There are many types of ovens and more ways to categorize them. However, they are generally categorized according to the source of heat energy, structural type and so on.

According to the oven heat source classification

According to the different heat sources, the oven can be divided into coal, gas, oil and electric furnaces.

1, coal oven

Coal-fueled oven is called coal oven. It also has a variety of types, the combustion equipment of this oven is simple, safe operation, and cheaper fuel, easy to obtain. It is suitable for small and medium-sized food factory baking a variety of food. The disadvantage is that the sanitary conditions are poor, the labor intensity of the workers, and the temperature adjustment is difficult, the body of the furnace is bulky, not suitable for transport.

2、Gas Oven

To gas, natural gas, liquefied petroleum gas and other fuels as the oven collectively referred to as the gas oven. Gas oven temperature regulation is easier than coal oven, in the high temperature area can be installed more nozzle, low temperature area can be installed less nozzle, if the local overheating, you can also close the corresponding nozzle. Gas furnace is much smaller than the external dimensions of the coal furnace, and can reduce heat loss, improve the labor conditions of workers.

3、Electric furnace

Electric furnace refers to electricity as a heat source of the oven. According to the different wavelengths of radiation, and is divided into ordinary electric oven, far-infrared electric oven and microwave ovens.

Electric oven has a compact structure, covers an area of small, easy to operate, easy to control, high production efficiency, good baking quality and other advantages. One of the most prominent far-infrared electric roaster, which uses the characteristics of far-infrared rays, improve the thermal efficiency, saving but electricity, in large, medium and small food factories are widely used.

Classification by structure type

Food oven according to the structure of different types, can be divided into two categories of box furnace and tunnel furnace.

1, box-type furnace

Box-type furnace shape such as a box, according to the food in the furnace in different forms of movement, can be divided into the baking plate fixed box-type furnace, windmill furnace and horizontal rotary furnace. One of the baking pan fixed box type furnace is the most simple structure in this type of oven, the most common use, the most representative of a, so often referred to as the box type furnace.

(1) box-type furnace. Box-type furnace chamber wall mounted on a number of layers of brackets to support the baking pan, radiation elements and baking pan between the arrangement, in the entire baking process, the baking pan in the food and radiation elements with no relative motion. This oven is intermittent operation, so the output is small. It is more suitable for small and medium-sized food factory baking all kinds of food.

(2) windmill oven. Windmill oven is named because there is a shape similar to the windmill inside the rotating basket device, its structure as shown in Figure 2-41.

This kind of oven mostly use anthracite, coke, gas, etc. as a heat source, can also be used for electric and far infrared heating technology, coal-fueled windmill oven, most of the combustion chamber is located in the drying room below. Because the fuel is burned in the baking room, the heat is directly through the radiation and convection baking food, so the thermal efficiency is very high. Windmill oven also has a small footprint, relatively simple structure, the advantages of a larger output. Currently still used in bread production. The disadvantage of the windmill furnace is manual loading and unloading of food, the operation is tense, labor-intensive.

(3) horizontal rotary furnace. Horizontal rotary oven is equipped with a horizontally arranged rotary baking pan support, baking pan with billet placed on the rotary support. Baking, because the food in the oven rotary, the temperature difference between the surface of the billet is very small, so the baking is uniform, the production capacity is larger. The disadvantage is that the manual loading and unloading of food, labor-intensive, and more bulky furnace, Figure 2-42 for the horizontal rotary furnace structure schematic.

2, tunnel furnace

Tunnel furnace is a very long body, baking room for a long, narrow tunnel, in the baking process of food and heating elements between the relative movement of the oven. Because of the movement of food in the oven, as if through a long tunnel, so called tunnel oven.

Tunnel oven according to drive the food in the oven movement of different transmission devices. Can be divided into steel belt tunnel furnace, mesh belt tunnel furnace, baking pan chain tunnel furnace and hand-push baking pan tunnel furnace.

