Oven lengths vary typically between 25m and 100m long. The conveyor band material is a wire-mesh or a carbon steel sheet, which turns around large cylindrical drums at each end of the oven. The conveyor is driven by a variable speed drive at the oven end which allows the operator to adjust the baking time.
The baking chamber may be heated directly with gas burners or electric heaters or by an indirect system using heat exchangers. Direct heating systems use gas or electricity, indirect systems may also use diesel oil fuel as the products of combustion do not enter the baking chamber.
The baking chamber is divided into zones along the length of the oven. Usually, each control zone is between 8m and 20m long. Each zone has independent control of temperature and humidity. This allows conditions throughout the baking process to be optimized for achieving the biscuit structure, moisture content, and color as the dough pieces travel through the oven.
The control of the humidity in the baking chamber and the removal of moisture from the dough pieces are accomplished by an extraction system in each zone. This consists of ducts which draw air and moisture from the baking chamber through a fan and expel the air through vertical flues (chimneys) to atmosphere.
In some ovens this wet air removed from the baking chamber can be diverted either to the flue or back into the baking chamber. This provides moving humid air within the baking chamber which can aid heat transfer and contribute to even baking conditions across the width of the oven. These systems are called “turbulence” systems and are mainly used on ovens which have relatively still air in the baking chamber, for example indirect radiant ovens and direct gas fired ovens.
Ovens are designed to optimise the heat transfer to the dough pieces in different ways using radiant heat, conduction and convection.
Direct gas fired ovens, electric ovens and indirect radiant (“cyclotherm”) ovens
Image 1: Baker Pacific Direct gas fired / Indirect Radiant oven
Infrared radiant energy penetrates the dough pieces and creates the volume and texture of the products. It is essential in the first third of the baking process. The penetration of the dough pieces is determined by the wavelength of the radiation. For food products including dough pieces the maximum absorption is achieved at Near Infrared (NIR) wavelengths from 2.7 – 5.9 microns.
Ovens with pre-heated compound balanced weave bands and steel conveyor band conduct heat into the base of the dough pieces. The heat is then conducted within the products.
Image 2: Cake baked on a steel band
Direct and indirect convection ovens transfer the heat by blowing hot air jets onto the products. Also “Re-circ” ovens which combine convection and radiant heat transfer.
Image 3: Baker Perkins Indirect convection oven
The convective air impinges on the surface of the products and is effective in evaporating moisture from the surface.
Direct gas fired ovens are very widely used throughout the biscuit baking industry. They offer versatility to bake all types of biscuits, cookies and crackers,
Image 4: Baker Pacific Direct Gas Fired Oven
The direct gas fired oven has a simple baking chamber of box section with the oven band supported through the middle of the chamber. Ribbon gas burners are located above and below the band. A gas/air mixture is supplied to the burners, and this is ignited by a spark electrode and burns on a strip or ribbon across the width of the oven conveyor band.
The heat transfer in a direct gas fired oven is primarily by radiation from the gas burners and from the oven top, base and walls of the baking chamber.
Most types of gas may be used including natural gas, town gas (manufactured from coal) and LPG (liquid petroleum gas).
The burners operate with a “zero gas pressure” gas system. The pressure of the combustion air supplied to the burners is controlled by motorised valves or variable speed blowers to increase or decrease the flame intensity and heat input. Various designs of corrugated stainless steel strips (or ribbons) are used to give a range of heat ratings. Woven wire-mesh (metal fibre) strips are also used for high infra-red heat ratings.
This type of oven can achieve high heat inputs per square metre of band surface. Heat input of 45,000 kcal/m2 of oven band area may be used for cracker baking. Direct gas fired ovens can successfully utilise any type of baking band, compound balanced weave, open wire-mesh bands and steel bands. Direct gas fired burners are used to supply band pre-heat where this is required for cracker baking.
The heat mass and stable radiant heat can be increased by installing heavy tiles at the top and bottom of the baking chamber. This arrangement is successfully used in the baking of water biscuits and crackers requiring very high heat inputs and stable radiant heating.
