The hottest flame known to man is the electric arc, with a temperature of 3,500 degrees centigrade. Electric furnaces are used not only for melting metal, but also for making glass, pottery, aluminium, and other substances requiring high temperatures in their manufacture
ROTARY ELECTRIC KILN used for firing enamelled colours on porcelain ware. The ware is packed on trays of heat-resisting steel, in a circular cage which is rotated through the furnace revolving once in eighteen hours. Nickel-chromium wire elements are used. The ware is heated by stages to 800°C as it passes through the kiln and is allowed to cook by stages until it emerges again.
THE use of electricity for heating purposes is well established in the homes of millions of people. Electric light itself is dependent on the heating properties of the electric current. Heating by electricity, too, is the basis of an engineering process immensely important in industry — such as the craft of welding.
Yet another application of electrical heating is in the operation of furnaces, large and small. The electric furnace covers a wide range of work from the heat treatment of tools to the melting of steel ingots and the manufacture of aluminium (see the chapter “Aluminium Production”). The design of the electric furnace is not confined to any one type. Electric furnaces may use the hottest flame known to man, the electric arc hissing and spluttering at a temperature of 3,500°C.
Another type operates on the induction principle. Let the human hand (devoid of any metal rings) be thrust into an electric induction furnace and the mighty force there concentrated remains unnoticed. But place a crucible of steel within and immediately the current induced in the metal will begin to generate heat that will suffice ultimately to reduce the contents of the crucible to a molten mass. In other types of furnace the requisite melting heat is furnished by the resistance to the passage of an electric current. This resistance is obtained by wire or carbon resistances, or by the metal itself.
Industrial electric heating is not confined to operations at the highest temperatures. Soldering and branding irons, drying plates and enamelling ovens, all provide instances of electric heating. Generally speaking, however, an oven is worked at a temperature not exceeding 300° C. Above that temperature a heating chamber is generally termed a furnace. From the electric iron to the great furnace holding many tons of steel, the basic operating principle is the same, namely the heating effect produced in a conductor by the passage of an electric current. The resistances used as heating elements in electric furnaces are made, therefore, of substances that conduct electricity but offer sufficient resistance to ensure an adequate rise in temperature.
The simplest electric furnace is the resistance type that differs but little in its heating elements from the domestic iron. The elements are composed of nickel-chromium wire or strip, wound round or within a heating chamber made of firebrick or other refractory material. The nickel-chromium wire is insulated with mica or silica, and the heating chamber is lagged with non-conducting materials such as magnesia, to reduce heat losses through the furnace walls. Temperatures up to 900° or even 1,150° C. are attained in resistance furnaces with heating elements of nickel-chromium. Furnaces of this type are widely used in the heat treatment of tools and for hardening steel.
Resistances of platinum, molybdenum and tungsten give higher ranges of temperature (1,500° to 2,000° C), but these metals are costly. Fortunately, however, there is another substance that has an extremely high melting point and is cheap. This is carbon, familiar for a score of different purposes in electrical work, from telephone receivers to arc lamps. Carbon, however, burns in air at about 550° C. Accordingly in industrial carbon resistance furnaces working at temperatures up to 1,400° C., the resistances are encased in recrystallized “Carborundum”. This material, known also as silicon carbide, is itself produced in an electric furnace and, apart from its heat-resisting qualities, it is widely used as an abrasive. Resistances of “Carborundum”, connected to protected carbon plates, are used also at temperatures up to 1,500° C, but above that heat the material shows signs of mechanical weakness.
A small carbon resistance furnace can be made by sawing a carbon tube into the form of a helix. The intervening spaces are filled in, and the helix is coated inside and out, with fireproof cement. Such a furnace would serve only for laboratory purposes, but large carbon resistance furnaces are used extensively for research work, especially in metallurgy. Coke is another well known substance used as a resistance in electric furnaces.
