Heating elements(heaters)

Wire zigzag heaters are hung on the walls and roof of the furnace on heat-resistant hooks, hearth heaters are laid loosely on shaped bricks.

Spiral heaters in low-temperature furnaces are suspended on shaped ceramic bushings on ceramic tubes 2 or on lining shelves. In medium-temperature furnaces, spiral heaters are also placed in grooves 3 of the lining.

Tape heaters (made from tape or cast) are mounted on the walls and ceiling, usually on special ceramic hooks; They are placed on the hearth on ceramic supports.

Materials for heating elements

Heating elements, like heat-resistant ones, operate in high-temperature zones. In electrical furnace construction, in addition to the above, a number of requirements are imposed on them related to their electrical properties. Thus, these materials must have:

1. Heat resistance, i.e. they should not oxidize under the influence of oxygen air or high temperatures.

2. Sufficient heat resistance may not be enough for the heaters to support themselves.

3. High resistivity. This is explained by the fact that thin and long heaters are not durable, not structurally convenient, and have a short service life.

The heater consists of a metal tube, along the axis of which there is a nichrome spiral 2, welded to the output ends 5 of the heater. The tube is filled with crystalline magnesium oxide (periclase). Lead insulators are fixed at the ends of the tube.

The tube bends easily, so heating elements are produced various shapes(including ribbed ones - for electric heaters).

Heaters made of zigzag wire are installed on the walls and dome of the furnace using heat-resistant hooks; hearth heaters are placed randomly on shaped bricks. Heaters with a spiral in furnaces with low temperatures mounted using shaped ceramic bushings, ceramic tubes or lining flanges. Installation in medium-temperature furnaces is carried out in a similar way, in the grooves of the lining. Tape heaters (both cast and made from tape) are mounted on walls and arches, usually using specialized ceramic hooks; on the hearth they are placed on ceramic supports.

Materials for heating elements

Similar to heat-resistant, heating elements are exposed to high temperatures. In production electric ovens high demands are placed on the properties of materials associated with their electrical conductivity:

• Materials should not be subject to a high degree of oxidation due to atmospheric oxygen and elevated temperatures
• Thermal resistance must be sufficient for self-supporting
• High ratio resistivity, since heaters with a small cross-sectional diameter have a short service life
• To reduce inrush current, materials must have low value temperature coefficient of resistance (TCR), since shocks can be long-lasting and reach 4-5 times the value
• Stable electrical properties of heaters
• Constant size of heating elements
• Materials should be easy to process

Nowadays, nichrome (an alloy of nickel, iron and chromium) is mainly used for the manufacture of heating elements, since their use is allowed at temperatures up to 1100 ° C. At temperatures up to 600 °C, it is advisable to use fechral and constant. In furnaces where operating temperatures reach 1100-1150 °C and higher, non-metallic heating elements are used, such as carborundum rods (extremely permissible temperature 1300-1400°C), molybdenum disilicide (1400-1500°C). Vacuum high-temperature furnaces with operating temperatures reaching up to 3000°C usually contain heaters made of tantalum, molybdenum, tungsten, carbon (graphite or carbon). Molybdenum (up to 2000°C in a protective environment) and tungsten (up to 2500°C in a protective environment) heating elements for high-temperature furnaces are widely used.

The electrical power consumed by the heating elements is a few kilowatts (for small stoves). This value for high-volume furnaces reaches thousands of kilowatts and higher.

Tubular electric heaters (TEHs)

In ovens that have electric heaters or salt baths, where operating temperatures below 600°C tubular electric heaters are used.

The design includes a metal tube, along the axis of which there is a nichrome spiral, connected by welding to the lead edges of the heater. Lead insulators are fixed at the ends of the tube and the cavity of the tube is filled with crystalline magnesium oxide (periclase). The tube has high performance plasticity, therefore it is available in a variety of forms (including a ribbed form intended for electric heaters).

resistance electric furnaces

Heating elements have the highest temperature in the furnace and, as a rule, determine the performance of the installation as a whole.

The following requirements apply to these materials:

1. Sufficient heat resistance (scale resistance).

2. Sufficient heat resistance - mechanical strength at high temperatures necessary for the heaters to support themselves.

3. Large specific electrical resistance. The lower the electrical resistivity, the longer the length of the heater and the smaller its cross-section. The cross-section of the heater must be large enough to ensure the required service life. It is not always possible to place a long heater in an oven. Thus, it is desirable that the heating element materials have a high electrical resistivity value.

4. Small temperature coefficient resistance. This requirement must be carried out so that the power generated by the heaters in hot and cold states is the same or differs slightly. If the temperature coefficient of resistance is high, to turn on the furnace in a cold state, it is necessary to use transformers that initially provide a reduced voltage.

