How continuous annealing and galvanising lines work
Continuous annealing and hot dip galvanising lines run cold rolled strip at high speed through a long vertical or horizontal furnace. Typical lines have a preheating section heated by furnace exhaust gas, a direct-fired or non-oxidising section where gas burners fire straight at the strip, a radiant tube section under a hydrogen-nitrogen atmosphere that brings the strip to about 700 to 850°C, and jet cooling before the zinc pot or the overageing section. The direct-fired section runs slightly fuel-rich to keep the strip bright, so its exhaust carries unburnt CO and hydrogen, which are burnt with added air before heat recovery. Radiant tube exhaust is collected in a separate header. Both streams pass recuperators or a strip preheater, then a waste gas ID fan to the stack.
Where the FD fan sits
- Inlet filter and silencer
- Combustion air (FD) fan
- Recuperators
- Air headers
- Direct-fired and radiant tube burners
The fan supplies an air main that passes through the recuperators and then splits into headers for the direct-fired section and the radiant tube zones. The fan pressure covers recuperators, headers, ratio control valves and burners, typically 400 to 800 mmWC. On many lines a separate fan supplies post-combustion air, while on others the main fan does both. The fan is usually installed with a standby, since a combustion air trip stops the furnace with strip inside it.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperators | 300 to 550°C, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Turndown | Wide, following line speed and strip size |
| 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 continuous annealing and galvanising lines
| Blade form | Backward curved, steep curve for stable pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter, silencer and a standby fan for continuous lines |
What this FD fan has to cope with
Precise ratio for the direct-fired section
The direct-fired section must run slightly rich to avoid oxidising the strip. Air main pressure swings upset the ratio control valves, giving oxidation or excess combustibles. A fan that holds main pressure steady across rapid firing changes is essential for strip quality. A steep fan curve, a fast speed loop and enough pressure margin at the ratio valves let the control hold the ratio within the narrow band the strip needs.
Recuperator protection
Loss of combustion air while flue gas is still hot leaves the recuperator tubes with no cooling on the air side. Automatic changeover to the standby fan, and interlocks that cut firing on low air flow, protect both the recuperators and the strip in the furnace.
Rapid load changes
Coil welds, thickness changes and speed changes move the firing rate quickly and repeatedly through each shift. The fan speed control must follow these changes without overshoot, or the ratio control valves downstream will hunt. Tuning the main pressure loop together with the ratio loops avoids interaction between them.
Energy and control
These lines run continuously for weeks between stops, so the combustion air fan is a steady, significant load. Efficient blading and variable speed control on air main pressure save power when the line runs thin gauges or slower speeds. Correct air to fuel ratio, especially in the direct-fired section, matters more for fuel use and for strip surface quality than the fan power itself.
ID fan and FD fan in continuous annealing and galvanising lines
| ID fan | FD fan | |
|---|---|---|
| Position | After recuperators, at the stack | Before the recuperators |
| What it handles | Waste gas from direct-fired and radiant tube sections | Ambient air |
| Temperature at the fan | 250 to 400°C | Ambient |
| Main design concern | Precise furnace pressure at changing line speed | Steady pressure for precise air to fuel ratio |
| Control | Furnace pressure | Air main pressure |
See also the Induced Draft (ID) Fan for continuous annealing and galvanising lines.
Questions buyers ask
What is the combustion air fan on a galvanising line?
It is the forced draft fan supplying combustion air to the direct-fired and radiant tube burners on a continuous annealing or galvanising line. The air is preheated in recuperators downstream of the fan. On many lines the same fan, or a separate one, also supplies post-combustion air for the direct-fired section exhaust. The fan itself handles cool, filtered air.
Why is air pressure stability so important on a CAL furnace?
The direct-fired section runs at a precise air to fuel ratio. Pressure swings upset the ratio and can oxidise the strip or leave excess combustibles. A stable combustion air fan protects strip quality. The fan should hold air main pressure within a narrow band during speed and thickness changes.
Is a standby combustion air fan needed?
Continuous annealing and galvanising lines usually specify one. Losing combustion air stops the furnace with strip inside, can overheat the recuperators, and leads to a long restart with scrap strip. A standby fan with automatic changeover and non-return dampers keeps the line running through a fan fault.
Information needed to quote an FD fan for continuous annealing and galvanising lines
- 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.
- Burner groups. Firing rates of direct-fired and radiant tube sections, and whether post-combustion air is from the same fan.
- Recuperators. Air side pressure drop and hot air temperature.
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.