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 ID fan sits
- Ribbon burners and baking chamber
- Zone extraction hood
- Extraction duct and damper
- Grease drain
- ID fan
- Roof stack
Each zone or pair of zones has a roof extraction duct, with a damper the baker uses to set zone humidity, and an extraction fan before the stack. The fan draws directly from the baking chamber at 150 to 250°C with no collector upstream, so it sees everything the oven gives off: steam, fat aerosol, crumbs and burner gases. Suction is low, typically 10 to 40 mmWC, and the chamber is held close to neutral so heat does not spill from the band entry and exit.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C at the zone extraction |
| Maximum or upset | Up to 300°C at the first zones during high-fire start-up |
| Dust at the fan | Fine crumbs and flour, with fat aerosol that sticks to cooler surfaces |
| Moisture and dew point | High steam content in the first zones; dew point can reach 60 to 80°C, so a cold casing condenses water and fat |
| Corrosive species | None significant; fat and sugar deposits are the concern |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 62% 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 biscuit ovens
| Blade form | Radial or backward curved, with smooth blades that shed fat deposits |
| Material of construction | Mild steel, or SS 304 for the wet extraction from the first zones |
| Drive and arrangement | Belt driven or direct coupled, mounted on the oven roof |
| Bearings and cooling | Shaft cooling disc, with bearings clear of the hot casing |
| Flow control | Zone damper set by the baker, or variable speed drive on zone humidity |
| Also specified | Insulated casing, grease drain at the lowest point, and a cleaning door |
What this ID fan has to cope with
Fat deposits and duct fires
Fat aerosol condenses on duct walls and fan blades, building a layer that can be ignited by a burner flame or a spark. Insulated ducts kept above the condensation point, grease drains at low points and at the fan casing, cleaning doors and a fixed degreasing schedule prevent duct and fan fires.
Condensate dripping back onto the band
When the extraction duct or fan casing is cold, steam condenses and runs back into the oven, marking the product. Insulation on duct and casing, a drain at the fan, and ducts that fall away from the oven towards a drain keep condensate out of the baking chamber.
Zone humidity drifting between cleans
Biscuit spread and texture depend on humidity in the early zones. As deposits build in the fan and duct, extraction falls, humidity rises and product dimensions change. Measuring zone humidity or pressure, and a fan on speed control that holds flow as deposits build, keep the product consistent between cleans.
Energy and control
Extraction carries heat away, and every extra cubic metre removed is gas the burners must reheat. Bakers often open extraction dampers wide and leave them there. Replacing fixed dampers with speed control set from zone humidity reduces both fan power and gas use. On larger ovens, heat recovered from the extraction air is used to preheat burner air or process water.
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 Forced Draft (FD) Fan for biscuit ovens.
Questions buyers ask
Why is the extraction fan on a biscuit oven important for product quality?
The extraction fans set the humidity in each zone of the biscuit oven. In the first zones humidity controls how the dough spreads and sets; later it affects drying and colour. Too little extraction gives soft, pale biscuits, while too much dries the surface early, limits spread and raises gas use.
How do you prevent fat fires in biscuit oven exhaust ducts?
Keep the ducts and fan warm with insulation so fat vapour does not condense, fit grease drains at low points and at the fan casing, provide cleaning doors, and clean on a fixed schedule. The fan should have smooth blades without ledges where fat collects, and extraction should be proven before the burners light.
What temperature does a biscuit oven exhaust fan handle?
Typically 150 to 250°C at the zone extraction, with the first zones hottest during start-up. The fan is designed for the upset temperature, with a shaft cooling disc and bearings clear of the casing, and its rating is corrected for hot, humid air, which is lighter than ambient air.
Information needed to quote an ID fan for biscuit 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.
- Zones. Number of extraction points, the zones each fan serves, and whether those zones are direct gas fired or convection.
- Humidity control. Whether extraction is set by hand dampers or controlled from zone humidity or pressure.
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.