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This article takes an in-depth look at ceramic heaters.
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Ceramic heaters are electric heaters that utilize a positive temperature coefficient (PTC) ceramic heating element and generate heat through the principle of resistive heating. Ceramic materials possess sufficient electrical resistance and thermal conductivity to generate and conduct heat as current flows through them. They have high strength and durability. Hence, they perform well when used as a heating element. The heating element of ceramic heaters are made of pure ceramic material, but most are composite materials that consist of the encapsulation of metal and ceramic materials. The ceramic material in the latter acts as an insulator but conducts heat to the surroundings simultaneously, which minimizes the heat and energy losses associated with uninsulated resistance wire.
Ceramic heaters are used in a wide range of industrial processes such as drying, boiling, molding, and melting products. They are widely utilized in space heating. They provide a quick, safe, and clean source of heat.
This chapter will discuss the important scientific principles involved in the design and operation of ceramic heaters. We will start first by tackling resistive heating. As mentioned earlier, ceramic heaters work through the principle of resistive heating or also known as Joule or Ohmic heating. Resistive heating is the phenomenon during which heat is generated due to resistive losses as electrical current passes through a material. It is a form of transformation of electrical energy into thermal energy. This transformation is beneficial for electric heaters as resistive losses raise their efficiency. However, this effect is undesirable in some cases, like in electric power transmission and distribution and running most types of electronic devices and equipment.
Joule’s first law, or the Joule-Lenz law, mathematically shows the relationship of the thermal energy generated concerning the input electrical parameters. This law states that the heat per unit time, i.e., heating power (P), is proportional to the product of the square of the current (I) and resistance (R), which is represented by the mathematical equation: P = I2R.
The phenomenon of resistive heating can be explained by viewing what happens within the material during the flow of current at the molecular level.
When there is a difference in electric potential between two points in a conductor, an electric field is generated, accelerating the free electrons at their outermost shells to move from atom to atom, giving these electrons kinetic energy. The electrons move from a point with a higher potential to a lower one. The rate of electron flow is referred to as the current, an essential electrical parameter. Current is directly proportional to the voltage (V), the potential difference between the two points, and inversely proportional to resistance; this relationship is shown by the mathematical equation: I = V/R.
As these electrons flow to a point with lower potential, they collide with atoms, other electrons, and impurities that constitute the material, which causes the vibration of molecules. Aside from these, opposing forces are present that hinder the flow of electrons. These collisions and opposing forces create friction as the electron flows to a lower potential. To overcome the friction during the movement of electrons, the electrons have to perform work corresponding to the heat generated by the material. The heat generated by the material (or in this context, the heating element) is harnessed to raise the temperature of its surrounding bodies.
Resistance is an extrinsic property of a material that refers to the opposition to the flow of current or electrons. Being an extrinsic property, it depends on the length (l) and the cross-sectional area (A) of the material, and its value can be calculated by R = ρL/A. In this equation, ρ is the resistivity which is an intrinsic property and varies with the temperature of the material.
All materials, except for superconductors, possess a certain extent of electrical resistance. For the material to be classified as a good heating element, it must possess sufficient internal resistance. Materials with higher resistance impede the flow of current more effectively and generate more heat. However, the resistance should not be extremely high for the material to act as an insulator.
Ceramic heaters transfer heat to the surroundings by either conduction, convection, or radiation. Conductive heat transfer involves the transfer of heat between two objects in contact. Convective heat transfer involves the transfer of heat between two fluids (liquids or gasses). In convective space heaters, the air flows through the hot ceramic heating element and increases the ambient temperature. Lastly, in radiative heat transfer, thermal energy through electromagnetic radiation is emitted directly to the objects or persons nearby.
The resistivity and the resistance vary with temperature. If the resistance of the material increases with increasing temperature, the material has a positive temperature coefficient. Ceramics are semiconducting materials and have a positive temperature coefficient.
