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 where
is the electrical resistance of a uniform specimen of the material is the resistivity of the material is the length of the specimen 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:
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)
Tubular or sheathed elements (also referred to by their brand name, Calrods) normally comprise a fine coil of resistance wire surrounded by an electrical insulator and a metallic tube-shaped sheath or casing. Insulation is typically a magnesium oxide powder and the sheath is normally constructed of a copper or steel alloy. To keep moisture out of the hygroscopic insulator, the ends are equipped with beads of insulating material such as ceramic or silicone rubber, or a combination of both. The tube is drawn through a die to compress the powder and maximize heat transmission. These can be a straight rod (as in toaster ovens) or bent to a shape to span an area to be heated (such as in electric stoves, ovens, and coffee makers).
Screen-printed elements Screen-printed metal–ceramic tracks deposited on ceramic-insulated metal (generally steel) plates have found widespread application as elements in kettles and other domestic appliances since the mid-1990s.
Radiative elements Radiative heating elements (heat lamps) are high-powered incandescent lamps that run at less than maximum power to radiate mostly infrared instead of visible light. These are usually found in radiant space heaters and food warmers, taking either a long, tubular form or an R40 reflector-lamp form. The reflector lamp style is often tinted red to minimize the visible light produced; the tubular form comes in different formats:
Gold-coated HeLeN quartz infrared heat lamps as originally patented and manufactured by Philips. A gold dichroic film is deposited on the inside that reduces the visible light and allows most of the short and medium wave infrared through. These tubular quartz lamps are designed for services other than illumination. Ruby-coatedSame function as the gold-coated lamps, but at a fraction of the cost. The visible glare is much higher than the gold variant. ClearNo coating and mainly used in production processes.
These heaters can also be halogen lamps. They comprise a transparent or translucent quartz exterior, with halogen gas and a tungsten wire. The high temperature of the wire causes it to emit high amounts of infrared radiation which is used as heat for cooking in some household toaster ovens and microwave ovens. They are also used in industrial processes where they are used for radiant heating in ovens such as those used to dry paint in car manufacturing, whereby only the surface being dried is heated and not the air surrounding the workpiece. Their low thermal inertial makes it possible to set the heaters to a low heat output and only increase it when a workpiece is dried, saving energy.
Removable ceramic core elements Removable ceramic core elements use a coiled resistance heating alloy wire threaded through one or more cylindrical ceramic segments to make a required length (related to output), with or without a center rod. Inserted into a metal sheath or tube sealed at one end, this type of element allows replacement or repair without breaking into the process involved, usually fluid heating under pressure.
Etched foil elements Etched foil elements are generally made from the same alloys as resistance wire elements, but are produced with a subtractive photo-etching process that starts with a continuous sheet of metal foil and ends with a complex resistance pattern. These elements are commonly found in precision heating applications like medical diagnostics and aerospace.
Polymer PTC heating elements
Resistive heaters can be made of conducting PTC rubber materials where the resistivity increases exponentially with increasing temperature. Such a heater will produce high power when it is cold, and rapidly heat itself to a constant temperature. Due to the exponentially increasing resistivity, the heater can never heat itself to warmer than this temperature. Above this temperature, the rubber acts as an electrical insulator. The temperature can be chosen during the production of the rubber. Typical temperatures are between .
It is a point-wise self-regulating and self-limiting heater. Self-regulating means that every point of the heater independently keeps a constant temperature without the need of regulating electronics. Self-limiting means that the heater can never exceed a certain temperature in any point and requires no overheat protection.
Thick-film heaters
Thick-film heaters are a type of resistive heater that can be printed on a thin substrate. Thick-film heaters exhibit various advantages over the conventional metal-sheathed resistance elements. In general, thick-film elements are characterized by their low-profile form factor, improved temperature uniformity, quick thermal response due to low thermal mass, high energy density, and wide range of voltage compatibility. Typically, thick-film heaters are printed on flat substrates, as well as on tubes in different heater patterns. These heaters can attain power densities of as high as 100 W/cm2 depending on the heat transfer conditions. The thick-film heater patterns are highly customizable based on the sheet resistance of the printed resistor paste.
These heaters can be printed on a variety of substrates including metal, ceramic, glass, and polymer using metal- or alloy-loaded thick-film pastes. The most common substrates used to print thick-film heaters are aluminum 6061-T6, stainless steel, and muscovite or phlogopite mica sheets. The applications and operational characteristics of these heaters vary widely based on the chosen substrate materials. This is primarily attributed to the thermal characteristics of the substrates.
There are several conventional applications of thick-film heaters. They can be used in griddles, waffle irons, stove-top electric heating, humidifiers, tea kettles, heat sealing devices, water heaters, clothes irons and steamers, hair straighteners, boilers, heated beds of 3D printers, thermal print heads, glue guns, laboratory heating equipment, clothes dryers, baseboard heaters, warming trays, heat exchangers, deicing and defogging devices for car windshields, side mirrors, refrigerator defrosting, etc.