How glass furnace waste heat boilers work
Container and float glass furnaces melt batch of sand, soda ash, limestone and cullet at about 1,500 to 1,600°C, fired with natural gas or fuel oil. Regenerative furnaces reverse firing between two regenerator chambers every 20 to 30 minutes, and the waste gas leaves the regenerators at about 400 to 550°C. Traditionally a tall chimney gives the draft. A waste heat boiler fitted after the regenerators takes away that natural draft and adds its own resistance, so an ID fan becomes necessary. The gas carries fine sodium sulphate and other condensed batch volatiles, with SO₂ from fuel and batch sulphate, and on many plants passes a dry scrubber and ESP or bag filter before the ID fan.
Where the ID fan sits
- Melting furnace
- Regenerators
- Reversing valve
- Waste heat boiler
- Dry scrubber and ESP or bag filter
- ID fan
- Chimney
The fan sits after the boiler and gas cleaning, before the chimney, with a bypass that returns the furnace to chimney draft if the boiler or fan is lost. It holds the furnace at a small positive pressure at the crown, typically below 1 mmWC, while overcoming the regenerators, boiler, scrubber and filter. Gas at the fan is about 180 to 220°C. Each reversal briefly changes flow and temperature, and the fan must ride through without disturbing the melt.
Gas at the ID fan
| Gas temperature, normal | 180 to 220°C after the boiler and gas cleaning |
| Maximum or upset | Up to 450 to 550°C regenerator exit gas if the boiler is bypassed through the fan route |
| Dust at the fan | Fine sodium sulphate and reaction products from the dry scrubber; low after the filter |
| Moisture and dew point | Moderate water vapour; acid dew point from SO₃ must be kept below the gas temperature |
| Flow pattern | Short swings at each reversal, every 20 to 30 minutes |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 furnace waste heat boilers
| Blade form | Backward curved for efficiency on cleaned gas, with blades that resist fine sticky deposits |
| Material of construction | Mild steel for normal duty; heat-resisting steel impeller where the fan can see bypassed regenerator gas |
| Drive and arrangement | Direct coupled, arrangement 8, often duty and standby |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace pressure, coordinated with the chimney bypass damper |
| Also specified | Fail-safe bypass that opens on fan trip, insulated casing and casing drain |
What this ID fan has to cope with
Furnace pressure through reversals
At each reversal the firing side changes and the flow and temperature to the boiler swing. If the fan reacts slowly, the crown pressure moves, which affects refractory life, batch carryover and glass quality. A speed loop tuned for these swings, with a feed-forward signal from the reversal sequence, holds the pressure.
Never stopping the furnace
A glass furnace cannot simply be stopped for a fan repair. If the fan trips, the bypass must return the furnace to chimney draft at once. Many plants also install a standby fan. Vibration monitoring and planned bearing changes on the running fan avoid unplanned trips.
Sodium sulphate deposits
Condensed sodium sulphate and scrubber reaction products are fine and can build up on blades and in the casing where gas cleaning is weak. A blade form that sheds deposits, access doors for cleaning during switchover to the standby fan, and in-situ balancing keep the fan smooth.
Energy and control
A glass furnace runs at nearly constant pull for years, so the fan runs at a steady point. Selecting the impeller close to its best efficiency at that point, with a variable speed drive to trim for regenerator fouling and seasonal changes, minimises power. Every kilowatt saved on the fan adds to the net benefit of the waste heat boiler.
Questions buyers ask
Why does a glass furnace need an ID fan after adding a waste heat boiler?
Without a boiler, the chimney provides enough draft for the furnace and regenerators. The boiler cools the gas, which reduces the chimney's natural draft, and adds its own resistance. An ID fan makes up the difference and holds the furnace pressure. The chimney remains as the bypass route.
What happens if the glass furnace ID fan trips?
The bypass damper must open to send the gas straight to the chimney so the furnace returns to natural draft. Without that, furnace pressure would rise within seconds. The bypass is designed to fail open, and many plants also keep a standby fan. The changeover is tested before commissioning.
How does regenerator reversal affect the ID fan?
Every 20 to 30 minutes the firing changes side, and for a short time the gas flow and temperature reaching the boiler change. The fan control must react quickly enough to keep the crown pressure steady through each reversal. A feed-forward signal from the reversal timer to the fan drive helps the loop anticipate each swing instead of only reacting to it.
Information needed to quote an ID fan for glass furnace waste heat boilers
- 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.
- Furnace data. Furnace type, pull rate, fuel, regenerator exit gas flow and temperature, and reversal interval.
- Bypass and redundancy. Chimney bypass arrangement and whether a standby fan is required.
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.