When the temperature of the ceramic heating element increases to its setpoint temperature due to the uptake of electrical current, the resistance will increase up to infinity which ceases the flow of current and the production of heat. The setpoint temperature depends on the composition of the ceramic. Hence, ceramic heaters can adjust to the ambient temperature and produce less heat for hotter environments. They provide only sufficient heat without raising the surrounding temperature excessively. Thus, ceramic heaters are self-regulating, and this characteristic is not present in metal heating elements. The self-regulating property makes them safer to operate.
The types of ceramic heaters are the following:
Cartridge heaters are tube-shaped electric heaters consisting of resistance wires (made from nichrome) wound around a ceramic core, insulated with magnesium oxide. These components are contained and sealed in a tubular metal sheath. The resistance wire is close to the outer sheath, with the gap filled with insulation.
Cartridge heaters are inserted into the built-in holes of dies, molds, and platens to heat them. A larger hole can be drilled to fit larger cartridge heaters. They can also be used in immersion heating. They generate large amounts of heat despite their small size. Cartridge heaters are used in laboratory equipment, food processing, oil heating, stamping, laminating, and molding.
Ceramic band heaters consist of a set of wound resistance wires embedded in ceramic fiber insulation, which are contained in ceramic bricks. The ceramic bricks and the components inside them sit in the inner circumference of the circular metal sheath. The metal sheath is typically made from stainless steel and aluminum; it can be coated with a suitable finishing material for better corrosion resistance and durability. The metal sheath provides mechanical stability, strength, and flexibility to the composite heating element. The insulation blanket prevents heat losses of the resistance wire by 25-30%. The heat produced by the resistance wire is transferred by conduction or radiation.
Ceramic band heaters are used to heat the contents of cylindrical tanks and vessels from the outside walls. They are clamped around the walls by a barrel nut which can be adjusted to compensate for the slight oversizing of the tank diameter. There are several clamping constructions for the ceramic band heaters to effectively cling around the outside walls of the tank. Ceramic band heaters come in one- and two-piece construction. These heaters can also provide heat to curved surfaces.
Ceramic band heaters are commonly used in heating the barrels, and thereby melting the feed resins, of plastic injection molding machines, extruders, and blow molding equipment.
Space heaters are equipment that provides heat to warm small to medium-sized enclosed areas. They can be used to supplement central heating systems in heating large spaces or facilities. There are different types of technologies and power sources used in the operation of space heaters. Space heaters can be powered by fuel or electricity.
Space heating is one of the applications of ceramic heaters. Ceramic heaters as space heaters provide a quick, clean, and efficient source of heat. They are compact and very portable; hence, they can be transported and placed almost anywhere across a room, as long as there is an electrical outlet available nearby. They are convenient to operate. With the appropriate specifications, a ceramic space heater can heat an entire enclosed space quickly in just a few minutes. Ceramic space heaters are a popular type of space heaters in homes, offices, and commercial spaces.
Ceramic strip heaters consist of a resistance wire coil encapsulated inside a ceramic core filled with magnesium oxide for maximum heat transfer. All these components are contained in a metal sheath. These heaters are thin, lightweight, and are available in a variety of shapes and widths.
Ceramic strip heaters are used in heating flat and slightly curved surfaces. The application of these heaters includes hot plates, hot stamps, hot sealing equipment, kettles, ovens, food warmers, incubators, and others.
The types of ceramic space heaters based on this application and the heat transfer mechanism involved in delivering heat to the surroundings are the following:
Convective ceramic space heaters consist of a ceramic heating element embedded in aluminum fins and baffles. The heat is transferred to the surroundings by convection; the cool air molecules will go down while the hot air molecules will rise above them. Convective heating is accomplished by drawing cool air from the surroundings and forcing it against the heated parts, causing the air to gain heat and raise its temperature. A fan is installed to heat the room quickly, which blows and disperses the heated air into the space. The heated air touches the objects in the surroundings of the convective ceramic space heater, which gives a warm feeling to the persons nearby.
Radiative ceramic space heaters use ceramic plates as a heating element that transfers heat directly to the objects in proximity by radiation. Radiative heat transfer involves the propagation of electromagnetic waves which carry thermal energy through space. The electromagnetic waves emitted by all radiative space heaters are not harmful; radiative space heaters are even beneficial to human health.
