Technology & Engineering
Insulators
Insulators are materials that do not conduct electricity easily. They are used to prevent the flow of electric current between conductors and to protect people and equipment from electric shocks. Examples of insulators include rubber, glass, and plastic.
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5 Key excerpts on "Insulators"
eBook - PDF- Stephan A. Roosa, Steve Doty, Wayne C. Turner(Authors)
- 2020(Publication Date)
- River Publishers(Publisher)
C HAPTER 15 I NSULATION S YSTEMS JAVIER A. MONT, PH.D., CEM Abbott Vascular Santa Clara, California MICHAEL R. HARRISON Manager, Engineering and Technical Services Johns-Manville Sales Corp. Denver, Colorado Thermal insulation is a mature technology that has evolved over the last decade. Insulation has an important role in energy management applications for buildings and thermal systems. The use of insulation is mandatory for the efficient operation of any hot or cold system. When insulation is used, the entire energy requirements of a system are reduced. Most insulation systems reduce the unwanted heat transfer, either loss or gain, by at least 90% as compared to bare surfaces exposed to atmosphere. Since insulation is vital to the energy-efficient operation of systems, the proper selection and applica-tion of that insulation is very important. This chapter describes the various insulation materials commonly used in industrial applications and explores the criteria used in selecting the proper types of Insulation prod-ucts. In addition, methods for determining the proper insulation thicknesses are discussed, accounting for the economic trade-offs between insulation costs and energy savings. 15.1 FUNDAMENTALS OF THERMAL INSULATION DESIGN THEORY The basic function of thermal insulation is to retard the flow of unwanted heat energy either to or from a specific location. To accomplish this, insulation products are specifically designed to minimize the three modes of heat transfer. The efficiency of an insulation is measured by an overall property called thermal conductivity. 15.1.1 Thermal Conductivity The thermal conductivity , or k value, is a measure of the amount of heat that passes through one square foot of one-inch-thick material in one hour when there is a temperature difference of 1°F across the insulation thick -ness.
eBook - PDF- Relva C. Buchanan(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
1 Ceramic Insulators Relva C. Buchanan University of Cincinnati Cincinnati, Ohio, U.S.A. 1. INTRODUCTION The primary function of Insulators in electrical circuits is to provide physical separation between conductors and to regulate or prevent current flow between them. Important other functions include providing mechanical support, heat dissi-pation, thermal shock resistance, and a chemically stable environment for the conductors. Ceramic materials that provide primarily these functions are classified as ceramic Insulators. They include most glasses, porcelains, oxide and nitride materials, as well as mica. The main advantage of ceramics as Insulators is their capability for high-temperature operation without hazardous degradation in chem-ical, mechanical, or dielectric properties. In particular, the ceramic Insulators must satisfy use specifications where high electrical resistance and dielectric strength, as well as relatively low dielectric constant and loss, are essential requirements [1-3]. Insulators of the type described above belong to the class of materials known as linear dielectrics. These are materials in which the electric displacement (D) increases in direct proportion to the electric field (E), where the proportional-ity constant is the relative dielectric constant (e r ), a characteristic material prop-erty. The relationship is given in Eq. (1): D = e 0 £ a = eo ZrE (1) 1 2 Buchanan The relative dielectric constant e r is a measure of the ability of the material to store charge, relative to the permittivity or dielectric constant of vacuum, given ase 0 = 8.85 X l(T 12 F/m [1]. In addition to the dielectric constant (e r ), three other material properties are important in determining the insulating characteristics of the material. These are the electrical (volume) resistivity (p), the dissipation factor (tan S), and the dielectric strength (DS).
eBook - PDF- Paul Rosenberg, Robert Gordon Middleton(Authors)
- 2004(Publication Date)
- Audel(Publisher)
Chapter 2 Conductors and Insulators A conductor is a substance that carries electric current. An insula-tor is a substance that does not carry electric current. Because no conductor is perfect, and because any conductor has at least a small amount of resistance, it is better to define a conductor as a substance with a very low resistance. We will also find that no insulator is per-fect, and because no insulator has an infinite resistance, it is better to define an insulator as a substance with a very high resistance. Therefore, conductors, resistors, and Insulators are all basically re-sistive substances. However, they are classified into different groups because a practical conductor has extremely low resistance, a load resistor has moderate resistance, and a good insulator has extremely high resistance. Classes of Conductors The substances listed in Table 2-1 have different conductivities. The best conductors are listed in the first column in the order of decreas-ing conductivity. For example, silver is the best conductor, lead has less conductivity, carbon has still less conductivity, moist earth is a poorer conductor than carbon, and slate has such a high resis-tance that it is called an insulator. Of the Insulators listed, dry air is the best. A high vacuum is a better insulator than dry air; how-ever, a vacuum can be used only in special devices such as rectifier tubes. Therefore, we will be concerned in this chapter only with Table 2-1 Conductors and Insulators Good Conductors Fair Conductors Insulators Silver Charcoal and coke Slate Copper Carbon Oils Aluminum Acid solutions Porcelain Zinc Sea water Dry paper Brass Saline solutions Silk Platinum Metallic ores Sealing wax Iron Living vegetable Ebonite Nickel substances Mica Tin Moist earth Glass Lead Dry air Note: In each column, the best conductor is at the top, the best insulator at the bottom. 35 36 Chapter 2 the more common Insulators used in electrical work.
