Heating Elements And Alloys
A heating element converts electricity into heat through the process of Joule heating. Electrical current running through the element encounters resistance, resulting in heating of the element.
Many heating equipment utilise electrical energy to produce the heat. In these equipment the heating element is used to convert the electrical energy in the form of heat. The working of heating elements is based on heating effect of electric current.
Heating Element Materials
Most heating elements use Nickel-chrome(NiCr) wire or ribbon as the conductor. Nickel-chrome(NiCr) is an ideal material as it is inexpensive, has relatively high resistance, and does not break down or oxidize in air in its useful temperature range.
The performance and life of heating element depend on properties of the material used for heating element. The required properties in material used for heating elements:
- High melting point.
- Free from oxidation in open atmosphere.
- High tensile strength.
- Sufficient ductility to draw the metal or alloy in the form of wire.
- High resistivity.
- Low temperature coefficient of resistance.
Following material are used for manufacturing heating element
Commercial Heating Elements
There are five kinds of commercial heating elements:
- Bare NiCr wire or ribbon: Either straight or coiled, usually found in toasters and hair dryers.
- Screen printed metal/ceramic tracks deposited on ceramic insulated metal (generally steel) plates. These elements have found widespread application for kettles and other domestic applicances since the mid 1990s.
- Calrod (sealed element): a fine coil of Nickel-chrome wire in a ceramic binder, sealed inside a tough metal shell. These can be a straight rod (as in toaster ovens) or curved to fit in a smaller space (such as in electric stoves, ovens, and coffee makers).
- Heat lamp: 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 R40 reflector-lamp form. The reflector lamp style is often tinted red to minimize the visible light produced; the tubular form is always clear.
- PTC ceramic: This material is named for its Positive Thermal Coefficient of resistivity. Most ceramics have a negative coefficient; most metals, a positive one. While metals do become slightly more resistive at higher temperatures, this class of ceramics (often barium titanate and lead titanate composites) has a highly nonlinear thermal response, so that it becomes extremely resistive above a composition-dependent threshold temperature. This behavior causes the material to act as its own thermostat, since current passes when it is cool, and does not when it is hot. Thin films of this material are used in automotive rear-window defrost heaters, and honeycomb-shaped elements are used in more expensive hair dryers and space heaters.
High-Temperature Heating Elements
Heating elements for high-temperature furnaces are often made of exotic materials, including platinum, molybdenum disilicide, and silicon carbide. Silicon carbide igniters are common in gas ovens.
For many applications, heating elements may be made from wire or strip in Nickel-Chromium or Iron-Chromium-Aluminium alloys. These can generate furnace temperatures up to 1280°C (2336°F). For furnace temperatures up to 1600°C (2912°F) or in lower temperatures where high power inputs are required, Silicon Carbide heating elements are often ideal. For furnace temperatures up to 1800°C (3272°F) and in some other applications, Molybdenum Disilicide heating elements can provide the answer.
- Metal alloys in Wire/Strip/Rod: For furnace temperatures up to 1250°C, nickel-chromium alloy (NiCr) or iron-chromium-aluminium alloys are often ideal and low-cost solutions. JLC Electomet in India keeps a wide range of these materials on hand to be able to produce these elements in-house.
- Silicon Carbide: For furnace temperatures up to 1600°C, silicon carbide elements can often provide rapid heating with very long service life. To obtain the best performance from SiC elements, the power supply must be properly designed for the application. Some means of power adjustment is usually required to cater for the resistance changes that will occur with this type of element.
- Molybdenum Disilicide: For furnace temperatures reaching up to 1700 or 1800°C, molybdenum disilicide elements provide a reliable heat source. Processes at much lower temperatures may find them advantageous in some circumstances.