The Baker Pacific Direct Gas Fired oven can be converted to electric heating. The gas burners are removed and replaced by electric heating elements mounted on frames which slide into the baking chambers.
Image 8: Replacement of gas burners with electric heating elements
Image 9: Electric ovens
The electric oven provides infrared radiant energy from electric elements in the baking chamber above and below the band. The infrared radiant energy penetrates the dough pieces and creates the volume and texture of the products.
The electric oven has several features contributing to high efficiency:
Electric ovens are constructed in a similar way to direct gas fired ovens, but use electric heating elements in place of the gas burners. These ovens have been widely used in the baking industry in some countries where industry had adequate electricity supply, but lacked gas, for example China. However most countries, including China, now use gas fuel predominantly in the baking industry, which is currently substantially cheaper than electricity.
Electrical heating has also been used in the first zones of ovens which required high heat inputs at the start of the baking process and where diesel oil with an indirect heating system was the preferred fuel for the main part of the oven.
Image 11: Baker Pacific Indirect Radiant oven
The Indirect Radiant ovens, (also known as “cyclotherm” ovens), are constructed in separate zones. Each zone is typically 10 – 20m long and has a single burner, heat exchanger and circulation system for the hot gases from the burner.
Each zone has a burner firing into a burner tube. The hot burnt gases are drawn from the burner tube through ducts to rows of steel tubes, or ducts, at the top and bottom of the baking chamber. These radiant tubes, or ducts, run the whole length of the zone. The hot gases travel through the tubes or ducts which radiate heat to the products from above and below. At each end of the zone, the hot gases are collected in a return duct through which they travel back to the circulating fan and from there to the burner tube to be re-circulated. It is essentially a closed, circulating system with a single burner, circulating fan and radiant tubes to heat the products from above and below.
A flue with natural convection is used to balance the pressure in the system resulting from the ingress of combustion air at the burner. The continuous re- circulation of the hot gases ensures a good efficiency. Fresh air is only drawn into the system at the burner for combustion and this is balanced by the natural extraction through the burner flue. Since the products of combustion do not enter the baking chamber, the burner may use diesel oil or gas. This system has been commonly used where oil was the most economic fuel, for example in India.
The indirect radiant baking system bakes by radiation with a high heat mass providing stable baking conditions. It is versatile, capable of baking all types of biscuit, cookies and some crackers. High rate crackers require the first zones with direct heating. The system is favoured by many bakers for producing a high quality of biscuit structure, texture and colour. It is an ideal system for achieving colour contrasts on rotary moulded and cracker products.
The heat transfer in radiant heating systems in Direct Gas Fired ovens, Electric ovens and Indirect Radiant ovens is enhanced by the movement of humid air in the baking chambers. This is provided by turbulence systems.
In all ovens moist air is extracted from the baking chambers to atmosphere as the moisture content of the dough pieces is reduced. The turbulence systems divert some of this moist air to ducts or tubes across the baking chamber above and below the oven band. Dampers allow the proportion of this moist air to be adjusted to top or bottom turbulence or to the extraction flue to atmosphere.
Image 14: Turbulence system
As noted above, steel baking bands and heavy mesh bands conduct heat rapidly into the base of the dough pieces. These types of band can be used in any of the oven designs, direct gas fired, indirect radiant ovens and convection ovens.
Steel bands are made of carbon steel, usually 1.2mm thick. They are principally used for the baking of cookies with high sugar and fat contents, which flow on the oven band in the first part of the oven. Snack cakes are also deposited and baked on steel bands. Traditionally steel bands are also used for the baking of “Marie” biscuits.
Compound balanced weave baking bands, mainly type CB5, are woven with a tight “herring bone” pattern providing a solid, thick, heavy mesh. These bands are pre-heated to 120oC – 150oC and they conduct heat immediately into the base of the dough pieces as soon as they are deposited onto the band. This is a major baking method, being used throughout the industry for the baking of soda crackers and saltines. These bands are also versatile and can be used for a wide range of crackers, hard sweet and rotary moulded products.
