How glass tableware and lighting furnaces work
Pressed and blown tableware, borosilicate kitchenware, lighting glass and tubing are melted in smaller furnaces, typically 20 to 150 tonnes per day. Recuperative unit melters dominate, with some small end-port regenerative furnaces and many all-electric or electrically boosted melters. Glass types vary widely: soda-lime for tumblers, borosilicate for ovenware and lamp tubing, opal glass made with fluorides for lighting, and lead or lead-free crystal. A unit melter fires many burners along both side walls, and waste gas leaves at about 1,300 to 1,400°C. Dilution air cools it to about 1,000°C before a metallic recuperator heats combustion air to 600 to 800°C, and gas leaves the recuperator at 600 to 800°C towards the chimney, a scrubber and filter, and the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Metallic recuperator
- Hot air main
- Burner valves
- Side wall burners
The fan stands near the recuperator and pushes ambient air through its tubes, then along an insulated hot air main to burners on both sides of the melter. Each burner or burner group has a hot air valve. The fan must overcome the recuperator air side, the hot main and the burner, typically 300 to 600 mmWC. A separate dilution air fan or opening is used on the gas side of the recuperator.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 600 to 800°C, downstream of the fan |
| Dust | Batch and cullet dust; filtered intake |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Minimum flow | A minimum air flow must pass the recuperator whenever the furnace is hot |
| 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 tableware and lighting furnaces
| Blade form | Backward curved, medium to high pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, with a standby fan on continuous furnaces |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air main pressure, ratioed to fuel |
| Also specified | Inlet filter, silencer, minimum flow bleed and emergency power |
What this FD fan has to cope with
Recuperator tube burn-out
Without enough air flowing through its tubes, a metallic recuperator overheats within minutes. At low firing or during a fan trip, the tubes are exposed. A minimum flow bleed to atmosphere after the recuperator, a standby fan and power backup protect the tubes when burner demand is low or a fan fails.
Hot air valve leakage
Hot air valves at the burners wear and leak, so air reaches burners that are turned down or off. The fan then supplies more air than the ratio calls for, and the melter runs with excess air. Regular valve checks and flue oxygen measurement catch this.
Unstable burner ratio at low pull
When the forming line runs slowly, the furnace fires at low rate and the fan operates far down its curve, where main pressure can become unsteady and burner ratios drift. A fan sized for the real maximum air demand, not an inflated one, running on speed control with a sensible minimum speed, keeps the ratio stable at low pull.
Energy and control
Unit melters run continuously but change pull with the forming lines. Speed control on air main pressure cuts power at lower pull. The air must still pass the recuperator at a minimum rate, so the minimum flow arrangement should bleed hot air as little as needed. Accurate air ratio matters more for fuel use than the fan's own power.
ID fan and FD fan in glass tableware and lighting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the chimney | Before the recuperator |
| What it handles | Diluted waste gas with borates and fluorides | Ambient filtered air |
| Temperature at the fan | 180 to 220°C, 400°C plus without cooling | Ambient |
| Main design concern | Corrosive deposits and dilution air | Recuperator tube protection |
| Control | Furnace pressure | Air main pressure ratioed to fuel |
See also the Induced Draft (ID) Fan for glass tableware and lighting furnaces.
Questions buyers ask
What does the FD fan on a unit melter do?
It is the combustion air fan. It blows ambient air through the metallic recuperator, where it is heated to 600 to 800°C, and then along a hot air main to the burners on both side walls of the unit melter. It also protects the recuperator by keeping air moving through the tubes.
Why does a recuperative glass furnace need a standby combustion air fan?
If the fan stops while the furnace is hot, air no longer cools the recuperator tubes, and they can overheat and fail in minutes. A standby fan with automatic changeover, or an emergency air supply, prevents this. The furnace also loses firing until air is restored.
What pressure does a glass tableware furnace combustion air fan need?
Typically 300 to 600 mmWC, enough to overcome the recuperator air side, the insulated hot air main, the hot air valves and the burners. The recuperator maker's air side pressure drop at maximum flow and the burner pressure requirement are the key figures for selection, with a margin for fouling of the tubes.
Information needed to quote an FD fan for glass tableware and lighting 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.
- Recuperator data. Air side pressure drop and minimum air flow for tube protection.
- Burner layout. Number of burners and hot air pressure needed 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.