What Are the Advantages of Hot Elements？

Most labs use at least one type of heating device, such as ovens, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, hot-air guns and microwave ovens. Steam-heated devices are generally preferred whenever temperatures of 100o C or less are required because they do not present shock or spark risks and can be left unattended with assurance that their temperature will never exceed 100o C. Ensure the supply of water for steam generation is sufficient prior to leaving the reaction for any extended period of time.

General Precautions

When working with heating devices, consider the following: 

• The actual heating element in any laboratory heating device should be enclosed in such a fashion as to prevent a laboratory worker or any metallic conductor from accidentally touching the wire carrying the electric current. 
• If a heating device becomes so worn or damaged that its heating element is exposed, repair the device before it is used again or discard of the device. 
• Use a variable autotransformer on a laboratory heating device to control the input voltage by supplying some fraction of the total line voltage, typically 110 V.
• Locate the external cases of all variable autotransformers where water and other chemicals cannot be spilled onto them and where they will not be exposed to flammable liquids or vapors. 

Fail-safe devices can prevent fires or explosions that may arise if the temperature of a reaction increases significantly because of a change in line voltage, the accidental loss of reaction solvent or loss of cooling. Some devices will turn off the electric power if the temperature of the heating device exceeds some preset limit or if the flow of cooling water through a condenser is stopped owing to the loss of water pressure or loosening of the water supply hose to a condenser.

Ovens

Electrically heated ovens are commonly used in the laboratory to remove water or other solvents from chemical samples and to dry laboratory glassware. Never use laboratory ovens for human food preparation.

• Laboratory ovens are constructed such that their heating elements and their temperature controls are physically separated from their interior atmospheres.
• Laboratory ovens rarely have a provision for preventing the discharge of the substances volatilized in them. Connecting the oven vent directly to an exhaust system can reduce the possibility of substances escaping into the lab or an explosive concentration developing within the oven. 
• Do not use ovens to dry any chemical sample that might pose a hazard because of acute or chronic toxicity unless special precautions have been taken to ensure continuous venting of the atmosphere inside the oven. 
• To avoid explosion, rinse glassware with distilled water after rinsing with organic solvents before being dried in an oven.
• Do not dry glassware contianing organic compounds in an unvented oven.
• Bimetallic strip thermometers are preferred for monitoring oven temperatures. Do not mount mercury thermometers through holes in the top of ovens so that the bulb hangs into the oven. If a mercury thermometer is broken in an oven of any type, turn off and close the oven immediately.  Keep it closed until cool. Remove all mercury from the cold oven with the use of appropriate cleaning equipment and procedures in order to avoid mercury exposure.

Hot Plates

Laboratory hot plates are normally used for heating solutions to 100o C or above when inherently safer steam baths cannot be used. Ensure any newly purchased hot plates are designed in a way that avoids electrical sparks. Older hot plates pose an electrical spark hazard arising from either the on-off switch located on the hot plate, the bimetallic thermostat used to regulate the temperature or both.

In addition to the spark hazard, old and corroded bimetallic thermostats in these devices can eventually fuse shut and deliver full, continuous current to a hot plate. 

• Do not store volatile flammable materials near a hot plate 
• Limit use of older hot plates for flammable materials. 
• Check for corrosion of thermostats. Corroded bimetallic thermostats can be repaired or reconfigured to avoid spark hazards. Contact EHS for more info. 

Heating Mantles

Heating mantles are commonly used for heating round-bottomed flasks, reaction kettles and related reaction vessels. These mantles enclose a heating element in a series of layers of fiberglass cloth. As long as the fiberglass coating is not worn or broken, and as long as no water or other chemicals are spilled into the mantle, heating mantles pose no shock hazard.

• Always use a heating mantle with a variable autotransformer to control the input voltage. Never plug them directly into a 110-V line.
• Be careful not to exceed the input voltage recommended by the mantle manufacturer. Higher voltages will cause it to overheat, melt the fiberglass insulation and expose the bare heating element. 
• If the heating mantle has an outer metal case that provides physical protection against damage to the fiberglass, it is good practice to ground the outer metal case to protect against an electric shock if the heating element inside the mantle shorts against the metal case. 
• Some older equipment might have asbestos insulation rather than fiberglass. Contact EHS to replace the insulation and for proper disposal of the asbestos. 

