Electric Heating: Methods of Electric Heating | Electrical Engineering

In this article we will discuss about the methods of electric heating. The methods are:- 1. Power Frequency Heating 2. High Frequency Heating.

Method # 1. Power Frequency Heating:

I. Resistance Heating:

As we know that whenever current passes through some resistance, power loss takes place which appears in the form of heat, e.g Resistance oven.

Properties of Resistance Heating Elements:

1. High Resistivity:

It should have high specific resistance so that small quantity of wire is required to produce certain amount of heat.

2. Low Temperature Co-Efficient of Resistance:

The heating element material should possess low temperature coefficient of resistance, so that resistance may not vary with the change in temperature

3. High Melting Point:

The melting point of the material used should be very high; so that high temperature can be obtained.

4. Free from Oxidation:

It should be free from oxidation to ensure a long life of heating element.

Most commonly material used for heating element is either alloy of nickel and chromium or alloy of nickel, chromium and iron.

Types of Resistance Heating:

a. Direct Resistance Heating:

In this type of heating, electric current passes through the charge itself. This current produces I²R losses in the form of heat within the body itself.

This principle is made use in resistance welding and in heating water by means of electrode boiler. In case of electrode boiler, the electrodes are lowered into the tank filled with water. The current flows through electrodes into water and the water gets heated by I²R losses. For temperatures up to 100°C, mild steel electrodes are used.

Figure 3.1 shows a simple sketch of direct resistance heating furnace.

AC supply is used having voltage varies from 2 to 20 volts and currents upto 2500 amps. Automatic stirring action is produced in the charge to be heated and no external method of stirring is required to get uniform heating.

b. Indirect Resistance Heating:

In indirect resistance heating the current does not flow through the body to be heated but it flows through the resistance elements which get heated up. The heat is then transferred from heating element to the charge mainly by radiation or convection.

Figure 3.2 shows a simple sketch of indirect resistance heating oven. In this method heating does not pass through the charge, hence natural stirring action is not there. So some external stirring apparatus are employed to get uniform heating. The appliances that work on this principle include immersion rod, electric kettles, electric iron and cooking oven etc.

II. Electric Arc Heating:

The arc furnaces depend upon the principle of heat generated by electric arc.

The electric arc heating may be used in the following different ways:

(i) By Strucking the Arc between the Charge and Electrode:

In this method the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnace. These furnaces are used for production and refining of various grade of steel.

(ii) By Strucking the Arc between Two Electrodes:

In this method the heat is transferred to the charge by radiations. The furnaces operating on this principle are known as indirect arc furnaces. These types of furnaces are used for melting of non-ferrous metals such as brass, copper and zinc.

There are two types of arc furnaces namely:

(i) Direct Arc furnace

(ii) Indirect Arc furnace

Method # 2. High Frequency Heating:

I. Induction Heating:

The induction heating works on the transformer principle. Moreover it is also known as eddy current heating! The currents are induced by the principle of electromagnetic induction. The induction heating may be low frequency as in case of core type induction furnace or high frequency as the case with coreless induction furnaces.

Types of Induction Heating:

(i) Direct Induction Heating

(ii) Indirect Induction Heating

(i) Direct Induction Heating:

In direct induction heating the eddy currents are produced within the material itself that is to be heated. The examples of direct induction heating are the high frequency eddy current heating used for case hardening or tempering of various machine parts, annealing of steel strip and soldering. The core type induction furnace used for melting non-ferrous metals such as copper, zinc, brass. Coreless induction furnace used for preparing various high grade steels also work on same principle.

(ii) Indirect Induction Heating:

The example of indirect induction heating is the indirect induction oven which is in direct competition with resistance oven and is preferred over it due to its fine temperature control. It is used for the same purpose as the resistance oven.

Moreover in the indirect induction heating method the eddy current are induced in the heating elements by electromagnetic induction which produces heat in heating elements. The heat thus produced is transferred to the body to be heated by radiation.

II. Dielectric Heating:

When non-metallic parts such as wood, plastics, bones ceramics are subjected to an alternating electrostatic field, dielectric loss occurs. These losses are used in dielectric heating which appears in the form of heat. The material to be heated is placed as a slab between two metallic electrodes across which high frequency voltage is applied.

To ensure sufficient loss and to give an adequate amount of heating, frequencies between 10 to 30 mega cycles per second must be used and the voltage needed may be as high as 20 KV. The necessary high-frequency supply voltage is obtained from a valve oscillator.

The current drawn by the capacitor, when an a.c supply voltage is applied across its two plates, does not lead the supply voltage by 90° exactly. Due to this component of current, heat is always produced in dielectric material placed in between the two plates of the capacitor.

