Methods of electric heating

Electric heating converts electrical energy into heat. The main methods are classified as power-frequency and high-frequency heating.

                 Electric heating
          +-------------+-------------+
   Power frequency               High frequency
   |-- Resistance                |-- Induction (coreless)
   |     direct / indirect       |-- Dielectric
   |     (incl. infrared)        |-- Microwave
   |-- Arc
   |     direct / indirect
   |-- Induction (core type)

1. Resistance heating

Heat is produced by $I^2R$ loss.

• Direct: current passes through the charge itself (e.g. salt-bath furnace, electrode boiler, resistance welding).
• Indirect: current passes through a heating element (nichrome, Kanthal) and heat reaches the charge by radiation and convection. Examples: room heaters, electric irons, kettles, water heaters, cooking ranges, ovens (domestic) and resistance ovens for annealing and drying (industrial).

2. Arc heating

An electric arc between electrodes produces about 3000–3500°C.

• Direct arc: arc between electrodes and charge; current flows through the charge (steel-making arc furnace).
• Indirect arc: arc between two electrodes above the charge; heat reaches it by radiation (melting of brass, copper).
• Also used in arc welding.

3. Induction heating

Eddy currents (and hysteresis loss) are induced in a conducting charge by an alternating field.

• Core type (low frequency): Ajax-Wyatt furnace for melting brass and non-ferrous metals.
• Coreless (high frequency): steel and alloy melting, surface hardening, brazing; induction cooktops in homes.

4. Dielectric heating

Non-conducting materials between electrodes at 10–50 MHz are heated by dielectric loss, uniformly through the volume. Used for gluing plywood, welding plastics, drying textiles and food processing.

5. Infrared heating

A form of indirect resistance heating: tungsten-filament (infrared) lamps radiate heat onto a surface. Used for drying paints and varnishes, and in some domestic heaters.

6. Microwave heating

A magnetron (2.45 GHz) heats water-containing material from inside. Used in microwave ovens at home and for industrial food drying.