How hot dip galvanising kettle furnaces work
In a hot dip galvanising plant the zinc bath, held at about 440 to 460°C, sits in a steel kettle enclosed by a refractory furnace. High velocity gas or oil burners fire into the space around the kettle so that the flame never touches the steel, and the hot gas circulates round the kettle walls. Flue gas leaves the furnace at 550 to 750°C. Most modern plants send it through a heat exchanger that heats the degreasing, pickling or flux tanks and the pre-dryer before the ID fan and chimney. The kettle runs continuously, at full firing during dipping shifts and at a low holding rate overnight and on holidays. Fume from the bath surface is a separate stream, taken by an enclosure or lip hood to a bag filter.
Where the ID fan sits
- Burners
- Combustion space round kettle
- Flue outlets
- Heat recovery exchanger
- ID fan
- Chimney
The fan is placed after the heat recovery exchanger, where the gas has cooled to about 200 to 300°C. Without heat recovery it sees 550 to 750°C and needs dilution air or heat-resisting construction. The fan works with flue dampers on each side of the kettle so that suction pulls gas evenly round the walls. Total suction is small, about 30 to 80 mmWC, but the exchanger resistance grows as soot and dust collect on its tubes.
Gas at the ID fan
| Gas temperature, normal | 200 to 300°C after the heat exchanger |
| Maximum or upset | 550 to 750°C with the exchanger bypassed or tanks at temperature |
| Dust at the fan | Low; some soot on oil firing and refractory dust |
| Moisture and dew point | Above dew point in normal running; condensation possible during long holding at low fire |
| Corrosive species | SO₂ on furnace oil firing; zinc chloride fume only if the bath hood leaks into the furnace |
| Density at the design temperature | About 0.64 kg/m³ at 280°C, 53% 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 hot dip galvanising kettle furnaces
| Blade form | Backward inclined, robust plate blades |
| Material of construction | Mild steel for gas below about 350°C; heat-resisting impeller if the exchanger bypass is used |
| Drive and arrangement | Belt or direct drive, arrangement 8 with cooling disc |
| Bearings and cooling | Pedestal bearings with heat slinger; no water cooling at normal temperature |
| Flow control | Variable speed drive on combustion chamber pressure |
| Also specified | Bypass damper and temperature trip for the exchanger, and drain point in the casing |
What this ID fan has to cope with
Hot spots on the kettle
If one flue outlet has more suction than the others, hot gas short-circuits along one wall and overheats that part of the kettle while another side runs cool. Iron-zinc attack accelerates on the hot area, and kettle failure means a zinc run-out. Balanced flue dampers and a fan that holds steady chamber pressure keep heat distribution even.
Acid condensation at holding load
Overnight the burners drop to holding fire, the flue gas volume falls and the gas cools further in the heat exchanger. On oil firing this can bring the fan below the acid dew point. Bypassing part of the exchanger at low fire, or limiting its duty, keeps the fan above dew point.
Exchanger fouling
Soot and dust settle on the heat exchanger tubes, increasing pressure drop and reducing the suction available at the furnace. The fan needs a pressure margin for a fouled exchanger, and the exchanger needs access for cleaning.
Energy and control
A galvanising kettle spends a large share of its hours at holding fire, keeping the zinc molten with no work going in. A fixed speed ID fan throttled by a damper wastes most of its power then. Speed control on chamber pressure brings fan power down with the burners, and tight pressure control also stops cold air entering round the kettle lip, which is a direct loss of heat from the zinc.
ID fan and FD fan in hot dip galvanising kettle furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the heat recovery exchanger | Feeding the burner main round the kettle |
| What it handles | Flue gas from round the kettle | Ambient air, ducted from clean intake |
| Temperature at the fan | 200 to 300°C, higher on bypass | Ambient |
| Main design concern | Even suction round the kettle; dew point at holding fire | Burner velocity across full and holding fire |
| Control | Combustion chamber pressure | Air main pressure |
See also the Forced Draft (FD) Fan for hot dip galvanising kettle furnaces.
Questions buyers ask
Does a galvanising furnace need an ID fan?
Small kettles with a short flue and a chimney often run on natural draft. Once a heat recovery exchanger is added to heat the pre-treatment tanks, its resistance usually needs an ID fan to keep the furnace pressure steady and the gas flowing evenly round the kettle.
Is the galvanising fume extracted by the furnace ID fan?
No. Zinc ammonium chloride fume rising from the bath surface is collected by a separate enclosure or lip hood and taken to a bag filter with its own fan. It must be kept out of the furnace flue because it is corrosive and would foul the heat exchanger.
What temperature does the galvanising kettle ID fan handle?
With heat recovery to the process tanks, typically 200 to 300°C. Without it, the flue gas is at 550 to 750°C and the fan needs either heat-resisting construction or dilution air ahead of it to bring the temperature down.
Information needed to quote an ID fan for hot dip galvanising kettle 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.
- Kettle size and firing. Kettle dimensions, number of burners and fuel, and holding load as a share of full firing.
- Heat recovery. Exchanger type, the tanks it heats, and its gas side pressure drop clean and fouled.
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.