Three modes of heat transfer are used in baking biscuits: radiation, conduction and convection. The most important is infrared radiation, which has the following advantages:
Penetrative heat transfer: Infrared radiation penetrates biscuit dough by approximately 4mm, (depending on wavelength and moisture content). It is the only heat transfer mode to truly bake the product from the centre. This is the key advantage of baking by infrared radiation.
Biscuit structure: because radiation penetrates the dough pieces, it is essential to achieving good structure with optimum volume and texture and is always the main mode of heat transfer in the first part of the baking process
Even moisture content: radiant baking ensures a low moisture gradient from centre to the outer surface of the biscuit. It is the best heat transfer mode to avoid “checking” (cracks appearing in the biscuit after baking, due to the moisture gradient).
Efficiency: heating of the surrounding air in the baking chamber is not necessary, which lowers energy consumption.
Colouring: radiation enables highlighted colour contrasts for crackers and rotary moulded products, whereas convection gives an overall, bland, even colour.
Versatility: infrared baking is suitable for all types of biscuit
All objects above a temperature of absolute zero radiate energy to their surroundings. This energy or radiation is emitted as electromagnetic waves which travel at the speed of light. The waves may travel through a vacuum or other medium. When they impact an object, they are partially absorbed and partially reflected. Good emitters are also good absorbers of thermal radiation.
Infra-red radiation is in the wavelength band of 0.7-300 microns (above visible light). Higher temperatures produce shorter wave lengths. Typical wavelengths in a radiant oven are around 4.6 – 6.4 microns, which provides good heat penetration of the dough pieces.
Since the radiant energy is proportional to the temperature to the power of 4, a small increase in temperature will give a large increase in radiation. This contributes to the fast response of DGF ovens and high efficiency.
DGF ovens bake with infrared radiation. High rate burners enable rapid baking of crackers with excellent “lift”, open and flaky texture. For cracker baking pre-heat burners also increase conduction to the dough pieces in the first oven zone.
Indirect ovens provide fuel security as gas or oil can be used. The burners fire into a heat exchanger and the hot air is circulated through radiant tubes above and below the baking band. Baking is by infrared radiation from the hot radiant tubes above and below the baking band. The heating system is essentially a closed system and the energy required from the burner is only sufficient to maintain the baking temperature.The burner draws in air for combustion and the excess is exhausted by a natural flue. This hot air / burnt gas in the flue can be used in a heat recovery system and this ensures a high efficiency.
All gas burners draw in a large amount of air for combustion. 1.0 m3 of gas requires 3.0 m3 of oxygen (approximately 15 m3 of air) for complete combustion. This air is exhausted through the extraction system of a DGF oven and through the natural draught burner flue of an Indirect oven.
The hot air and burnt gas in the burner flues of an Indirect Radiant oven is at high temperature, typically over 200°C and this hot air can be recovered and used for baking in a Heat Recovery System.
The hot air can be diverted from the flues to a collection pipe running along the top of the oven. The hot air is drawn down the length of the oven by a fan which blows the hot air into a final radiant zone. This zone does not require a burner and is heated by radiant ducts above and below the band.
Independent tests were carried out on 3 ovens in the same factory producing identical rotary moulded biscuits with the same baking time. The tests measured the oven efficiency by calculating the energy usage (gas) in kWh (kilowatt hours) to produce one kilo of baked biscuit.
The Baker Pacific Indirect Radiant oven was 18% more efficient than the DGF/cyclotherm oven and 6% more efficient than the DGF/convection oven. The savings in gas consumption per 8 hour shift (23 tonnes of biscuits) are approximately 212 m3 of gas compared to the DGF/cyclotherm and 62 m3 compared to the DGF/convection oven.