Biscuit baking process and engineering Example of a training session presentation Biscuit baking process and engineering Oven designs

Baking process and engineering Oven designs Biscuit baking ovens have long conveyors which carry the products through a heated baking chamber. The conveyor carries the products on a wire-mesh or steel band, which is driven by a variable speed inverter control which allows the baking time to be adjusted. The baking chamber is divided into separately controlled zones. In each zone the temperature and humidity can be controlled to establish a “baking profile” to provide the optimum conditions for the development of structure, moisture loss and colouring of the biscuits

Baking process and engineering Oven designs Radiant heating Direct gas fired ovens Indirect Radiant (cyclotherm) Conduction Baking with conduction Convection Direct convection ovens Indirect convection Combination ovens Dielectric baking Hybrid ovens

Baking process and engineering Oven designs Direct fired systems Direct heating systems have multiple gas burners or electric elements inside the baking chamber. Only gas fuel or electricity is used in direct fired ovens In a direct fired convection oven the hot gases from the burner and blown into the baking chamber through ducts at the top and bottom of the baking chamber. The gases from the burner enter the baking chamber in all direct baking systems

Baking process and engineering Oven designs Indirect heating systems Indirect heating systems use heat exchangers. The burner (one in each zone) fires into the heat exchanger and the hot gases circulate through tubes. In a radiant (cyclotherm) oven the heat transfer to the product is by radiation.. Indirect convection ovens use heat exchangers through which the air is blown by a circulating fan. The heated air is then blown into the baking chamber

Baking process and engineering Oven designs Extraction systems All ovens require a system to remove moisture from the baking chamber as the dough pieces are dried to a low final moisture content. The air and moisture in the baking chamber are drawn through an extraction fan on top of the oven to a flue (chimney) which expels the wet air from the bakery. The amount of extraction from the baking chamber is controlled by a damper.

Baking process and engineering Oven designs Turbulence systems A turbulence system divides the wet air extracted from the baking chamber: part is extracted through the flue and part is returned to the baking chamber. This air is directed through tubes or ducts to provide moving (turbulent) air around the products. This enhances heat transfer and promotes an even moisture and colour Baker Perkins turbulence system for a radiant (cyclotherm) oven

Baking process and engineering Oven designs Zone integrity systems When wet air is extracted from a zone of the baking chamber it must be replaced. Normally air is drawn into the zone from adjacent zones, thus affecting the baking conditions in several zones. A zone integrity system provides an air inlet controlled in conjunction with the extraction system to balance the air pressure in the zone and ensure that it operates independently. Baker Perkins zone integrity system

Baking process and engineering Oven designs Direct gas fired ovens Direct gas fired ovens are very widely used. They offer versatility to bake any type of biscuits, cookies and crackers. Heat transfer is by radiation from the burners and the baking chamber walls The baking chamber is a simple box section with “ribbon” burners installed above and below the oven band. Cracker ovens have heat input ratings up to 45,000 kcal/m2 and baking temperatures may reach over 350oC

Baking process and engineering Oven designs Direct gas fired ovens Direct gas fired burners operate on a “zero gas pressure” system and they are controlled by an increase or decrease of air to the gas/air mixers, installed at the burner heads. The air is supplied by fans mounted on top of the oven and the pressure is adjusted by motorised valves. Temperatures are automatically controlled from settings made at the zone control panels on Omron units..

Baking process and engineering Oven designs Electric ovens Electric ovens are constructed in a similar way to direct gas fired ovens, but are heated by electric elements in place of the gas burners. They are not widely used now as gas is available in most countries at a considerably lower cost than electricity. Electric ovens are clean, versatile, can be used for most biscuits, crackers and cookies. They are accurately controlled by thyristors, which vary the current in the heater elements to adjust the baking temperature.

