The complex of thermophysical, biochemical and colloidal processes taking place in the working chambers of the baking ovens determines the quality of the products produced: appearance, baking and volumetric yield of baked bread.
Bakery ovens can be classified in several ways.
for technological purposes: universal ovens for baking a wide assortment and specialized ones for productivity:
ovens of ultra low productivity (for bakeries), low productivity (with a hearth area up to 25 m2) and high productivity (with a hearth area 25 m2 and higher);
design features: dead-end and tunnel furnaces;
the method of heating the baking chamber of the furnace: heat; with channel heating; with recirculation of combustion products; with steam-water heating; with electric heating; with combined heating.
The process of baking bread. The process of baking bread consists of three stages: the first is hygrothermal processing; the second is the formation and consolidation of form; the third is baking.
At the first stage, the workpieces are moistened with steam, which, condensing onto the relatively cold surface of the dough. The resulting thin film of condensate contributes to the formation of a thin glossy crust. A certain amount of steam penetrates into the dough pieces, is sorbed by them, as a result of which large-volume products with well loosened are obtained
The duration of the dough in the steam humidification zone of the furnace compared with the duration of baking is small and amounts to 120 ... 180 s. In order to create conditions on the test surface for condensation of the maximum amount of steam (approximately 100 ... 150 g of steam on the 1 m2 surface), in the steam humidification zone, maintain a temperature of not more than 100 ... 120 ° С and maximum relative humidity 70 ... 85%.
After wetting, the dough pieces fall into the heating zone, where heat is supplied with the highest possible intensity. This zone of the furnace is directly adjacent to the zone of steam humidification. In the heating zone, the highest permissible temperature is maintained, giving more heat to the channels of this zone.
At the second stage of baking, the gases in the pores of the preforms expand, as a result of which the volume and height of the dough pieces increase. Then the growth of the dough pieces is stopped, and their shape is fixed with a formed crust.
The third stage of baking, called baking, is characterized by a noticeable decrease in the amount of heat supplied to the dough pieces. Due to the evaporation of moisture, the surface layers of the workpieces turn into a crust, and their mass decreases. To reduce the pitch and thickness of the crusts, the temperature at this stage is maintained at a relatively low level.
At the third stage, the heating of the inner layers of the dough pieces continues. When the temperature of the crumb in the central layers reaches 97 ... 98 ° С, it is considered to be completely baked, and the baking process ends there.
The baking mode of each type of product has its own characteristics. It is influenced by the baking properties of flour, the formulation of products, the duration of the proofing and other factors. For example, prefabricated weak flour or long proofers are baked at a higher temperature to prevent
If the products are baked from the dough with a short ripening time, then the temperature of the baking chamber medium is reduced, and the baking time is increased to extend the ripening processes that will continue in the billet during baking. Products having a small mass and thickness bake faster and at a higher temperature. Below are the baking modes of some bread products.
When baking baton-shaped products from 1 grade wheat flour, an intensive and lengthy process of hygrothermal treatment is required at a relative humidity in the steam humidification zone of 80% and a temperature of 100 ° C in it. Under these conditions, it is possible to obtain products with a glossy surface and well loosened crumb with uniform porosity. Subsequently, as the baking process of such products proceeds, the temperature in the baking chamber is maintained around 220 ... 230 ° C and then gradually reduced to about 190 ° C by the end of the baking process.
When baking products on the surface of which as a result of a knife cut of dough pieces during planting, a scallop is formed, for example, a city roll, a scallop, optimal steam and humidity modes are as follows: temperature in the zone of hygrothermal processing of workpieces 130 ... 140 ° С at the same time with high relative humidity. Such parameters in the steam humidification zone are necessary for a complete
Subsequently, as the baking process proceeds, the temperature in the working chamber of the furnace is maintained at approximately the same or slightly lower level as in the case of baking
The greatest difficulties are the creation of thermal conditions when baking rye-wheat and rye hearth products. Dough made from rye flour has weak form-holding properties, so the dough pieces are prone to spreading. In the process of baking such products, after hygrothermal processing of the dough pieces, they must be subjected to intensive heat treatment at relatively high temperatures of the baking chamber: up to 250 ... 260 ° C, and in some cases up to 270 ° C. This process of high-intensity heat supply is called frying, and the initial portion of the baking chamber is called frying.