Image 17: Baker Perkins direct gas fired cracker oven with pre-heat burners below the return band
Convection ovens are constructed in zones, each zone having a single burner and circulation fan. The fan blows the air around the burner tube or through the heat exchanger, where it is heated and then through ducts along the length of the zone. These ducts, located above and below the baking band, have slots or nozzles through which jets of hot air are blown onto the products and the oven band. Hot air from the baking chamber is drawn back to the fan to be re-circulated through the system. Each zone has an extraction fan and flue to remove moisture from the baking chamber and this system will also extract some of the hot air from the heating system.
Convection ovens generally have fixed speed fans. Typically, the discharge velocity of the air from the nozzles is a maximum of approximately 20m per second at a maximum temperature of 310-320°C.
The control of the baking process is therefore by temperature by modulating the burner and by adjusting the proportion of convective air diverted to the top or bottom ducts. The airflow to the top and bottom of the oven is controlled by separate top and bottom manual or motorised dampers.
Temperature control is by thermocouples detecting the temperature of the convective air above and below the oven band. The thermocouples are connected to temperature controllers in each zone which will automatically adjust the burners to increase/decrease the heat input.
The hot gases from the burner are combined with the re-circulated air from the baking chamber and blown through the ducts in the baking chamber and directly onto the products. As the products of combustion are blown directly onto the products, diesel oil fuels are unsuitable for direct convection baking.
The burner fires into a burner tube connected to a multi-pass heat exchanger. The products of combustion are circulated within the heat exchanger and do not enter the baking chamber. Air is drawn from the baking chamber through the circulating fan and then passed through the heat exchanger, where it picks up heat, before being blown through the ducts in the baking chamber and onto the products. This system can utilise gas or diesel oil fuels.
Convection ovens can also employ electrical heater units. The duct heaters replace the gas burners and heat exchangers.
These ovens were developed in USA as a versatile pet food and biscuit oven. The system is a direct convection oven, but the volume of hot gases blown directly onto the products can be adjusted. When this is reduced, the hot gases circulate through the ducts at the top and bottom of the oven and return to the burner tube, with less hot air being blown onto the products. The system can therefore balance the heat transfer by convective air or by radiation from the ducts.
Re-circ ovens may be used for products requiring lower heat inputs. For the first zones, the oven is operated in a mainly radiant mode to avoid “skinning” the dough piece. However the heat transfer in this mode is low and the biscuit structure is relatively slow to form. In the convective mode the heat transfer is increased and the system operates as a direct convection oven.
It has become common practice to combine different oven types into a “hybrid” or “combination” oven. This allows the baker to use different heat transfer modes at different stages of the baking process.
Products such as crackers and semi-sweet biscuits require high heat inputs in the first part of the baking process to establish good structure and volume. This can only be provided effectively by a direct heating system and a direct gas fired oven section is normally specified. This system also minimises the drying and skinning of the surface of the dough pieces, which would prevent the lift and expansion of the dough pieces.
The length of this direct fired section is usually one third of the total length of the oven and the power input of the direct fired section is one half of the total power input of the oven.
The indirect radiant oven section will contribute to the optimum development of texture and colour of a wide range of crackers and biscuits.
Image 23: Baker Pacific Hybrid Oven: Direct Gas Fired / Indirect Radiant
This specification again uses the benefits of the direct gas fired oven in the first third of the oven. The convection section will effectively remove moisture from the dough pieces and achieve a low and even moisture content for the final product. Colour will be even and bland, without contrasts, which is suitable for a range of hard sweet biscuits.
Image 24: Dingson Food Machinery hybrid Direct Gas Fired / Indirect Convection oven
Changing the type of belt in tunnel ovens
As a former product manager and now a consultant for tunnel oven belts, I would be interested to ask for your experience in changing the principal type of belt used in your tunnel oven and for what reason. Did you change from solid steel belt to mesh belt or (multiple spiral) CB5-belt to Z-belt (rolled baking oven belt)? Or was it vice versa? Or are there intentions and thoughts to do something like this? Looking forward to your comments and replies.
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