Types of Heating Elements
A heating element converts electrical energy into heat through the process of Joule heating. Joule heating occurs when an electric current passing through the electrical element encounters resistance, resulting in the heating of the electrical element. This process is independent of the direction of current passing through it.
The different types of heating elements can be classified based on the material used to make them which each gives them their repective characteristics
- Basic Heating Elements Types:
- Metal Heating Elements
- Ceramic and Semiconductor Heating Elements
- Thick Film Heating Elements
- Polymer PTC Heating Elements
- Composite Heating Elements
- Combination Heating Element Systems
Metal Heating Elements
Resistance Wire Heating Elements
Metallic resistance heating elements are usually a coil, ribbon (straight or corrugated), or strip of wire that gives off heat much like a lamp filament. They are used in common heating devices like floor heating, roof heating, toasters, hair dryers, industrial furnaces, pathway heating, dryers, etc. The most common classes of materials used include:
- Nickel-Chrome Alloy: Most resistance wire heating elements use nichrome 80/20 (80% nickel, 20% chromium) in wire, ribbon, or strip form. NiCr 80/20 is an ideal material because it has relatively high resistance and forms an adherent layer of chromium oxide when heated for the first time. Material beneath this layer does not oxidize thus preventing the wire from breaking or burning out.
- FeCrAl Alloy: FeCrAl Alloys or Iron Chromium Aluminium Alloys are ferromagnetic alloys whose electrical resistance properties are similar to those of Nickel-Chromium alloys making them suitable for electrical heating applications. Although the absence of nickel makes them cheaper than Nickel-Chromium alloys, it also makes them more prone to corrosion. These FeCrAl range of electric heating elements have the widest market.
- CuNi Alloy: CuNi Alloy or Copper-Nickel alloys are characterized by low electrical resistivity and low temperature coefficient of resistance. They provide good resistance against oxidation and chemical corrosion and are used for low temperature heating.
- Etched Foils: Etched foil heating elements are made from the same alloys as resistance wire elements but are produced using a subtractive photo-etching process. This process starts with a continuous sheet of metal foil and ends with a complex resistance pattern for the heating element. These heating elements are commonly found in precision heating applications like medical diagnostics and aerospace.
Ceramic and Semiconductor Heating Elements
- Molybdenum Disilicide Heating Elements: Molybdenum disilicide (MoSi2) an intermetallic compound, a silicide of molybdenum, is a refractory ceramic primarily used in heating elements. It has a moderate density, a melting point of 2030 °C and is electrically conductive. At high temperatures it forms a passivation layer of silicon dioxide, protecting it from further oxidation. The applications of these type of heating elements include heat treatment furnaces, glass manufacturing, ceramic sintering and semiconductor furnaces.
- Silicon Carbide Heating Elements: Silicon carbide heating elements offer increased operating temperatures compared to metallic heaters. Silicon carbide heating elements are used today in heat treatment of metals, the melting of glass and non-ferrous metal, production of ceramics, float glass production, electronics components manufacturing, pilot lights, gas heater igniters, etc.
- PTC Ceramic Heating Elements: PTC ceramic materials are named such for their positive thermal coefficient of resistance. A positive temperature coefficient of heating materials, often barium titanate and lead titanate composites, means that their resistance increases upon heating. While most ceramics have a negative temprature coefficient, these materials, have a highly nonlinear thermal response. Above a composition-dependent threshold temperature their resistance increases rapidly when heated. This behavior causes the material to act as its own thermostat because current passes when it is cool and does not when it is hot.
- Quartz Halogen Elements: Quartz halogen heaters are also used to provide radiant heating and cooling. These emitters heat up and cool down within seconds making them particularly suitable for systems requiring short cycle times. Heat output is also very high making these heaters useful in high heat demand or in fast moving processes such as paper, processes etc.
