Joule heating
A heating element called an electrical device that converts thermal energy into electrical energy ( = heat), and is a special form of electrical resistance.
Construction
The heating resistor as a component of a resistance heater, in its simplest form a high-resistance wire or heating element, represents the basic form of electric heat and identifies this over other electric heaters; Systematics Main article: Electric heat.
This is called depending on the shape and design of the immersion heater ( a cylindrical metal cartridge as a housing with a coiled heating coil inside), the heating coil, the heating element, the heating tape, the heating sleeve (usually as flexible collar for example for heating of machinery and equipment in cold working environments ), the heat mat or heat register, in which the heated wire emits its heat directly to a passing stream of air or simply by the electric heater.
Principle
The heat is generated by applying a conductive material adapted to the purpose, is flowed through the high electrical resistance of the current and is thereby heated. Principle, can use any current-carrying metal wire for this purpose.
The heat capacity depends on the resistance value according to Ohm's law (depending on the resistivity of the material, the cross -section and length ) and the electric current flow and thus dependent on the applied voltage. The current flow must not be so high that the melting temperature of the conductive material or the limit temperature of the insulating is obtained.
For power control several resistors can be connected in parallel or in series, see, among others, seven clock circuit.
Execution
As the material special heating element alloys or alloys resistance (DIN 17471 ) are usually used, which have a over wide temperature ranges approximately constant electrical resistivity (eg, constantan, manganin ), have a very high melting point (eg tungsten for filaments in the absence of oxygen) or resistant to oxidation in air, oxygen (for example, Kanthal and nickel-iron alloys).
In order to obtain sufficient heat, a large length of wire is often needed, the (for example, in a kiln ) or spiral ( for example, a hot plate ) usually wound as a helix or a double helix, and depending on the use of a cylindrical coil.
And embedding the shape also depends on the nature of the heat transfer (heat radiation, conduction, or convection).
Heating coils are used both self-supporting and wrapped around a heat-resistant core or embedded in electrically insulating materials.
Application Examples
- Electric Soldering Irons have a cylindrical cartridge heater with a coiled heating coil inside or a self-regulating heating element ( PTC, PTC short )
- Immersion heaters and heating elements in oven and grill devices consist of a tube and a coiled, insulated against the tube heating element inside
- Heating elements of sealers have a flat insulated heating tape
- Heater, clothes dryer, heat guns and hair dryers have heating coils in the air stream of a fan or heater, partly free strained heating wires, but often encapsulated metallic heating resistors.
- Xerox copiers and laser printers use heating elements similar to a halogen bulb or thick-film heating elements for fixing
- Thermal printer in fax machines or cash registers have a line of individually controllable small resistor elements
- Electric furnaces have heating elements for high temperature silicon carbide
- In the production of aluminum by fused-salt electrolysis in the Hall -Heroult process, a portion of the high current (up to 330,000 amperes ) needed to maintain the melting temperature of the alumina - cryolite melt by resistance heating
- The resistance heated boats of evaporation systems often consist of tungsten sheet
In switchboards, switchgear and other equipment for outdoor use, but also in electric boilers heating elements with a thermostat (temperature switch) are combined. For freeze protection or in cabinets often self-regulating heating resistors ( consist of or contain a PTC thermistor ) is used.
Also on the resistance heating is based Inductive heating.
Efficiency
A heating resistor has in principle an efficiency of 100 percent, but it also may only a small portion of the heat generated at the target object. The efficiency strongly depends on the design used and the intended use. An immersion heater or kettle generates unused heat only to its own heating or for the heating of the pot, he has an efficiency of about 90 percent. A cooking plate in an electric cooker, the heating coil is embedded in a metal or ceramic construction, has a significantly greater heat capacity, so that a not inconsiderable part of the energy input is not used for heating of the food. Another part of the heat output also goes by heat conduction in the hearth area.
Especially remains in this approach, the efficiency in the generation of electrical energy from the primary energy considered, even though it is far less than 50% thermal power plants. However, the efficiency of heating appliances shall consider only the conversion of electrical energy into heat used. Despite the high efficiency of an electric heater so it is a very inefficient and expensive type of heat generation. It is therefore generally limited to situations where no other energy sources can be used.