(1) steel belt tunnel furnace steel belt tunnel. Furnace refers to food as a carrier, and along the tunnel movement of the oven, referred to as steel belt furnace. Steel belt by respectively located at both ends of the furnace body, the diameter of 500-1000mm hollow roller drive. The baked products are output from the end of the oven and fall into the cooling conveyor belt in the subsequent process. The shape of the steel belt furnace is shown in Figure 2-43.

Since the steel belt only circulates in the furnace, there is little heat loss. Usually the steel belt furnace using speed control motor and food molding machinery synchronous operation, can produce bread, cookies, cookies and snacks and other food products. The disadvantage is that the steel belt is more difficult to manufacture and the deflection device is more complicated. This type of oven is usually natural gas, gas, fuel oil and electricity as a heat source.

(2) Mesh Belt Tunnel Oven. Mesh belt tunnel furnace referred to as mesh belt furnace, its structure is similar to the steel belt furnace, but the carrier used to transmit the surface of the blank is a mesh belt. Mesh belt is compiled from metal wire. The mesh belt can be supplemented when it is damaged after long-term use, so it has a long service life. Due to the large mesh gap in the mesh belt, the water at the bottom of the product is easy to evaporate during the baking process, and will not produce oil puddle and concave bottom.

Mesh belt is not easy to produce slipping in the process of running, the phenomenon of deflection is also easier to control than the steel belt. The heat source of the mesh belt is basically the same as that of the steel belt furnace. Mesh belt furnace baking output, heat loss is small, mostly used for baking cookies and other foods. The furnace is easy to form a continuous production line with food molding machinery. The shortcomings of the mesh belt furnace is not easy to clean, the dirt on the mesh is easy to stick in the bottom of the food, affecting the appearance of food quality.

(3) chain tunnel furnace. Chain tunnel furnace refers to the food and its carrier in the furnace movement by the chain drive to realize the oven, referred to as the chain furnace. The main transmission part of the motor, transmission, reducer, drive shaft, sprocket and so on. Chain furnace structure sketch shown in Figure 2-44, the furnace body in and out of the two ends of a horizontal horizontal axis, the axis were equipped with active and passive sprocket. Chain driven food carrier along the track movement. According to the baking of different food varieties, the chain furnace carrier is roughly two kinds, namely, baking pan and baking basket. The baking tray is used to carry cookies, pastries and fancy breads, while the baking basket is used to bake hear-shaped breads.

Chain tunnel ovens are generally equipped with a baking pan steering device and a turning device at the exit end so that the finished product enters the cooling conveyor and the carrier is returned to the entry end by the oven’s outer conveyor. Since the baking tray circulates outside the oven, the heat loss is large, which is not favorable to the working environment and wastes energy.

According to the number of different carriers entering the furnace side by side at the same time, the chain furnace is divided into single chain furnace and double chain furnace. Single chain furnace with a pair of chain, once into the furnace a baking dish or a row of baking baskets. Double chain furnace with two pairs of chain, side by side into the furnace at the same time two baking trays or two baking baskets.

Chain oven is generally used in conjunction with molding machinery, and form a continuous production line, its production efficiency is high. Because the speed of the drive chain is adjustable, so it is widely used, and can be used to bake a variety of foods.

(4) hand-pushed baking pan tunnel oven. Hand-push baking pan tunnel furnace without mechanical transmission, carrier movement in the furnace is dependent on human-powered. This oven is equipped with a pair or several pairs of flat steel made of track on the bottom of the oven, food baking pan on the track.

Operation, import and export each need an operator to complete the task of loading and discharging the furnace. This tunnel furnace body is shorter, simple structure, wider application, mostly used in small and medium-sized food factories. The disadvantage is that more workers need to operate, labor intensity, food in the furnace is not easy to control the speed of movement, food baking quality is not easy to grasp, and can not be formed with food molding machinery supporting the composition of a continuous production line. Its shape is shown in Figure 2-45.

According to the location of the heat source heater classification

1、Heat source inside the oven chamber

At present, the domestic are used in this type. And the most widely used is the heat source in the furnace chamber inside the tubular electric heaters and tubular gas atmospheric burner two.

