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 ID fan sits
- Direct-fired and radiant tube sections
- Post-combustion chamber
- Strip preheater
- Recuperators
- Waste gas (ID) fan
- Stack
The fan sits after the last recuperator, at the stack. Upstream are the post-combustion chamber, the strip preheater section and the combustion air recuperators, giving suction of about 150 to 300 mmWC. Gas reaches it at about 250 to 400°C. The fan, with dampers in the exhaust legs, holds the direct-fired section at a precise small pressure so that atmosphere does not flow the wrong way between sections and air is not drawn in at the strip entry seal.
Gas at the ID fan
| Gas temperature, normal | 250 to 400°C after heat recovery |
| Maximum or upset | 500 to 600°C if strip stops in the preheater or a recuperator is bypassed |
| Dust at the fan | Very low; clean gas firing with occasional refractory dust |
| Moisture | High from natural gas, coke oven gas and hydrogen in the exhaust |
| Combustibles | Possible unburnt CO and hydrogen if post-combustion is incomplete |
| Density at the design temperature | About 0.57 kg/m³ at 350°C, 47% 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 continuous annealing and galvanising lines
| Blade form | Backward curved, for clean hot gas and precise control |
| Material of construction | Boiler quality plate to about 400°C; heat-resisting steel where upset temperature is high |
| Drive and arrangement | Direct coupled, double bearing, with sliding supports |
| Bearings and cooling | Cooling disc and heat shield; cooled bearings on hot duties |
| Flow control | Variable speed drive on furnace pressure, with fast trim dampers |
| Also specified | Slow turning on shutdown, inlet temperature trip, insulated casing |
What this ID fan has to cope with
Pressure control at changing line speed
When line speed changes, for a coil weld or a thickness change, the firing rate and the exhaust flow change quickly. If furnace pressure swings, atmosphere flows between sections and the strip surface suffers. A fan with a stable curve over the full range, a fast speed loop and trim dampers give the precision these lines need.
Unburnt gas from the direct-fired section
The direct-fired section runs rich, so its exhaust contains CO and hydrogen. If the post-combustion air is short or mixing is poor, combustibles reach the recuperators and fan. Monitoring and controlling post-combustion air, and designing the system to avoid pockets where gas collects, prevent damage.
Strip stop overheating
If the strip stops, the preheater no longer absorbs heat and the gas reaching the recuperators and fan rises sharply. Interlocks that cut firing, dilution air, and a fan designed for the upset temperature protect the equipment until the line restarts. The strip preheater tubes or rolls are also at risk, so the fan control and the dilution damper should act together within seconds of a line stop.
Energy and control
Continuous annealing and galvanising lines run around the clock, but line speed and strip size vary from coil to coil. A variable speed waste gas fan follows these changes while holding furnace pressure, saving power compared with damper control. Keeping post-combustion and recuperators in good order reduces fuel use and the volume the fan must move.
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 Forced Draft (FD) Fan for continuous annealing and galvanising lines.
Questions buyers ask
What does the ID fan do on a galvanising line furnace?
On a continuous annealing or galvanising line the waste gas ID fan draws exhaust from the direct-fired and radiant tube sections through post-combustion, strip preheating and recuperators, and holds furnace pressure. Precise pressure control keeps the atmosphere in each section stable and protects strip surface quality.
Why does furnace pressure matter on a continuous annealing line?
Each section has its own atmosphere. If pressure swings, gas can flow from the direct-fired section into the radiant tube section, or air can enter at the strip seals, oxidising the strip. Stable pressure from the induced draft fan and dampers prevents this. The fan must respond within seconds to line speed changes.
What temperature does the waste gas fan on a CAL or CGL handle?
Typically 250 to 400°C after heat recovery. If the strip stops or a recuperator is bypassed, the temperature can rise to 500 to 600°C, so the impeller and bearings should be designed for that upset. The gas is also wet from hydrogen-rich fuel and atmosphere, so the casing should be insulated to avoid condensation during low-speed running and start-up.
Information needed to quote an ID fan for continuous annealing and galvanising lines
- 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 sections. Direct-fired and radiant tube firing rates, post-combustion air and exhaust gas composition.
- Line operation. Line speed range, strip sizes and the furnace pressure set points.
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.