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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
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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 eachzone) 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 isessentially a
closedradiant 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 circulatedthrough
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 requiredirect 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