In some furnaces, used in the heat treatment of steel and for melting brass and similar metals, the heating element comprises a trough filled with coke to which the supply of current is led through carbon rods. The coke is allowed to burn with a restricted air supply so that an atmosphere of carbon monoxide is formed, and this prevents oxidation of the metallic charge in the furnace. Fresh coke is added as required Coke resistance furnaces are also used for the reheating or annealing of ingots and forgings on a large scale with furnace charges weighing up to 100 tons Resistance furnaces for the heat treatment of steel forgings and other work are built in sizes as much as 30 feet deep or more.
The operating current tor a resistance furnace is generally at a low pressure, rarely exceeding 400 volts, but the power used ranges from 10 to 300 kilowatt hours for every ton of the charge, according to the material and the temperature at which it is being treated. Either direct or alternating current can be used in the operation of a resistance furnace, and regulation of the heat is possible within narrow limits. This is a valuable consideration in a large number of processes, especially in the heat treatment of steel. Heat control is obtained by the use of additional resistances for direct current, and by variable tapping transformers where alternating current is used. The regulation of furnace temperature is generally done by a thermostatic device and a relay switchgear so that the heat of the furnace automatically controls the amount, of current according to requirements.
RESISTANCE FURNACES consist generally of a heating chamber made of firebrick, or other refractory material, and lagged with non-conducting materials to prevent loss of heat through the walls. The furnaces shown are used in the manufacture of tiles for decorative purposes. The elements, of nickel-chromium wire, are hairpin-shaped and their ends may be seen in the open furnace.
Resistance electric furnaces have proved economical for continuous operation. In one type of continuous furnace a conveyer belt takes a supply of material in at one end of the furnace and out at the other end through doors or fireproof curtains. Another type of furnace has a rotary action and the charge is carried on a revolving plate between inner and outer circular walls lined with resistances.
In the same way as resistances are heated by the passage of an electric current, the furnace charge or its containing crucible may be used as conductors and so may become heated to the required temperature. Furnaces of the conductor type consist of a crucible made of special material, generally with a graphite base, that permits its use as a combined container and resistance.
The crucible is lagged on its outer surface with refractory material, and is lined with an insulating substance that prevents contact between the crucible and its metallic contents. The crucible is also fitted with a heat-resisting cover or lid and is suspended, bell fashion, in a U-shaped steel framework. A geared quadrant and pinion wheel provide means for tilting and pouring out the contents of the crucible when the charge is melted. Air-cooled rings, making contact with extensions at the top and bottom of the crucible, serve as terminals for the current supply. This may be direct or alternating, and a heavy current is necessary with, however, a low voltage. A furnace of this type will melt some 250 lb. of brass in about an hour with an input of 30 kW.
Another example of heating by the conduction of a current through the charge is found in the “salt bath” furnace used for the heat treatment of delicate steel components. The salt is contained in a crucible fitted with two or more main electrodes. When cold, the salt is non-conducting and an auxiliary rod, attached by a cable to one of the main electrodes, is brought into contact with the opposite electrode, near which a molten salt pool is formed.
Heating by Induced Current
The pool of molten salt extends to the other electrode as the rod is drawn away and finally the whole of the salt in the crucible melts, attaining a temperature of about 1,350° C. with alternating current. The steel to be treated is immersed in the salt, and is there heated to within closely predetermined limits of temperature. During heating the steel is protected by the molten salt from the effects of oxidation. In some instances the salt bath is resistance wound, but is then capable of giving a temperature of only 800° to 900° C., with direct or alternating current. The foregoing methods of direct heating by electricity are also applicable to other processes, apart from those carried out in furnaces. A particularly interesting application of the heating properties of the electric current is in the hardening and tempering of enormous lengths of steel wire. The untreated wire is run off a storage drum or coil on to a power-operated hauling drum. Between the two drums the current is led into and out of the moving wire by roller electrodes. After having reached the required temperature the wire is passed through a quenching tank. The heat treatment of steel tubes is carried out by the use of clamp-type electrodes that hold the work at either end. When the tube has been heated to a predetermined temperature, with corresponding increase in length by expansion, the clamps are automatically released.