5. Consistency electrical properties. Some materials, such as carborundum, age over time, i.e., they increase electrical resistance, which complicates their operating conditions. Requires transformers with big amount stages and voltage range.

6. Machinability. Metal materials must have ductility and weldability so that they can be used to make wire, tape, and from the latter - heating elements with complex configurations. Non-metallic heaters are pressed or molded so that the heater is a finished product.

The main materials for heating elements are alloys based on iron, nickel, chromium and aluminum.

These are, first of all, chromium-nickel, as well as iron-chromium-aluminum alloys. The properties and characteristics of these alloys are presented in.

Double alloys consist of nickel and chromium (chromium-nickel alloys), triple alloys - of nickel, chromium and iron (iron-chromium-nickel alloys). Ternary alloys are a further development of chromium-nickel steels, since X23N18, X15N60-N are used up to approximately 1000°C.

Double alloys are, for example, X20N80-N. They form on the surface protective film from chromium oxide. The melting point of this film is higher than that of the alloy itself; the film does not crack when heated and cooled. These alloys have good mechanical properties both at low and high temperatures, they are creep-resistant, ductile, well processed, and welded.

Chromium-nickel alloys have satisfactory electrical properties, do not age, and are non-magnetic. Their main disadvantage is their high cost and scarcity, primarily of nickel. Therefore, iron-chromium-aluminum alloys containing iron, chromium and up to 5% aluminum were created. These alloys can be more heat-resistant than chromium-nickel alloys, i.e. they can operate up to 1400°C (for example, Kh23Yu5T alloy). However, these alloys are quite brittle and fragile, especially after exposure to temperatures above 1000°C. Therefore, after operating the heater in the furnace, it cannot be removed and repaired. These alloys are magnetic and can rust in a humid atmosphere at normal temperatures. They have low resistance creep, which must be taken into account when designing heaters from them. The disadvantage of these alloys is also their interaction with fireclay lining and iron oxides. In places where these alloys come into contact with the lining at operating temperatures above 1000°C, the lining must be made of high-alumina brick or coated with a special high-alumina coating. During operation, these heaters elongate significantly, which must also be taken into account during design, i.e. it is necessary to provide the possibility of their extension.

Representatives of these alloys are Kh15Yu5 (application temperature - about 800°C); X23Yu5 (1200°C); Kh27Yu5T (1300°C) and Kh23Yu5T (1400°C).

IN Lately alloys of the type Kh15N60Yu3 and Kh27N70YuZ were developed, i.e., with the addition of 3% aluminum, which significantly improved the heat resistance of the alloy, and the presence of nickel practically eliminated the disadvantages of iron-chromium-aluminum alloys.

Alloys Kh15N60YUZ, Kh27N60YUZ do not interact with fireclay and iron oxides, are fairly well processed, mechanically strong, and non-fragile.

High-temperature furnaces use non-metallic heaters: carborundum and molybdenum disilicide.

For furnaces with a protective atmosphere and vacuum, coal and graphite heaters are used. Heaters in this case are made in the form of rods, pipes and plates.

In high temperature vacuum furnaces and furnaces with a protective atmosphere, heaters made of molybdenum and tungsten are used. Molybdenum heaters can operate up to 1700°C in a vacuum, and up to 2200°C in a protective atmosphere. The application temperature in a vacuum is lower, which is explained by the evaporation of molybdenum. Tungsten heaters can operate up to 3000°C.

IN in some cases Heaters made of niobium and tantalum are used.

Most heating elements industrial furnaces made either from tape or wire (Fig. 3.4 – 3.7). Typically, for the manufacture of heaters for industrial furnaces, wire with a diameter of up to mm is used. However, for furnaces with operating temperatures of C and higher, wire with a diameter of less than mm should be used. The relationship between the pitch of the spiral and its diameter and the diameter of the wire is chosen in such a way as to facilitate the placement of heaters in the furnace, ensure their sufficient rigidity and at the same time not make it too difficult to transfer heat from them to the products.

Basic theory electric heating

Topic: Types of electric furnaces

In foundry production, a large number of different electric furnaces are used: for melting and holding metal, heat treatment and drying. It is convenient to classify electric furnaces according to the method of converting electrical energy into heat and the method of supplying heat to the heated body. In electric furnaces heating is carried out using the resistance method, electric arc, induction method and electron beam.

The domestic industry produces a large number of different electric furnaces. The designation of electric furnaces produced by the domestic industry is based on the heating method, taking into account technological characteristics. Thus, for melting furnaces, the designations consist of three main letters, several numbers and auxiliary letters.

