Heating is required for domestic purposes such as cooking and heating of buildings, as well as for industrial purposes such as melting of metals, hardening and tempering, case- hardening, drying and welding. Practically all the heating requirements can be met by some form of electric heating equipment.
Modes of Heat Transfer:
A heated substance will give off heat to another substance at a lower temperature.
The different modes by which heat is transferred are conduction, convection and radiation:
In this mode of transfer of heat, one molecule of the substance gets heated and transfer the heat to the adjacent one and so on. Thus heat is transferred through a substance from one part to another, or between two substances in contact.
The rate of conduction of heat along a substance depends upon the temperature gradient. It may be expressed in MJ per hour per square metre per metre or in watts per square centimetre per centimetre while dealing electric heating.
In a plate of thickness t metres, having x-sectional area of its two parallel faces A square metres and temperature of its two faces T1 and T2 °C absolute the quantity of heat passed through it during T hours is given by-
Q = kA/t (T1 – T2)T …(5.1)
Where, k is the coefficient of thermal conductivity for the material in MJ/m2/m/°C/hour.
Heat is transferred by convection in case of immersion type water heater or in case of low temperature heating equipment for buildings. The air in contact with a heated radiator element in a room receives heat from contact with the element. The heated air expands and rises, cold air flowing into take its place. Thus there is a constant flow of air upwards across the heating element.
This process is called convection. These convection currents give up some of their heat to the colder parts of the room. The room and its contents are thus gradually heated by this means. A similar action takes place in an electric water heater, a continuous flow of water passing upwards across the immersed heater element, with the result that the whole of the water in the tank becomes hot.
The quantity of heat absorbed from the heater by convection depends mainly upon the temperature of the heating element above the surroundings and upon the size of the surface of the heater. It also depends partly on the position of the heater. Heat dissipation is given by the following expression.
Heat dissipation, H = a(T1 – T2)b W/m2 …(5.2)
Where, a and b are constants, whose value depend upon the heating surface facilities for heating etc. T1 and T2 are the temperatures of the heating surface and the fluid in °C absolute respectively.
For vertical surfaces in air, the above expression becomes
Heat dissipation, H = 3.875 (T1 -T2)1.25 watts/m2 …(5.3)
In furnaces heat transfer by convection is not to any great extent.
In this mode of heat transfer the heat reaches the substance to be heated from the source of heat without heating the medium in between. Rate of heat radiation is given by Stefan’s law, according to which-
where T1 is temperature of source of heat in °C absolute, T2 is temperature of substance to be heated in °C absolute, k is a constant known as radiant efficiency, k is unity for a single element and between 0.5 and 0.8 for several elements placed side by side and e is emissivity, which is unity for black body and is equal to 0.9 for resistance heating elements.
Since radiation is proportional to the difference of fourth powers of temperatures, we can have very efficient heating at high temperatures.
Classification of Electric Heating Methods:
Electric heating can be broadly classified as:
(i) Power frequency heating and
(ii) High frequency heating.
Power frequency heating can be further classified as:
(i) Resistance heating and
(ii) Arc heating.
Resistance heating can be further classified as:
(i) Direct resistance heating,
(ii) Indirect resistance heating and
(iii) Infrared or radiant heating.
Similarly arc heating can be further classified as:
(i) Direct arc heating and
(ii) Indirect arc heating.
High frequency heating can be classified into:
(i) Induction heating and
(ii) Dielectric heating.
Induction heating can further be classified as:
(i) Direct induction heating and
(ii) Indirect induction heating.
1. Direct Resistance Heating:
Electric current is made to pass through the body to be heated. This principle of heating is employed in resistance welding and electrode boiler for heating water.
2. Indirect Resistance Heating:
Electric current is made to pass through a wire or other high resistance material forming a heating element; heat so developed is transferred from the heating element to the body by the agency of radiation or convection. Normally this method is used in immersion heaters, resistance ovens, domestic and commercial cooking and heat treatment of metals.