When choosing thermal conditions, one must take into account that intensification of the heating of dough pieces and a reduction in the duration of baking lead to a decrease in the content of aromatic substances in bread, since intensification of physical processes does not cause intensification of biochemical processes, the amount of aromatic substances depends on the speed of their course.
The device is a modern baking oven. A modern baking oven is an aggregate that includes the main elements: a heat generator, a baking chamber, under the oven, heat transfer devices, fencing, auxiliary devices and instrumentation.
The heat generator for most baking ovens is furnace devices, which are of two types: for burning solid fuels (coal, firewood, peat, etc.) and for burning gaseous or liquid fuels (gas, oil, fuel oil
The furnace device of a baking oven for burning solid fuel consists of the following main parts: grate (on which fuel is burning); the furnace space where the combustion of volatile fuel components occurs; a blower (ash pan) through which air is supplied to the furnace and where the ash formed during combustion falls through.
Solid fuel is thrown onto the grate through the combustion door. A door is provided for cleaning the ash pan.
The grate consists of individual grates, which are cast-iron plates with ribs. Grid-irons are laid on the arm-grate beams. In the grate there are holes designed to supply air necessary for combustion.
The furnace device of a baking oven for burning gaseous fuel in furnaces with recirculation of combustion products consists of coaxially arranged cylindrical combustion chambers (furnaces) and mixing. Between them there is an annular gap for the passage of recirculating gases. In the cylinder of the mixing chamber, combustion products and recirculating gases are mixed. The combustion process in the furnace is accompanied by heat transfer to the recirculating gases washing it and radiation from the torch through the outlet openings of the furnace cylinder into the mixing chamber.
The furnace device of the recirculation-heated furnace (Fig. 3.23) consists of a 2 heat-resistant cylinder connected to one side with a 7 metal cone, and the other with four 9 plates with an 3 cylinder. The outer surface of the chamber is assembled from three metal cylinders; 3 distance rings are installed between the 4 and 5 cylinders. The 4 cylinder has a nozzle for supplying recirculated gas. The open left end of the combustion chamber is connected to the 6 pipe, which exhausts gas into the heating channels.
The Dinax refractory mass is packed into a metal cone so that three holes 10 ... 12 remain for the burner, igniter and inspection hatch, respectively.
Gas burns in the 2 heat-resistant cylinder, the inner surface of which is lined with rings of heat-resistant mass. Recirculating gases flow through the 8 pipe and then, moving between the 3 and 4 cylinders, cool the first, then go around
Fig. 3.23. The furnace furnace with recirculation heating.
its end and move to the nozzle <5, which diverts gases to the heating channels; while touching the outer wall of cylinder 2, they
Combustion of combustion products and recirculating gases takes place in the 3 cylinder. To remove combustion products and recirculating gases at the outlet of the chamber, a vacuum is maintained.
During the operation of the chamber, the Dinaks refractory mass is heated to glow and irradiates the gas combustion zone, which ensures a stable temperature and complete combustion.
In the gas distribution pipe, where the gases are directed from the combustion chamber, an 7 safety valve is installed.
Two types of gas burners are used for burning gas in furnaces: injection and internal mixing with forced air supply. The choice of the type of burner is made depending on the gas flow rate, the design of the furnace unit, the combustion device, the gas pressure in the network, etc. To burn liquid fuel, nozzles with a steam and air spray are used.
Injection burners are simple in design, easy to maintain and can operate at low gas pressure without special installations and energy costs for the supply of primary air. They provide a short transparent torch with a high temperature, which decreases along the length of the torch.
The gas injection burner of medium pressure (Fig. 3.24), consisting of the 5 nozzle, the 4 mixer, the 3 gas nozzle, the 2 washer for regulating the air mounted on the 7 gas supply pipe, is most widely used in the bakery industry.
In low pressure burners, part of the air required for combustion is injected; the missing part (secondary air) is sucked in through special openings due to rarefaction in the furnace. Before each burner, a shut-off valve is installed in the gas pipeline. The burner steadily works without separation and slip
Fig. 3.24. Гgas injection torch of average pressure.
flame in a wide range of regulation of pressure and gas flow. The burner unit is equipped with automatic devices that provide gas shut-off in the event of a torch detaching or extinguishing the flame of a constantly operating igniter.
