Heating resistor
A heating resistor is a resistor that has the purpose of converting electrical energy into thermal energy ( heat ).
construction
A heating resistor as a component of resistance heating is in its simplest form a metal wire, also called a heating conductor. For heating using Joule heat, see also electric heat .
Even poorly conductive metals usually result in a long required wire length in order not to overheat with common voltages ( e.g. 230 V mains voltage), which is why the wires are often twisted or laid in a meander shape.
If semiconductors such as silicon carbide or graphite are used as heating conductors, the heating elements are more compact and have the shape of rods or tubes. See also Globar .
If they cannot be exposed, heating coils are inserted into insulating moldings or z. B. fixed in metal pipes or plates by means of sand or cement. Insulating bodies or pipes wrapped with wires or tapes can be found e.g. B. in a soldering iron or in a toaster .
The shape and embedding also depends on the type of heat transfer ( heat radiation , heat conduction or convection ).
Depending on the application, heating cartridges (cylindrical metal sleeve as a housing with a heating coil inside), heating tapes, heating jackets, heating mats or heating registers (heating wires give off heat directly to the air flowing past) are manufactured. Electric radiators distribute the heat from resistance heating by means of thermal conduction over a large area or e.g. B. by means of convection to a large volume ( oil radiator ).
principle
The heat is generated by current flowing through a conductive material and heating up by the Joule heat. In principle, any current-carrying metal wire can be used for this purpose, but materials and alloys are selected that can reach a particularly high temperature without melting or oxidizing. The specific resistance should be as high as possible so that shorter heating conductors can be used. A positive temperature coefficient that is as low as possible (metal alloys based on iron, nickel, chromium, cobalt) or even a negative temperature coefficient of the specific electrical resistance (semiconductors such as SiC or graphite) is also useful. This avoids local overheating.
According to Ohm's law , the heat output is dependent on the resistance value (dependent on the specific resistance of the material, its cross-section and length) and the electrical current and thus dependent on the voltage applied . The current flow must not be so high that the melting temperature of the conductor material or the limit temperature of the insulating support is reached.
Several resistors can be connected in parallel or in series for power control, see e.g. B. Seven-step circuit for hotplates.
materials
The material used is usually special heating conductor alloys or resistance alloys (DIN 17471) made of austenitic CrFeNi alloys or ferritic CrFeAl alloys, which have an approximately constant electrical resistance over a wide temperature range and have a particularly high melting point (e.g. tungsten for incandescent filaments for Absence of oxygen) or are resistant to oxidation in atmospheric oxygen (e.g. Kanthal and nickel-iron alloys). The high temperature resistance of these materials is based on the formation of a protective oxide skin on the surface.
Semiconducting materials such as silicon carbide , molybdenum disilicide and graphite have a resistance that decreases when the temperature rises and can therefore often not be operated in an uncontrolled manner.
Heating coils are both cantilevered used and wound around a heat-resistant core or embedded in electrically insulating materials. Insulating materials are for example porcelain , glass, quartz glass , mica , fireclay , steatite (electroporcelain) or rock wool.
Application examples
- Electric cookers for food preparation
- Electric water heater for water heating
- Electric soldering irons have a cylindrical heating cartridge with a coiled heating coil inside or a self-regulating heating element (PTC thermistor)
- Immersion heaters and heating elements in kettles as well as in baking and grilling devices consist of a pipe and a coiled heating conductor inside that is insulated from the pipe
- electric underfloor heating
- Foil sealing device heating elements have a flat, insulated heating band
- Fan heaters , tumble dryers , heat guns and hair dryers have heating coils or heating registers in the air flow of a fan, sometimes also freely tensioned heating wires, but often also metallic encapsulated heating resistors.
- Xerox copiers and laser printers use heating rods similar to a halogen lamp or thick-film heating resistors for fixing
- Thermal printers in fax machines or cash registers have a line of individually controllable small resistance elements
- Electric furnaces for high temperatures have silicon carbide heating elements
- Boats heated by current flow for steaming are often made of tungsten sheet metal
In switch cabinets, switchgear and other devices for outdoor use, but also in electric boilers, heating resistors are combined with a thermostat ( temperature switch ). Self-regulating heating resistors (consist of or contain a PTC thermistor ) are often used for frost protection or in control cabinets .
Inductive heating is also based on resistance heating . Here, the material to be heated forms the heating resistor in the form of a short-circuit coil (eddy current).
The following processes also use electricity heat when electricity flows in the goods to be heated:
- In the production of aluminum using the Hall-Héroult process by melt-flow electrolysis , part of the high current strength (up to 330,000 amperes ) is required to maintain the melting temperature of the aluminum oxide - cryolite melt by means of resistance heating
- Smelting reduction furnaces for melting products for the metallurgical industry .
Efficiency
In principle, a heating resistor has an efficiency of 100 percent, but under certain circumstances only a small part of the heat generated reaches the target object. The efficiency therefore depends heavily on the design used and the intended use. An immersion heater or kettle only generates unused heat for its own heating or to heat the pot, it has an efficiency of approx. 90 percent. A hotplate in an electric stove , the heating coil of which is embedded in a metal and ceramic construction, has a much greater heat capacity , so that a not inconsiderable part of the energy used is not used to heat the food. Another part of the heat output is transferred to the stove surface through heat conduction.
Above all, however, this consideration does not take into account the efficiency of generating electrical energy from primary energy , although it is far below 50% in thermal power plants. However, the efficiency of heating devices only takes into account the conversion of electrical energy into used heat. Despite the high efficiency of electrical heating, it is a very inefficient and expensive type of heat generation. It is therefore generally limited to cases in which no other energy source can be used.
literature
Reference books
- Hans Fischer: Materials in electrical engineering. 2nd Edition. Hanser, Munich / Vienna 1982, ISBN 3-446-13553-7
- Günter Springer: Expertise in electrical engineering. 18th edition. Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9
- Adolf Senner: Electrical engineering. 4th edition. Europe teaching material, Wuppertal 1965, DNB 451091205 .
Technical brochures and articles
- Dieter Brockers: Lexicon resistances . Gino Ese Electrotechnical Factory, Bonn 1998/2006.
Individual evidence
- ↑ Flexible heating jackets. Accessed in 2012 .
- ↑ Carl Kramer, Alfred Mühlbauer: Practical Guide Thermoprocess Technology Basics . Vulkan-Verlag, 2002, ISBN 978-3-8027-2922-5 , p. 288 ( limited preview in Google Book search).
- ↑ Volkmar M. Schmidt: Elektrochemische Verfahrenstechnik , ISBN 978-3-527-62362-4 .
- ↑ Scriptum Elektrochemie (PDF; 1.3 MB) of the University of Siegen, page 184.