The electric energy dissipated in the form of heat energy in the dielectric material is known as dielectric loss. The dielectric loss is directly proportional to the frequency of ac supply.

This method of heating is also employed for drying of textiles, manufacture of plywood, paper etc. The overall efficiency in case of dielectric heating is about 50%.

In insulators or non-conducting materials, the amount of heat produced by dielectric heating can be calculated as follows:

The material to be heated may be considered imperfect dielectric of a condenser and may be, therefore represented as a capacitance placed in parallel with a resistance as shown in Fig. 3.20 (a). The phasor diagram of the circuit is shown in Fig. 3.20 (b). If Vis the supply voltage, f is the supply frequency in Hz, cos ϕ is the power factor of the load and C is the capacitance of the condenser in farads.

where ε₀ = Permittivity of vacuum and its value is 8.854 x 10^–12 F/m

ε _r = Relative permittivity or dielectric constant

A = Surface area of electrodes in m²

d = distance between two electrodes in meter.

Now current through the capacitor,

Current drawn from supply,

where C is in farads and V is in volts

Advantages of Dielectric Heating:

1. Since the heat is produced throughout the whole mass of material, we get uniform heating. By conventional method of heating, it is not possible to achieve this.

2. Short time is required to complete the process as compared to other method.

3. Materials heated by this method are non-conducting, so by other methods heat cannot be conducted to inside so easily.

Disadvantages of Dielectric Heating:

1. Only those materials can be heated which have high dielectric loss.

2. The cost of equipment required for dielectric heating is so high that it is employed only where other methods are impracticable.

Applications of Dielectric Heating:

1. It is used in drying tobacco, paper, wood, gluing and bonding of woods

2. Welding of PVC

3. Sterilization of medical supplies.

4. For producing artificial fibers, heating of bones and tissues etc.

5. Food processing.

Example:

A slab of insulating material 150 cm² in area and 1 cm thick to be heated by dielectric heating. The power required is 400 W at 30 MHz. Material has permittivity of 5 and power factor 0.05. Determine voltage necessary. Absolute permittivity = 8.854 x 10^–12 F/m.

Solution:

The capacitance of the capacitor formed by insulating material,  

III. Infra-Red Heating and Its Applications:

Infra-red heating is suitable for low and medium temperatures. In this method of heating, heating elements consist of tungsten filament lamps of rating 250 and 1000 Watts. The lamps are operated at 115 V. The filament of the lamps are operated at 2300°C instead of 3000°C giving greater proportion of infra-red radiations and a longer life reflectors are also used with lamps to direct the whole of the heat emitted to the charge.

Plant sizes range from a single lamp to chambers containing several hundred KW of lamps. With this method, temperature between 200°C to 300°C can be obtained. For obtaining best results, the infra-red lamps should be located at a distance of 25-30 cm from the object to be heated. It is also called Radiant Heating.

Advantages:

(1) Rapid heating.

(2) Compactness of heating unit.

(3) More flexibility.

(4) More safe than other type of heating.

(1) Drying of painted surface.

(2) Drying of radio-cabinets and wood furniture.  

(3) Drying of foundry moulds.

(4) Pre-heating of plastics prior to moulding.  

(5) Drying of pottery, paper textiles etc. where moisture contents is not large.

(6) Drying of enamel on enamelled copper wire.

Microwave Heating:

Microwave technology uses electromagnetic waves that pass through material and cause its molecules to oscillate, generating heat. In conventional heating, the material’s surface heats first and then the heat moves inward. Microwave heating generates heat within the material and heat the entire volume at about the same rate.

Industrial microwave furnaces have traditionally used fixed-frequency electromagnetic waves that can cause uneven heat distribution. New technology varies the frequency, eliminating hot and cold spots. Microwave technology can be combined with conventional heating and drying units and is easily automated. The microwave heating is frequently used in microwave ovens for efficiently and uniformly heating and drying. Example of microwave heating is Micro oven.

Advantages:

1. Energy required as compared to conventional heating is less 10 to 30% for fuel fired processes

2. Rapid, even heating

3. It requires 20-30% less floor space than conventional units.

4. Instant on and off

5. Low drying temperatures

6. Additional cost savings comes from better production speed, waste reduction and product quality.

Disadvantages:

1. It is costly process

2. More chances of failure

3. Lack of technology awareness

4. Risk of accidents

5. Unhealthy for human being. 

Uses:

1. Food processing

2. Rapid wood drying

3. Paper making

4. Polymer and epoxy drying and textile drying.