Oil, Salt and Sand Baths

Electrically heated oil baths are often used to heat small or irregularly shaped vessels or when a stable heat source that can be maintained at a constant temperature is desired. For temperatures below 200 °C, a saturated paraffin oil is often used; for temperatures up to 300 °C, a silicone oil should be used. Care must be taken with hot oil baths not to generate smoke or have the oil burst into flames from overheating.  Molten salt baths, like hot oil baths, offer the advantages of good heat transfer, but have a higher operating range (e.g., 200 to 425oC) and may have a high thermal stability (e.g., 540oC).There are several precautions to take when working with these types of heating devices:

• When using oil, salt, or sand baths, do not spill water or volatile substances into the baths. Such an accident can splatter hot material over a wide area and cause serious injuries.
• Take care with hot oil baths not to generate smoke or have the oil burst into flames from overheating.
• Always monitor oil baths by using a thermometer or other thermal sensing devices to ensure that its temperature does not exceed the flash point of the oil being used. 
• Fit oil baths left unattended with thermal sensing devices that will turn off the electric power if the bath overheats. 
• Mix oil baths well to ensure that there are no “hot spots” around the elements that take the surrounding oil to unacceptable temperatures. 
• Contain heated oil in a vessel that can withstand an accidental strike by a hard object. 
• Mount baths carefully on a stable horizontal support such as a laboratory jack that can be raised or lowered without danger of the bath tipping over. Iron rings are not acceptable supports for hot baths. 
• Clamp equipment high enough above a hot bath that if the reaction begins to overheat, the bath can be lowered immediately and replaced with a cooling bath without having to readjust the equipment setup. 
• Provide secondary containment in the event of a spill of hot oil. 
• Wear heat-resistant gloves when handling a hot bath. 
• The reaction container used in a molten salt bath must be able to withstand a very rapid heat-up to a temperature above the melting point of salt. 
• Take care to keep salt baths dry since they are hygroscopic, which can cause hazardous popping and splattering if the absorbed water vaporizes during heat-up.

Hot Air Baths and Tube Furnaces

Hot air baths are used in the lab as heating devices. Nitrogen is preferred for reactions involving flammable materials. Electrically heated air baths are frequently used to heat small or irregularly shaped vessels. One drawback of the hot air bath is that they have a low heat capacity. As a result, these baths normally have to be heated to 100oC or more above the target temperature. Tube furnaces are often used for high-temperature reactions under pressure. Consider the following when working with either apparatus:

• Ensure that the heating element is completely enclosed. 
• For air baths constructed of glass, wrap the vessel with heat resistant tape to contain the glass if it should break. 
• Sand baths are generally preferable to air baths. 
• For tube furnaces, carefully select glassware and metal tubes and joints to ensure they are able to withstand the pressure. 
• Follow safe practices outlined for both electrical safety and pressure and vacuum systems.

Heat Guns

Laboratory heat guns are constructed with a motor-driven fan that blows air over an electrically heated filament. They are frequently used to dry glassware or to heat the upper parts of a distillation apparatus during distillation of high-boiling materials.

Read the Heat Gun Advisory for more information on proper selection and use of a heat gun for research operations.

Microwave Ovens

Use microwave ovens specifically designed for laboratory use. Domestic microwave ovens are not appropriate. Microwave heating presents several potential hazards not commonly encountered with other heating methods: extremely rapid temperature and pressure rise, liquid superheating, arcing, and microwave leakage. Microwave ovens designed for the laboratory have built-in safety features and operation procedures to mitigate or eliminate these hazards. Microwave ovens used in the laboratory may pose several different types of hazards.

• As with most electrical apparatus, there is the risk of generating sparks that can ignite flammable vapors.
• Metals placed inside the microwave oven may produce an arc that can ignite flammable materials.
• Materials placed inside the oven may overheat and ignite.
• Sealed containers, even if loosely sealed, can build pressure upon expansion during heating, creating a risk of container rupture.

To minimize the risk of these hazards, 

• Never operate microwave ovens with doors open in order to avoid exposure to microwaves.
• Do not place wires and other objects between the sealing surface and the door on the oven’s front face. The sealing surfaces must be kept absolutely clean.
• Never use a microwave oven for both laboratory use and food preparation.
• Electrically ground the microwave. If use of an extension cord is necessary, only a three-wire cord with a rating equal to or greater than that for the oven should be used.
• Do not use metal containers and metal-containing objects (e.g., stir bars) in the microwave. They can cause arcing.
• Do not heat sealed containers in the microwave oven. Even heating a container with a loosened cap or lid poses a significant risk since microwave ovens can heat material so quickly that the lid can seat upward against the threads and containers can explode.
• Remove screw caps from containers being microwaved. If the sterility of the contents must be preserved, use cotton or foam plugs. Otherwise plug the container with kimwipes to reduce splash potential.
• Do not modify a microwave for experiemental use. 