Baking process and engineering Oven designs Indirect radiant ovens Indirect radiant (cyclotherm) ovens are constructed in separate zones. Each zone is 10-20m long and has a single burner, heat exchanger and circulation system for the hot gases from the burner. The hot gases are circulated by a fan through radiant tubes above and below the band and then are returned via the fan to the burner tube. It is essentially a closed radiant heat transfer system

Baking process and engineering Oven designs Indirect radiant ovens Heat transfer is by radiation from tubes above and below the oven band. The burner (gas or oil fuel) fires into the burner tube from where the hot gases are circulated through the radiant tubes The heat is retained in the system. Fresh air is only drawn into the system at the burner and this is balanced by natural extraction through the burner flue. This hot flue gas can be used in a Heat Recovery System

Baking process and engineering Oven designs Indirect radiant ovens

Baking process and engineering Oven designs Indirect radiant ovens Baker Pacific Radiant Oven 1.2m x 100m Indirect radiant ovens provide a stable, penetrative radiant heat, preferred by many bakers The “closed” circulation system retains the hot gases from the burner and contributes to good fuel efficiency. Suitable for Heat Recovery System Suitable for all types of products, except for some crackers, which require direct heat in the first zones Suitable for all types of fuel including diesel oil

Baking process and engineering Oven designs Convection Ovens Convection ovens bake by blowing hot air onto the product surface. The convection system is efficient in drying biscuits to even low moisture contents. The rapid drying occurs from the surface of the biscuit and may cause a moisture gradient within the biscuit leading to “checking” The system cannot be used for development of structure and volume and normally a direct gas fired system will be used for the first zones of the oven Convection baking provides a bland, even biscuit colour Direct convection ovens are low cost to build and control

Baking process and engineering Oven designs Direct Convection Ovens Convection ovens are constructed in zones, each zone having a single burner and circulation fan. The fan blows air around the burner tube where it is heated and then through ducts above and below the oven band. From these ducts jets of hot air are blown on to the products. Hot air from the baking chamber is drawn back to the fan to be re-circulated. Each zone has an extraction fan and flue to remove moisture from the baking chamber together with hot air from the heating system. Direct Convection Oven

Baking process and engineering Oven designs Direct Convection Ovens

Baking process and engineering Oven designs Indirect Convection Ovens The burner fires into a burner tube connected to a multi-pass heat exchanger. Air is drawn from the baking chamber by the fan and is passed through the heat exchanger. The hot air and gases from the burner are re-circulated through the heat exchanger and do not enter the baking chamber

Baking process and engineering Oven designs Re-circ Ovens Re-circ ovens were developed in USA as low cost, versatile ovens for biscuits and cookies. The heating system is similar to a direct convection oven, but the volume of gases blown into the baking chamber can be adjusted. When this volume is reduced, the gases circulate through ducts at the top and bottom of the oven back to the circulating fan and burner. The oven combines a direct convection system with a simple radiant system and the proportion of convection to radiation can be adjusted.

Baking process and engineering Oven designs Re-circ Ovens FPA / Werner Lehara Re-Circ oven

Baking process and engineering Oven designs Combination (hybrid) Ovens Combination ovens allow different heat transfer modes to be used at successive stages of the baking process: forming the structure / drying / colouring Direct gas fired / Indirect Radiant ovens The DGF section (1/3 in length – 1/2 in power) is ideal for forming the structures of biscuits and crackers. The indirect section will maintain optimum development and colour Direct gas Fired / Convection ovens The convection section will reduce the moisture level efficiently and give an even colour, suitable for many hard sweet biscuits and cookies Meincke DGF / convection combination oven

Baking process and engineering Oven designs Dielectric heating Dielectric dryers from Strayfield Co in UK have been used in biscuit baking for over 50 years. They are effective in reducing moisture content to very low, even levels. They are particularly effective for hard sweet biscuits which are prone to checking. The Strayfield units are positioned immediately after the conventional oven. They consist of a baking chamber, conveyor with synthetic band and a drying system. The dielectric system uses direct heating from a radio frequency energy source with a high frequency, typically 27.12MHz. As the products pass between the electrodes, they are subject to a reversal of polarity, which causes the water molecules to heat up. Dielectric heating has a high capital and operating cost. Its’ use is limited to high output lines where reduction in checking is a major and quantifiable benefit Strayfield dryer with synthetic conveyor, variable inductance electrodes above and below the band

Baking process and engineering Oven designs Biscuit baking process and engineering In our next training session we will cover: Oven bands Oven conveyor design Oven specifications: direct heating systems Oven specifications: indirect heating systems

Baker Pacific on-site training This is an example of one training presentation. There are 14 presentations, which can be presented in one week. Day 1 Introduction / the biscuits / baking process / biscuit design and output l Day 2 Heat transfer / oven designs and specifications   Day 3 Oven bands and conveyor design Day 4 Oven control / safety systems / oven operation Day 5 Oven maintenance / inspection and audit / oven efficiency / heat recovery system Please contact us for a quotation