Detailed analysis and development of intermediate frequency induction furnace

The development of intermediate frequency induction furnaces is very rapid, and the development of industrial frequency induction furnaces to large capacity has stopped. The reasons are as follows:

1. Static frequency converter

　　 The development of an intermediate frequency induction furnace benefits from the use of static frequency converters. Compared with magnetic frequency converters, this kind of frequency converter has efficiency as high as 95% to 98%. The rated power of the inverter used as an induction furnace continues to increase. Recently, a 9000 kW inverter has been put into production. Connect it to a furnace with a capacity of 12t. The productivity of molten iron can reach 18 t/h; the power density of the intermediate frequency induction furnace The melting capacity per ton is increased to 1000 kW, which can shorten the melting period to 35 minutes. The melting rate of the induction furnace changes with the capacity of the furnace. Generally, the melting rate of the intermediate frequency induction furnace for melting cast iron is 0.4 to 35 t/h. For example, a furnace with a capacity of 2 t can obtain a melting rate of 2 to 2.38 t/h, and a furnace with a 12 t capacity can achieve a melting rate of 18 to 21 t/h; while a power frequency induction furnace is used to melt cold materials. The rate is 0.75 t/h for 1.5 t furnace, 1.5 t/h for 3 t furnace, 2.5 t/h for 5 t furnace, and 4 t/h for 10 t furnace. It can be seen that the melting rate of the intermediate frequency induction furnace far exceeds that of the power frequency induction furnace. This is a creation for the choice of cast iron production smelting equipment to replace the larger one with a smaller capacity intermediate frequency induction furnace instead of a larger capacity power frequency induction furnace. Conditions, the replacement of the industrial frequency furnace by the intermediate frequency furnace not only reduces the land and investment but also ensures the continuous supply of molten iron, which is very beneficial to the continuous operation and the production of cast iron with large production capacity. If the intermediate frequency induction furnace is used for continuous casting or centrifugal casting of molten iron in the production of ductile iron pipes, it can replace the cupola or double-connect with blast furnaces and cupolas, and its production capacity will be fully utilized. For example, there is a domestic manufacturer of centrifugal ductile iron pipes that uses a 10t intermediate frequency induction furnace and a blast furnace dual process to heat up and adjust the composition of the molten iron, and raise the stored blast furnace molten iron from 1300 ℃ to 1520 ℃, which is about It takes 27 min. The furnace has a frequency of 100-200 Hz and a power of 2500 kW.

　　The medium frequency induction furnace has high electrical and thermal efficiency, which not only improves the melting rate and shortens the melting time, but also reduces its unit power consumption. Compared with an industrial frequency induction furnace, its power consumption can be reduced from 700 kW.h/t to 515～580 kW.h/t. Relevant data shows that, taking into account the energy loss required for slag melting and overheating, the unit power consumption of the intermediate frequency induction furnace is about 582 kW.t/h during cold start, and the unit power consumption is 505 ~ 545 kW when the hot furnace is operated. .h/t, if continuous feeding operation, the unit power consumption is only 494 kW.h/t.

2. Production flexibility

　　 IF induction furnace has greater flexibility in production arrangements and greater flexibility in melting operations. For industrial frequency induction furnaces, it is required to work continuously without interruption, because the intermittent will increase the cold start, which not only increases the melting time and energy consumption but also must use the start block every time. When the intermediate frequency induction furnace is used intermittently for a short period, it can be cold-started without using the starting block. It is very convenient to replace the charge, and the molten iron can be completely emptied, which can realize the material replacement in a short time, which is beneficial to the production organization and arrangement. This brings favorable conditions for single-piece and small-batch production. For example, the industrial machinery repair plant in the iron and steel metallurgical enterprises mainly undertakes the production of spare parts, spare parts, and bulk consumables required by the iron and steel enterprises, as well as the manufacture of some non-standard equipment, and most of them are single-piece and small-batch production, with many varieties. , Material requirements often change. In this regard, the production flexibility of the intermediate frequency induction furnace makes up for the shortcomings of the industrial frequency induction furnace and other smelting furnaces.

3. Furnace structure

With the continuous improvement of the power density of the intermediate frequency induction furnace, the requirements for the safe operation of the furnace, the improvement of the life of the furnace lining and the reduction of noise have become higher and higher, and the rationality of the furnace structure has become more and more important. The steel shell furnace has many advantages such as strong durability, high efficiency, high productivity, low noise, and easy maintenance.

