How tunnel baking ovens work
A tunnel baking oven carries bread, buns, cakes or rusk on a steel band, mesh belt or tray conveyor through a series of zones, each held at its own temperature, typically 180 to 280°C. Many bread ovens are indirectly fired: a gas or oil burner fires into a combustion chamber, and the hot combustion gas, at about 400 to 600°C, is circulated by a recirculation fan through radiator tubes running above and below the band. The product never touches the flue gas. A small part of the circulating gas is bled off continuously by a flue exhaust fan and replaced by fresh combustion products. Separately, vapour extraction ducts with dampers remove the steam released by the dough from the baking chamber.
Where the ID fan sits
- Burner and combustion chamber
- Recirculation fan
- Radiator tubes
- Return manifold
- Flue bleed damper
- ID fan
- Stack
Each heating zone or group of zones has its own circuit, and the flue exhaust fan draws a bleed from the return manifold after the gas has given up heat in the radiator tubes. It is usually mounted on the oven roof or a platform beside it, close to the stack. The suction it develops is small, typically 20 to 80 mmWC, but it must hold the radiator circuit slightly negative so that combustion gas never leaks through tube joints into the baking chamber.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C at the return manifold |
| Maximum or upset | Up to 450°C if the recirculation fan trips while the burner fires |
| Dust at the fan | Negligible with gas or light oil firing |
| Moisture and dew point | Combustion water vapour; condensation only during a cold start |
| Corrosive species | SO₂ when firing diesel or furnace oil; matters at cold start when the casing is below the acid dew point |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 tunnel baking ovens
| Blade form | Backward curved, suited to a small, clean, hot flow |
| Material of construction | Mild steel casing with external insulation; impeller in mild steel up to about 350°C, heat-resisting steel where upsets exceed about 400°C |
| Drive and arrangement | Direct coupled, overhung on a pedestal, with the casing free to expand |
| Bearings and cooling | Shaft cooling disc and a heat shield between casing and bearing housing |
| Flow control | Bleed damper set at commissioning, or variable speed linked to burner firing |
| Also specified | Gas-tight insulated casing and expansion joints on the inlet and stack connections |
What this ID fan has to cope with
Combustion gas leaking into the bake chamber
If the flue fan loses suction, the radiator circuit goes positive and combustion gas leaks through tube joints and access plates into the baking chamber, where it can taint the product. A fan with suction margin, a low-draft alarm on the circuit and an interlock that stops the burner on fan failure prevent it.
Overheating after a recirculation fan trip
When the recirculation fan stops but the burner fires on for a few seconds, the flue fan draws undiluted gas straight from the combustion chamber. The impeller is designed for this upset temperature, not the normal one, and the burner is interlocked to prove recirculation flow before and during firing.
Zone interaction on a shared flue fan
Several zones often share one flue fan. Opening one zone's bleed damper steals suction from the others and shifts their temperatures. Balanced branch ducts with individual circuit pressure measurement, or one smaller fan per zone group, keep the zones independent. Zone temperatures then respond to their own burners rather than to a neighbour's damper.
Energy and control
Bakery ovens run long, steady shifts with occasional product changes. The flue fan is small, but the gas it bleeds carries heat away, and every extra cubic metre exhausted means more burner fuel. Setting the bleed to the minimum that keeps the circuit negative, and using a variable speed drive that follows burner firing, saves gas. Larger bakeries recover heat from the flue to proving rooms or process water.
ID fan and FD fan in tunnel baking ovens
| ID fan | FD fan | |
|---|---|---|
| Position | Oven roof, on the flue bleed to the stack | At each zone burner |
| What it handles | Combustion gas from the radiator circuit | Clean hall or outside air |
| Temperature at the fan | 250 to 350°C, up to 450°C upset | 30 to 50°C |
| Main design concern | Keeping the circuit under suction | Steady air-fuel ratio |
| Control | Circuit suction and burner firing | Burner modulation |
See also the Forced Draft (FD) Fan for tunnel baking ovens.
Questions buyers ask
What is the flue exhaust fan on a tunnel baking oven for?
On an indirectly fired oven the burner heats gas that circulates through radiator tubes. The flue exhaust fan, which is the ID fan of the oven, draws off part of this gas to the stack so fresh combustion products can enter, and keeps the circuit under slight suction so flue gas cannot leak into the baking chamber.
What temperature does a tunnel oven flue fan handle?
Typically 250 to 350°C at the return manifold of the radiator circuit, with upset peaks to about 450°C if the recirculation fan trips. The fan needs a shaft cooling disc, an insulated casing and an impeller rated for the upset temperature; above about 400°C heat-resisting steel is used for the wheel.
Is the vapour extraction fan the same as the flue fan on a bakery oven?
No. Vapour or steam extraction removes moisture released by the dough from the baking chamber itself, through dampered ducts that are often natural draft or served by a separate low-temperature fan. The flue fan handles only combustion gas from the indirect heating circuit. The two systems are kept apart so flue gas never reaches the product.
Information needed to quote an ID fan for tunnel baking ovens
- 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.
- Oven firing. Number of burners and zones, fuel fired, and whether zones share a flue fan.
- Circuit pressure. Required suction in the radiator circuit and the duty of the recirculation fan.
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.