A heating element is a device used for conversion of electric energy into heat, consisting of a heating resistor and accessories. Heat is generated by the passage of electric current through a resistor through a process known as Joule heating. Heating elements are used in household appliances, industrial equipment, and scientific instruments enabling them to perform tasks such as cooking, warming, or maintaining specific temperatures higher than the ambient. Heating elements may be used to transfer heat via conduction, convection, or radiation. They are different from devices that generate heat from electrical energy via the Peltier effect, and have no dependence on the direction of electrical current.
Principles of operation
Resistance & resistivity
Materials used in heating elements have a relatively high electrical resistivity, which is a measure of the material's ability to resist electric current. The electrical resistance that some amount of element material will have is defined by Pouillet's law as R = ρ ℓ A {\displaystyle R=\rho {\frac {\ell }{A}}} where
R {\displaystyle R} is the electrical resistance of a uniform specimen of the material
ρ {\displaystyle \rho } is the resistivity of the material
ℓ {\displaystyle \ell } is the length of the specimen
A {\displaystyle A} is the cross-sectional area of the specimen The resistance per wire length (Ω/m) of a heating element material is defined in ASTM and DIN standards. In ASTM, wires greater than 0.127 mm in diameter are specified to be held within a tolerance of ±5% Ω/m and for thinner wires ±8% Ω/m.
Power density Heating element performance is often quantified by characterizing the power density of the element. Power density is defined as the output power, P, from a heating element divided by the heated surface area, A, of the element. In mathematical terms it is given as:
Φ = P / A {\displaystyle \Phi =P/A}
Power density is a measure of heat flux (denoted Φ) and is most often expressed in watts per square millimeter or watts per square inch. Heating elements with low power density tend to be more expensive but have longer life than heating elements with high power density. In the United States, power density is often referred to as 'watt density.' It is also sometimes referred to as 'wire surface load.'
Components
Resistance heater
Wire
Resistance wires are very long and slender resistors that have a circular cross-section. Like conductive wire, the diameter of resistance wire is often measured with a gauge system, such as American Wire Gauge (AWG). It is possible to use bare exposed wire, for example wrapped around a ceramic core or wrapped around the edge of a mica star-like support structure such as in hair dryers, possibly in coiled or jagged form for higher power density.
Ribbon Resistance ribbon heating elements are made by flattening round resistance wire, giving them a rectangular cross-section with rounded corners. Generally ribbon widths are between 0.3 and 4 mm. If a ribbon is wider than that, it is cut out from a broader strip and may instead be called resistance strip. Compared to wire, ribbon can be bent with a tighter radius and can produce heat faster and at a lower cost due to its higher surface area to volume ratio. On the other hand, ribbon life is often shorter than wire life and the price per unit mass of ribbon is generally higher. In many applications such as in toasters, resistance ribbon is wound around a mica card or on one of its sides.
Coil Resistance coil is a resistance wire that has a coiled shape. Coils are wound very tightly and then relax to up to 10 times their original length in use. Coils are classified by their diameter and the pitch, or number of coils per unit length.
Insulator Heating element insulators serve to electrically and thermally insulate the resistance heater from the environment and foreign objects. Generally for elements that operate higher than 600 °C, ceramic insulators are used. Aluminum oxide, silicon dioxide, and magnesium oxide are compounds commonly used in ceramic heating element insulators. For lower temperatures a wider range of materials are used.
Leads Electrical leads serve to connect a heating element to a power source. They generally are made of conductive materials such as copper that do not have as high of a resistance to oxidation as the active resistance material. The leads can be insulated with heat resistant, braided fiberglass.
Terminals Heating element terminals serve to isolate the active resistance material from the leads. Terminals are designed to have a lower resistance than the active material by having with a lower resistivity and/or a larger diameter. They may also have a lower oxidation resistance than the active material.
Types Heating elements are generally classified in one of three frameworks: suspended, embedded, or supported.
In a suspended design, a resistance heater is attached at two or more points to normally either a ceramic or mica insulator. Suspended resistance heaters can transfer heat via convection and radiation, but not conduction as they are surrounded by air. In an embedded heating element, the resistance heater is encased in the insulator. In this framework the heater can only transfer heat via conduction to the insulator. Supported heating elements are a combination of the suspended and embedded frameworks. In these assemblies, the resistance heater can transfer heat via conduction, convection, or radiation.
Tubes (Calrods)
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