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 FD fan sits
- Inlet filter and silencer
- FD fan (quench blower)
- Air ducts
- Upper and lower plenums
- Quench nozzles
- Glass sheet
The blower stands beside the quench section, often in an acoustic enclosure, and supplies upper and lower nozzle plenums through large ducts. For thick glass the quench needs only a few hundred mmWC; for 3 to 4 mm glass it needs 1,500 mmWC or more at the nozzles. Plants processing thin glass use one or more high pressure blowers, sometimes in series, to reach this pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Glass dust in the processing hall; filtered intake essential |
| Pressure at the fan outlet | From a few hundred mmWC for thick glass to 1,500 mmWC or more for 3 to 4 mm glass |
| Load pattern | Cyclic, with each load quenched for a short burst then cooled at lower flow |
| Motor size | Large, often the biggest electrical load on the tempering line |
| 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 tempering furnaces
| Blade form | Backward curved or radial, at high tip speed for the pressure |
| Material of construction | High strength steel impeller for the tip speed |
| Drive and arrangement | Direct coupled, arrangement 4 or 8, high speed |
| Bearings and cooling | Heavy duty bearings with vibration monitoring |
| Flow control | Variable speed drive, set by recipe for each glass thickness |
| Also specified | Inlet silencer and filter, acoustic enclosure and fast acceleration capability |
What this FD fan has to cope with
Thin glass not tempering
If the quench blower cannot deliver enough pressure, thin glass does not reach the required surface compression and fails fragmentation tests. Selecting the blower for the pressure needed by the thinnest glass in the plan, at the nozzles, with duct and plenum losses added, avoids this.
Uneven quench and optical distortion
If air reaches the nozzle boxes unevenly, one area of the sheet cools faster than another. The result is stress patterns visible as anisotropy or iridescence, uneven fragmentation, and in the worst case breakage in the quench. Balanced plenum and duct design, with a blower that holds steady pressure through the quench, keeps delivery even across the sheet.
Blower response time
The blower must reach quench pressure in the few seconds between the load leaving the furnace and entering the quench. A blower stopped between loads cannot do this, while one at full speed all the time wastes energy. A drive and motor sized for rapid acceleration, with the blower idling at reduced speed between loads, balances response and energy.
Energy and control
The quench blower is often the largest power user on a tempering line, but it runs at full pressure only during the short quench of thin glass. Variable speed control set by recipe, idling between loads and cutting speed for thick glass, saves substantially compared with running at full speed and dumping air.
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 Induced Draft (ID) Fan for glass tempering furnaces.
Questions buyers ask
What is the quench blower on a tempering furnace?
It is the fan that forces ambient air through the quench nozzles onto both faces of the hot glass sheet, cooling the surfaces rapidly so they set in compression. It is the forced draft or FD fan of a tempering furnace and usually the largest motor on the line.
What pressure is needed to temper thin glass?
Thin glass needs much higher quench pressure than thick glass. For 3 to 4 mm glass, quench pressure at the nozzles is often 1,500 mmWC or more; for 10 mm and thicker, a few hundred mmWC is enough. The blower pressure must include duct and plenum losses.
Can the quench blower use a variable speed drive?
Yes, and it should. Each glass thickness needs a different quench pressure, so a variable speed drive set by recipe delivers the right pressure for each load and idles the blower between loads. This saves a large share of the blower's energy and reduces noise, compared with running at full speed and dumping or throttling air.
Information needed to quote an FD fan for glass tempering 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.
- Glass thickness range. Thinnest and thickest glass to be tempered and the quench pressure each needs.
- Quench section. Nozzle box size, plenum arrangement and any existing blower data.
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.