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 employed across various industrial processes, including drying, boiling, molding, and melting. They are also commonly used for space heating. Known for their quick, safe, and clean heating capabilities, ceramic heaters offer an efficient solution for a range of applications.
This chapter will explore the fundamental scientific principles behind the design and operation of ceramic heaters, starting with resistive heating. Ceramic heaters operate based on resistive heating, also known as Joule or Ohmic heating. This phenomenon occurs when electrical current passes through a material, generating heat due to resistive losses. Essentially, resistive heating transforms electrical energy into thermal energy, enhancing the efficiency of electric heaters. However, this effect can be problematic in other contexts, such as in electric power transmission and distribution, and for most electronic devices and equipment.
Joule’s first law, also known as the Joule-Lenz law, mathematically describes the relationship between the thermal energy generated and the electrical parameters involved. According to this law, the heating power (P) is proportional to the product of the square of the current (I) and the resistance (R), expressed by the equation: P = I²R.
Resistive heating can be understood by examining what occurs at the molecular level within the material when current flows through it.
When an electric potential difference exists between two points in a conductor, an electric field is established. This field accelerates free electrons in the material, causing them to move from atom to atom and gain kinetic energy. Electrons travel from a region of higher potential to one of lower potential. The rate of this electron flow is known as the current, which is a key electrical parameter. Current is directly proportional to the voltage (V) – the potential difference between the two points – and inversely proportional to the resistance (R). This relationship is described by the equation: I = V/R.
As electrons move toward the lower potential, they collide with atoms, other electrons, and impurities within the material. These collisions cause vibrations in the material’s molecules. Additionally, there are opposing forces that resist the flow of electrons. The friction created by these collisions and resistances requires the electrons to perform work, which translates into heat. This heat is generated within the material (or heating element) and is used to raise the temperature of its surrounding environment.
Resistance is an extrinsic property of a material that describes its opposition to the flow of electrical current or electrons. As an extrinsic property, resistance depends on both the length (l) and the cross-sectional area (A) of the material, and can be calculated using the formula R = ρL/A. In this equation, ρ represents the resistivity, which is an intrinsic property and varies with the material's temperature.
All materials, except superconductors, exhibit some degree of electrical resistance. For a material to be effective as a heating element, it must have sufficient internal resistance. Materials with higher resistance impede the flow of current more effectively, thereby generating more heat. However, if the resistance is too high, the material may act as an insulator rather than a conductor, which is not desirable for heating applications.
Ceramic heaters transfer heat to their surroundings through three main mechanisms: conduction, convection, and radiation. Conductive heat transfer occurs when heat is transferred between two objects in direct contact with each other. Convection involves the transfer of heat between fluids (liquids or gases); in convective space heaters, air flows through the hot ceramic element, warming the surrounding air and increasing the ambient temperature. Radiative heat transfer involves emitting thermal energy through electromagnetic radiation directly to nearby objects or individuals.
The resistivity and resistance of ceramic materials change with temperature. If the resistance increases as the temperature rises, the material is said to have a positive temperature coefficient (PTC). Ceramics, being semiconducting materials, exhibit this PTC behavior.
As the ceramic heating element reaches its setpoint temperature due to electrical current, its resistance increases significantly, eventually reaching a point where it effectively halts the current flow and heat production. The setpoint temperature is determined by the ceramic's composition. This allows ceramic heaters to adjust their heat output based on the ambient temperature, producing less heat in warmer environments. Consequently, ceramic heaters provide only the necessary heat without excessively raising the surrounding temperature. This self-regulating characteristic enhances their safety compared to metal heating elements, which lack such automatic adjustment.
The various types of ceramic heaters include:
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 pre-drilled holes in dies, molds, and platens to provide localized heating. For larger cartridge heaters, bigger holes can be drilled. They are also suitable for immersion heating applications. Despite their compact size, cartridge heaters produce significant amounts of heat and are utilized in various fields including 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 designed to heat the contents of cylindrical tanks and vessels by applying heat to their outer walls. They are secured around the tank using a barrel nut, which can be adjusted to accommodate slight variations in the tank's diameter. These heaters come in both one-piece and two-piece constructions and are available in various clamping designs to ensure a secure fit around the tank's exterior. Ceramic band heaters are particularly effective for heating curved surfaces.
They are commonly employed in applications such as heating the barrels of plastic injection molding machines, extruders, and blow molding equipment, where they melt feed resins efficiently.
Space heaters are devices designed to warm small to medium-sized enclosed areas. They can supplement central heating systems or provide heat in large spaces or facilities. Space heaters come in various types, utilizing different technologies and power sources, including both fuel and electricity.
Ceramic heaters are a popular choice for space heating due to their quick, clean, and efficient performance. They are compact and portable, allowing them to be easily moved and placed anywhere in a room, provided there is an electrical outlet nearby. With the right specifications, a ceramic space heater can rapidly warm an entire enclosed space within minutes. Their convenience and effectiveness make ceramic space heaters a favored option for homes, offices, and commercial environments.
