Heating Elements

Chapter 1: What is a Heating Element?

A heating element is a material or device that converts electrical energy into heat through a process called Joule heating. Joule heating occurs when an electric current flows through a conductor, causing electrons or charge carriers to collide with the conductor's atoms or ions. These collisions generate friction at the atomic level, which is experienced as heat. The amount of heat produced by this process is described by Joule's first law (or Joule-Lenz law), which can be expressed as:

P = IV or P =I²R

According to these equations, the heat generated depends on the current, voltage, and the resistance of the conductor. When designing heating elements, the resistance of the conductor is a crucial factor.

Joule heating occurs in all conductive materials to varying degrees, except in superconductors. In general, materials with low electrical resistance produce less heat because charge carriers flow more easily through them, whereas materials with high resistance generate more heat. Superconductors allow electrical current to flow without producing any heat. Typically, heat generated by conductors is considered an energy loss. Electrical energy used to power equipment often results in unwanted heating, known as copper loss, which does not contribute to useful work.

Electrical heating elements are nearly 100% efficient in converting electrical energy into heat, as virtually all the supplied energy is transformed into thermal energy. These elements may also emit energy in the form of light and radiation. However, this efficiency is ideal only for resistors. Small losses can occur due to the material’s inherent capacitance and inductance, which convert electrical energy into electric and magnetic fields, respectively. Additionally, overall system efficiency can be affected by heat dissipation into the external environment from the process fluid or the heater itself. Therefore, to maximize the utilization of generated heat, the heating system must be well-insulated.

Chapter 2: What are the properties of heating elements?

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:

• Resistivity: To produce heat, the heating element must have enough electrical resistance. However, the resistance must not be so high that it becomes an insulator. Electrical resistance is equal to the resistivity multiplied by the length of the conductor divided by the conductor cross-section. For a given cross-section, to have a shorter conductor, a material with a high resistivity is used.

  

  
  
• Oxidation Resistance: Heat generally accelerates oxidation in both metals and ceramics. Oxidation can consume the heating element which can decrease its capacity or compromise its structure. This limits the life of the heating element. For metallic heating elements, alloying with an oxide former, helps in resisting oxidation by forming a passive layer. For ceramic heating elements, protective oxidation resistant scales of SiO2 or Al2O3 are most common. Heating element types not suitable for use in oxidizing environments, such as graphite, are most often used in vacuum furnaces, or furnaces containing non-oxidizing atmosphere gases, such as H2, N2, Ar or He, where the heating chamber is evacuated of air.
• Temperature Coefficient of Resistance: Note that the resistivity of the material changes with temperature. In most conductors, as temperature increases, resistance also increases. This phenomenon has a more significant effect on some materials than others. A higher temperature coefficient of resistance is mostly used for heat-sensing applications. For heat generation, it is usually better to have a lower value. Though in some instances where the change in resistance can be accurately predicted, a sharp increase in resistance is desirable to deliver more power. To make the system adjust for the changing resistivity, control or feedback systems are employed.

  

  
  
• Mechanical Properties: Rigid heating elements can deform when used at high temperatures. As the material approaches its molten or recrystallization phase, the material can weaken and deform more easily as compared to its state at room temperature. A good heating element can maintain its form even at high temperatures. On a different note, ductility is also a desired mechanical property, especially for metallic heating elements. Ductility enables the material to be drawn into wires and formed into shape without compromising its tensile strength.
• Melting Point: Aside from the temperature where oxidation significantly increases, the material‘s melting point also limits its operating temperature. Ceramics generally have higher melting points than metallic heaters.

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Chapter 3: What materials are used for heating elements?

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 (Ni-Cr) Alloy

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.

Iron-Chromium-Aluminum (Fe-Cr-Al) Alloy

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 (MoSi2)

Molybdenum disilicide (MoSi₂) 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. MoSi₂ heating elements are manufactured through various energy-intensive methods, including mechanical alloying, combustion synthesis, shock synthesis, and hot isostatic pressing.

MoSi₂ 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, MoSi₂ 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 MoSi₂ 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 (SiC)

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

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.

Molybdenum, Tungsten, and Tantalum

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.

Positive Thermal Coefficient (PTC) Materials

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.

Chapter 4: What are the different types of heating elements?

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.

• Air Process Heaters: As the name suggests, this type of heater is used to heat up flowing air. Air process heaters are basically a heated tube or pipe wherein one end is for introducing cold air while the other end is the hot air exit. Along the walls of the pipe are coils of heating elements insulated by ceramics and non-conducting gaskets. These are typically used in high-flow, low-pressure applications. Applications for air process heaters are heat shrinking, laminating, adhesive activation or curing, drying, baking, etc.

  

  
  
• Cartridge Heaters: In this type of heater, the resistance wire is coiled around a ceramic core, typically made of compacted magnesium oxide. Rectangular configurations are also available where the resistance wire coils pass three to five times along the length of the cartridge. The resistance wire or the heating element is situated near the walls of the sheathing material for maximum heat transfer. To protect the internals, the sheath is usually made of corrosion resistant materials like stainless steel. The leads are usually flexible with both of their terminations located on one end of the cartridge. Cartridge heaters are used in die or mold heating, fluid heating (immersion heaters), and surface heating.

  

  
  
• Tubular Heaters: Tubular heaters‘ internals is the same as that of cartridge heaters. Its main difference from cartridge heaters is that the lead terminals are on the opposite ends of the tube. The whole tubular construction can be bent into different forms to suit the heat distribution required by the space or surface to be heated. Also, these heaters can feature fins that are mechanically bonded onto the sheath surface to aid in an effective heat transfer. Tubular heaters are as versatile as cartridge heaters and are used in similar applications.