Device that converts electricity into heat

A heating element is a device used for conversion of electric energy into heat, consisting of a heating resistor and accessories.[1] 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 than devices that generate heat from electrical energy via the Peltier effect, and have no dependence on the direction of electrical current.

Principles of operation

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Resistance & resistivity

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A piece of resistive material with electrical contacts on both ends

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}}}

• R {\displaystyle R}

• ρ {\displaystyle \rho }

• ℓ {\displaystyle \ell }

  length of the specimen

• A {\displaystyle A}

  cross-sectional area of the specimen

where

The resistance per wire length (Ω/m) of a heating element material is defined in ASTM and DIN standards.[2]: 2 [3][4] 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

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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.[5] 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.[6]

In the United States, power density is often referred to as 'watt density.' It is also sometimes referred to as 'wire surface load.'

Components

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Resistance heater

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Wire

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A coiled heating element from an electric toaster

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).[7]

Ribbon

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Resistance ribbon heating elements are made by flattening round resistance wire, giving them a rectangular cross-section with rounded corners.[8]: 54  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.[8]: 55  In many applications, resistance ribbon is wound around a mica card or on one of its sides.[8]: 57 

Coil

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Resistance coil is a resistance wire that has a coiled shape.[8]: 100  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

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Heating element insulators serve to electrically and thermally insulate the resistance heater from the environment and foreign objects.[9] Generally for elements that operate higher than 600 °C, ceramic insulators are used.[8]: 137  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

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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.[8]: 131–132 

Terminals

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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.[8]: 131–132 

Types

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Heating elements are generally classified in one of three frameworks: suspended, embedded, or supported.[8]: 164–166 

• 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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Tubular electric heater.

1. Resistance heating element
2. Electrical insulator
3. Metal casing

Tubular oven heating element

Tubular or sheathed elements (also referred to by their brand name, Calrods®[10]) 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

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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

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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 – Made famous by the patented Phillips Helen lamp. 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. Mainly for heating people. A number of manufacturers now manufacture these lamps and they improve constantly.
• Ruby-coated – Same function as the gold-coated lamps, but at a fraction of the cost. The visible glare is much higher than the gold variant.
• Clear – No coating and mainly used in production processes.

Removable ceramic core elements

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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

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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

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A flexible PTC heater made of conductive rubber

Resistive heaters can be made of conducting PTC rubber materials where the resistivity increases exponentially with increasing temperature.[11] 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 0 and 80 °C (32 and 176 °F).

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

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A thick-film heater printed on a mica sheet Thick-film heaters printed on a metal substrate

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.[12] 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.[12] 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.[13]

For most applications, the thermal performance and temperature distribution are the two key design parameters. In order to maintain a uniform temperature distribution across a substrate, the circuit design can be optimized by changing the localized power density of the resistor circuit. An optimized heater design helps to control the heating power and modulate the local temperatures across the heater substrate. In cases where there is a requirement of two or more heating zones with different power densities over a relatively small area, a thick-film heater can be designed to achieve a zonal heating pattern on a single substrate.

Thick-film heaters can largely be characterized under two subcategories – negative-temperature-coefficient (NTC) and positive-temperature-coefficient (PTC) materials – based on the effect of temperature changes on the element's resistance. NTC-type heaters are characterized by a decrease in resistance as the heater temperature increases and thus have a higher power at higher temperatures for a given input voltage. PTC heaters behave in an opposite manner with an increase of resistance and decreasing heater power at elevated temperatures. This characteristic of PTC heaters makes them self-regulating, as their power stabilizes at fixed temperatures. On the other hand, NTC-type heaters generally require a thermostat or a thermocouple in order to control the heater runaway. These heaters are used in applications which require a quick ramp-up of heater temperature to a predetermined set-point as they are usually faster-acting than PTC-type heaters.

Liquid

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An electrode boiler uses electricity flowing through streams of water to create steam. Operating voltages are typically between 240 and 600 volts, single or three-phase AC.[14]

Laser heaters

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Laser heaters are heating elements are used for achieving very high temperatures.[15]

Materials

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Materials used in heating elements are selected for a variety of mechanical, thermal, and electrical properties.[9] Due to the wide range of operating temperatures that these elements withstand, temperature dependencies of material properties are a common consideration.