Ceramic strip heaters feature a resistance wire coil encapsulated within a ceramic core, which is filled with magnesium oxide to maximize heat transfer. These components are enclosed in a metal sheath. Ceramic strip heaters are known for their thin, lightweight design and are available in various shapes and widths.
These heaters are ideal for heating flat and slightly curved surfaces. Common applications include hot plates, hot stamps, hot sealing equipment, kettles, ovens, food warmers, and incubators, among others.
Based on their application and heat transfer mechanisms, ceramic space heaters can be categorized into the following types:
Convective ceramic space heaters feature a ceramic heating element mounted on aluminum fins and baffles. They operate by transferring heat through convection, where cool air is drawn in, heated by the ceramic element, and then circulated as warm air rises. A built-in fan helps distribute the heated air more rapidly throughout the room, creating a comfortable warmth for nearby occupants.
Radiative ceramic space heaters use ceramic plates to emit heat directly to nearby objects through radiation. This method involves the propagation of electromagnetic waves that carry thermal energy. These electromagnetic waves are not harmful; in fact, radiative heaters can be beneficial to human health.
Unlike convective ceramic heaters, radiative heaters do not rely on fans or blowers. Instead, they heat objects directly, bypassing the need to first warm the surrounding air. This results in a quicker sense of warmth. Radiative ceramic heaters provide a more natural and long-lasting heat without increasing humidity or reducing oxygen levels, thereby preventing mold and mildew growth.
Common styles of ceramic space heaters include:
Immersion heaters are specifically designed for heating liquids and gases directly within tanks and vessels. These heaters typically feature a series of tubular heating elements bent into a hairpin shape. For flanged immersion heaters, the tubular elements consist of a resistance wire encased in ceramic insulation, all contained within a metal sheath. As the fluid comes into contact with the metal sheath, it heats up through convective heating.
The metal sheath material can be selected to ensure compatibility with the liquid being 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 Solutions |
For improved control and monitoring of the heating process, these heaters can be fitted with thermostats, thermocouples, and RTD sensors.
Immersion heaters can be categorized based on their installation method as follows:
Mica band heaters are a type of ceramic band heater that use mica as an insulating material. Mica, a group of silicate minerals known for their softness and lightweight properties, is utilized for its electrical and thermal insulating qualities.
In mica band heaters, a resistance wire ribbon is wound around mica insulation. This mica insulation is then shaped into a circular band using a die. The circular mica sheet, along with the resistance wire, is enclosed within a stainless steel or aluminum sheath. Mica band heaters are commonly used to heat the contents of cylinders, nozzles, and pipes. They are also employed in various applications, including injection molding, blow molding, and extrusion machines.
Mica strip heaters are made by embedding resistance wire ribbons within mica insulation and enclosing these elements inside a metal sheath. Similar to other strip heaters, they are designed to deliver heat to flat and slightly curved surfaces.
Radiant heaters feature ceramic elements that emit heat via electromagnetic waves. These waves carry thermal energy, determined by their frequency and wavelength, and directly transfer heat to objects or products without relying on a convective medium like air. The reflectors in radiant heaters are designed to optimize the direction and intensity of these waves to enhance heating efficiency.
Radiant heaters are available in forms such as panels and sheathed heating elements. They are used in various applications, including the drying and curing of paints and powders, melting and thawing of food products, and heating plastic sheets for thermoforming.
Tubular heaters feature a resistance wire encased in a ceramic insulator, all contained within a tubular metal sheath. The ceramic insulator, known for its high dielectric strength and excellent thermal conductivity, provides mechanical stability to the resistance wire while protecting it from oxidation and corrosion. This design not only enhances the safety and effectiveness of the tubular heater but also minimizes fire risks.
The resistance wire in tubular heaters is typically made from high-quality nichrome and is coiled and connected to terminal pins for a secure electrical connection. When current passes through the wire, it generates heat through resistive heating. This heat is then conducted through a bed of insulation—commonly magnesium oxide—toward the outer metal sheath. The heat on the sheath's surface is transferred to the surroundings via conduction, convection, or radiation. For radiant heat transfer applications, quartz is often used as the insulating material.
Tubular heaters are highly versatile and can be customized by manufacturers to fit specific applications. They can be bent or coiled into various shapes to suit different heating needs. Common applications include heating liquids, air, gases, and oils. Tubular heaters are used in diverse settings such as soldering and desoldering equipment, dehumidifiers, heat sealing tools, copiers, valve heaters, and space heaters.
The advantages of ceramic heaters include:
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...
A cartridge heater is a cylindrical tubular heating device that provides concise and precise heating for various forms of materials, machinery, and equipment. Unlike an immersion heater, a cartridge heater is inserted into a hole in the item to be heated to furnish internal radiant heat...
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