They have long conveyors which carry the dough pieces through a heated box section baking chamber. Oven lengths vary typically between 25 m and 100 m 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 tunnel oven. The conveyor is driven by a variable speed drive at the tunnel 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 electric energy, indirect systems may also use diesel oil fuel as the products of combustion do not enter the baking chamber.
The temperature and humidity control is divided into zones along the length of the tunnel oven, usually each control zone is between 8 m and 20 m long. This enables the temperature and humidity to be set and controlled throughout the baking process to optimise the conditions for the establishment of the biscuit structure, moisture content and colour 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 is 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 air within the baking chamber which can aid heat transfer and contribute to even baking conditions across the width of the tunnel 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. We can group the basic designs into the following:
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.
The direct gas fired oven has a simple baking chamber of box section with the oven band supported through the middle of the chamber. Above and below the band are ribbon gas burners. A gas/air mixture is supplied to the burner tubes 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 flames 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 on a “zero gas pressure” system and the pressure of the 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 very high infra-red heat ratings.
This type of oven can achieve high heat inputs per square metre of band surface (up to 40,000 kcal/m2 may be used for cracker baking) and can successfully utilise any type of baking band, heavy mesh, 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.
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 predominantly in the baking industry, which is invariably 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.
The indirect radiant ovens (also known as “cyclotherm” ovens) are constructed in separate zones. Each zone is typically 10 – 20m long and it 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 then radiate heat to the products from above and below. At the 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 oil or gas. This system is commonly used where oil is the most economic fuel, for example in India.
This 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 a first zone of 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.
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 tunnel oven designs, direct gas fired, indirect radiant ovens and convection ovens.
Steel bands are made of carbon steel, usually 1.2 mm 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. Traditionally steel bands are also used for the baking of “Marie”.
Heavy mesh baking bands, mainly Ashworth type CB5, are woven with a tight “herring bone” pattern providing a solid, thick, heavy mesh. These bands are pre-heated to 120°C – 150°C and they conduct heat immediately into the base of the dough piece as soon as it is deposited on 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.
“Convection” ovens are constructed in zones, each zone having a single burner and circulation fan.
The fan blows the air around the burner tube, 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 the hot air from the heating system.
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.
Indirect convection ovens
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 blow through the ducts in the baking chamber and onto the products. This system can utilise gas or diesel oil fuels.
These ovens were developed in USA as a versatile cookie and biscuit oven. The system is basically 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 tunnel 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 is 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 hard 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 when it is required.
The length of this direct fired section is usually one third of the total length of the tunnel 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.
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.
Ovens utilising three systems, for example direct gas fired/indirect radiant/convection have been installed. However the theoretical benefits are outweighed by the complexity of set up and control and the relatively shorter sections of each system.
Dielectric dryers have been manufactured by the Strayfield Company in UK and used in the biscuit baking industry for almost 50 years. These are very effective in reducing moisture content and in achieving very even moisture contents across the width of the oven band. They are particularly effective on hard sweet biscuits which are prone to checking (cracking after baking due to stresses created by moisture gradient between the outer surface and the centre of the biscuit).
The Strayfield units are positioned immediately after the conventional tunnel oven and consist of a separate baking chamber, conveyor with synthetic band, and drying system. The drying system comprises a transformer, rectifier, oscillator, circuit with variable inductance and electrodes above and below the product band.
The dielectric system subjects the products to direct heating from a radio frequency energy source. As the products pass between the electrodes they are subject to a reversal of the polarity which causes the molecules of water to rub together and heat up. The heating rate increases as the frequency of the reversal of the charge increases. High frequencies are used, typically 27.12 MHz.
While the technology is effective, it is also relatively costly in terms of both capital cost and operating cost. It is therefore used mainly on high output lines where elimination of checking is a major and quantifiable benefit.
Microwave units have been successfully tested in conventional ovens. The addition of microwave units significantly speeds texture and volume development and aids moisture loss from the centre of the product. However the cost of generating the microwaves and the safety issues involved have prevented the wider application of microwave baking in the industry.
Tony Coral: “Radio Frequency Heating and Post Baking” 2004: Strayfield Ltd. www.strayfield.co.uk
Iain Davidson: “A Baker’s Guide to Modern Biscuit Ovens” 1989: Japanese Biscuit Makers Association