Band Heaters
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A band heater is a heating device that clamps onto objects to provide external heat using radiant and conductive heating. The different mounting methods of band heaters makes it possible to secure them tightly and...
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This article presents a comprehensive guide about heating elements. Read further to learn more about:
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. Joule heating is the phenomenon where a conductor generates heat due to the flow of electric current. As the electric current flows through the material, electrons or other charge carriers collide with the ions or atoms of the conductor creating friction at an atomic scale. This friction then manifests as heat. Joule‘s first law (Joule-Lenz law) is used to describe the amount of heat produced from the flow of electricity in a conductor. This is expressed as,
P = IV or P =I²R
From these equations, the amount of heat generated depends upon the current and the voltage or the conductor resistance. In the design of heating elements, the resistance is the more important factor.
Joule heating is evident in all conducting materials in varying intensities, except for a special type of material known as superconductors. Generally, for electrically conductive materials, less heat is generated since the charge carriers can easily flow through; while for materials with high electric resistance, more heat is generated. Superconductors, on the other hand, allow the flow of electricity but do not produce any heat. Usually, heat from conductors is classified as energy loss. Electrical energy used to drive powered equipment generates unnecessary heating in the form of copper loss which ultimately does not produce any useful work.
Electrical heating elements, in a sense, are almost 100% efficient since all supplied energy is converted into its intended form. Heating elements may not only conduct heat but also transfer energy through light and radiation as well. However, this is only true for ideal resistors. Small losses can be derived from the inherent capacitance and inductance of the material which converts the electrical energy into electric and magnetic fields, respectively. Considering the whole heater system, losses are from the dissipation of heat into the external environment from the process fluid or from the heater itself. Thus, the system must be isolated to utilize all the heat generated.
Almost all conductors are capable of generating heat when an electric current is passed through. However, not all conductors are suited to be made into heating elements. The right combination of electrical, mechanical, and chemical properties is required. Enumerated below are the properties significant to heating element design.
The material properties mentioned in the previous chapter narrows down the selection into a few materials. The most common materials are nickel-chromium alloy, iron-chromium-aluminum alloy, molybdenum silicide, and silicon carbide. These materials can operate at high temperatures due to their resistance to high-temperature oxidation. Another group is composed of graphite, molybdenum, tungsten, and tantalum. These materials oxidize at high temperatures and are only used in a vacuum environment or in furnaces where the atmosphere is devoid of oxygen.
This type is one of the most widely used materials for heating elements due to its ductility, high resistivity, and oxidation resistance even at high temperatures. The most common composition of nickel-chromium alloys is 80/20 or 80% nickel, 20% chromium. Other compositions are available depending on the manufacturer. Due to its high ductility, it is usually drawn into wires when used as a heating element. A common application that exhibits this property is on hot-wire foam cutters. Maximum heating temperatures achieved by nickel-chromium wires are around 1,100 to 1,200°C.
This type is popularly known under the trademark Kanthal. Kanthal ferritic iron-chromium-aluminum alloys typically have a chemical composition of 20 to 24% chromium, 4-6% aluminum, and iron as the balance. Iron-chromium-aluminum heaters are used for their pliability and lower gravity compared to Ni-Cr. They can also generate higher temperatures than nickel-chromium wire, which is around 1,300 to 1,400°C. By having iron as the base metal, this alloy has less price volatility than Ni-Cr, which is composed mostly of nickel. The downside of using iron-chromium-aluminum alloys is their decreased strength at higher temperatures.
Iron-chromium-aluminum alloys may be made better by a process known as powder metallurgy. In this process, the alloy ingot is turned into powder and compressed into a die. It is then sintered or hot-pressed (hot isostatic pressing) in a temperature-controlled atmosphere to create a metallurgical bond without completely melting the powdered metal. Dispersoids are added into the alloy mix to reinforce the mechanical properties of the material to impart additional strength and toughness at higher temperatures.