Heating by induced current provides a convenient method of carrying out an important industrial operation, the fitting of steel tyres to railway carriages. In this instance, however, the current is not led direct to the tyre, but is induced in it by a transformer. The primary or input windings of the transformer are connected to an alternating current supply and the tyre takes the place of the ordinary secondary or output windings. The tyre is placed concentric with the primary windings, within which is a rectangular section iron yoke or core. Part of this yoke is removable to admit the tyre, which requires between 40 and 50 kVA to bring it to the correct temperature (about 250° C) for shrinking on the wheel.
The process is similar to that used in fitting tyres to locomotive wheels. Locomotive wheels, however, are not required in such large quantities as coach wheels and it is found more economical to heat their larger tyres by gas. In another type of induction heater the work is made part of a magnetic circuit produced by alternating current, so that the metal is heated by what are known as eddy currents. The iron core of a transformer, as used for instance in a radio receiver, is composed of a number of separate plates or laminations. This method of construction is adopted to avoid the eddy currents that would be set up in a solid core with consequent overheating. It is this heating property that is turned to good account, not only for the heating of massive pieces of iron or steel, but also in the induction furnace used for melting.
Low and High Frequency
Induction furnaces are divided into two main classes, low and high frequency, both of which have their own technical advantages for various types of work. Low-frequency induction furnaces operate on alternating current at frequencies ranging from 5 to 50 cycles a second. The high-frequency types use frequencies between 500 and 50,000 cycles a second. Both types of induction furnace display considerable variety in design, according to the purpose for which they are intended.
A typical low-frequency furnace, used for the melting of steel, comprises a primary coil, cooled by an air blast, surrounded by a circular hearth made of refractory material. In the upper surface of the hearth is a deep annular channel that contains the charge of metal. The bottom of the channel is provided with an outlet for drawing off the molten charge. Passing through the middle of the primary coil is a laminated core that completely encircles part of the circular hearth and the charge contained in it.
A supply of alternating current having been connected to the primary windings, eddy currents are induced in the charge and the steel or other metal is melted by the heat produced. A furnace of this type, holding, say, five tons of steel, would operate at about 1,800° C on a power consumption of some 800 kW. In some low-frequency furnaces the circular hearth is slightly tilted and is revolved by an electric motor to aid the melting process. Another type of low-frequency furnace, used mainly for melting brass and other non-ferrous metals, consists of a crucible, the lower part of which is V-shaped and contains the coil of the primary windings. Passing through the horizontal primary coil and encircling the crucible on either side are the yokes of a transformer coil. From the hollow upper part of the crucible a V-shaped channel of rectangular section passes under and on either side of the primary coil. The V-channel serves as the secondary winding of the transformer and the molten metal in it rises to heat the main contents of the crucible by convection. Such furnaces vary considerably in power and capacity, but a 75-kW furnace of this type would melt about 700 lb. of brass.
The high-frequency induction furnace requires no iron core in the primary coil, but the frequency of the alternating current supply is necessarily much higher than in the furnaces described above. The operating principles of both classes of furnace are, however, substantially the same. The absence of an iron core permits the use of an ordinary crucible that can be placed conveniently within the primary coil of the high-frequency furnace. An additional advantage of this type of furnace is that it can be completely enclosed for the purpose of carrying out melting operations in an oxidizing or reducing atmosphere or in a vacuum, if required.
FORCED AIR CIRCULATION FURNACES for the annealing of brass articles. The circular lid is swung aside by means of a lever and the work to be treated is lowered into the furnace in a perforated metal “ basket.” The heating elements are of nickel-chromium wire and an effective distribution of heat is ensured by a centrifugal fan fitted in the bottom of the furnace.
High-frequency furnaces are thus of great utility in research, especially as extremely high temperatures are possible in their operation. The temperature is limited only by the melting point of the charge, and 3,000° C is often attained. The oscillating current for high-frequency furnace operation is taken either from a special alternator, a thermionic oscillator, or from the discharge of condensers that are charged from commercial alternating supply. The high-frequency furnace, apart from its use in laboratory research, is used in the making of special tool steels, for graphitizing carbon and in the preparation of alloys and highly refractory materials.
