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 FD fan sits
- Inlet filter
- FD fan
- Air damper
- Zone burner
- Combustion chamber
- Radiator circuit
Most zone burners are packaged with their own combustion air fan, mounted at the burner head on the side of the oven. Larger ovens sometimes use a central combustion air fan feeding a manifold with a damper at each burner. Either way the fan draws air from the bakery hall or from outside and pushes it through the burner against the pressure of the combustion chamber, typically 100 to 250 mmWC at full fire.
Air at the FD fan
| Air temperature at the fan | Hall air beside the oven, 30 to 50°C |
| Dust | Flour dust in the bakery air; a fine inlet filter is needed |
| Pressure at the fan outlet | Typically 100 to 250 mmWC at the burner |
| Turndown | Burners modulate with production rate and zone set points |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 tunnel baking ovens
| Blade form | Backward curved, quiet and stable on clean air |
| Material of construction | Mild steel, painted; stainless steel where hygiene rules require |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; fan kept clear of oven casing heat |
| Flow control | Characterised damper linked to the fuel valve, or variable speed drive |
| Also specified | Inlet filter against flour dust, air pressure switch and inlet silencer |
What this FD fan has to cope with
Flour dust on the impeller and burner head
Bakery air carries flour that settles on the blades and inside the burner head, slowly cutting air flow and upsetting fan balance. An inlet filter rated for fine dust, an air intake from a clean location away from mixers and dividers, and periodic cleaning keep the fan and burner right.
Hot hall air reducing capacity
Air beside the oven can reach 45 to 50°C, which lowers its density and the mass of oxygen the fan delivers. A fan sized on hall temperature at full fire, or ducted from outside the building, avoids burners running short of air on summer afternoons.
Noise in the production hall
A row of burner fans along the oven adds to the noise in a hall where people work all shift. Backward curved fans at moderate speed with inlet silencers, or a central fan in a plant room feeding a manifold, bring noise down. Flexible connections stop fan vibration travelling into the oven frame.
Energy and control
Burner air fans run whenever the oven is hot, including through product changeovers and idle periods. A burner with good turndown, and a fan whose air delivery follows the fuel through a linked damper or speed control, avoids heating surplus air. Checking flue oxygen at each zone at commissioning and after burner maintenance keeps gas use down.
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 Induced Draft (ID) Fan for tunnel baking ovens.
Questions buyers ask
What does the FD fan do on a bakery tunnel oven?
It is the combustion air fan of the zone burner. It supplies air so the burner can fire into the combustion chamber that heats the radiator tubes. Each zone usually has its own burner and fan, so the duty is small, but the forced draft fan must hold a steady air-fuel ratio as the burner modulates.
Why does the tunnel oven burner trip on low air pressure?
Common causes are a clogged inlet filter, flour dust on the impeller, a fan drawing very hot air from beside the oven, or a slipping belt on older units. Each reduces air pressure at the burner until the safety switch opens. Cleaning the filter and fan, and checking the fan curve against the burner requirement, usually cures it.
Can one combustion air fan serve all burners of a baking oven?
Yes. A central fan with a manifold and a damper at each burner is used on large ovens and is quieter and easier to maintain than many small fans. The manifold must be sized so each burner still gets enough pressure at full fire, and the fan needs a stable curve as burners modulate independently.
Information needed to quote an FD fan for tunnel baking ovens
- 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, rating of each and the air pressure each needs at full fire.
- Air source. Whether air is drawn from the bakery hall or ducted from outside, and the hall temperature near the oven.
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.