Unlike conductive ceramic heaters, radiative ceramic space heaters do not utilize fans or blowers. Instead of heating the ambient air first before warming the objects, radiative ceramic heaters heat the surrounding objects with lower temperatures directly. Hence, the warmth is felt more quickly. Radiative ceramic space heaters give off more natural and long-lasting heat and do not increase the humidity level and reduce the oxygen content of ambient air and promote the growth of molds and mildews.
The common styles of ceramic space heaters are:
Immersion heaters are intended and designed for heating liquids and gasses directly in tanks and vessels. These heaters consist of a set of tubular heating elements that are bent in a hairpin shape. The tubular heating elements of flanged immersion heaters also consist of a resistance wire packed in a bed of ceramic insulators contained in a sheath. The fluids come in contact with the metal sheath and increase their temperature through convective heating.
The material for the metal sheath can be selected to ensure compatibility with the liquid to be heated:
Fluids for Different Sheath Metal Materials Sheath Material Fluid Copper Potable Water Steel Oils, Gasoline, and Fuels Stainless Steel Mild Acids, Deionized and RO water, and Process water Incoloy 800 Water, Mild alkaline solutions, Air, and Gases Incoloy 600 Water, Strong alkaline solutions, and High temperature air and Gases Titanium Seawater, Alkaline solutions, and some Acid SolutionsTo offer better control and monitoring of the heating process, these heaters can be equipped with thermostats, thermocouples, and RTD sensors.
The types of immersion heaters based on their installation are the following:
Mica band heaters are ceramic band heaters that utilize mica as an insulating material. Mica is a group of silicate minerals that are characterized by their softness and lightweight. Sheet and block mica are used as electrical and thermal insulators.
Mica band heaters consist of a resistance wire ribbon wound around mica insulation. The mica insulation is bent over a die to form a circular band. The circular mica sheet, together with the encapsulated resistance wire, is then contained in a stainless steel or aluminum sheath. Mica band heaters are used in heating the contents of cylinders, nozzles, and pipes. They also can provide heat in injection molding, blow molding, and extruding machines.
Mica strip heaters are constructed by embedding resistance wire ribbons inside mica insulation and encapsulating these components inside a metal sheath. Like any strip heaters, they provide heat to flat and slightly curved surfaces.
Radiant heaters can have a ceramic heating element that transfers heat across space through electromagnetic waves. Those electromagnetic waves have thermal energy associated with them, depending on their frequency and wavelength. The heat produced by these heaters is directed from the heating element to the object or product without using a convective medium such as air. The reflectors of radiant heaters are designed such that the waves strike the product optimally.
Radiant heaters come in the form of panels and sheathed heating elements. The applications of radiant heaters include drying and curing of paints and powders, melting and thawing of food products, and heating plastic sheets for thermoforming.
Tubular heaters consist of a resistance wire packed with a ceramic insulator which is contained inside a tubular metal sheath body. The ceramic insulator has high dielectric strength and thermal conductivity. It provides mechanical stability to the resistance wire and protects it from oxidation and corrosion. The insulator also makes the tubular heater safer, effective, and reduces the risk of fire.
The resistance wire is typically made from a premium-grade nichrome. It is coiled and connected to a terminal pin to ensure a positive electrical connection. The current flows through the resistance wire. The heat generated from the resistive heating of the wire is conducted across the bed of insulation (typically made of magnesium oxide) to the outer sheath. The heat on the surface of the sheath is transferred to the surroundings by either conduction, convection, or radiation. For radiative heat transfer, quartz is the preferred insulating material.
Tubular heaters are versatile electric heaters and can be customized by the manufacturer depending on the application. They are bent or coiled in various forms. Tubular heaters are used in heating liquids, air, gasses, and oils. They can be found in soldering and desoldering equipment, dehumidifiers, heat sealing tools, copiers, valve heaters, and space heaters.
The advantages of ceramic heaters are the following:
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