heat boiler, dry sorbent injection and an ESP or bag filter before an ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Reversing valve
- Regenerator or recuperator
- Ports and burners
- Melter
The fan stands near the regenerator or recuperator and blows ambient air into it through a reversing valve or a cold air main. Air pressure needs are modest because the regenerators are large: typically 150 to 400 mmWC at the fan. Recuperative furnaces need more because of the recuperator and burner. Furnaces also carry separate fans for electrode and throat cooling, dilution air and forehearths, which are not the FD fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after heat recovery | 1,100 to 1,300°C after regenerators; 600 to 800°C after a recuperator |
| Dust | Batch house dust around the furnace; filtered intake needed |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Duty pattern | Continuous for the whole campaign, with reversals 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 glass melting furnaces
| Blade form | Backward curved, for efficiency on continuous duty |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8, with a full standby fan |
| Bearings and cooling | Standard bearings with condition monitoring |
| Flow control | Variable speed drive on combustion air flow, ratioed to fuel |
| Also specified | Inlet filter, silencer, automatic standby changeover and emergency power |
What this FD fan has to cope with
Reliability over a long campaign
The combustion air fan must run for years without a planned stop. A bearing failure that stops firing chills the glass and can crack the refractory. Two fans with automatic changeover, condition monitoring on both and a supply from an emergency generator are the normal arrangement, with the standby fan run regularly so that it is known to be ready.
Air flow during reversal
During reversal the air flow briefly stops or switches sides as the valve moves. A fan working against a closed valve can surge, and the pressure spike stresses ducts and expansion joints. A fan with a stable curve close to shut-off, a short slowdown of the drive during reversal, or a relief path keeps the fan steady while the valves move.
Batch dust in the intake
Fine batch dust from the batch house and doghouse blocks inlet filters and is carried into the regenerators, where it adds to the checker plugging that already limits furnace life. Taking air from a clean location, through a filter with a pressure gauge across it, protects both the fan and the checkers at little cost.
Energy and control
The combustion air fan runs every hour of a long campaign. Controlling it by speed rather than by a throttle saves steadily, especially when pull rate is reduced. More important for cost is accurate air-fuel ratio: excess air through regenerators wastes fuel and raises NOx, so the fan must deliver precise, measured flow.
ID fan and FD fan in glass melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the filter, before the chimney | Before the regenerators or recuperator |
| What it handles | Waste gas with sulphate dust and acid gases | Ambient filtered air |
| Temperature at the fan | 180 to 400°C | Ambient |
| Main design concern | Acid dew point, deposits and reliability | Reliability and accurate air flow |
| Control | Furnace crown pressure | Air flow ratioed to fuel |
See also the Induced Draft (ID) Fan for glass melting furnaces.
Questions buyers ask
Does an oxy-fuel glass furnace need an FD fan?
No. An oxy-fuel furnace burns its fuel with oxygen supplied from a VPSA unit or liquid storage, so there is no combustion air fan. It still needs small fans for electrode and throat cooling, quench or dilution air and the forehearths, and an ID fan or ejector to remove the waste gas from the melter.
What pressure does a glass furnace combustion air fan need?
For a regenerative furnace, typically 150 to 400 mmWC, because the checker chambers are large with low velocity. Recuperative furnaces need more, to overcome the recuperator and burners. The figure is set from the regenerator or recuperator resistance at maximum pull and the burner requirement.
Why do glass furnaces have a standby combustion air fan?
The furnace runs for 10 years or more without stopping, and losing combustion air means losing firing. A second fan with automatic changeover lets one fan be serviced while the furnace keeps running, and protects the glass and refractory if the duty fan fails. Both fans are normally on the emergency power supply.
Information needed to quote an FD fan for glass 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.
- Heat recovery type. Regenerative or recuperative, and air side pressure drop at maximum pull.
- Standby and power. Whether a standby fan and emergency power supply are required.
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.