Metal alloys

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Resistance heating alloys are metals that can be used for electrical heating purposes above 600 °C in air. They can be distinguished from resistance alloys which are used primarily for resistors operating below 600 °C.[8]

While the majority of atoms in these alloys correspond to the ones listed in their name, they also consist of trace elements. Trace elements play an important role in resistance alloys, as they have a substantial influence on mechanical properties such as work-ability, form stability, and oxidation life.[8] Some of these trace elements may be present in the basic raw materials, while others may be added deliberately to improve the performance of the material. The terms contaminates and enhancements are used to classify trace elements.[9] Contaminates typically have undesirable effects such as decreased life and limited temperature range. Enhancements are intentionally added by the manufacturer and may provide improvements such as increased oxide layer adhesion, greater ability to hold shape, or longer life at higher temperatures.

The most common alloys used in heating elements include:

Ni-Cr(Fe) alloys (AKA nichrome, Chromel)

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Ni-Cr(Fe) resistance heating alloys, also known as nichrome or Chromel, are described by both ASTM and DIN standards.[2][4] These standards specify the relative percentages of nickel and chromium that should be present in an alloy. In ASTM three alloys that are specified contain, amongst other trace elements:

• 80% Ni, 20% Cr
• 60% Ni, 16% Cr
• 35% Ni, 20% Cr

Nichrome 80/20 is one of the most commonly used resistance heating alloys because it has relatively high resistance and forms an adherent layer of chromium oxide when it is heated for the first time. Material beneath this layer will not oxidize, preventing the wire from breaking or burning out.

Fe-Cr-Al alloys (AKA Kanthal®)

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Fe-Cr-Al resistance heating alloys, also known as Kanthal®, are described by an ASTM standard.[3] Manufacturers may opt to use this class of alloys as opposed to Ni-Cr(Fe) alloys to avoid the typically relatively higher cost of nickel as a raw material compared to aluminum. The tradeoff is that Fe-Cr-Al alloys are more brittle and less ductile than Ni-Cr(Fe) ones, making them more delicate and prone to failure.[16]

On the other hand, the aluminum oxide layer that forms on the surface of Fe-Cr-Al alloys is more thermodynamically stable than the chromium oxide layer that tends to form on Ni-Cr(Fe), making Fe-Cr-Al better at resisting corrosion.[16] However, humidity may be more detrimental to the wire life of Fe-Cr-Al than Ni-Cr(Fe).[8]

Fe-Cr-Al alloys, like stainless steels, tend to undergo embrittlement at room temperature after being heated in the temperature range of 400 to 575 °C for an extended duration.[17]

Other alloys

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Ceramics & semiconductors

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Applications

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Toaster with red hot heating elements

Heating elements find application in a wide range of domestic, commercial, and industrial settings:

• Home Appliances: Common household appliances such as ovens, toasters, electric stoves, water heaters, and space heaters rely on heating elements to generate the necessary heat for their functions.
• Industrial Processes: In industries, heating elements are integral to processes such as metal smelting, plastic molding, and chemical reactions that require controlled temperatures.
• Scientific Instruments: Laboratories use heating elements in various equipment, including incubators, furnaces, and analytical instruments.
• Automotive Industry: Heating elements are utilized in vehicles for applications like heated seats, rear window defrosters, and engine block heaters.

Life cycle

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The life of a heating element specifies how long it is expected to last in an application. Generally heating elements in a domestic appliance will be rated for between 500 and 5000 hours of use, depending on the type of product and how it is used.[8]: 164 

A thinner wire or ribbon will always have a shorter life than a thicker one at the same temperature.[8]: 58 

Standardized life tests for resistance heating materials are described by ASTM International. Accelerated life tests for Ni-Cr(Fe) alloys[22] and Fe-Cr-Al alloys[23] intended for electrical heating are used to measure the cyclic oxidation resistance of materials.

Packaging

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Resistance wire and ribbon are most often shipped wound around spools.[8]: 58–59  Generally the thinner the wire, the smaller the spool. In some cases pail packs or rings may be used instead of spools.

Safety

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General safety requirements for heating elements used in household appliances are defined by the International Electrotechnical Commission (IEC).[24] The standard specifies limits for parameters such as insulation strength, creepage distance, and leakage current. It also provides tolerances on the rating of a heating element.

See also

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References

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