How biscuit ovens work
Biscuit, cracker and cookie dough pieces travel through a long continuous oven, often 40 to 100 m, on a steel band or wire mesh. The front zones are usually direct gas fired: rows of ribbon burners above and below the band, fed with a premixed air-gas mixture, release their combustion products straight into the baking chamber. Later zones are often convection, with hot air blown onto the product to dry and colour it. Dough at roughly 15 to 30% moisture leaves at about 1 to 4%, so the oven releases large quantities of steam, together with fat vapour and fine crumbs. Zone extraction fans draw this steam and the burner products out through roof ducts to stacks.
Where the FD fan sits
- Inlet filter
- FD fan
- Air manifold
- Air-gas mixers
- Ribbon burners
- Baking chamber
A central combustion air fan, beside the oven or in a plant room, feeds an air manifold running along the burner zones. At each zone an air-gas mixer proportions gas to the air, and the premix is piped to the ribbon burners above and below the band. The fan must hold a stable manifold pressure, typically a few hundred mmWC, while individual zones are turned up and down by the baker or the controller.
Air at the FD fan
| Air temperature at the fan | Hall or outside air, 30 to 50°C |
| Dust | Flour and sugar dust in the hall; a fine inlet filter is needed |
| Pressure at the fan outlet | Steady manifold pressure, typically a few hundred mmWC |
| Flow variation | Zones modulate independently, so total flow varies widely |
| 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 biscuit ovens
| Blade form | Backward curved, with a flat pressure curve across the flow range |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive holding manifold pressure |
| Also specified | Inlet filter, pressure switch on the manifold, and inlet silencer |
What this FD fan has to cope with
Manifold pressure dips upsetting flames
When several zones go to high fire together, manifold pressure falls if the fan curve droops, and every ribbon burner in the oven shortens its flame at once. A fan with a flat curve, sized for all zones at high fire, and speed control holding manifold pressure keep flames stable along the band.
Blocked burner ports from dusty air
Flour and sugar dust drawn in with the air passes the mixers and lodges in the fine ribbon burner ports, leaving gaps in the flame and pale stripes across the biscuits. A good inlet filter, and an intake away from mixing and sheeting areas, protect the burners.
Ratio change with air temperature
Hot hall air in summer is lighter, so a fan running at fixed speed delivers less oxygen and the premix runs rich. Speed control on manifold pressure, an intake ducted from outside where practical, and seasonal checks of the air-gas ratio keep combustion clean all year.
Energy and control
The combustion air fan runs whenever the oven is hot, often 20 hours a day or more. Speed control on manifold pressure means the fan develops only the pressure the zones need, instead of throttling against a closed damper or a bypass. The correct air-gas ratio also saves gas, since excess air in a direct fired zone is heated and then removed by the extraction fans.
ID fan and FD fan in biscuit ovens
| ID fan | FD fan | |
|---|---|---|
| Position | Oven roof, on each zone extraction | Beside the oven, feeding the burner manifold |
| What it handles | Steam, fat vapour and burner gases | Clean hall or outside air |
| Temperature at the fan | 150 to 250°C | 30 to 50°C |
| Main design concern | Fat deposits and zone humidity | Steady manifold pressure |
| Control | Zone humidity | Manifold pressure |
See also the Induced Draft (ID) Fan for biscuit ovens.
Questions buyers ask
What pressure does a biscuit oven combustion air blower need?
It depends on the burner and mixer design, but ribbon burner systems typically need a steady manifold pressure of a few hundred mmWC. The fan should hold that pressure with every zone at high fire, with some margin, and keep it steady as zones turn down. The burner maker's mixer data sets the figure.
Why are there pale stripes on biscuits from a direct gas fired oven?
Pale stripes usually come from gaps in the ribbon flame where burner ports are blocked, often by dust carried in with the combustion air or by soot from a rich mixture. Cleaning the burners, improving inlet filtration on the combustion air fan and resetting the air-gas ratio usually cure them.
Should a biscuit oven have a central combustion air fan or one per zone?
Both are used. A central forced draft fan with a manifold is common on direct gas fired ovens with premix mixers, and is easier to filter and silence. Separate fans per zone give independence but more units to maintain. Either way the fan must hold steady pressure as zones modulate.
Information needed to quote an FD fan for biscuit 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.
- Burner system. Ribbon burner rating per zone, number of mixers and the air pressure the mixers need.
- Hall conditions. Air temperature and dust near the fan, and whether air can be ducted from outside.
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.