Resistance Furnaces: Types and Application | Electrical Engineering

In this article we will discuss about:- 1. Types of Resistance Furnaces 2. Application of Resistance Furnaces 3. Temperature Control.

Types of Resistance Furnaces:

1. Salt-Bath Furnace:

Salt-bath furnaces are used for the purposes of tempering, quenching and hardening of steel tools. The characteristics of salt-bath heating are rapidity and uniformity and selective localised heating combined with protection from oxidation.

This type of furnace consists of a bath of some salt such as molten sodium chloride and two electrodes immersed in it. When the current between electrodes immersed in the salt is passed, heat is developed and the temperature of the salt bath may be in the range between 1,000°C and 1,500°C depending upon the type of salt used. In this bath the material to be heated is dipped and the necessary heat treatment, as required for tool steel, is given to it. The heat transfer in this case is by conduction and by better contact of the body with the heated salt, which is in molten state.

As dc would cause electrolysis of the salt, therefore, alternating current is used. The voltages up to 20 volts and currents up to 3,000 amperes depending on the size of the furnace are used. Voltage on the secondary side is changed by tap changing gear included in the primary of the transformer.

The resistance of the salt decreases with the increase of temperature of the salt therefore, in order to maintain constant power input it is necessary that the salt furnaces are started with highest tap and as the temperature rises, this is gradually brought to lower taps. Control of power input is also affected by varying the depth of immersion and distance between electrodes. It must be ensured that the current flows through the salt and not through the job being heated.

2. Infrared or Radiant Heating Furnace:

In the ordinary resistance furnaces the heat is transferred from heating elements to the charge partly by radiation and partly by convection, the latter predominating at low and medium temperatures but in this form of heating suitable for low and medium temperature heating purely radiant or infra­red heating is used. In this method of heating, heating elements consist of tungsten filament lamps together with reflectors to direct the whole of the heat emitted on to the charge.

The lamps are operated at 2,300°C instead of 3,000°C giving greater proportion of infrared radiation and a longer life. The reflectors are plated with rhodium which, being harder, has a longer life and is easier to maintain. The lamps employed are usually of rating between 250 and 1,000 watts operating at 115 volts. The operation at low voltage results in a robust filament.

Plant sizes range from a single lamp to chambers containing several hundred kW of lamps. With this arrangement charge temperature between 200°C and 300 C can be obtained. Heat emission intensities up to 7,000 watts/square metre of the chamber surface can be obtained.

These are much higher than those obtained with ordinary resistance ovens (1,500 watts/m²). In radiant heating heat absorption remains practically constant whatever the charge temperature is where as it falls off rapidly as the temperature of charge rises in the ordinary resistance furnace.

In this process the lamps with their reflectors are grouped close together to form the walls and possibly the top of a heating chamber; the lamps being mounted on a light angle- iron framework, no heat insulation is required as all heat is directed inwards. Individual reflectors for each lamp or trough reflectors may be used, the latter enabling a greater number of lamps to be installed in the given area.

The precise shape of the chamber depends on the size and shape of the charge to be heated and the framework can easily the made adjustable. Continuous processes can be had, by carrying the articles to be heated through the chamber on a conveyor belt.

Infrared heating possesses the advantages of:

(i) Rapid heating,

(ii) Compactness of heating units,

(iii) Flexibility and

(iv) Safety and finds application in:

(a) Paint stoving

(b) Drying of wood furniture

(c) Preheating of plastics prior to moulding

(d) Softening of thermoplastic sheets and

(e) Drying of pottery, paper, textiles etc. where moisture content is not large.

For obtaining best results, the infrared lamps should be located at a distance of 25-30 cm from the object to be heated.

Application of Resistance Furnaces:

The most important applications of resistance ovens are air- circulation oven and bright annealing furnace.

1. Air-Circulation or Convection Type Furnaces:

The charge may not get uniformly heated in an ordinary furnace in which heat is transferred to the charge almost entirely by radiation. If uniform distribution of heat is necessary and the temperature is not to exceed about 600°C, such furnaces are used. The common applications are drawing and hardening of steel, heat treatment of aluminium and light metals. In such furnaces, air is passed over the heating elements and this heated air is circulated over or through the charge imparting its heat uniformly to all parts of the charge by conduction.

