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 FD fan sits
- Inlet filter
- FD (cooling air) fan
- Supply ducts
- Zone dampers
- Cooling zone nozzles or plenums
- Lehr tunnel
The fan stands beside the lehr, usually near the cold end, and supplies ambient air through ducts and dampers to plenums or nozzles in the cooling zones. In indirect cooling zones the air passes through tubes or a muffle and does not touch the ware; in direct zones it is blown onto the ware. On gas-fired lehrs a small combustion air blower supplies the burners. Pressure is moderate, typically 100 to 300 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Cullet and batch dust in the forming area; filtered intake recommended |
| Pressure at the fan outlet | Typically 100 to 300 mmWC |
| Load pattern | Continuous, varying with job, ware weight and line speed |
| 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 annealing lehrs
| Blade form | Backward curved, for even delivery at moderate pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on cooling zone temperature, with zone dampers |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Wrong cooling rate
Cooling too fast through the strain range leaves stress in the ware; too slowly wastes lehr length and limits line speed. A fixed-speed fan with dampers cannot hold the curve as ware weight changes. Variable speed control on zone temperature lets the cooling rate follow each job.
Uneven air across the belt
If cooling air reaches one side of the belt more than the other, ware on that side cools faster and carries more stress. Balanced plenum design and a fan with stable pressure keep delivery even across the width, which matters most on wide lehrs carrying several rows of containers.
Dust blown onto ware
Unfiltered air drawn from the forming hall carries cullet fines, swabbing compound residues and oil mist. Blown directly onto hot containers or tableware in the direct cooling zones, it marks the surface and can leave scratches that weaken the ware. A filtered intake taken from a clean location, with a pressure gauge across the filter, prevents this at little cost.
Energy and control
Cooling air demand varies with ware weight and line speed. A variable speed cooling air fan delivers only what the cooling curve needs, rather than running at full speed and throttling. Less excess cooling air also means less heat drawn back towards the heating zones, which reduces burner or element load.
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 Induced Draft (ID) Fan for glass annealing lehrs.
Questions buyers ask
Does a glass lehr have an FD fan?
A lehr has cooling air fans that force ambient air into the cooling zones, and gas-fired lehrs also have a combustion air blower for the burners. Together these are the forced draft side of the lehr. The exhaust fan on the other side is the lehr's ID fan.
How is the cooling rate controlled in an annealing lehr?
The cooling rate is set by the temperature profile in each cooling zone, controlled by the volume of cooling air and by the zone dampers. A variable speed cooling air fan, controlled on zone temperature, lets the cooling curve follow changes in ware weight and line speed without manual damper changes.
Why does uneven cooling air cause breakage?
Uneven cooling leaves residual stress in the ware, higher on the side that cooled faster. Stressed containers break on filling lines, in pasteurisers or in transport, and stressed tableware fails in dishwashers. Even cooling air delivery across the belt width, from a stable fan and balanced plenums, keeps the stress uniform and low.
Information needed to quote an FD fan for glass annealing lehrs
- 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.
- Cooling zones. Number of direct and indirect cooling zones and the air flow to each.
- Ware range. Range of ware weight and line speed the lehr must handle.
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.