Molybdenum disilicide is a refractory cermet (ceramic-metallic composite) primarily used as a heating element material. This is a desirable material for high-temperature furnaces due to its high melting point and good corrosion resistance. Molybdenum silicide heating elements are produced by various energy-intensive processes such as mechanical alloying, combustion synthesis, shock synthesis, and hot isostatic pressing.
MoSi₂ type heaters can achieve heating temperatures up to 1,900°C. Downsides for using molybdenum silicide are its low toughness at ambient conditions and high-temperature creep. Its brittleness at room temperature necessitates very careful handling. Increased toughness is achieved at its brittle-ductile transition temperature around 1,000°C. A higher creep rate, on the other hand, causes the heating element to easily deform at high temperatures. The most common type of MoSi2 element is a 2-shank hairpin design, that is usually suspended through the roof of a furnace, and located around the furnace walls. Other shapes are available often combined with ceramic insulation formers that provide both mechanical support and thermal insulation as an integrated package.
This is a type of ceramic produced by recrystallization or reaction bonding of SiC grains at temperatures above 2,100°C. Silicon carbide heating elements are porous bodies (typically 8-25%) where the furnace atmosphere can react through the cross-section of the material. The whole heating element may be gradually oxidized which leads to an increase in the electrical resistance properties of the elements over time (commonly referred to as "aging") A variable voltage supply is usually required to maintain the desired power output from the elements by gradually increasing the voltage to the elements during their lifetime. This aging eventually limits the life and performance of the heating element.
Silicon carbide has many properties that make it suitable for making heating elements for very high service temperatures. This ceramic has no liquid phase. Meaning that elements will not sag or deform due to creep at any temperature, and no supports are required inside the furnace. SiC directly sublimates at temperatures around 2,700°C. Moreover, it is chemically inert from most process fluids and has high rigidity and a low coefficient of thermal expansion. Silicon carbide heaters can achieve around 1,600 to 1,700°C heating temperatures.
Graphite is a mineral composed of carbon wherein the atoms are arranged in a hexagonal structure. This mineral, also its synthetic form, is a good thermal and electric conductor. Graphite can generate heat at temperatures greater than 2,000°C. At high temperatures, its electric resistance significantly increases. Moreover, it can withstand thermal shocks and does not become brittle even after rapid cycles of heating and cooling. The main disadvantage of using graphite is its tendency to oxidize at temperatures around 500°C. Continued use at this range eventually results in the consumption of the material. Graphite heating elements are typically used in vacuum furnaces where oxygen and other gases are evacuated from the heating chamber. The absence of oxygen not only prevents oxidation of the molten metals, but also the heating element itself.
These are refractory metals with similar properties as graphite when used as heating elements. Among these metals, tungsten has the highest operating temperature but also more expensive. In terms of viability, molybdenum is more popular since it is the least expensive but is still more expensive than graphite. Like graphite, they can only be used in vacuum conditions since they have a strong bonding affinity with oxygen and even hydrogen and nitrogen. They begin to oxidize at temperatures around 300 to 500°C.
Typical PTC material is rubber but can be ceramics as well. PTC rubber is made of polydimethylsiloxane (PDMS) with carbon nanoparticles. PTC heaters have a unique property in which the heater maintains or limits the current flow by having an increased electrical resistance as the temperature increases. This makes the material safe and suitable for use in clothing. Initially, the heater draws full power and heats up due to its resistivity. The material‘s resistance increases with the rising heat and then acts as an insulator. This is achieved without the need for any feedback loop.
The heating element alone does not comprise the entire heating system. 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. Enumerated below are the most common heaters and their applications.
Heating elements technically operate the same way but several factors determine its performance and service life. Typical heater ordering specifications are the power or wattage, maximum operating temperature, type of process fluid, sheath material, and power supply (voltage and frequency). However, there are additional factors that need to be considered such as fluid flow and temperature control.
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