First letter denotes the heating method: D - arc; I - induction; C - resistance; E - electronic.

Second letter determines the metal for which the furnace is intended for melting: C - steel; C - cast iron; A - aluminum; M - copper and its alloys, etc.

Third letter determines the main design feature of the furnace: for open arc furnaces P - rotary arch; for open induction furnaces T - crucible; for resistance furnaces K - chamber, etc.

Number after letter designation for electric melting furnaces indicates the capacity in tons.

Examples of notation: IAT-6 - induction crucible furnace for melting aluminum with a capacity of 6 tons; IST-0.4 - induction steel-smelting crucible furnace with a capacity of 0.4 tons; DSP-12 is an arc steel-smelting furnace with a rotary roof with a capacity of 12 tons.

Arc furnaces successfully used for melting metals, because high concentration of energy ensures rapid melting of the metal. Industrial arc furnaces operate on alternating current, so during each period, as a result of a change in the direction of the current, the surface of the end of the electrode and the surface of the metal alternately become the cathode and the anode.

Current transfer in an arc mostly carried out moving electrons. When electrons pass through, the gas in the arc gap becomes ionized. The electric arc discharge is maintained by the emission of electrons from the cathode, which increases sharply with increasing temperature. Electrons emitted by the cathode are accelerated under the influence of voltage applied to the arc gap, rush to the anode and give it their kinetic energy, causing it to heat up and destroy.

Thus, an arc column is created between the graphitized electrodes and the metal, consisting of a mixture of neutral gas particles of electrons, ions and atoms of the electrode and metal vapor. As a result, metal circulation occurs in the liquid bath, which accelerates the transfer of heat from the arc to the metal, promotes mixing of the metal and equalization of its temperature throughout the volume of the bath.

The electric arc is an intense high-temperature radiation source, the arc temperature is in the range of 4900-5800ºС. Electrodes must have high electrical conductivity, elevated oxidation onset temperature in air, high mechanical strength and good machinability. Electrodes are made with a diameter of 100-710 mm and a length of 1000-1800 mm. The extension is carried out using special graphite nipples with conical or cylindrical thread. Electrode consumption on average is 5-8 kg/t.

Electron beam furnaces They work on the principle of converting the energy of an electron beam into heat when it interacts with the surface of a heated body. Electrons emitted by the cathode are accelerated electric field to high speeds and are directed towards the heated body. The creation and acceleration of electron beams is effective only in high vacuum conditions.

Reaching the surface of the metal, electrons are introduced into its crystal lattice or liquid structure of the melt. Passing through the metal, electrons interact with both crystal lattice, and with individual atoms, molecules and electrons. As a result, part of the energy of the electron beam goes into thermal energy, due to which the temperature of the metal increases. Thus, the metal in the region of incidence of the electron beam heats up. Heat then spreads through the metal by conduction.

Electron melting plants(EPU) are used to produce ingots of especially pure metals and remelt waste of these metals.

Electron gun is a device in which a beam of electrons emitted by the cathode is formed by electrical and magnetic fields, accelerates in electric field, is discharged through holes in the anode and directed to the heated metal.

Resistance heating elements

Resistance heating elements (heaters) can be metal or ceramic. Metal resistance heating elements are mainly used in thermal furnaces; they are conductors made of a special alloy that has high electrical resistance and long service life at temperatures corresponding to heat treatment. If a potential difference is created at the ends of the conductor, an electric current will flow through the conductor, the strength of which will depend both on the voltage at the ends of the conductor and on the electrical resistance of the conductor itself, i.e.

where I is the current in the conductor, A; V- voltage at the ends of the conductor, V; R- electrical resistance of the conductor, Ohm.

When flowing through a conductor electric current electrical energy is converted into thermal energy. The amount of electrical energy converted into heat in 1 s can be expressed by the formula R= V*I, where_ R- amount of energy in 1 s or power, W.

By adjusting the voltage and resistance of the conductor, you can achieve isolation required quantity thermal energy. The voltage is regulated by a transformer. On thermal furnaces, the voltage can vary from 5 to 380 V. For this purpose, furnace transformers are used, designed to transmit large quantities electrical energy. The resistance of the conductor (heater) is changed by increasing or decreasing its length and cross-section.

The electrical resistance of a conductor can be calculated using the formula

ρ-specific electrical resistance of the conductor material, Ohm-m; l- conductor length, m; S is the cross-sectional area of ​​the conductor, m2.

