Thermal insulators aging

Selected references that include thermal insulation data arc listed in the bibliography at the end of this article. Table 2 contains nominal thermal resistivities of ten commonly used insulations. The thermal resistivities iii Table 2 are at 23.9°C (75°F) and include the effects of aging and settling. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

General-purpose polymers, being thermal insulators, cannot dissipate heat generated by mechanical work or by electronic devices and so on. If the temperature rises, the mechanical properties of the polymer decrease and ageing speeds up. Eventually, the temperature can reach the melting point. 

Translucent sandwich panels for the side-facades were also made of glass-fiber-reinforced polyester composites (see Figure 1.8). The sandwich panels consisted of two layers separated by a composite fiber sheet with trapezoidal corrugations. The surface of the facade panels was finished with fleeces that also provide resistance to aging and U V radiation. As the main function of these facade elements was thermal insulation, the sandwich panels were filled with aerogels. They were therefore able to provide a K-value of 0.4 W m with a panel thickness of only 50 mm [18]. In terms of building fire considerations, a sprinkler system was installed as an active fire protection. 

The effects of these parameters and cross-linking in polymer cable insulations, aged in radiation and thermal enviromnents, were investigated. The results were then used to recommend standards for an OIT methodology suited for practical use, including the nuclear power industiy. Techniques to estimate error in (O.l.T.) thermograms interpretation and reproducibility were also developed (Mason and Reynolds 1997). 

Even if supercritical drying permits to obtain perfectly monolithic and transparent sihca-based thermal insulators, the process to obtain such crack-free large plates remains stUl too far from industrial large-scale commercialization. This was the initial reason why different processes have been studied to develop subcritical routes to access rapid massive commercialization. Among the various studies, aging of the gels in silica precursor containing solution has permitted to reach room temperature thermal conductivities as low as... 

The ageing of polymer foams is due to a more complex mechanism because it can be produced by matrix degradation, by changes in gas composition or because of both. During ageing the foam structure and the main properties such as thermal conductivity in the case of thermal insulation, are modified. 

Thermal Conductivity and Aging. Thernial performance is governed by gas conduction and radiation (18—20). In most ceUular plastic insulations, radiation is reduced because normal densities of use ate 4-50 kg/m and the average cell size is <0.5 mm. For open-ceU and other materials containing air (at 24°C, 7 = 0.025 W/(m-K)) this results in total values of X at 0.029-0.0039 W/(m-K). 

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. 

The life of the insulation will also be affected by an excessive operating temperature. It is halved for every 11°C rise in temperature over its rated value and occurs when a machine is occasionally overloaded. Sometimes the size of the machine may be only marginal when it was initially chosen and with the passage of time, it may be required to perform duties that are too arduous. Every time the machine overheats, the insulation deteriorates, and this is called thermal ageing of insulation. Figure 9.1 illustrates an approximate reduction in life expectancy with a rise in operating temperature. 

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... 

Insulation systems were first classified according to the material used, and permissible temperatures were established based on the thermal aging characteristics of these materials. For example. Class B insulation was defined as inorganic materials such as mica and glass with organic binders 130°C was the allowable maximum operating temperature. The present definition of insulation system Class B stipulates that the system be proven. . by experience or accepted tests. .. to have adequate life expectancy at its rated temperature, such life expectancy to equal or... 

IEC 60216-4, Sections 1-3, Guide for the determination of thermal endurance properties of electrical insulating materials. Part 4 Ageing ovens, 1990-2000. 

Thermal ageing in air, followed by extrapolation of data to lower temperatures using Arrhenius formula, has been applied widely in the design of electrical insulation for use at service temperatures, typically between 80 °C and 150 °C. The methodology is defined in detail in IEC 60216 [9]. 

The American standard ASTM D-2307 [12] provides a test method for measuring Relative thermal endurance properties of film insulated magnet wire . The ageing philosophy is similar to that required by IEC 60216 [9] and is based on the Arrhenius ageing model. Oven ageing is carried out at three specific temperatures. The samples are twisted wire... 

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