　　The heavy-duty steel shell furnace is different from the frame furnace. It is a high-strength ring-shaped steel shell with multiple larger inspection openings. The inspection openings are closed when the furnace is running, and each inspection opening can be opened during maintenance. The heavy-duty steel shell furnace has a strong internal structure, which can avoid possible deformation caused by tilted iron casting and prolong the life of the furnace lining. Moreover, due to the enclosed solid furnace shell and the addition of sound-absorbing insulation materials inside, the working noise is greatly reduced. The strong steel shell can also effectively protect the induction coil from the danger of splashing metal so that the furnace has the greatest safety during operation. To effectively heat insulation and increase the life of the furnace lining, the heavy steel shell furnace is also equipped with cooling rings at the top and bottom, which can uniform the temperature of the furnace lining and reduce thermal expansion. The stainless steel cooling ring with low energy consumption and high strength greatly improves the efficiency of the furnace.

　　Heavy-duty steel shell furnace not only has a strong steel shell, but also designed a thick-walled tube structure coil specially used for induction melting, and by correctly selecting the distance between the turns of the induction coil, the coil has the highest conversion efficiency and the smallest resistance. On the one hand, the thick-walled induction coil has a large current-carrying cross-section, uniform wall thickness, and is not prone to damage caused by arc and expansion. On the other hand, the coil adopts a special support system, and each turn is firmly locked with good rigidity. The vibration caused by electromagnetic force is reduced, and the service life of the furnace lining is greatly extended. In addition, 50% of the magnetic yoke is covered around the induction coil, which effectively changes the distribution of the magnetic field. At the same time, the isolation cushion also reduces noise and vibration and increases the efficiency of the yoke. The heavy-duty steel shell furnace also adopts an elongated coil design that extends the induction coil beyond the furnace substrate, so that the coupling between the charge load and the magnetic field from the top to the bottom of the furnace body is very uniform, which improves the energy conversion efficiency and reduces the agitation caused by the magnetic field. The influence of force on the bottom lining.

　　Reasonable furnace structure is very important. Heavy steel shell furnace is only one of them. Many manufacturers are working hard to improve the furnace structure to adapt to the application and development of intermediate frequency induction furnaces.

4. Deslagging and furnace repair

　　The productivity of the intermediate frequency induction furnace is high, and the melting period can be shortened to about 35 min. To maximize the power of the furnace, the slag must be removed as quickly as possible. Using a skimmer or manual slag removal, the effect is poor and the time is long, and the working conditions are bad. To solve this very time-consuming and arduous operation, a method of dumping the slag behind the furnace has emerged, that is, pour the slag into the transport truck through the slot at the top of the furnace body. This method is quick and convenient. The backward tilt angle of the furnace is generally 20°～25°.

　　 After one campaign of the intermediate frequency induction furnace, it must be dismantled and repaired. To shorten the furnace shutdown time for the replacement of refractory linings, mechanization must be considered, and the vibrating furnace building and the lining push-out machine have become the main accessories of large-scale induction furnaces. The lining push-out machine can push out the refractory lining before it is completely cooled, further shortening the repairing time and improving the working environment.

5. Dual power supply and control system

　　 A unique development of the intermediate frequency induction furnace is to use one power supply system to supply power to two furnaces, realizing a working system with no production interval. The practice has proved that the total effective power of an induction furnace is usually not fully utilized during the entire melting period. When measuring the temperature of molten iron, sampling, removing slag, and tapping, especially in the case of pouring, the power must be reduced or the power supply must be cut off. . If the pouring cycle is long, the utilization rate is only about 50%, which indicates that to achieve the required productivity, the rated power of the power supply device needs to be 1.8 times the utilization rate of 90%. To solve this problem, a dual power supply system was successfully developed. The system uses two identical converters and capacitor banks, each furnace uses one set, but they all use a common rectifier and transformer to supply power. Each converter can be individually controlled, and the total effective power can be in any proportion. Assigned to 2 stoves. In addition to providing sufficient power for the insulation of one furnace, this system can be used for the second furnace for molten iron smelting. Once the smelting furnace is ready for pouring, the power of its converter is immediately reduced to the insulation level; at this time, the other furnace is emptied, and then refilled with a cold material, the power of the converter is increased, and a new smelting is started. If necessary, the total effective power can be connected to one furnace and the other furnace is deactivated; or the total effective power can be evenly distributed to two furnaces to make them smelt at the same time. For example, the dual power supply and control system of China. can supply power to two furnaces at the same time, and the power delivered to the two furnaces is stepless distributed. The use of this power supply can simultaneously deliver melting power to one furnace and heat preservation power to another furnace so that the power utilization rate can reach 100%. This kind of power supply sends power to two furnaces at the same time, completely avoiding the switch or adding power supply, and it is not necessary to switch the power supply to the holding furnace during smelting to maintain the necessary pouring temperature so that the intermediate frequency furnace can achieve melting and insulation 2 functions. When a furnace is under maintenance, the power supply can be isolated from this furnace, and only the other furnace is powered, which also increases safety.

