Indirect Radiant ovens bake by infrared radiation, providing a stable penetrative heat transfer, creating excellent volume, texture and flavour. The ovens are suitable for all types of products except high rate crackers, particularly suitable for all hard sweet biscuits, soft dough biscuits and cookies. The Indirect Radiant oven is also ideal for cakes, for example layer cakes

Hybrid DGF / Indirect Radiant ovens are the most versatile and the optimum specification for most biscuit baking.

Heating System

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 run the whole length of the zone. The hot gases travel through the tubes, 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 diesel oil or gas. This system is commonly used where oil is the most economic fuel, for example in India.

This baking system bakes by infrared 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.

Heat Recovery System

The Heat Recovery System (HRS) uses the waste heat from the burner flues. This may be used to heat one or two final zones of the oven. These zones would not require burners, giving a saving in capital and running costs.

All gas burners draw in a large amount of air for combustion. 1.0m³ of gas requires 3.0m³ of oxygen, (approximately 15m³ of air) for complete combustion. This air is exhausted through the extraction system of a direct gas fired 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 are at a high temperature, typically up to 200^oC in the first zones and this hot air can be recovered and used for baking in a Heat Recovery System.

A proportion of the hot gases in the burner flues are diverted to an HRS collection pipe which runs along the top of the oven. Hot gases are collected from each zone with a burner. The hot gases are drawn along the collection pipe by a fan and blown into radiant ducts in the final oven zone.

The burner flue (on the right) is connected to the HRS collection pipe and the flow of hotgases is controlled by dampers.   One damper controls the exhaust flue and one damper controls the connecting pipe from the burner flue to the collection pipe. These dampers are set by the commissioning engineer to allow sufficient quantity of heat for the final zone of the oven.

The HRS zone is constructed with radiant ducts above and below the oven band. The hot gases recovered from the burner flues are fed by the fan to the ducts. The fan is located on top of the oven at the end of the collection pipe.

The final zone of the oven has radiant ducts above and below the oven band. These ducts are divided along their length into 3 sections (control side, centre and non-control side).  The flow of hot air into each section is controlled by a damper. These dampers may be adjusted to ensure the optimum heat balance top to bottom and across the width of the oven.

The Heat Recovery System improves the efficiency of the baking process providing a substantial saving in fuel cost. Independent tests carried out in one factory with three ovens baking an identical product with the same baking time showed that the Indirect Radiant oven with the Heat Recovery System improved efficiency measured as the energy required to bake one kg of biscuit.

Summary

1. Indirect Radiant ovens provide a very stable, infrared radiant heat transfer, preferred by many bakers.
2. The “closed” circulation system retains and re-circulates the hot gases from the burner and contributes to good fuel efficiency
3. Suitable for all types of products, except for the first zone(s) of ovens for baking high rate crackers (for example soda crackers) which required direct heating
4. Suitable for all types of fuel including diesel oil
5. Hot air and burnt gas may be recovered from the burner flues and used to heat the final zone of the oven, providing improved fuel efficiency.

The 2^nd edition of Biscuit Baking Technology: Processing and Engineering Manual is a guide for designers and operators of the biscuit oven-baking technology.

• Thoroughly explores the engineering of baking, details biscuit baking equipment, oven specifications, installation, operation and maintenance
• Provides details of best industry practice for safety, hygiene and maintenance of ovens
• Contains explanations of heat transfer and all the types of biscuit oven design with clear pictures and drawings
• Gathers all the information on how to select and specify an oven to be purchased for a particular range of biscuits

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About the Author

Iain Davidson graduated from the School of Industrial Design (Engineering) at Royal College of Art in London in 1965 and joined Baker Perkins Ltd. He was Industrial Design Engineer, working in the Technical Department on the design of new biscuit, bakery and candy processing machines until 1975, gaining a thorough technical knowledge of the machines and processes. In 1990 Iain was appointed Regional Manager Asia Pacific for Baker Perkins and re-located to Indonesia and later in 1997 to China. His appointments included Managing Director of Baker Perkins (Hong Kong) Ltd. and Director of Baker Perkins Japan KK. Iain was responsible for a substantial increase in the company’s biscuit business in Asia During this period the company’s Asian biscuit machinery business grew by over 3 times. Iain established a successful manufacturing facility for biscuit ovens in Dalian, China in 1990 for Baker Perkins and subsequently continued a manufacturing capability for Baker Pacific Ltd. In China, India and Indonesia.