The direction of flow in air-circulation furnaces is sometimes reversed periodically in order to make the distribution of heat more uniform. If this were not done, the temperature of that part of the charge which comes across the hot air first will be higher than that which comes across it afterwards. However, this difference can be minimised by circulating large volume of air. Blowers and fans are used for circulating the gas or air. The advantage of the circulating air or gas is to impart all the parts of the charge a uniform application of the atmosphere. Generally the charge is to be cooled in the furnace itself and to achieve this gases are circulated after being cooled.

Gases are let through cooling pipes during the cooling process. Nitriding is an important application. In this process the steel after being heated by hot air, is exposed to ammonia. The steel absorbs nitrogen and gets hardened. An important item in the design of such an oven is the characteristic of the fan. A centrifugal fan is used, but it must be of such a design that a change in the resistance of the air path due to charges of different size and shape does not appreciably affect the volume of air delivered or kW required by the motor.

2. Resistance Furnace for Bright Annealing:

The process of cooling a heated job slowly for removing its brittleness is known as annealing. In the usual process of annealing the job is left covered with a scale of oxide which must be removed in order to have bright finish. The formation of oxide scale can be prevented if annealing process is carried out in an atmosphere free from oxygen, water vapour and carbon dioxide.

Such a condition can be achieved by heating the job in an air tight furnace to which is connected a non-­return air valve. During the period of heating air and any oil absorbed by the job are expelled through the non-return valve due to their expansion. During cooling the non-return valve closes, thereby preventing access of oxygen or other gases to the job. Thus the surface of the job under annealing process is maintained bright.

Temperature Control of Resistance Ovens/Furnaces:

Temperature control is necessary in resistance ovens/furnaces- temperature may have to be kept constant or varied according to requirements. Control may be manual or automatic.

In resistance ovens/furnaces heat developed depends upon I² R t or V²/R t. So there are three ways in which the temperature can be controlled. Firstly, by varying the applied voltage to the elements or current flowing through the element; secondly, by varying the resistance of elements and thirdly, by varying the ratio of on and off times of supply.

Voltage across the oven can be controlled by changing the transformer tappings. This is economical and most suitable if the transformer is to be used for stepping down the voltage for supply to ovens or furnaces, but such conditions do not arise usually. Auto-transformer or induction regulator can also be used for variable voltage supply. In case of large furnace, there may be an independent generating set for the supply to the furnace and in that case variable voltage supply can be provided.

Alternatively voltage across the oven or furnace can be controlled by varying the impedance connected in series with the circuit. But this method is not economical as power is continuously wasted in the controlling resistance. Its use is, therefore, limited to small furnaces.

Temperature can also be controlled by switching the various combinations of groups of resistances used in the ovens or furnaces in the following ways:

1. Use of Variable Number of Elements:

In this method, the numbers of heating elements in working are changed; so total power input or heat developed is changed. This method does not provide uniform heating unless the numbers of heating elements in the circuit at any particular instant are distributed over the surface area, which requires complicated wiring.

2. Change of Connections:

In this method the elements are arranged to be connected either all in series or all in parallel or combination of both or in star or in delta by means of switches at different instants according to the requirements. This is the simplest and most commonly used method of control.

An on-off switch can also be employed for temperature control but its use is restricted to small ovens. The time duration for which the oven is connected to the supply and the time duration for which it remains cut off from supply will determine the temperature. Here an oven is supplied through a thermostat switch which makes and breaks the supply connections at particular temperature. The ratio of-

is an indication of temperature. The higher the ratio, the larger will be the temperature of the oven. Advantage of this method is that it is more efficient than series impedance method.

Normally the temperature is required to be maintained between limits. This necessitates some type of automatic control of the thermostat type. When the temperature of the oven exceeds or falls below certain predetermined values, the thermostat connected in the circuit operates a relay which varies the current in the elements by any one of the methods given above. The relay may even switch the oven out of service if so required. The actual switching is carried out by contactors.

For large capacity ovens, some protection is to be provided against overloading, excessive temperatures, and in some cases against the possibility of workmen loading or unloading the furnace with the power on.

The only possibility of overload is due to a part of a metallic charge falling against and short circuiting the elements. An instantaneous overload relay is provided and set to trip the circuit at 10 or 15 per cent, above normal current. This adequately guards against damage due to such a contingency.

In addition to above, fuses are also provided either in the main oven circuit or in the hold-on coil of the energizing contactors to provide protection in case of failure of automatic control system. For the protection of nickel-chrome wire in this way a gold fuse is employed, because it has a sharp melting point of 1,060°C; and does not oxidise. Fuses melting at a lower temperature can be had, but although of some service are not completely reliable.