Types of Insulation

Tests on Cable Constructions. The Association of Edison Illumination Companies (AEIC) has approved an accelerated cable hfe test in which typical underground distribution power cables can be statistically compared based on their resistance to water treeing (number of days to fail). The comparison can be made by varying the type of insulation and/or other cable layers in an environment that contains hot water (90°C) under 8V/fi (200 V/mil) voltage stresses (four times the typical power cables operating voltages). 

Thermal Conductivity (K Factor) Depending on the type of insulation, the thermal conductivity K factor) can vary with age, manufacturer, moisture content, and temperature. Typical published values are shown in Fig. 11-65. Mean temperature is equal to the arithmetic average of me temperatures on both sides of the insulating material. 

Types of Insulation Ciyogenic insiilations have generally been divided into five general categories high vacuum, multilayer insulation, powder, foam, and specif insulations. Each is discussed in turn in the following sections. 

Multilayer Insulation Miiltilayer insulation consists of alternating layers of highly reflec ting material, such as aluminum foil or aluminized Mylar, and a low-conduc tivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net, all under high vacuum. When properly applied at the optimum density, this type of insulation can have an apparent thermal conduc tivity as low as 10 to 50 jlW/m-K between 20 and 300 K. 

Powder Insulation A method of reahzing some of the benefits of multiple floating shields without incurring the difficulties of awkward structural complexities is to use evacuated powder insulation. The penalty incurred in the use of this type of insulation, however, is a tenfold reduction in the overall thermal effectiveness of the insulation system over that obtained for multilayer insulation. In applications where this is not a serious factor, such as LNG storage facihties, and investment cost is of major concern, even unevacuated powder-insulation systems have found useful apphcations. The variation in apparent mean thermal conductivity of several powders as a function of interstitial gas pressure is shown in the familiar S-shaped curves of Fig. 11-121. ... 

The common types of insulating materials in use for electric motors are E and B for small motors and F for medium sized and large ones. General industrial practice, however, is to limit the temperature to class B limits, even if class F insulation is used. 

Temperature rise at the guaranteed output to ascertain the adequacy of the insulating material and life of the motor. If the temperature rise is more than permissible for the type of insulation used, it will deteriorate the insulating properties and cause thermal ageing. As a... 

The compounds so formed have excellent thermal stability and are self-extinguishing and even completely fire-retardant. Their properties are given in Table 13.14. A few common types of insulators and supports are shown in Figure 13.31. 

Thermal insulation in use today generally affects the flow of heat by conduction, convection, or radiation. The extent to which a given type of insulation affects each mechanism varies. In many cases an insulation provides resistance to heat flow because it contains air, a relatively low thermal conductivity gas. Ill general, solids conduct heat the best, liquids are less conductive, and gases are relatively poor heat conductors. Heat can move across an evacuated space by radiation but not by convection or conduction. 

Table 10-45 presents types of insulation material. Foo gives insulation thermal conductivity, k, at 100°F mean as ... 

Glass-reinforced aluminum foil with either a bright polished or white lacquer surface is utilized with most types of insulant. Primarily it is used as a vapor control layer or as a means of upgrading the fire properties of plastic foams, but it does give a semi-decorative finish to the insulation. It is therefore often use where the insulation is open to view but located away from direct risk of mechanical damage. 

It is not possible to detail all the many different types of insulation used in production, service and building industries. The following attempts to give a summary of the composition, properties and major areas of use of a representative range of insulation types. 

Provide a similar blanket of insulation on top of any suspended ceiling which is 300 mm below the flat roof or an external hot liquid spray-on type of insulation to the roof finish. 

The current-carrying capacity of the wire is not directly related to the dielectric. This is determined by the conductor resistance and the heating effect that it produces in the wire. The required current-carrying capacity determines the size of the wire and thus the size of the insulator. The temperature rise caused by the current flow determines the type of insulation to be used. If the wire is limited to 140°F (60°C) service, the insulation can be one of those discussed above. If the wire is to operate at 300°F (150° C), another specification for plastic wire with better heat resistance such as TP polyester or PTFE is used. 

There are a number of suggested and implemented ways to store the snow/ice. If the snow is stored underground it is not necessary with thermal insulation. Otherwise a more or less insulated building or insulation directly on the snow is needed. There are different types of insulations loose fill, sheets and superstructures, with different advantages and disadvantages. 

Both types of insulation act to suppress thermal radiation by the intermediate shield principle. The insulation also acts to reduce the effective cell size for any residual gas in the vacuum space, thereby suppressing the thermal conductivity of the gas. In a typical commercial superinsulated dewar, there are about 50 layers of superinsulation, corresponding to a thickness of about one inch. The first few layers are the most effective in the attenuation of thermal radiation however the subsequent layers are important for the suppression of thermal conductivity of any residual gas. One can define an effective thermal conductivity for these insulations, which in the case of superinsulation is about 10 6 W/(cmK) between 300 and 4K. 

The white matter is a covering for the nerve fiber known as the myelin sheath. Myelin is a sphingolipid produced by oligodendrocytes (Schwann cells). It acts as a special type of insulation which allows for a much more rapid transmission of the nerve signal. The myelin cover is interrupted periodically. This "bare" area is called a node of Ranvier. It is very important to rapid nerve conduction. See Figure 21. 

The grade determines whether the mica can be used in high technology elecironic instruments, e.g.. computer-aided tomography (CATl scan, or in low technology devices, e.g.. a toaster. Many types of insulators, as well as the base for electronic circuits, are formed from the high quality sheets of mica by a punch pressing operation. 

LNG tanks are all double-walled because an evacuated space is required to achieve sufficient insulation to keep the LNG from vaporizing too quickly. A hold time (time from when the tank is filled to when pressure build-up requires vapor to be vented) of at least five days is required for practical LNG tanks. In addition to the evacuated interstitial space, several different types of insulation may be used. The goal is to make the conductive, convective, and radiative heat transfer into the LNG as low as possible. 

The various types of insulation used in the storage and transfer of cryogenic liquids can be divided into five categories (1) vacuum, (2) multilayer, (3) powder and fibrous, (4) foam, and (5) special. The boundaries between these... 

The AIT of a substance may be reduced below ideal laboratory conditions by as much as 100-200°C for surfaces that are insulated with certain types of insulation, or are contaminated by dust. 

Determination of the most economical type of insulation and its optimum thickness. 

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