How glass annealing lehrs work
An annealing lehr is a long tunnel with a mesh belt or rollers that takes formed glass from the forming machine and removes its stresses by controlled cooling. Containers and tableware enter at about 500 to 600°C, are reheated or held just above the annealing point, around 540 to 560°C for soda-lime glass, then cooled slowly through the strain range and finally quickly with air. Heating is by gas burners, firing directly into the tunnel or into a muffle, or by electric elements. Inside, recirculation fans drive hot air across the ware in each zone. Ambient air is blown into the cooling zones, and an exhaust fan removes the hot air and combustion products through a stack. Hot end coating and pharmaceutical dealkalisation treatments add HCl or sulphur compounds to the lehr atmosphere.
Where the ID fan sits
- Lehr entrance and heating zone
- Controlled cooling zones
- Exhaust hoods or ducts
- Zone dampers
- Exhaust (ID) fan
- Stack
The fan sits on the lehr roof or beside it, connected to exhaust hoods or ducts on the heating and controlled cooling zones, each with a damper. It pulls hot air and the small volume of combustion gas out of the tunnel, while cooling air fans blow fresh air into the later zones. The balance between supply and exhaust decides whether hot air leaks out of the entrance or cold air is drawn in. Suction is low, typically 20 to 60 mmWC.
Gas at the ID fan
| Air temperature, normal | 150 to 300°C, mixed exhaust from heating and cooling zones |
| Maximum or upset | Up to 400 to 450°C when cooling air fails or the belt runs empty and hot |
| Dust at the fan | Very low; occasional glass fragments and belt scale |
| Moisture and dew point | Low; mostly air with a small share of combustion products |
| Corrosive species | HCl from hot end coating where it enters the lehr; SO₂ or SO₃ where sulphur dealkalisation is used |
| Density at the design temperature | About 0.67 kg/m³ at 250°C, 56% 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 annealing lehrs
| Blade form | Backward curved, for quiet, even extraction of clean hot air |
| Material of construction | Mild steel up to about 350°C; stainless steel where HCl or sulphur treatment gases are present |
| Drive and arrangement | Belt or direct drive, arrangement 8 or 9, often roof mounted |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive, with zone dampers for distribution |
| Also specified | Insulated casing, compact layout for roof mounting and low vibration to protect the lehr structure |
What this ID fan has to cope with
Uneven temperature across the belt
Extraction that is stronger on one side of the lehr pulls hot air that way and leaves cold spots on the other side. The ware then carries uneven stress, which appears in polariscope checks and as breakage in filling lines. Balanced exhaust ducts, zone dampers and a fan with a stable curve at its working point keep temperatures even across the belt width.
Acid fumes from coating and treatment
Hot end coating with tin chloride compounds releases HCl near the lehr entrance, and pharmaceutical vials treated with sulphur for dealkalisation give off sulphur oxides. On a mild steel exhaust fan these condense when the lehr cools at weekends and pit the casing. Stainless construction, or a separate coating hood exhaust, avoids the damage.
Entrance draught and cold air ingress
If the exhaust fan pulls too hard, cold air enters at the lehr mouth and chills the ware arriving from the forming machine, causing checks and breakage. Speed control linked to zone pressure or temperature, rather than a fixed damper setting, holds the balance as the load changes.
Energy and control
A lehr runs whenever its forming line runs, and the load changes with job changes, weight of ware and line speed. Exhaust demand changes with it. A variable speed exhaust fan cuts power when the belt is lightly loaded and avoids over-extraction, which wastes the heat the burners or elements have just supplied. The heat saving from correct balance is usually larger than the fan power saving.
ID fan and FD fan in glass annealing lehrs
| ID fan | FD fan | |
|---|---|---|
| Position | Exhaust hoods on heating and cooling zones | Cold end, supplying cooling zone plenums |
| What it handles | Hot air with combustion products and treatment fumes | Ambient filtered air |
| Temperature at the fan | 150 to 300°C | Ambient |
| Main design concern | Balanced extraction and acid fumes | Even, adjustable cooling air |
| Control | Lehr pressure balance | Cooling zone temperature |
See also the Forced Draft (FD) Fan for glass annealing lehrs.
Questions buyers ask
What fans are used on a glass annealing lehr?
A lehr usually has recirculation fans inside each heating zone, cooling air fans supplying ambient air to the cooling zones, and an exhaust fan drawing hot air and combustion products to the stack. The exhaust fan is the lehr's ID fan; it sets the pressure balance and so the temperature profile along the tunnel.
What temperature does a lehr exhaust fan handle?
Usually 150 to 300°C, a mix of hot air from the heating zone and warm air from the controlled cooling zones. It can reach 400°C or more if the cooling air fails or the lehr runs empty and hot. The fan material and bearings are chosen for that upset temperature.
Why are lehr exhaust fans made of stainless steel?
Where hot end coating releases HCl at the lehr entrance, or sulphur is used to dealkalise pharmaceutical glass, the exhaust carries acid gases. When the lehr cools, they condense and corrode mild steel. Stainless steel resists this. Lehrs without such treatments can use mild steel.
Information needed to quote an ID fan for glass annealing lehrs
- 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.
- Lehr size and zones. Lehr width and length, number of heating and cooling zones and present exhaust points.
- Surface treatments. Whether hot end coating or sulphur dealkalisation fumes reach the lehr exhaust.
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.