3. Infrared or Radiant Heating:
Heat energy from an incandescent lamp is focused upon the body to be heated up in the form of electromagnetic radiations. This is employed to dry the wet paints on an object.
4. Arc Heating:
The arc drawn between two electrodes develops high temperature (about 3,000-3,500°C) depending upon the electrode material.
The electric arc may be used in the following different ways:
(i) By striking the arc between the charge and the electrode or electrodes. In this method the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnaces.
(ii) By striking the arc between the two electrodes. In this method the heat is transferred to the charge by radiation. The furnaces operating on this principle are known as indirect arc furnaces.
(iii) By striking an arc between an electrode and the two metallic pieces to be joined, as in arc welding.
5. Direct Induction Heating:
In this method of heating the currents are induced by electromagnetic action in the body to be heated. The induced currents when flowing through the resistance of the body to be heated develop the heat and thus raise the temperature. In induction furnace heat is used to melt the charge and eddy current heaters used for heat treatment of metals are other forms of direct induction heating.
6. Indirect Induction Heating:
In this method of electric heating the eddy currents are induced in the heating element by electromagnetic action. Eddy currents set up in the heating element produce the heat which is transferred to the body to be heated up, by radiation and convection. Certain types of induction ovens used for heat treatment of metals operate on this principle.
7. Dielectric Heating:
In this method of electric heating use of dielectric losses is made to heat the non-metallic materials. Non-metallic material to be heated is placed between two metal electrodes across which a high voltage having high frequency is applied, the heat is developed owing to the dielectric losses taking place.
High Frequency Power Supply Sources for Electric Heating:
The power supply for coreless induction furnace is usually obtained from ordinary supply system and its frequency is converted to higher value either by means of motor-generator set with salient pole alternator (suitable for frequencies up to about 1,000 Hz and for any output required) or by means of motor-generator set with inductor alternator (suitable for frequencies up to about 10,000 Hz and output of 1 tonne) or by means of valve oscillators (suitable for very small furnaces which necessitates frequencies up to 1 million Hz).
The oscillator which is employed in induction heating differs from the oscillator used in radio transmitters. The induction heaters are rugged, compact, and portable and are almost automatic in operation They are to be handled by unskilled operators and in comparatively dirty atmosphere. They are also subjected to overload and vibration. Hence while designing an oscillator for high frequency current supply for induction heating the above factors should be given a proper consideration.
The roundabout value of frequency used for induction heating is 400 kHz, the circuit being used is shown in Fig. 5.17.
The ac supply is stepped up by the transformer initially, the stepped up voltage is rectified by using a bridge rectifier circuit, and the rectified voltage is applied to the oscillator to obtain high frequency currents. The capacitances and inductances (including that of work piece) decide the frequency of supply to work piece.
The vacuum tube oscillators are less efficient as compared to SCR, the voltage drop across the SCR being very small (of the order of 1 V). The operating efficiency of SCR is about 90%. The SCRs are fired by pulses of gate current produced by a UJT. The circuit is shown in Fig. 5.18. The frequency of supply to work piece depends upon the value of R and C. The smaller the product of R and C, higher is the frequency across the work piece.
Sometimes spark gap oscillators are also employed to provide high frequency supply. The basic principle of operation of a spark gap converter is the alternate charging and discharging of a capacitor.
The transformer step ups the voltage, say to about 500 V, the tank condenser gets a voltage build-up till the spark gap breaks down and flashover takes place. The value of inductance and capacitance in the discharge circuit decides the frequency of current through the gap. Frequencies up to 1 MHz can be obtained.
