How glass tempering furnaces work
A flat glass tempering furnace heats cut and edged glass sheets on ceramic rollers to about 620 to 650°C, then moves them into a quench section where high pressure air jets from both sides cool the surfaces rapidly, leaving them in compression. Most furnaces are electrically heated with radiant elements and forced convection; some are gas-fired. Heating takes roughly 35 to 45 seconds per millimetre of thickness, with the load oscillating on the rollers. Bending furnaces for automotive and architectural glass follow the same principle. The dominant fans are the quench blowers, which supply ambient air to the quench nozzles at high pressure, and exhaust fans that remove the hot quench air and furnace vent gas from the building. Convection systems inside the furnace use compressed air or internal fans.
Where the ID fan sits
- Heating chamber
- Quench section
- Quench exhaust hood
- Exhaust ducting
- Exhaust (ID) fan
- Discharge outside the building
The fan sits outside or on the roof, connected to a hood over the quench section and to vents on the heating chamber. It must remove the quench air, which is the largest volume in the system, so that hot air does not fill the building and spill back onto the furnace and control panels. Suction is low, typically 30 to 80 mmWC, but the flow follows the quench blower output and changes with glass thickness.
Gas at the ID fan
| Air temperature, normal | 60 to 120°C from the quench hood |
| Maximum or upset | Up to 200 to 250°C from the furnace vent during heating chamber purging |
| Dust at the fan | Glass fragments and fine glass dust when sheets break in the quench |
| Moisture and dew point | Low; mostly ambient air heated by the glass |
| Flow variation | Large; follows the quench blower output, highest for thin glass |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 tempering furnaces
| Blade form | Backward curved or backward inclined, for efficient handling of large volume at low pressure |
| Material of construction | Mild steel; wear plate at the blade inlet where glass particles are carried |
| Drive and arrangement | Direct coupled or belt driven, arrangement 4, 8 or 9 |
| Bearings and cooling | Standard bearings; cooling disc where vent gas is handled |
| Flow control | Variable speed drive linked to the quench blower or recipe |
| Also specified | Coarse screen or knock-out at the hood to stop large glass pieces, and acoustic treatment |
What this ID fan has to cope with
Glass fragments in the exhaust
When a sheet breaks in the quench, fragments and fine glass dust are blown up into the hood. Large pieces damage the impeller; fine dust slowly abrades it. A screen or knock-out box at the hood, wear plate at the blade inlet and a blade form that tolerates particles protect the fan.
Heat building up in the shop
If the exhaust fan cannot match the quench air volume on thin glass, hot air spills into the building, raising temperatures around the furnace and control panels and affecting operators. Sizing the exhaust for the highest quench flow, with speed control linked to the quench, keeps the building cool.
Noise
Quench exhaust fans move large volumes and add to the noise already coming from the quench blowers and nozzles, in a hall where operators load and unload glass beside the line. Inlet and outlet silencers, acoustic lagging of the casing and ducts, and placing the fan outside the building where possible keep noise at the workplace and the site boundary down.
Energy and control
Quench air flow changes with glass thickness, from large flows for thin glass to small flows for thick glass. An exhaust fan on a variable speed drive linked to the quench recipe extracts only what is needed for each load. Running the exhaust flat out for every load wastes power, especially on plants processing mostly thick glass.
ID fan and FD fan in glass tempering furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | Quench exhaust hood and furnace vent | Feeding the quench nozzle plenums |
| What it handles | Warm quench air with glass particles | Ambient filtered air at high pressure |
| Temperature at the fan | 60 to 120°C | Ambient |
| Main design concern | Volume and glass fragments | Pressure for thin glass and even quench |
| Control | Linked to quench recipe | Recipe-based speed for each thickness |
See also the Forced Draft (FD) Fan for glass tempering furnaces.
Questions buyers ask
Does a glass tempering furnace need an ID fan?
Not in the combustion sense, as most tempering furnaces are electric. An exhaust fan, which plays the ID fan role, removes the large volume of hot quench air and the heating chamber vent so that heat does not build up in the building. Gas-fired tempering furnaces also need it to remove combustion products.
What temperature is the air from a tempering quench?
The quench air picks up heat from glass at 620 to 650°C, but the air volume is large, so the exhaust is typically 60 to 120°C. The heating chamber vent can be hotter, up to 200 to 250°C during purging, so the fan is checked for that temperature.
How is the tempering exhaust fan protected from broken glass?
A screen or knock-out box at the quench hood catches large fragments before they reach the fan, and wear plate at the blade inlet resists fine glass dust. The blade form is chosen to tolerate particles, and inspection doors allow the impeller to be checked after a heavy breakage, before small damage turns into imbalance.
Information needed to quote an ID fan for glass tempering 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.
- Quench blower data. Quench blower flow and pressure for the thinnest and thickest glass processed.
- Furnace heating. Electric or gas-fired, and the vent flow from the heating chamber.
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.