Heating sheet is widely used in a variety of heating equipment, the core of which relies on the heating element, so what is the heating element? What are the properties of heating elements? This article will take you to understand.
1. What is a heating element?
A heating element is a material or device that converts electrical energy directly into heat or thermal energy through a principle called joule heating. Joule heating is a phenomenon in which a conductor generates heat due to the flow of electric current. When an electric current flows through the material, electrons or other charge carriers collide with the ions or atoms of the conductor, creating friction at the atomic scale. This friction then manifests itself as heat. Joule's first law (Joule-Lenz law) is used to describe the heat generated by an electric current in a conductor. This is expressed as,
P = IV or P = I²R
According to these equations, the heat generated depends on the current, voltage, or resistance of the conductor material. In the design of the entire heating element, resistance is an important factor.
The principle of heating the original
Joule heating is evident in all conductive materials of varying intensity, except for a special material called a superconductor. In general, for conductive materials, less heat is generated because charge carriers flow through easily; For materials with high resistance, more heat will be generated. Superconductors, on the other hand, allow current to flow without generating any heat. In general, heat from a conductor is classified as energy loss. The electrical energy used to drive power equipment generates unnecessary heat in the form of transmission losses and ultimately does not produce any useful work.
In a sense, the efficiency of the electric heating element is almost 100%, since all the energy supplied is converted to its intended form. The heating element not only conducts heat, but also transfers energy through light and radiation. However, this only applies to some ideal resistors. The material's inherent capacitance and inductance convert electrical energy into electric and magnetic fields, respectively, resulting in slight losses. Considering the entire heater system, the loss comes from the heat dissipated from the process fluid or the heater itself to the external environment. Therefore, the system must be isolated to utilize all the heat generated.
Second, the heating element properties
When current passes through, almost all conductors can generate heat. However, not all conductors are suitable for heating elements. The right combination of electrical, mechanical and chemical properties is required. The following are some of the features that are important for heating element design.
Resistivity: To generate heat, the heating element must have sufficient resistance. However, the resistance cannot be high enough to become an insulator. Resistance is equal to resistivity multiplied by conductor length divided by conductor cross-section. For a given cross-section, in order to obtain a shorter conductor, a material with high resistivity is used.
Oxidation resistance: Heat usually accelerates the oxidation of metals and ceramics. Oxidation consumes the heating element, reducing its capacity or damaging its structure. This limits the life of the heating element. For metal heating elements, alloys are formed with oxides, which help resist oxidation by forming a passivation layer. For ceramic heating elements, protective anti-oxidation scale of SiO2 or Al2O3 is the most common. Types of heating elements that are not suitable for use in oxidizing environments, such as graphite, are most commonly used in vacuum furnaces, or furnaces containing non-oxidizing atmosphere gases such as H2, N2, Ar, or He, where there is no air in the heating chamber.
Temperature coefficient of resistance: Note that the resistivity of the material changes with temperature. In most conductors, the resistance increases as the temperature increases. This phenomenon affects some materials more pronounced than others. The high temperature coefficient of resistance is mainly used in thermal applications. For fever, it is usually preferable to use a lower value. Although changes in resistance can be accurately predicted in some cases, a sharp increase in resistance is required to provide more power. To adapt the system to changing resistivity, control or feedback systems are employed.
Mechanical properties: Rigid heating elements deform when used at high temperatures. As the material approaches its melting or recrystallization stage, the material becomes more likely to weaken and deform compared to its state at room temperature. A good heating element retains its shape even at high temperatures. On the other hand, ductility is also an ideal mechanical property, especially for metal heating elements. Ductility enables a material to be drawn into a thread and shaped without affecting its tensile strength.
Melting point: In addition to the significantly increased temperature of oxidation, the melting point of a material also limits its operating temperature. Ceramics generally have a higher melting point than metal heaters.