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 ID fan sits
- Unit melter
- Dilution air inlet
- Metallic recuperator
- Gas cooler
- Dry scrubber and bag filter
- ID fan
- Chimney
The fan stands after the bag filter or ESP. Before it, the gas has been diluted with ambient air at the recuperator inlet, cooled through the recuperator, then cooled again by air bleed or water spray to about 180 to 220°C to suit a bag filter. On furnaces without gas cleaning, a hot gas fan or air ejector at the chimney base handles gas at 400 to 600°C. Suction is typically 150 to 300 mmWC with filtration.
Gas at the ID fan
| Gas temperature, normal | 180 to 220°C after a bag filter |
| Maximum or upset | 400 to 500°C if cooling fails or the filter is bypassed |
| Dust at the fan | Low after the filter; fine borates, fluorides and sulphates |
| Moisture and dew point | Moderate; dew point 50 to 60°C, higher with water spray cooling |
| Corrosive species | HF from opal and some borosilicate batches, boric acid fumes, SO₂ and HCl; strongly corrosive in condensate |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 tableware and lighting furnaces
| Blade form | Backward curved, for clean gas after the filter |
| Material of construction | Mild steel where gas stays well above the dew point; stainless steel for opal and fluoride-bearing glasses |
| Drive and arrangement | Direct coupled, arrangement 8, insulated casing |
| Bearings and cooling | Cooling disc and heat shield; bearings rated for the bypass upset |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Chimney bypass, casing drain and inspection door; insulation against acid condensation |
What this ID fan has to cope with
Borate and fluoride deposits
Borosilicate and opal glasses evaporate boron and fluorine compounds from the melt. These condense as fine, glassy, sticky dust that coats ducts, recuperator tubes and fans and is corrosive when damp. An efficient filter ahead of the fan, keeping the gas above its condensation range in ducts, and stainless parts where fluoride levels are high are the defence.
Recuperator overheating
The metallic recuperator survives only if the gas entering it is limited to about 1,000 to 1,100°C. The ID fan draws the dilution air that achieves this. If the fan output drops, the recuperator inlet runs hot and the tubes fail. The fan must be sized for full dilution air at maximum firing, with an interlock on recuperator temperature.
Product changes
Tableware plants change glass types and colours, and pull rates vary with the forming line. The waste gas volume and composition change with them. A fan with good efficiency over a wide range, on variable speed, copes better than one sized for a single point.
Energy and control
Smaller glass furnaces change pull often, and electric boost shifts the balance between fuel and electricity. A variable speed ID fan on furnace pressure follows these changes. Correctly controlled dilution air also matters: too much adds gas volume and fan power, too little endangers the recuperator, so the dilution damper and fan speed should be controlled together.
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 Forced Draft (FD) Fan for glass tableware and lighting furnaces.
Questions buyers ask
Why is dilution air needed on a recuperative glass furnace?
Waste gas leaves a unit melter at 1,300 to 1,400°C, too hot for a metallic recuperator. Ambient dilution air drawn in at the recuperator inlet brings it down to about 1,000 to 1,100°C. The ID fan or chimney must handle the extra volume, so dilution air is a large part of the fan duty.
What material should the ID fan be for a borosilicate or opal glass furnace?
Borosilicate and opal glasses release boron and fluorine compounds that are corrosive when they condense. Where the gas stays hot and the filter is efficient, mild steel with full insulation may serve. Where condensation or fluoride levels are significant, stainless steel construction is used for the gas-wetted parts.
Can a small glass tableware furnace run without an ID fan?
Yes, many run on chimney draft with a dilution air ejector at the recuperator. Once a bag filter or scrubber is added, its resistance requires an ID fan. The fan also gives steadier furnace pressure control than a chimney alone, which helps glass quality and colour consistency on smaller furnaces.
Information needed to quote an ID fan for glass tableware and lighting 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.
- Glass types. Glass compositions melted, especially borosilicate, opal or lead glasses, and any fluoride content.
- Recuperator and dilution. Recuperator inlet temperature limit and the dilution air quantity at maximum firing.
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.