The advantages of low pressure burners include automatic mixing of certain amounts of gas and air, the absence of blowing devices and ease of maintenance. However, along with this, low-pressure burners have a number of disadvantages: noise during operation and the need to dismantle the burner and masonry from refractory bricks on the grate of the furnace during the transition to the reserve solid fuel.
In furnace furnaces of a baking oven for burning liquid fuel, nozzles with a steam or air atomizer are most widely used.
The nozzle with a universal atomizer (Fig. 3.25) consists of a 7 metal body, inside of which is horizontally located the barrel of the 10 nozzle, assembled from two tubes (one into the other), the 9 tip, the spray nozzle 8 and the 7 nozzle. 6 incendiary cone is laid out in the masonry of the firebox brick wall. The nozzle body is bolted to the masonry of the furnace wall.
Fig. 3.25. Universal atomizer nozzle
Fig. 3.26. Electric heaters: a - direct; b - U-shaped
Air is supplied to the nozzle through a pipeline connected to the 5 pipe, fuel to the 3 pipe, and reserve fuel (in case of transition from air sawing to steam) to the 2 pipe. An 4 needle with a handwheel is provided to control the fuel supply.
In addition to furnace devices, heat generators in baking ovens can be electric heaters (Fig. 3.26), as well as devices based on the use of infrared radiation and high-frequency currents. In baking ovens tubular elements are used straight (see fig. 3.26, a) and U-shaped (see fig. 3.26, b). They consist of resistance spirals / made of nichrome or fechral wire and enclosed in steel or brass thin-walled tubes 2 with diameter 12,5 ... 25 mm, filled with insulating heat-conducting material - magnesite 3. Both ends of the wire end with 4 insulators and 5 terminals for connecting to the power supply.
For baking small-sized bakery and flour confectionery products as heat generators, devices based on infrared radiation and high-frequency currents (mirror lamps and quartz emitters), which are usually installed in the upper zone of the baking chamber, have spread.
When using infrared radiation, baking time (almost twice), loss from baking (by 60 ... 70%) and energy consumption are significantly reduced compared to other ovens. When using a high-frequency current, heat is generated inside the baked product, and the baking process is independent of the ambient temperature.
The configuration and dimensions of the baking chamber depend on many factors: the purpose and productivity of the furnace, the type of products being produced and the organization of the production process.
During baking in the baking chamber, heat to the billets is transmitted by radiation (70 ... 90%) from the heating surfaces, by convection from the vapor-gas medium of the baking chamber and thermal conductivity from the hearth of the oven to the bottom surface of the test
Baking chambers of ovens are dead-end, in which the dough pieces are planted on under and the finished products are unloaded through one window (mouth), and tunnel ones, in which they are planted on one side of the baking chamber, and unloading on the other.
Under the oven, on which baking is carried out in a baking oven, it can be stationary or conveyor.
Currently, in bakeries, fixed hearth ovens are not widely used.
Conveyor hearths can be divided into cradle-hearth.
In the cradle-hearth conveyor hearths, chains made of angle steel with two pendants and fingers that are inserted into the inner bushings of the chain chains are pivotally suspended between the chains. For baking hearth products, a steel sheet (hearth) with a thickness of 1 ... 2 mm is placed inside the cradle.
In tunnel kilns, two types of conveyor hearths are used - plate and mesh.
The conveyor belt for the plate type consists of two roller plate chains. Frames overlapped by steel plates are attached to the side planks of the chains. Talochlorite or ceramic tiles are attached on top of the plates in some conveyors, which improves storage
The conveyor belt under the mesh type is performed in two versions. In the first version, the conveyor consists of two drums: a drive and a tension drum, the axes of which are located horizontally, and an endless spiral-rod mesh worn on them. The upper working branch of the hearth is held in horizontal position on steel rods or wire, and the lower idle branch is on rollers. The disadvantage of this design is the need to regulate the position of the mesh on the drums and the use of special devices for this.