2, the heat source in the oven chamber outside

This type of forced heating, the medium is heated outside the furnace chamber and then pass into the furnace. At present, this type is more widely used in foreign countries. There are mainly the following types.

(1) convection radiation oven. This type of oven is the simultaneous use of convection heating and radiation heating. Figure 2-46 shows the principle of this type of oven.

Highly heated flue gas through the burner 4 after heating, through the circulation fan 6 and flue damper 7 into the heating tube. The heating tubes are divided into upper and lower parts, and the oven belt 8 passes between the upper and lower heating tubes. Upper and lower heating tube can be divided into radiation tube 2 and blowing tube 1, in the front half of the heating tube near the furnace belt for the radiation tube, the tube surface is not open; away from the furnace belt in the back half of the heating tube for the blowing tube, in the direction of the direction of facing the belt open with small holes.

Radiation tube puts out radiant heat to bake cookies, and the air blowing tube blows the hot smoke flowing from the radiation tube to the furnace belt from the holes to form a good convection in the furnace chamber to achieve the purpose of uniformly heating the cookies. Part of the flue gas accumulated in the upper part of the oven chamber is recycled and reused after gas-water separation, and part of it is discharged out of the oven chamber through the exhaust fan 3 and the exhaust pipe 5.

There is a connecting pipe between the flue inlet and the exhaust pipe, so that when there is too much flue gas in the flue, it can be discharged into the atmosphere through the connecting pipe into the exhaust pipe. Usually a furnace zone is equipped with a convection radiant heating system, each of which can be individually and automatically controlled.

The convection heating system is characterized by uniform heat transfer and high thermal efficiency, which can be increased by about 20% compared with the usual ovens. The baking time can be shortened, which improves the economic efficiency of the fuel. But the adaptability of the baking cookie varieties is poor, the technical requirements are also higher.

(2) indirect radiation heating oven. In some of the use of gas baking oven, some fluffy cookies can adsorb the harmful ingredients in the gas, so that the cookies deteriorate, especially for some high-grade products, adsorption will produce an odor. If the use of indirect radiation heater baking oven, the flue gas generated by the combustion of gas is not in direct contact with the food, so as to ensure the quality of baking.

Figure 2-47, 48 for the indirect radiation heater sketch and working principle diagram. A fixed amount of gas under a certain pressure into the burner 1, combustion in the combustion chamber 2, air introduced by the induced draft fan 7 combustion chamber, the resulting high-temperature flue gas through the flue gas distribution room 3 into the upper and lower rows of radiant heating tubes 4 and 5, the furnace belt in the upper and lower heating tube running between the heating tubes, the heating tubes release radiant heat baking cookies, the diameter of the heating tubes are mostly 100mm.

The flue gas from the heating tubes, after passing through the flue gas return chamber 6 most of the flue gas is discharged from the chimney, and a small portion of the flue gas is repeated cycle, and then sent back to the combustion chamber by the induced draft fan. Since the temperature of the flue gas is much higher than the air temperature, the repeated circulation of part of the flue gas can improve the thermal efficiency of the oven. However, in order to ensure that the combustion chamber has enough air, you must control the amount of repeated circulation of flue gas, the general amount of flue gas back to 25-30%. Indirect radiation heater circulation system must ensure that there is a good seal to prevent flue gas leakage contact with food.

In addition, an attempt should be made to operate the flue gas in the heating circulation system under negative pressure. In this way, even in the case of small leaks in the heating tubes, the product quality will not be affected by the escape of fumes. Before the flue gas enters the upper and lower heating tubes, a regulating valve should be set in the flue gas distribution room to regulate the amount of flue gas entering the upper and lower heating tubes, so as to control the heat required on the lower part of the furnace belt. If there is too much flue gas in the flue gas distribution room, the exhaust valve of the distribution room can be opened to pump the flue gas elsewhere.

Since the indirect radiant heater uses indirect heating, this limits the oven temperature to a certain degree, not as high as a directly heated oven, so it is not suitable for baking soda type cookies. In addition, its thermal efficiency is lower than that of direct heating, so it is only applied when special requirements are met.