The largest electric furnaces are those that use the arc, with carbon electrodes that produce a crater attaining an enormous temperature — over 3,000° c.
There are three main classes of arc furnace — the arc-radiation type, in which the arc is arranged above the charge; the direct-arc furnace, in which the arc is struck between an electrode and the charge; and the arc-resistance type of furnace, in which the arc is struck between an electrode and the charge through which the current then flows to a submerged electrode.
The arc-radiation furnace consists of an enclosed refractory hearth, in the roof of which two electrodes are directed downwards in the form of a V. Where three-phase current is used three electrodes are arranged Ʇ-fashion. The number of electrodes in an arc furnace is not limited to one pair or set. Rotating and rocking furnaces generally consist of a horizontal cylinder lined with refractory material and mounted on rollers. The electrodes project inwards from either end along the axis of the cylinder. In the direct-arc furnace the electrodes are arranged vertically in the roof of the hearth, and it is usual to provide them with water-cooled rings where they emerge from the casing. The current passes down one electrode and arcs across to the furnace charge through which it flows. The current then arcs across from the charge to the other electrode, thus completing the circuit.
The arc-resistance furnace is similar to the direct-arc furnace except that the vertical electrode or electrodes have their counterpart in an electrode submerged below the surface of the charge. In some types of arc-resistance furnace the lower electrode is not in contact with the charge, but is buried within a hearth made of material that becomes a conductor of electricity when hot.
Giants of their Type
Arc furnace electrodes are made of carbon, generally petroleum coke, gas-retort coke or anthracite. The material is ground into a fine powder, mixed with tar and moulded under hydraulic pressure. The electrodes are finally baked at a temperature of 1,300° C.
Arc furnaces rank among the giants of electric furnaces, and the title is borne out by the proportions of the electrodes used in a 100,000-amperes furnace in France. In this instance the suspended electrode is 7½ feet in diameter and 4 feet long. Carbon electrodes 3 to 4 feet in diameter and 5 to 6 feet long are in common use. The electric furnace, apart from its use in the melting of metal, has scores of applications in modern industry, for the making of glass, pottery, calcium carbide, aluminium and many other substances requiring high temperatures in their manufacture.
Two other uses of the electric furnace provide an interesting contrast — the making of diamonds from carbon and the abstraction of nitrogen from the atmosphere for the preparation of fertilizers. Diamonds, which, however, can be made only of minute size, are obtained by dissolving carbon in molten iron in an electric furnace. The crucible is plunged into water and the outer layer of iron solidifies, exerting enormous pressure on the still molten mass within. The carbon crystallizes out as diamond specks in this molten iron, which also solidifies eventually. The whole mass is then acid-dissolved, leaving the diamonds exposed.
In the making of nitrogen fertilizer an enormous electric arc is “blown” by an electro-magnet into a great sheet of flame against the furnace wall. A powerful fan drives a blast of air (nitrogen and oxygen) through this flame, and the intense heat causes the oxygen and nitrogen to combine and form nitric oxide. The nitric oxide gas is absorbed by lime to form the nitrate of lime used as fertilizer.
CONTINUOUS ALUMINIUM ALLOY BILLET-HEATING FURNACE. One of the largest in Great Britain, the furnace chamber is 16 ft. 6 in. long, 7 ft. 6 in. wide and 10 feet high. Its capacity is forty billets (12-in. diameter) an hour, the temperature being 450 degrees centigrade.
The operating current tor a resistance furnace is generally at a low pressure, rarely exceeding 400 volts, but the power used ranges from 10 to 300 kilowatt hours for every ton of the charge, according to the material and the temperature at which it is being treated. Either direct or alternating current can be used in the operation of a resistance furnace, and regulation of the heat is possible within narrow limits. This is a valuable consideration in a large number of processes, especially in the heat treatment of steel. Heat control is obtained by the use of additional resistances for direct current, and by variable tapping transformers where alternating current is used. The regulation of furnace temperature is generally done by a thermostatic device and a relay switchgear so that the heat of the furnace automatically controls the amount, of current according to requirements.