  

  
  
• Band Heaters: These heaters are designed to wrap around cylindrical metal surfaces or containers such as pipes, barrels, drums, extruders, and so forth. They feature bolted locking tabs to securely clamp onto the surface of the container. Inside the band, the heater is a thin resistance wire or ribbon typically insulated by a mica layer. The sheathing is made of stainless steel or brass. Another advantage of using band heaters is that it indirectly heats the fluid inside the vessel. This means the heater is not subjected to any chemical attack from the process fluid. Possible ignition is also prevented when used for oil and lubricant service.

  

  
  
• Strip Heaters: This type of heater is flat and rectangular in form and is bolted on to the surface to be heated. Its internals are similar to a band heater. However, the insulating material, aside from mica, can be ceramics such as magnesium oxide and fiberglass. The typical use of strip heaters is surface heating of dies, molds, platens, tanks, ducts, etc. Aside from surface heating, they can also be used for air or fluid heating by having finned surfaces. Finned strip heaters are seen in ovens and space heaters.

  

  
  
• Etched Foil Heaters: Etched foil heaters can also be referred to as thin-film heaters. In this type, the resistive heating material is etched and bonded onto a foil usually made of aluminum. If more flexibility and tear resistance is required, the substrate can also be made of heat-resisting synthetic rubber or thermoplastic polyurethane (TPU). In addition to its flexibility, another advantage is the tight spacing of the heating elements. This is the inherent advantage of photochemical etching. Even heat distribution with a larger heat density can be achieved in such small forms. Its applications are more specialized in comparison with the conventional wire heaters. Etched foil heaters are usually seen in medical devices, electronics and instrumentation, aerospace, and clothing. One side can be lined with an adhesive layer for easy mounting.

  

  
  
• Ceramic Heaters: These heaters use ceramics with a high melting point, high thermal stability, high-temperature strength, high relative chemical inertness, and small heat capacity. Note that these are different from ceramics used as an insulating material. Due to its good thermal conducting properties, it is used to conduct and distribute heat from the heating element. Notable ceramic heaters are silicon nitride and aluminum nitride. These are commonly used for rapid heating as seen on glow plugs and igniters. However, when subjected to quick high-temperature heating and cooling cycles, the material is prone to cracking due to fatigue caused by thermal stresses. A special type of ceramic heaters is a PTC ceramic. This type can self-regulate its power consumption which then prevents it from becoming red hot.

  

  
  
• Ceramic Fiber Heaters: In this type of heater, the ceramic fiber is used as an insulator to concentrate the heat into the surface to be heated to prevent system losses. Ceramic fiber pads have a resistance wire wound on one-side. This side is bonded on the surface to be heated which can reach up to 1,200°C.

  

  
  

Chapter 5: What factors should be considered when selecting a heater?

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.

• Watt Density: Watt density is the heat delivered of a heating element per unit area. The right watt density must be used for a specific application to fully utilize the service life of the heater. Note that for a given wattage, both high and low-density heaters will deliver the same amount of heat but at different temperatures. High-density elements can reach much higher temperatures which leads to premature burning or failure of the element. In selecting a heating element, check the manufacturer's recommended watt densities for a particular application.

  

  
  
• Temperature: The target operating temperature directly affects the watt density. There must be a balance between these two factors. In designing a process heater, the temperature is determined first which is usually a process parameter required by the system.
• Power Supply: The heating element must be able to operate with the available power supply. Check the voltage rating which is typically 120V or 240V. In selecting a target wattage, verify the amperage produced. Be careful not to exceed the power supply circuit breaker tripping point or the ratings of the power cables.
• Fluid Flow: From intuition, stagnant fluids are easier to heat with a controlled temperature than flowing fluids. Air or other gases do not generally absorb heat quickly because of their low density. A solution would be to slow down the flow, but most of the time, this is not an option. Thus, heaters with large surface areas are required. Finned surfaces and long wire coils (low-density heating elements) are the usual features of air heaters.

  

  
  
• Temperature Sensor Location: Conventional heaters come with a temperature sensor and a controller. In most applications, the sensing device only measures the temperature of the process fluid. Note that this does not usually represent the actual heating element temperature. Before installing the heater and the temperature sensing device, check if its location is appropriate for the heater unit. If the sensor is too far, the temperature reflected may be much lower due to heat dissipation and low heat transfer rate. This can lead to very high temperatures that can burn the heating element.

  

  
  
• Corrosion: Corrosion can be from the process fluid or the external environment. The sheathing material protects the heating element, leads, and insulators from chemical attack. Thus, the sheath must be able to maintain its strength in high temperatures while being resistant to corrosion. Widely used sheathing materials are stainless steel, brass, copper, and other special alloys such as Monel and Incoloy. Moreover, the integrity of the sheath and terminal sealing must be sufficient for the application. For demanding applications, hermetic sealing is the best option in providing complete protection from the process fluid.

  

  
  

Conclusion

• A heating element is a material or device that directly converts electrical energy into heat or thermal energy through a principle known as Joule heating.
• The most important heating element characteristics are sufficient resistivity, high oxidation resistance, low-temperature coefficient of resistance, high toughness, and high melting point.
• Widely used heating elements are nickel-chromium alloy, iron-chromium-aluminum alloy, molybdenum disilicide, and silicon carbide. These are followed by graphite and other refractory metals which generally have higher oxidation rates.
• Aside from the heating element, a heater consists of the terminations, leads, insulation, packing, sheath, and seals. These heaters have various forms and configurations to suit a particular application.
• Typical heater ordering specifications are the power or wattage, maximum operating temperature, type of process fluid, sheath material, and power supply (voltage and frequency).

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