Thick Film Heating Elements
Thick film heating elements are resistive heating elements which can be printed on a thin substrate. Thick film heating elements have an advantages over conventional metal-sheathed resistance elements. Thick film heating elements are characterized by their low profile form factor, improved temperature uniformity, quick thermal response because of low thermal mass, low energy consumption, high power density and a wide range of voltage compatibility. Typically, thick film heating elements are printed on flat substrates and on tubes in different heater patterns. 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, polymer using metal or alloy thick film pastes. The most common substrates used to print thick film heaters are aluminum, stainless steel and muscovite or phlogopite mica sheets. The operational characteristics and uses of these heaters widely vary based on what substrate materials are chosen. This is primarily due to the thermal characteristics of the heater substrate.
There are several conventional applications of thick film heaters. For most applications, the thermal performance and temperature distribution are the two key design parameters. In order to avoid any hotspots and maintain uniform temperature distribution, the circuit design can be optimized by changing the power density of the resistor circuit. An optimized heater design helps control the heater output and modulate temperatures. They can be used in waffle irons, thermal print heads, water heaters, stove-top electric heating, cloth steamers, tea kettles, humidifiers, boilers, heated beds, heat sealing devices, cloth irons, hair straighteners, 3D printers, clothes dryers, glue guns, laboratory equipment, defogging devices, car mirrors, deicing devices, warming trays, heat exchangers, etc.
Thick film heaters can largely be characterized under two subcategories- negative temperature coefficient (NTC) or positive temperature coefficient (PTC) based on the effect of temperature increase on the element's resistance.
- The NTC or negative temperature coefficient type heaters are characterized by a decrease in resistance as the heater temperature increases, giving higher power output at higher temperatures for a given input voltage. NTC type heaters generally require a thermostat or a thermocouple to control heater temprature runaway. NTC heaters are used where a quick ramp-up of heater temperature to a predetermined set-point is required.
- The PTC or positive temperature coefficient type heaters behave in the opposite manner with an increase in resistance and decrease in heater power at elevated temperatures. This characteristic of PTC heaters make them self regulating as their output power saturates at a fixed temperature.
Polymer PTC Heating Elements
Resistive heaters can be made of conducting PTC rubber materials whose resistivity increases exponentially with increasing temperature. Such resistive heaters produce high power they are cold and rapidly heat up to a constant temperature. Due to this exponentially increasing resistivity on heating, a PTC rubber resistive heater can never heat itself to be warmer than this temperature. Above this temperature, the rubber acts as an electrical insulator. This temperature can be chosen during the production of the rubber, typical temperatures being between 0 °C and 80 °C.
Polymer PTC heating elements are point-wise self-regulating heaters and self-limiting heaters. 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.
Composite heating elements
- Tubular Sheathed Heating Elements: Tubular or sheathed elements normally comprise of a fine coil of nickel chrome resistance heating alloy wire that is located inside a metallic tube of copper or stainless steel alloys such as NiCrFe Alloy) and insulated by magnesium oxide powder. To keep moisture out of the hygroscopic insulator, the ends of the element 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 the heat transmission. These heating elements can be in the shape of a straight rod as in toaster ovens or bent to a shape to span an area to be heated such as in electric ovens, electric stoves and automatic coffee makers.
- Screen-printed Heating Elements: These heating elements are screen-printed metal–ceramic tracks deposited on ceramic insulated metal (generally steel) plates. Screen-printed heating elements have found widespread application as elements in electric kettles and other domestic appliances since the mid-1990s.
- Radiative Heating Elements: Radiative heating elements or heat lamps are a high-powered incandescent lamp usually 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 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 - 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.
- 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 Heating Elements: Removable ceramic core heating elements use a coiled resistance heating alloy wire threaded through one or more cylindrical ceramic segments to make a required length which is related to the heater output, with or without a centre rod. Inserted into a metal sheath or tube sealed at one end, this type of heating element allows replacement or repair without breaking into the process involved, usually fluid heating under pressure.
Combination Heating Element Systems
Heating elements for high-temperature furnaces are often made of exotic materials, including platinum, tungsten disilicide, molybdenum disilicide, molybdenum used in vacuum furnaces and silicon carbide. Silicon carbide igniters are commonly used in gas ovens.
Laser heaters are also being used for achieving high temperatures.Article courtecy of Wikipedia - a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.