This article presents a comprehensive guide about heating elements. Read further to learn more about:
A heating element is a component or material designed to transform electrical energy into heat through a phenomenon known as Joule heating. This process happens when an electric current travels through a conductor, with electrons or charge carriers interacting with the atoms or ions within the conductor. These interactions create friction on an atomic scale, which manifests as heat. The heat produced is quantified by Joule's first law (or the Joule-Lenz law), which states:
P = IV or P = I²R
These formulas indicate that the heat generated is influenced by the current, voltage, and the conductor's resistance. The resistance of the conductor is a key consideration in the design of heating elements.
Joule heating happens across all conductive materials, with the exception of superconductors, albeit to varying extents. Materials with lower electrical resistance emit less heat as charge carriers pass through them with ease, whereas materials with higher resistance generate greater heat. Superconductors are unique in that they allow current to pass without producing heat. Generally, the heat generated in conductors is seen as energy loss. For instance, when powering equipment, some electrical energy results in unwanted heating, known as copper loss, which is not beneficial for practical work.
Electrical heating elements are close to 100% efficient in converting electrical energy into thermal energy since nearly all the energy supplied is turned into heat. These elements might also emit energy as light and radiation. However, this level of efficiency is particularly true for resistors. Minor losses occur due to the intrinsic capacitance and inductance of materials, transforming electrical energy into electric and magnetic fields, respectively. Moreover, the overall efficiency can decrease due to heat escaping into the surroundings from the heater or the process fluid. Therefore, to ensure optimal use of generated heat, the heating system should be insulated effectively.
Nearly all conductors generate heat when an electric current passes through them. However, not all conductors are suitable for use as heating elements. The ideal heating element material must possess a specific combination of electrical, mechanical, and chemical properties. The following are key properties essential for effective heating element design:
The material properties discussed earlier narrow down the selection to a few key materials. The most commonly used materials include nickel-chromium alloy, iron-chromium-aluminum alloy, molybdenum silicide, and silicon carbide, all of which are suitable for high-temperature applications due to their resistance to oxidation. Another category includes graphite, molybdenum, tungsten, and tantalum. These materials are prone to oxidation at elevated temperatures and are therefore typically used only in vacuum environments or furnaces where the atmosphere is free from oxygen.
Nickel-chromium alloys are among the most commonly used materials for heating elements, prized for their ductility, high resistivity, and resistance to oxidation even at elevated temperatures. Typically, these alloys are composed of 80% nickel and 20% chromium, though other compositions may be available from different manufacturers. Due to their high ductility, nickel-chromium alloys are often formed into wires for use as heating elements, such as in hot-wire foam cutters. These wires can reach maximum heating temperatures of approximately 1,100 to 1,200°C.
Often known by the trademark Kanthal, ferritic iron-chromium-aluminum alloys typically consist of 20 to 24% chromium, 4 to 6% aluminum, with iron making up the remainder. These alloys are favored for their pliability and lower density compared to nickel-chromium alloys. They can also achieve higher temperatures, reaching around 1,300 to 1,400°C. Iron-chromium-aluminum alloys tend to be less expensive due to the lower price volatility of iron compared to nickel. However, they have reduced strength at elevated temperatures compared to nickel-chromium alloys.
Iron-chromium-aluminum alloys can be enhanced through powder metallurgy. In this process, the alloy ingot is ground into a powder, which is then compressed into a die and sintered or hot-pressed in a controlled atmosphere. This process creates a metallurgical bond without fully melting the powder. Dispersoids are added to the mix to improve the material’s mechanical properties, increasing its strength and toughness at higher temperatures.
Molybdenum disilicide (MoSiâ‚‚) is a refractory cermet, a ceramic-metallic composite, used predominantly as a heating element material. It is well-suited for high-temperature furnaces due to its high melting point and excellent corrosion resistance. MoSiâ‚‚ heating elements are manufactured through various energy-intensive methods, including mechanical alloying, combustion synthesis, shock synthesis, and hot isostatic pressing.
MoSiâ‚‚ heaters can reach temperatures up to 1,900°C. However, they have some drawbacks, including low toughness at ambient temperatures and susceptibility to high-temperature creep. At room temperature, MoSiâ‚‚ is brittle and requires careful handling. Toughness improves significantly at its brittle-ductile transition temperature of around 1,000°C. Nevertheless, a higher creep rate can cause deformation at high temperatures. The most common MoSiâ‚‚ element design is the 2-shank hairpin type, which is often suspended from the furnace roof and positioned around the furnace walls. Other configurations are available and are frequently combined with ceramic insulation formers to provide both mechanical support and thermal insulation in a single package.
Silicon carbide heating elements are made from a ceramic produced by recrystallizing or reaction bonding SiC grains at temperatures above 2,100°C. These elements are typically porous (8-25%) allowing the furnace atmosphere to interact through the material. Over time, the heating element may undergo gradual oxidation, which increases its electrical resistance in a process known as "aging." To maintain consistent power output, a variable voltage supply is often used to incrementally raise the voltage as the element ages. This aging process eventually limits the heating element's lifespan and performance.
Silicon carbide is ideal for high-temperature applications due to several key properties. It lacks a liquid phase, which means it does not sag or deform due to creep at high temperatures, and no internal supports are necessary within the furnace. SiC sublimates directly at around 2,700°C., making it suitable for extreme conditions. Additionally, it is chemically inert to most process fluids, has high rigidity, and a low coefficient of thermal expansion. Silicon carbide heaters can achieve temperatures of approximately 1,600 to 1,700°C.
Graphite, a mineral with a hexagonal atomic structure composed of carbon, is an excellent conductor of both heat and electricity. It can generate heat at temperatures exceeding 2,000°C. At high temperatures, graphite's electrical resistance increases significantly. It also withstands thermal shocks well and remains resilient without becoming brittle during rapid heating and cooling cycles. However, graphite has a notable drawback: it tends to oxidize at around 500°C, leading to material degradation with prolonged exposure. Consequently, graphite heating elements are predominantly used in vacuum furnaces, where oxygen and other gases are removed from the heating chamber to prevent oxidation of both the molten metals and the heating element itself.
Refractory metals such as tungsten and molybdenum exhibit properties similar to graphite when used as heating elements. Among these metals, tungsten can operate at the highest temperatures but is also the most expensive. Molybdenum, while less costly and more commonly used, remains more expensive than graphite. Like graphite, these metals must be used in vacuum conditions because they have a strong affinity for oxygen, hydrogen, and nitrogen. They begin to oxidize at temperatures between 300 to 500°C.
Typical PTC (Positive Temperature Coefficient) materials include rubber and ceramics. PTC rubber is commonly made from polydimethylsiloxane (PDMS) infused with carbon nanoparticles. PTC heaters are distinguished by their ability to regulate current flow through an increase in electrical resistance as temperature rises. This characteristic makes them safe and suitable for applications such as clothing. Initially, the heater draws full power and heats up due to its resistivity. As the temperature increases, the material’s resistance grows, eventually acting as an insulator. This self-regulation occurs without the need for an external feedback loop.
A heating system includes more than just the heating element. It also comprises terminations, leads, insulation, packing, sheath, and seals. Heaters come in various forms and configurations to meet specific application needs. Below are some of the most common types of heaters and their applications.
While heating elements generally operate on the same principle, their performance and service life are influenced by several factors. Key specifications for heaters include power or wattage, maximum operating temperature, type of process fluid, sheath material, and power supply (voltage and frequency). Additionally, factors such as fluid flow and temperature control must also be considered to optimize performance.
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