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 FD fan sits
- Inlet filter
- FD fan
- Air main round furnace
- Burner air valves
- High velocity burners
The fan sits beside the furnace, away from the pickling area, and supplies an air main that runs round the kettle with a branch to each burner. Most kettle burners use cold air; where a recuperator preheats the air, it is placed in the flue before the heat recovery exchanger. Fan pressure is typically 400 to 800 mmWC for high velocity burners.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient, or 250 to 400°C with a recuperator |
| Dust and fumes | Can carry acid mist and zinc fume if the intake faces the pickling or dipping area |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Turndown | Holding fire at a fraction of full load overnight and on holidays |
| 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 hot dip galvanising kettle furnaces
| Blade form | Backward curved, high pressure design |
| Material of construction | Mild steel with epoxy coating where acid mist is present |
| Drive and arrangement | Direct coupled, two-pole speed |
| Bearings and cooling | Standard bearings, no cooling |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Intake ducted from clean air outside the building, inlet silencer |
What this FD fan has to cope with
Acid mist at the intake
Galvanising buildings carry HCl vapour from pickling tanks. A fan drawing room air corrodes its casing and impeller and sends acid into the burners. Ducting the intake from outside, or locating the fan away from the pre-treatment line, avoids this.
Burner velocity at holding fire
High velocity burners lose their stirring effect if air pressure falls too far at low fire. Some plants run pulse firing at holding load so that each burner fires at full velocity for part of the cycle. The fan and drive must support that mode with steady main pressure.
Loss of air with a full kettle
If the combustion air fan trips, the burners shut down and the zinc starts to cool. A full kettle holds a lot of heat and gives some hours, but a frozen kettle is a long and costly recovery and can damage the kettle as the zinc re-melts. A standby fan, or a quick restart arrangement with a spare motor, is often specified.
Energy and control
The kettle fires hard for perhaps two shifts and holds for the rest of the day. A variable speed drive on main pressure follows that pattern and saves substantially compared with a throttled damper. Excess air control matters too: every extra cubic metre of cold air takes heat that would otherwise keep the zinc at temperature.
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 Induced Draft (ID) Fan for hot dip galvanising kettle furnaces.
Questions buyers ask
What size combustion air fan does a galvanising kettle need?
It is sized from the total burner firing rate at full load, the air-fuel ratio and the burner pressure. A kettle of several metres length with high velocity burners usually needs a fan of a few thousand m³/h at 400 to 800 mmWC, but the burner data decides it.
Why do galvanising furnaces use high velocity burners?
High velocity burners drive hot gas round the kettle at speed, which evens out the wall temperature and avoids flame impingement on the steel. That needs combustion air at relatively high pressure from the forced draft fan.
Should the FD fan be inside the galvanising shop?
It can be inside, but its intake should take clean air from outside or from an area away from the pickling and fluxing tanks. Acid and flux fume in the combustion air corrodes the fan impeller, casing and burner parts. Where the fan must stand in the shop, an epoxy-coated fan with a ducted intake is the usual answer.
Information needed to quote an FD fan for hot dip galvanising kettle 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.
- Burners. Number of burners, burner model and the air pressure needed at full firing.
- Plant layout. Where the fan can stand and where clean intake air is available.
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.