In the second variant, under is a spiral-rod mesh attached to two traction roller-chain chains with a pitch of 100 mm. Asterisks (blocks) are installed on the drive and tension shafts. The upper branch moves along the base of the baking chamber, and in the lower part the traction chains move along the guides made of corner steel. The conveyor belt under the mesh type has low thermal inertia, which distinguishes it from the hearths of other designs.
Furnaces in which channels with flue gases moving in them are used as heat transfer devices are called channel ones. By configuration, the channels can be of rectangular cross-section with a flat or vaulted overlap, semicircular or circular cross-section.
Furnaces in which high pressure steam is used as a heat carrier, obtained in shielded furnaces or in tubular boilers of the G.P. Marsakov system, belong to steam-heated heating furnaces. Steam is transported to the heating sections located in the baking chamber through seamless steel pipes.
In steam-water and combined-heating furnaces, heat-transfer devices are widely used for heating steam-water seamless thick-walled pipes filled with distilled water on the 1 / 3, both ends of which are carefully welded. The ends of the pipes in the furnace are heated, as a result, steam is formed inside the pipes with a working pressure within 6 ... 11 MPa, which condenses by transferring heat through the pipe wall to the baking chamber. Condensate flows back to the furnace end, where it again turns into steam.
The baking and combustion chambers, channels (gas ducts) and other heat transfer systems are separated from the surrounding space by walls and ceilings, which are called fences
Depending on the design of the furnace, the fencing is made of brick or metal panels filled with insulating material. The latter are a box, the walls of which are made of sheet steel with a thickness of 1 ... 2 mm, and insulating material is strewn between the walls. The exterior wall cladding for some furnaces is made of sheet aluminum.
Auxiliary devices of the baking oven include steam humidification devices of the baking chamber and devices for its ventilation, heat recovery units, blast and traction devices of the heat generator.
Steam humidification devices of various designs are installed in the baking chamber, which include one or more perforated pipes located in the humidification zone. The amount of steam entering the humidifier is manually controlled using valves located on the pipes.
Steam is supplied (Fig. 3.27) from the 7 and 2 steam lines equipped with an 10 valve and an 77 pressure gauge through perforated 4 pipes through the side surface of the baking chamber.
An 7 water separator is installed outside the furnace, to which perforated pipes are attached. Each steam pipe has an 6 valve for regulating the steam supply and 5 handle, using
which by turning the pipe to give the steam jets the desired direction. The vapor pressure in the 4 pipes is controlled using the 3 pressure gauge.
Fig. 3.27. Steam humidifier.
The location of the steam humidifier in the area where the upper heating surfaces have a temperature of 300 ... 400 ° C leads to overheating of the steam and an increase in its flow rate, worsening condensation conditions and the quality of most types of products.
In a number of designs, to eliminate steam overheating in the area where the steam pipes are located, the top
Fig. 3.27. Steam humidifier. To remove the condensate formed in the steam lines, at the steam inlet to the furnace there is an 9 centrifugal water separator connected to the 8 condensate line.
As waste gas heat exchangers, water heaters and steam boilers, as well as tubular devices (steam generators) located in gas ducts, are most widely used in ducted heating furnaces. The heat of the exhaust gases can be used to generate steam and heat water to moisten the environment of the baking chamber, as well as for technological and sanitary needs and other purposes.
Mercury technical thermometers, thermoelectric pyrometers with millivoltmeters, and automatic systems are used as control and measuring instruments for monitoring the temperature of the baking chamber medium.
Modern bakery ovens are equipped with an automatic temperature control system (ASR) and automatic safety of burning gas or liquid fuels. Automation of the furnace unit provides for: control of the temperature of the medium in all areas of the baking chamber; two-position temperature control of the baking chamber with light alarm by adjusting the fuel consumption (“large” torch - “small” torch);
blocking the temperature rise of the mixture of flue and recirculation gases in the mixing chamber (protection against burnout of the metal channels of the heating system);
control of intermittent movement of the conveyor hearth of the furnace with light signaling.
Safety automation provides for automatic ignition of the furnace and the following procedure:
1) purge of gas ducts in the furnace before start-up for 1 ... 2 min;
ignition of fuel using ignition electrodes, to which a high voltage is supplied from the ignition transformer.
holding for 1 ... 2 minutes when warming up
shutting down the burner if the flame does not light up within 15 s after turning on the fuel supply.