Choice of Frequency for Electric Heating:
The selection of frequency for heating purposes is an important factor. Although, the selection of frequency has a great bearing on the work to be heated and the method of heating to be employed (induction heating or dielectric heating). Power frequency (50 Hz) furnaces can be of 1 MW capacity whereas medium frequency (500 Hz to 1,000 Hz) furnaces have capacity of 500 kW and high frequency (100 kHz to 2 MHz) furnaces have capacity from 200 kW to 500 kW.
i. Induction Heating:
For making a choice of frequency for induction heating the following factors are to be considered:
a. The thickness of surfaces to be heated. Higher the frequency, thinner the surface will get heated.
b. The time of continuous heating. Longer the duration, deeper the penetration of heat owing to conduction.
b. The temperature to be obtained, and duration of heating. If higher temperatures are to be obtained in shorter time, higher capacities of generator should be selected.
ii. Dielectric Heating:
The rate of heat produced in dielectric heating is given by the expression P = 2π/CV2 cos φ watts where V is the supply voltage, f is the supply frequency and C is the capacitance of the condenser formed, which depends upon the relative permittivity ϵr of the material.
Thus the rate of heat production α V2 x f x cos φ x ϵr
The voltage across any specimen is limited by its thickness, i.e., potential gradient, breakdown voltage, insulation and safety consideration. The voltages used for dielectric heating are usually from 600 V to 3,000 V, however voltage up to 20,000 volts is also sometimes used.
Alternatively higher rate of production of heat can be obtained by employing higher frequency but this is also limited due to the following considerations:
(a) Necessity of incorporating a special matching circuit at higher frequencies due to the fact that maximum power output is obtained from the supply oscillator only when the impedance of the oscillator match with that of load.
(b) Possibility of existing of a standing wave across the surface of electrodes having wavelength approximately equal to or more than one quarter of the frequency wavelength at any time.
It causes a variation in the voltage across the electrodes which results into uneven heating. This is avoided by imposing limits on the length of the electrode at a particular frequency.
(c) At very high frequency it is difficult for tuning inductance to resonate with the charge capacitance.
(d) At higher frequencies uniform voltage distribution cannot be obtained.
(e) The higher frequencies may disturb nearby radio station services due to radiation. So special care should be taken that there is no radiation etc.
Advantages of Electric Heating:
The main advantages of electric heating over other systems of heating (coal, oil or gas heating) are given below:
Electric heating is economical as electric furnaces are cheaper in initial cost as well as maintenance cost. It does not require any attention so there is a considerable saving in labour cost over other systems of heating. Electrical energy is also very cheap as it is being produced on large scale.
Since dust and ash are completely eliminated in electric heating system, it is a clean system and cleaning costs are rendered to a minimum.
3. Absence of Flue Gases:
Since no flue gas is produced in this system, there is no risk of atmosphere or objects being heated and operation is, therefore, hygienic.
4. Ease of Control:
Simple, accurate and reliable temperature control can be had either by hand operated or by fully automatic switches. Desired temperature or temperature cycle can be had accurately in electric heating system, which is not convenient in other heating systems.
5. Automatic Protection:
Automatic protection against over-currents or overheating can be provided through suitable switchgears in the electric heating system.
6. Upper Limit of Temperature:
There is no upper limit to the temperature obtainable except the ability of the material to withstand heat.
7. Special Heating Requirements:
Certain requirements of heating such as uniform heating of material or heating of one particular portion of the job without effecting others, heating of non-conducting materials, heating with no oxidation, can be met only in electric heating system.
8. High Efficiency of Utilization:
The overall efficiency of electric heating is comparatively higher, since in this system of heating the source can be brought directly to the point where heat is required, thereby reducing the losses. Further there is no product of combustion in which heat losses are involved.
It has been practically ascertained that 75 to 100% of heat produced by electric heating can be successfully utilised whereas in cases of gas, solid fuel and oil heating the efficiencies are 60%, 30% and 60% respectively.
9. Better Working Conditions:
Electric heating system produces no irritating noise and also the radiating losses are low. Thus working with electric furnaces is convenient and cool.
Electric heating is quite safe and responds quickly.