A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is independent of the direction of current.
Resistance wire: Metallic resistance heating elements may be wire or ribbon, straight or coiled. They are used in common heating devices like toasters and hair dryers, furnaces for industrial heating, floor heating, roof heating, pathway heating to melt snow, dryers, etc. The most common classes of materials used include:
Thick film heaters are a type of resistive heater that can be printed on a thin substrate. Thick film heaters exhibit various advantages over the conventional metal-sheathed resistance elements. In general, thick film elements are characterized by their low profile form factor, improved temperature uniformity, quick thermal response due to low thermal mass, low energy consumption, high watt density and wide range of voltage compatibility. Typically, thick film heaters are printed on flat substrates, as well as on tubes in different heater patterns. These heaters can attain watt densities of as high as 100 W/cm2 depending on the heat transfer conditions. The thick film heater patterns are highly customizable based on the sheet resistance of the printed resistor paste.
There are several conventional applications of thick film heaters. They can be used in griddles, waffle irons, stove-top electric heating, humidifiers, tea kettles, heat sealing devices, water heaters, clothes irons and steamers, hair straighteners, boilers, 3D printer heated beds, thermal print heads, glue guns, laboratory heating equipment, clothes dryers, baseboard heaters, warming trays, heat exchangers, deicing/defogging devices for car windshields, side mirrors, etc., refrigerator defrosting, etc.
For most applications, the thermal performance and temperature distribution are the two key design parameters. In order to avoid any hotspots and to maintain a uniform temperature distribution across a substrate, the circuit design can be optimized by changing the localized power density of the resistor circuit. An optimized heater design helps to control the heater output and modulate the local temperatures across the heater substrate. In cases where there is a requirement of 2 or more heating zones with different output power over a relatively small area, a thick film heater can be designed to achieve a zonal heating pattern on a single substrate.
Thick film heaters can largely be characterized under two subcategories- negative temperature coefficient (NTC) or positive temperature coefficient (PTC) based on the effect of temperature increase on the element's resistance. The NTC type heaters are characterized by a decrease in resistance as the heater temperature increases and thus have a higher output power at higher temperatures for a given input voltage. The PTC heaters behave in an opposite manner with an increase of resistance and decreasing heater power at elevated temperatures. This characteristic of the PTC heaters make them self regulating too, as their output power saturates at a fixed temperature. On the other hand, NTC type heaters generally require a thermostat or a thermocouple in order to control the heater runaway. These heaters are used in applications which require a quick ramp-up of heater temperature to a predetermined set-point as they are usually faster acting than the PTC type heaters.
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.
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 1,200C.
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.
This is a type of ceramic produced by recrystallization or reaction bonding of SiC grains at temperatures above 2,100C. 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 2,700C. 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 1,700C 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 2,000C. 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 500C. 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 500C.
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.
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... 59ce067264