Insulation system

BE-7S67 Fibre optics for remote monitoring of structural integrity of elevated temperature insulated systems (FORMS) Mr. Manuel Gomea Eitrada Nacional... 

One may now consider how changes can be made in a system across an adiabatic wall. The first law of thermodynamics can now be stated as another generalization of experimental observation, but in an unfamiliar form the M/ork required to transform an adiabatic (thermally insulated) system, from a completely specified initial state to a completely specifiedfinal state is independent of the source of the work (mechanical, electrical, etc.) and independent of the nature of the adiabatic path. This is exactly what Joule observed the same amount of work, mechanical or electrical, was always required to bring an adiabatically enclosed volume of water from one temperature 0 to another 02. 

Air-Atmosphere Furnaces. These furnaces are appHed to processes where the workload can tolerate the oxidation that occurs at elevated temperatures in air. In some special appHcations, the oxidation is not only tolerable but is desired. Some furnaces heat the work solely to promote oxidation. Furnaces designed for air operation are not completely gas-tight which results in somewhat lower constmction costs. There are no particular problems encountered in selecting the insulation systems because almost all refractory insulations are made up of oxides. Heating element materials are readily available for the common temperature ranges used with air atmospheres. 

High Temperature Radiation Furnaces. These furnaces ate similar in constmction to medium temperature radiation furnaces, but operate above 1150°C. The insulation system must be designed to withstand the high temperatures, and internal stmctural parts become critical. 

Thermal Insulation Systems—A Survey, NASA Report SP-5027, NASA, Washington, D.C., 1967. 

Corrosion under insulation is also a concern, particularly in refrigeration systems. The specification of the insulation system needs to include painting, vapor barriers, and external metal jackets (16). 

The effective therm conductivity values generally obtained in practice are at least a factor of two greater than the one-dimensional thermal conductivity values measured in the laboratoiy with carefully controlled techniques. This degradation in insulation thermal performance is caused by the combined presence of edge exposure to isothermal boundaries, gaps, joints, or penetrations in the insulation blanket required for structure supports, fill and vent hnes, and high lateral thermal conductivity of these insulation systems. 

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. ... 

Low-loss system. In a low-loss system the winding also resides in a reservoir of liquid helium but a very efficient insulation system enables the magnet to operate for long periods, typically I year or more, between liquid helium refills. An important feature of these systems is that they are relatively immune to short-term electrical power failures, which has enabled complete reliability even in extremely difficult environments. 

Rolaling electrical machines. Functional evaluation of insulation systems. General guidelines 7816/1991 BS EN 600.34-18-1/1994 -... 

This comprises composite materials on mica, glass fibre and asbestos bases, impregnated or glued together with silicone resins or silicone elastomer. These materials must not contain any organic fibrous materials such as paper or cloth backing, which is covered under class B and even F insulation systems. 

Irrespective of the class of an insulation system and its quality, it will have some leakage current through its dielectric circuit on application of a high voltage. For all practical piiiposes. therefore, we can consider an insulation system as an imperfect capacitor. 

A low tan S will mean a high degree of resin cure. Most insulation systems are composites of many materials. In practice, they almost always contain small voids. Consider a coil side with a single void. The voltage distribution across the insulation will be non-uniform, due to different permittivities of air and insulation. When a low voltage is applied, a proportion of this will appear across the... 

Figure 9.6 Representation of leakage current in an HT insulation system...

Impulse voltage withstand test of the insulation system for rated voltages 2.4 kV and above for machines wound with formed coils 11/261... 

This is a test to monitor the quality and dielectric behaviour of the insulating system of high-voltage machines, 5 kV... 

Recommended practices for therinnl evaluati[Pg.271]

Guide for functional evaluation of insulation system for large HT machines. 

Renganian, S.. Agrawal, MD. and Nema, R.S. Behaviour of high voltage machine insulation system in the presence of thermal insulation and electrical stresses, IEEE Trans, on El El 20. No. I. Feb. (1985). 

The rated frequency, insulation systems and the requirement of creepage distances will generally remain the same as for a voltage transformer (Section 15.4,1). For the remaining parameters, the following may be noted. 

The choice of any of the cables mentioned in Table A16.1 w ill depend upon the site conditions, fault level and the voltage rating of the system. A brief comparison of all these insulating systems is given in Table A16.2... 

Industrial Power Engineering and Applications Handbook Table A16.2 Comparative study of different insulating systems... 

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