eBook - PDFWarm Air Heating
International Series of Monographs in Heating, Ventilation and Refrigeration
- David Kut, N. S. Billington, E. Ower(Authors)
- 2014(Publication Date)
- Pergamon(Publisher)
CHAPTER 12 THERMAL INSULATION PRINCIPLES OF INSULATION The undesirable emission of heat from hot pipes and ducts must be avoided to conserve heat and fuel and to obviate discomfort. Usually, this requirement necessitates the application of thermal insulation. By definition, a thermal insulation material is one which offers a high resistance to the flow of heat, i.e. a material of low thermal conductivity. Insulating materials in most common use have a thermal conductivity of between 0.029 and 0.15 W/m °C (0.2 and 0.8 Btu in./hr ft 2 °F). It should be noted that the thermal conductivity of a material varies with temperature ; e.g. that of mineral wool varies from 0.03 W/m°C (0.25 Btu in./hr ft 2 °F) at a hot face temperature of 37.8°C (100°F) to 0.061 w/m°C (0.42 Btu in./hr ft 2 °F) at a hot face temperature of 371°C (700°F), and that of glass silk for these conditions from 0.039 to 0.096 W/m °C (0.27-0.655 Btu in./hr ft 2 °F). Traditional insulating materials are amosite asbestos, blue asbestos, plastic asbestos, cork, glass silk, 85 per cent magnesia, and mineral wool. More recently developed insulating materials for use with warm air heating systems are aluminium foil, expanded ebonite, expanded polystyrene, and Polyurethane. The thermal conductivity of still air at atmospheric temperature has the low value of 0.024 W/m °C (0.168 Btu/hr ft 2 °F); thermal insulating materials therefore generally rely on the presence of a large number of small pockets of still air to limit the flow of heat through the material. Thus the extension of glass into the fibres employed in the manufacture of glass fibre insulating slabs or mattresses results in the creation of very large numbers of air pockets within the insulating material, giving this a thermal conductivity of the order of 0.035 W/m °C (0.24 Btu in./hr ft 2 °F) at atmospheric temperature, compared to the equivalent thermal conductivity of solid glass of 0.69 W/m °C (4.8 Btu in./hr ft 2 °F).
eBook - PDF- George Murray, Charles V. White, Wolfgang Weise(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
309 10 Electrical Properties of Materials 10.1 INTRODUCTION The electron conductivity of solid materials gives an almost unambiguous way to classify them. Simply put, on the basis of electrical conductivity, materials are either Insulators, semiconductors, conductors, or superconductors. Superconduc-tors are a special class of materials that exhibit zero resistance below a certain temperature. They will not be considered here. The conductivity of all of the more common and widely used materials is shown in Figure 10.1. The range of conductivities is quite large. Where we draw the lines for these materials appears to be somewhat arbitrary, but we can define these three categories fairly precisely in terms of the number of electrons available for conduction. This number can be computed using the energy band structure for the valence electrons, a subject covered in the following section. Insulators and most polymers have a low conductivity because of their strong covalent bonds and the absence of free electrons, but in some polymers a conducting powder is mixed with the polymer to form a conducting composite. In a few others of the so-called conducting polymers, there exist some free electrons within the polymer structure, creating conductivity on the order of that found in crystalline semiconductors, and in some conducting polymers the conductivity approaches that of metals. There is a tremendously large variation in the conductivity of solids, being about a factor of 10 25 from conductors to Insulators. Ohm’s law can be used to express conductivity and its reciprocal, resistivity, which are not functions of specimen dimensions, and the conductance and resis-tance, which are functions of specimen dimensions. Resistance is related to resistivity by: R = ρ l /A where R = resistance ρ = resistivity (usually expressed in Ω · m) l = specimen length A = specimen area and in terms of conductivity, σ , σ = l /RA units are ( Ω · m) − 1
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