How glass fibre melting furnaces work
Glass fibre is made in two main forms. Continuous filament or textile fibre, usually E-glass for reinforcement, is melted in furnaces of about 50 to 200 tonnes per day, often oxy-fuel or air-gas recuperative with electric boost, at up to about 1,600°C, and the glass flows through forehearths to platinum bushings. E-glass batch contains colemanite or other boron sources and sometimes fluorspar, so the waste gas carries boron compounds and HF along with particulates. Glass wool for insulation is melted in electric or gas-fired furnaces and fiberised by spinners. Air-gas furnaces cool waste gas through a recuperator and dilution air; all types use a dry or wet scrubber and filter before the ID fan. The FD fan supplies the recuperator and burners.
Where the FD fan sits
- Inlet filter
- FD fan
- Recuperator
- Hot air main
- Melter burners
The fan sits near the recuperator and pushes ambient air through it to a hot air main feeding the melter burners. Forehearth burners usually have their own smaller combustion air fan or premix blower. Pressure at the fan is typically 300 to 600 mmWC. On oxy-fuel furnaces this fan is absent, and on all-electric glass wool melters there is no combustion air at all.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 500 to 700°C, downstream of the fan |
| Dust | Batch dust; filtered intake |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Duty pattern | Continuous and steady |
| 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 fibre melting furnaces
| Blade form | Backward curved, medium to high pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, with a standby fan |
| Bearings and cooling | Standard bearings with condition monitoring |
| Flow control | Variable speed drive on air main pressure, ratioed to fuel |
| Also specified | Inlet filter, silencer and emergency power |
What this FD fan has to cope with
Bushing temperature disturbances
Glass fibre quality depends on very steady glass temperature at the bushings. Fluctuations in combustion air change the flame temperature in the melter and travel through the forehearth, where they can show up as fibre breaks and diameter variation. A fan with stable flow, a responsive speed loop and accurate air flow measurement keep the melter steady.
Recuperator protection
As on any recuperative furnace, losing combustion air exposes the recuperator tubes to overheating within minutes, while the melter loses firing and the bushings start to cool. A standby fan with automatic changeover, both fans on emergency power and a minimum flow bleed for low firing are the standard protection for this duty.
Batch dust in the intake
Fibre batch is finely ground and includes boron minerals such as colemanite, and dust from the batch plant drifts around the furnace building. Drawn into the intake, it fouls filters, burner passages and the recuperator air side. A filtered intake taken from a clean location, with a pressure gauge across the filter, prevents this at little cost.
Energy and control
Fibre furnaces run continuously at steady pull, because the bushings need a constant flow of glass. The combustion air fan's speed control mainly trims small changes in firing and electric boost. Choosing a fan with high efficiency at that steady operating point is the main energy lever, together with tight air-fuel ratio control, which governs melter fuel use.
ID fan and FD fan in glass fibre melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber and filter | Before the recuperator |
| What it handles | Waste gas with borates and HF | Ambient filtered air |
| Temperature at the fan | 60 to 200°C depending on scrubber | Ambient |
| Main design concern | Corrosion and deposits | Steady flow and recuperator protection |
| Control | Furnace pressure | Air main pressure ratioed to fuel |
See also the Induced Draft (ID) Fan for glass fibre melting furnaces.
Questions buyers ask
Does a glass fibre furnace have a combustion air fan?
An air-gas recuperative glass fibre furnace has one, supplying air through the recuperator to the melter burners. Oxy-fuel fibre furnaces do not, because oxygen from a VPSA plant replaces air. All-electric glass wool melters need no combustion air at all. Forehearth burners usually have their own small blower in every case.
What pressure does a glass fibre furnace combustion air fan need?
Typically 300 to 600 mmWC, to overcome the recuperator air side, the hot air main, the control valves and the burners. The recuperator maker's air side pressure drop at maximum flow and the burner manufacturer's pressure requirement set the final figure, with some margin for fouling of the recuperator tubes over the campaign.
Why is steady air flow important on a glass fibre furnace?
Fibre drawing at the bushings needs constant glass temperature and viscosity. Fluctuations in combustion air change the melter temperature, and the disturbance carries through the forehearth to the bushings, where it can cause fibre breaks. Steady, measured air flow from the combustion air fan helps keep the whole glass path stable.
Information needed to quote an FD fan for glass fibre melting 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.
- Firing type. Air-gas recuperative or oxy-fuel, and the burner layout.
- Recuperator data. Air side pressure drop and minimum air flow.
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.