Different Types of Electric Heating:
Different types of 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:
- 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.
- 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.
- 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 types of electric 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 types 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 types 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.