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The Ultimate Guide to Oscillators

Introduction Oscillators are a core electronics component that control the timing of devices, ranging from your watch to your laptop and mobile processors. In a nutshell, oscillators can generate AC voltage signals for different waves from a simple DC voltage source. This means they can create AC voltage graphs that represent square, rectangular, and sine waves from a plain DC voltage graph. In this guide, we will learn the definitions, types, working principles, and applications of oscillators. We will also explore the different terminology associated with electronic oscillators. Figure 1. Structure of An Oscillator Circuit Source: m.youtube.com What is an Oscillator? An oscillator is an electronic circuit or device that uses a DC voltage to generate a periodic AC signal of a desired frequency. The output of the oscillator can be either a sinusoidal or non-sinusoidal signal, and it can generate audio frequencies that range from 20 Hz to 20 kHz or radio frequencies that range from 100 kHz to 100 GHz. In simple terms, the oscillator circuit is like an amplifier that gives positive feedback because the input voltage makes a total angle of 360° with the input signal, meaning the input voltage is in phase with the input signal. The main characteristic of an oscillator is its ability to control a steady waveform over time. This key feature is achieved through a combination of energy storage devices such as capacitors or inductors and an amplification process that restores the energy lost due to resistance. Oscillators are usually categorized based on the following factors: The type of waveform produced: this includes sinusoidal, square, triangular, and non-sinusoidal waves. The range of operating frequency: this includes audio, microwave, infrared, and radio wave frequencies. The primary components in their structures: this includes amplifiers, capacitors, inductors, resistors, and feedback networks. Basic Principles of Oscillator Operation Figure 2. The Working Process of Oscillators Source: theengineeringknowledge.com When an input sinusoidal signal is applied to the amplifier in the above diagram, the input signal will be increased by the gain of the amplifier, and the output signal will be increased by the input signal. This means that the output signal is given as an input to the feedback circuit, which is a frequency selective circuit represented by VF. This VF can be written as Aβ × Input Voltage, where β is the feedback fraction of the output voltage. When the phase shift introduced by the amplifier and feedback circuit is zero, the feedback signal will be in phase with the input signal. As the feedback signal gets added to the input signal, the input signal will be removed from the circuit. In that case, the possibility of a sustained oscillation is dependent on the loop gain of the oscillator (Aβ). If the loop gain (Aβ) is less than 1, the amplitude of the input signal will reduce, and the oscillations in the circuit will not be sustained. Similarly, whenever Aβ is greater than 1, the oscillations in the circuit will not be stable due to an exceptional increase in amplitude. However, if Aβ = 1, the feedback signal (VF) will be equal to the input signal, and the oscillations will be sustained at the output. Therefore, the following are Barkhausen’s criteria for sustained oscillations: The product of the loop gain should be equal to 1. The phase shift of the loop gain should be equal to 0. To prove this mathematically, let's assume that the output of the feedback circuit is VF, the input to the amplifier is Vin + VF, and the output voltage is A × (Vin + VF). By substituting VF = β × Vout into the output expression, we obtain: Vout= A × Vin + Aβ × Vout. Therefore, Vout/Vin = A/(1-Aβ). If the value of Aβ in the circuit is 1 and the phase shift introduced by the loop gain is 0°, the oscillation will be sustained at a constant amplitude. Achieving Sustained Output Without Input Figure 3. Sustaining Colpitts Oscillator with Op-Amp Source: geeksforgeeks.org In the above diagram, a constant oscillation is achieved at the output without the help of an input signal. How is that possible? It is done with the help of an operational amplifier. The thermal noise in every circuit contains all frequency components ranging from a few Hz to even hundreds of GHz. Whenever the oscillator is turned on, all the frequency components of the thermal noise will get amplified by the amplifier, and out of all the frequency components, the phase shift for one particular frequency will be equal to zero, while all other frequencies have a different phase shift. The frequency with a zero-phase shift will be added to the input noise, and the noise signal of the frequency will increase with time. Once the signal reaches a certain voltage, the loop gain (Aβ) of the circuit will become 1 as a result of the non-linear behavior of the feedback loop. That is how sustained oscillation is reached in the absence of an input signal. Types of Oscillators Oscillators come in various forms, each designed for specific applications and frequency ranges. The main types include: RC oscillators Crystal oscillators LC oscillators Voltage-controlled oscillators Relaxation oscillators Now, let's discuss the first three types of oscillators in full detail. RC Oscillators Figure 4. RC Oscillator Circuit Source: electronics-tutorials.ws This oscillator contains an RC circuit that uses its phase shift to generate stable sine waves with a low audio frequency range of 100-200 Hz. It also contains a transistor and an inverting op-amp that amplifies the electronic signal of the feedback network. To sustain the oscillations in the RC oscillator, the amplifier and feedback network both provide a 180° phase shift, which results in an overall phase shift of 0°. By changing the gain of the amplifier and feedback circuit, it is possible to achieve a loop gain that is equal to 1. The transfer function of the RC oscillator is the relationship between the Laplace transform of its output and input voltage. Interestingly, this transfer function is somehow related to the phase shift of the oscillator. Let's derive an expression for the transfer function and phase shift of the RC oscillator by using “j” as a constant: Vout/Vin = R/R-jXC where capacitive reactance (Xc) = 1/ωC. By substituting the value of XC in the previous equation, we obtain: R/R-j(ωC )-1 = 1/1-j(ωCR)-1. Phase shift (ø) = 0 - tan-1 (-1/ωCR). Therefore, the phase shift = tan-1(XC/R). In practical RC oscillators, more than three stages of the RC circuit are used to obtain a stable phase shift. For