It should be mentioned that the intermediate frequency induction furnace equipped with a dual power supply and control system can guarantee the normal production of the other furnace even when one furnace is drying, to realize the timely and continuous supply of molten iron. The frequency induction furnace is incomparable. The practice of 1 transformation and 2 furnaces in foundry proved that two 10t industrial frequency induction furnaces use one set of power supply. When one furnace is baking, the other furnace cannot be produced. If one oven takes about 3 days, it will not be able to produce for about 1 month in a year, which will have a great impact. To achieve the goal of continuous production, 1 is to increase the number of power frequency induction furnaces, and 2 is to add 1 set of oven power equipment. When the Foundry was building a centrifugal ductile iron pipe workshop, it added a power frequency induction furnace and corresponding power equipment based on the original 1 transformer and 2 furnaces; another domestic manufacturer of centrifugal ductile iron pipes was in the original 1 transformer and 2 furnaces. Based on the furnace, it is planned to add a set of power supply equipment for the oven. Regardless of the measures mentioned above, corresponding equipment and facilities must be increased, which also increases land occupation and investment. The application of the dual power supply and control system of the intermediate frequency induction furnace not only ensures the melting and heat preservation at the same time but also realizes the non-stop production of the oven.

6. Degree of automation

　　The intermediate frequency induction furnace adopts a computer control system with a high degree of automation. For high-power intermediate frequency induction furnaces, the melting period is very short and the overheating rate is very high. Only by using a computer control system can the potential of the equipment be realized and the most effective protection for the factory and operators can be provided. A furnace that overheats and melts the charge at a rate of 25 to 30 °C/min, left unattended for a few minutes at full power, is likely to cause a catastrophic accident. At present, the large-scale intermediate frequency induction furnace has been equipped with a relatively complete computer-aided monitoring and control system, the main contents of which include:

　　(1) When the molten iron in the furnace crucible reaches the pre-programmed temperature, ensure that the furnace power is automatically reduced;

　　(2) The furnace is fixed on a weight measuring instrument, which provides the computer with the quality data of the metal charge in the furnace;

　　(3) Monitor, calculate and control the temperature of molten iron;

　　(4) Provide automatic sintering program for new furnace lining;

　　(5) Determine the amount of carbon, silicon, and other added elements required to meet the required iron composition;

　　(6) Other useful and necessary functions including an automatic cold start device.

　　 In addition, the installation of an automated mechanical charging system is also indispensable to realize the full potential of the high-power intermediate frequency induction furnace. Generally, a vibrating feeding system that can be installed on a rail to move horizontally and vertically or rotate around a pivot is the most suitable.

　　The melting processing system of the high-efficiency intermediate frequency induction furnace is composed of a melting processor to monitor the functions and operating procedures required for the entire melting process. This processing system is composed of a microprocessor with a color monitor. At work, under the combined action of the furnace charging system and the weighing system, the power required for melting to the pouring temperature is automatically calculated, and the energy absorption required for the melting process of the furnace during the melting period is controlled. Under the action of the electric bridge, the automatic charging of the furnace melting process is realized; the molten iron reaches the pouring temperature after the smelting, and the processing system can automatically switch the heat preservation process, which can achieve the expected super heat with the minimum energy; through additional programs,

In the most conducive to protecting the furnace lining, automatic control of the furnace cold start and the sintering process of the furnace lining refractory materials.

　　The melting processing system can conduct a comprehensive self-diagnosis of the power supply and the furnace body. If the system alarm is activated in an abnormal state, the processing system can provide the problem and can store the fault information, or it can be connected to the printer for printing and retention.

　　 An intermediate frequency induction furnace has more advantages than a power frequency induction furnace. It is suitable for smelting all kinds of cast iron, especially suitable for smelting alloy cast iron and ductile iron. In recent years, it has been widely used in cast iron production. The furnace has high productivity, energy-saving, and power-saving, and greater production flexibility, which is extremely beneficial to the machine repair plant of industries such as steel and metallurgical enterprises and other single-piece small batch production; it has a high degree of automation and is equipped with dual power supply and control systems. With the functions of smelting and heat preservation, it is also very suitable for continuous operations such as the production of cast iron pipes, and its application prospects are bound to be promising.