How oxy-fuel glass furnaces work
An oxy-fuel glass furnace burns natural gas or oil with oxygen of about 90 to 99% purity instead of air, supplied by an on-site VPSA plant or liquid oxygen storage. With almost no nitrogen in the flame, waste gas volume falls to roughly a quarter to a third of an air-fired furnace, NOx drops sharply and no regenerators or recuperators are needed. The waste gas, however, leaves the melter at about 1,300 to 1,450°C, is roughly half water vapour and carries concentrated sodium sulphate, borates or other volatiles depending on the glass. It is quenched with ambient air or water, cooled further in a waste heat boiler or heat exchanger, often one that preheats batch, cullet or oxygen, and cleaned in a filter before an ID fan. Oxy-fuel is used for container, tableware, fibre and speciality glass furnaces.
Where the ID fan sits
- Oxy-fuel melter
- Flue port
- Quench chamber
- Heat exchanger or waste heat boiler
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the bag filter or ESP, with the quench chamber and gas cooler upstream. Ambient air or water spray in the quench brings the gas from about 1,400°C down to 450 to 600°C, and the cooler takes it to about 180 to 250°C for filtration. The quench air may double or triple the gas volume, so the fan often handles more air than furnace gas. Suction is typically 150 to 300 mmWC. Furnace pressure is held slightly positive at a small flue port.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after the filter |
| Maximum or upset | 400 to 500°C if quench water or cooling fails |
| Dust at the fan | Low after the filter; fine sulphates or borates, sticky when cool |
| Moisture and dew point | High; water dew point 70 to 85°C before quench dilution, lower after air quench |
| Corrosive species | SO₂ and SO₃ concentrated by the lack of nitrogen; HF and boron compounds on fibre and borosilicate glasses |
| Air ingress sensitivity | Leakage air at the flue port raises NOx and gas volume |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 oxy-fuel glass furnaces
| Blade form | Backward curved, for a smooth, controllable curve at modest flow |
| Material of construction | Mild steel where the gas stays well above the dew point; stainless steel where water quench or high moisture brings condensation risk |
| Drive and arrangement | Direct coupled, arrangement 8, insulated casing |
| Bearings and cooling | Cooling disc and heat shield; rated for the cooling failure upset |
| Flow control | Variable speed drive on furnace pressure with fast response |
| Also specified | Chimney bypass, casing drain, insulation and standby provision |
What this ID fan has to cope with
Wet gas condensation
Oxy-fuel waste gas is rich in water vapour, so its dew point is much higher than air-fired gas. With water quench, it rises further. Cold spots on the fan casing, inlet box or ducts condense acidic water that corrodes steel and turns sulphate dust to sludge. Full insulation and keeping the fan gas temperature well above the dew point are essential.
Furnace pressure sensitivity
An oxy-fuel furnace has a small flue port and low gas volume, so small changes in draft cause large swings in furnace pressure. Too much suction draws in air at peepholes and the doghouse, raising NOx and fuel use; too little pushes hot gas out. A fan with fine speed control and a fast pressure loop is needed.
Quench air control
The quench air sets the gas temperature and much of the volume at the fan. If the quench damper and fan speed are not coordinated, temperature at the filter swings and the fan hunts. Controlling the quench on temperature and the fan on furnace pressure, with interlocks between them, keeps both steady.
Energy and control
With no combustion air fan and a smaller flue gas volume, an oxy-fuel furnace uses less fan power than an air-fired one, but the quench air can bring the volume back up. Using water quench or a heat exchanger instead of air dilution, and running the ID fan on variable speed against furnace pressure, keeps fan power in proportion. Avoiding air ingress also saves oxygen and fuel.
Questions buyers ask
Does an oxy-fuel glass furnace need an ID fan?
Usually yes, once gas cooling and filtration are fitted. The waste gas must be drawn through the quench, cooler and filter while the furnace pressure is held slightly positive. Some small oxy-fuel furnaces use an ejector at the chimney, but an ID fan gives finer furnace pressure control.
Why is oxy-fuel waste gas so wet?
Burning natural gas produces water vapour and CO₂. In an air-fired furnace, nitrogen from the air dilutes these. With oxygen firing, there is almost no nitrogen, so water vapour may make up around half of the waste gas, with CO₂ most of the rest. Its dew point is therefore much higher.
Is there an FD fan on an oxy-fuel glass furnace?
No. Oxygen is supplied by a VPSA plant or liquid oxygen storage, not by a fan. The furnace still has fans for quench air, cooling of electrodes and throat, and forehearths, and the ID fan for waste gas, but there is no combustion air or forced draft fan.
Information needed to quote an ID fan for oxy-fuel glass 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 and cooling. Quench medium (air or water), quench air flow, and gas temperature after the cooler.
- Glass type. Glass composition and any boron, fluorine or high sulphate content.
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.