The durability of the heater depends on the temperature to which it is heated. How better conditions heat removal from the heater,

Rice. 9. Heating elements

the lower its temperature, other things being equal. In thermal furnaces, the heater temperature should not exceed the temperature of the furnace working space by more than 50-100 °C. JB in this case, uniform heating of the parts is ensured. If the heater temperature is much higher than the heating temperature of the parts, local overheating of the parts will occur, which will lead to product defects. The durability of the heater also depends on the surface heat flux density measured in W/m2. Permissible surface heat flux density for different temperatures oven space is different.

To ensure rapid heating of the furnace, the electrical energy consumption is assumed to be 1.5 times greater than that required during normal operation of the furnace.

Depending on the temperature in the working space of the furnace, appropriate heater materials are used: up to 1100 °C - mainly heaters made of chromium-nickel alloys, for higher temperatures (up to 1300 °C) - ceramic heaters. ."Majority thermal furnaces have heaters made of nichrome - an alloy of nickel and chromium, made of wire or tape. A wire heater can be made in the form of a spiral or zigzag.

In Fig. 9, A the heater (right) is shown in the form of a spiral and examples of its location in the furnace are given. The service life of the heater depends on its placement in the oven. Suitable heaters 2 place on the side walls on shelves / or 5. An open heater releases heat into working space furnace radiation. Heater 3 the vault is placed in a special shaped brick 4, having a channel with a slot. The conditions for heat removal from the heater in this case are worse. The roof heater is surrounded on all sides by refractory material, and only a narrow slot connects the channel with the working space of the furnace. Under such conditions, the service life of the roof heater can be increased only by reducing the surface heat flux density by 25-35%. Bottom heater 9 also works in difficult conditions. It is screened from the workspace by a hearth plate 8. The bottom heater must be protected from contact with scale and other objects. To the heaters located in the oven, Electric Energy supplied via special conductor-outputs 6, made of heat-resistant steel. They have a cross-section much larger than the cross-section of the heater conductor in order to avoid heating them by the current flowing in the circuit. Product 7 is placed on the hearth of the furnace.

In Fig. 9, b a heater (right) made of zigzag wire is shown and examples of its location in the oven are given. Zigzag heaters on the walls 4 hung on hooks 3, made from nichrome. Heaters/on the vault are also hung on hooks 2, the opposite ends of which are bent, which prevents them from falling out. Heaters 5 placed on the hearth of the furnace.

The opening area of ​​zigzag heaters is larger; they transfer heat well into the furnace space, which increases their service life. The service life of the heater also depends on the diameter of the wire from which the spiral or zigzag is made.

Nichrome tape heaters are usually made in the form of a zigzag. The arrangement of heaters in the furnace is the same as for zigzag wire heaters.

Cast heaters are manufactured special method casting into shell molds. Such heaters are used in furnaces when it is difficult to select heaters from rolled products due to the unacceptably high surface heat flux density and, therefore, more high temperature heater. Fed heaters are successfully used at furnace space temperatures of 950-1150 °C. The heaters have bosses at the bend, with the help of which they are secured in ovens on special loops and hooks.

For heating dryers, tubular heaters have become somewhat widespread. A tubular electric heater (TEH) consists of a heat-resistant or ordinary tube carbon steel, inside which is placed a spiral wound from nichrome wire. The spiral is located along the axis of the tube, and the space between the spiral and the tube wall is filled with magnesium oxide powder, which has good electrical insulating properties and high thermal conductivity. The length of the tube can be up to 1 m. At the ends of the tube, insulators are installed with leads located in them, to which a spiral is attached. Tube assemblies can be easily bent and given any shape. These heaters are used at operating temperatures up to 600 °C for heating air, water, oil, etc.

Calculation basis electric heaters the conditions for their heat transfer with the surrounding “space” are established. The service life of the heater depends on the temperature for each grade of steel or alloy from which the heater is made, there is optimal temperature. Overheating of the heater leads to its burnout.

The heater temperature depends on the surface density of the heat flux from the heater to environment. The lower the temperature in the working space of the furnace, the greater the surface heat flux density from the heater can be. For nichrome heaters, a surface heat flux density of 1.5-10 4 W/m2 at a temperature of 900 “C is recommended; 1-10 4 - at 1000 °C; 0.7-10 4 - at 1100 °C.

The dimensions of the heater are determined by determining the required power of the furnace using a thermal calculation and specifying the permissible surface density of the heat flux from the heater.

Dimensions of round steel heaters

Where d- wire diameter, m; ρ - specific electrical resistance of the heater, 10 -6 Ohm-m; R- power, kWt; V- voltage on the heater, V; l - heater length, m; q- surface heat flux density, W/m2.

For tape heaters with the ratio of the tape sides bla = t

The heat transfer conditions of the heater are influenced by the composition of the furnace atmosphere and mutual arrangement heaters.