Classification of industrial furnace equipment

Industrial furnaces are divided into two types according to heating methods: one is flame furnace (or fuel furnace), which uses the combustion heat of solid, liquid or gas fuel in the furnace to heat the workpiece; The second is an electric furnace, where electric energy is converted into heat energy for heating.

Industrial furnaces are divided into two types according to the thermal system: one is intermittent furnace, also known as periodic furnace, which is characterized by no temperature section in the furnace, one or two shifts of production, and the temperature changes in each heating cycle, such as various chamber furnaces, bench furnaces, well furnaces, hood furnaces, etc. The second type is continuous furnace, which is characterized by dividing the furnace into temperature zones, generally consisting of preheating, heating (high temperature) and soaking (heat preservation) zones. The furnace is produced in three shifts continuously, and the temperature of each zone can be regarded as constant during the heating process of industrial furnaces, such as two-stage or three-stage continuous heating furnace, push rod heating furnace and heat treatment furnace, annular furnace, walking beam furnace, vibrating bottom furnace, cupola, lime kiln and so on. Electric furnace-Shen Guang electric furnace, a metallurgical furnace which uses electrothermal effect to supply heat. Electric furnace equipment is usually complete, including electric furnace body, electric equipment (electric furnace transformer, rectifier, frequency converter, etc. ), switches, auxiliary appliances (chokes, compensation capacitors, etc. ), vacuum equipment, electrical instruments, thermal instruments and other detection and control instruments (see the figure below), automatic adjustment system, furnace machinery and equipment (feeding and discharging machinery, furnace tilting device, etc.). ). The power equipment and detection and control instruments of large electric furnaces are generally concentrated in the power supply room of electric furnaces. Compared with the oil-burning furnace, the advantages of electric furnace are: the atmosphere in the furnace is easy to control, and even vacuum can be pumped; The material is heated quickly, the heating temperature is high, and the temperature is easy to control; The production process is easy to realize mechanization and automation; Good labor hygiene conditions; High thermal efficiency; Good product quality, etc.

The heating capacity of industrial furnaces calculated by unit time and unit bottom area is called furnace productivity.

Part of the residual heat of flue gas is absorbed by the cold workpiece entering the furnace, which reduces the temperature of flue gas discharged from the furnace.

The basic measures to improve the thermal efficiency of the furnace are: fully improve the combustion efficiency and strengthen the heat transfer to the workpiece; Continuous production and full load work as much as possible; Set up a preheater to preheat air and gas and recover the waste heat of flue gas; Refractory materials with low specific heat capacity and low thermal conductivity are used to reduce the heat storage and heat dissipation losses of the furnace wall.

In order to make the furnace temperature constant and realize the specified heating rate, besides determining the excellent furnace structure according to the technological requirements, the types of preheaters and stoves, the types of fuels and combustion devices, and the exhaust mode of industrial furnaces, it is also necessary to adjust the controllable variables such as fuel and combustion air or electricity through various control units to realize the automatic control of the furnace temperature, furnace atmosphere or furnace pressure. It is necessary for industrial furnaces to adopt pulse combustion.

High-grade industrial products require high uniformity of temperature field in the furnace and high stability and controllability of combustion atmosphere, which cannot be realized by traditional continuous combustion control. With the appearance of large-capacity industrial furnace with wide cross section, pulse combustion control technology must be adopted to control the uniformity of temperature field in the furnace.

Advantages of pulse combustion control

Pulse combustion control is an intermittent combustion mode, which uses pulse width modulation technology to control the temperature of the kiln by adjusting the duty ratio (on-off ratio) of combustion time. The fuel flow can be preset by pressure adjustment, and once the burner works, it will be in a full load state, ensuring that the gas outlet speed of the burner will not change. When the temperature needs to be raised, the combustion time of the burner is prolonged and the intermittent time is reduced; When cooling is needed, the combustion time of the burner is reduced and the intermittent time is prolonged. The main advantages of pulse combustion control are high heat transfer efficiency and greatly reducing energy consumption. The uniformity of the temperature field in the furnace can be improved. Accurate control of combustion atmosphere can be achieved without on-line adjustment. The load regulation ratio of the burner can be improved. The system is simple, reliable and low cost. Reduce the production of nitrogen oxides. The regulation ratio of common burners is generally around 1: 4. When the burner works at full load, the gas flow, flame shape and thermal efficiency can reach the best state. When the burner flow is close to its minimum flow, the heat load is minimum, the gas flow is greatly reduced, the flame shape can not meet the requirements, and the thermal efficiency drops sharply. When the high-speed burner works below 50% full load flow, the above indexes are far from the design requirements.

This is not the case with pulse combustion. In any case, the burner has only two working states, one is working at full load, the other is not working, and the temperature can be adjusted only by adjusting the time ratio of the two states. Therefore, pulse combustion can make up for the defect of low regulation ratio of the burner, and the burner can still work in the best combustion state when low temperature control is needed. When the high-speed burner is used, the gas injection speed is fast, so that negative pressure is formed around it, so that a large amount of flue gas in the kiln is sucked into the main gas and fully stirred and mixed, which prolongs the residence time of flue gas in the kiln and increases the contact time between flue gas and products, thus improving the convective thermal efficiency. In addition, the flue gas and gas in the kiln are fully stirred and mixed, so that the temperature of the gas is close to the temperature in the kiln, the uniformity of the temperature field in the kiln is improved, and the direct thermal impact of the high-temperature gas on the heated object is reduced.

Application of pulse combustion control technology in industrial furnace

Pulse combustion control technology is widely used in industrial furnace industry, which consists of high-speed burner and industrial furnace control system. Pulse combustion technology is used to complete the heating and temperature control of industrial furnaces. For the internal temperature field and temperature wave force of gas kiln 2 C, for the internal temperature field and temperature fluctuation of fuel (diesel) kiln 3 C, the effect is better in the kiln with heavy diesel as fuel. When the internal temperature of the common burner is lower than the fuel spontaneous combustion temperature, the flame of the burner will be extinguished immediately after the fuel is interrupted, and the temperature in the furnace will not be affected, thus solving the flameout problem. Adopting the most advanced atomization technology-bubble atomization technology, the burner has better atomization effect and less atomization medium consumption. Kilns that used to burn light diesel oil can now burn heavy diesel oil.

In practical application, when the common pulse width modulation is used to adjust the combustion duty ratio, when the duty ratio is close to 0% or 100%, the intermittent or combustion time is too short, and the field operation effect is not ideal, so we introduce the concept of minimum time, set the minimum time of intermittent and combustion to 3 seconds, and extend the corresponding combustion and intermittent time when the duty ratio is close to 0% or 100%. As a new technology, pulse combustion has broad application prospects and can be widely used in ceramics, metallurgy, petrochemical and other industries. It will play an important role in improving product quality, reducing fuel consumption and reducing pollution. It is an innovation of automatic control in industrial furnace industry and will become the development direction of industrial furnace combustion technology in the future. Medium-frequency induction heating electric furnace plays an important role in actual production, especially the heating and quenching equipment before forging round steel.