example, if you use four stages of RC circuits, each stage will provide approximately 45° phase shift, and by combining all four stages, we can get a 180° phase shift. To calculate the maximum frequency obtained, we use the expression F = 1/2πRC√2N, where N is the number of RC stages used. Examples of RC oscillators include RC phase shift, twin-T, and Wien bridge oscillators. LC Oscillator Figure 5. LC Oscillator Circuit Source: brainkart.com The LC oscillator is a type of oscillator that is used for the generation of high-frequency signals, typically in the range of radio frequencies. The LC circuit is used in its feedback path because it oscillates at a resonant frequency of 800 to kHz. Whenever some finite voltage is applied to the LC tank circuit, its capacitor starts charging and reaches its peak in a few seconds. However, when the applied voltage is disconnected, the electrostatic energy of the capacitor will get stored across the inductor in the form of a magnetic field. According to Lenz law, the inductor produces a back emf so that the same amount of energy can continuously flow through it. As a result, the capacitor starts charging in the reverse direction, and the energy stored in the inductor will be converted into the electrostatic energy of the capacitor. Once the capacitor is fully charged, the same process discussed earlier will be repeated, and a sustained oscillation will be obtained in the output. In terms of phase shift, the LC oscillator is similar to the RC oscillator because the amplifier and feedback circuit both provide a 180° phase shift. This means that the overall phase shift of the LC oscillator is also 0°. The gain of the amplifier and feedback circuit is set in such a way that the loop gain (Aβ) = Vin × C2/C1. For the sustained oscillation, the loop gain is 1. Hence, Vin = C1/C2, and the feedback fraction = C2 /C1. Therefore, the frequency of the oscillation can be given by the expression: F = 1/2π√total LC. Where total LC = C1C2/C1+C2. Examples of LC oscillators include Armstrong, tuned base, Colpitts, and Hartley oscillators. Crystal Oscillator Figure 6. Crystal Oscillator Circuit Source: righto.com The crystal oscillator is a type of oscillator that uses piezoelectric crystals to provide a stable frequency reference. This is because the natural frequency changes regularly due to a change in temperature or a change in the power supply voltage. The piezoelectric crystal does not only provide a high level of stability; it also provides good selectivity for the crystal oscillator due to its high-quality factor. As a result, the crystal oscillator is used as an essential part of microcontrollers for generating clock signals. The stable frequencies generated by the piezoelectric crystal range from hundreds of kHz to even hundreds of MHz. The crystal oscillator works on the principle of the inverse piezoelectric effect, which states that whenever some external voltage is applied to certain materials, they produce mechanical deformation. This means that if we apply an AC signal of a certain frequency to the crystal oscillator, it will vibrate at the same frequency. Examples of naturally occurring crystals include rochelle salts, quartz, and tourmaline. Quartz is the cheapest crystal with medium piezoelectric activity and mechanical strength, which makes it the most preferred material in the crystal design. The electrically equivalent circuit of the quartz crystal is the RLC circuit, and it provides the most accurate frequency selectivity whenever it is used with the amplifier in the feedback circuit. The crystal oscillator is made up of series and parallel resonant frequencies. The series resonant frequency is given by Fs = 1/2π√LsCs, while the parallel resonant frequency is given by Fp = 1/2π√LsCtotal. Applications of Oscillators The following are the main applications of oscillators across different fields: Telecommunications: RF oscillators generate carrier signals for radio and television broadcasts. They also provide clock signals in digital communication systems. Tests and measurements: Relaxation oscillators are used as electric generators in electronics testing and measurement of frequency variables. Industrial control: Crystal oscillators are used in motor speed controls and the regulation of power inverters. Medical devices: The TX-508 temperature compensated crystal oscillator (TCXO) is used in the manufacture of ultrasound imaging devices due to its high frequency range and good sensitivity. Navigation systems: Oven-controlled crystal oscillators (OCXO) are used in GPS devices to provide precise timing for satellite ranging. Benefits of Oscillators Some of the main benefits of using oscillators include: Low-power design High frequency stability Efficient signal amplification Effective signal modulation and demodulation High-quality factor High durability Future Trends in Oscillator Technology As technology keeps advancing, the field of oscillator design continues to evolve because engineers are coming up with record-breaking ideas year in and year out. Some of the latest inventions in oscillator technology include: Microelectromechanical systems (MEMs) oscillators Photonic oscillators Software oscillators Optical oscillators Conclusion Oscillators provide the essential timing and frequency generation functions that support many modern technologies. In this guide, we have learned the definitions, working principles, types, and benefits of oscillators. We have also discussed the latest developments and future trends in oscillator technology. Feel free to explore our diverse range of oscillator products and discover how they can enhance your next project!

#Oscillators

Heating Elements

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Introduction

This article presents a comprehensive guide about heating elements. Read further to learn more about:

• What is a heating element and how does it work?
• Heating element properties
• Different heating element materials
• Types of heating elements
• And Much More...

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:

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• 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 (MoSi&#;) 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. MoSi&#; heating elements are manufactured through various energy-intensive methods, including mechanical alloying, combustion synthesis, shock synthesis, and hot isostatic pressing.

MoSi&#; 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, MoSi&#; 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 MoSi&#; 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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