Insulations are used to decrease heat flow and to decrease surface temperatures. These materials are found in a variety of forms, typically loose fill, batt, and rigid. Even a gas, like air, can be a good insulator if it can be kept from moving when it is heated or cooled.

A vacuum is an excellent insulator. Usually, though, the engineering approach to insulation is the addition of a low-conducting material to the surface.

While there are many chemical forms, costs, and maximum operating temperatures of common forms of insulations, it seems that when a higher operating temperature is required, many times the thermal conductivity and cost of the insulation will also be higher.

Loose-fill insulations include such materials as milled alumina-silica (maximum operating temperature of 1260 deg C and thermal conductivities in the range of 0.1 to 0.2 W/m2 K) and perlite (maximum operating temperature of 980 deg C and thermal conductivities in the range of 0.05 to 1.5 W/m2 K).

Batt-type insulations include one of the more common types — glass fiber. This type of insulation comes in a variety of densities, which, in turn, have a profound affect on the thermal conductivity. Thermal conductivities for glass fiber insulations can range from about 0.03 to 0.06 W/m2K.

Rigid insulations show a very wide range of forms and performance characteristics. For example, a rigid insulation in foam form, polyurethane, is very lightweight, shows a very low thermal conductivity (about 0.02 W/m2 K), but has a maximum operating temperature only up to about 120 deg C.

Rigid insulations in refractory form show quite different characteristics. For example, high-alumina brick is quite dense, has a thermal conductivity of about 2 W/m2 K, but can remain operational to temperatures around 1760 deg C.

Many insulations are characterized in the book edited by Guyer (1989). Often, commercial insulation systems designed for high-temperature operation use a layered approach.

Temperature tolerance may be critical. Perhaps a refractory is applied in the highest temperature region, an intermediate-temperature foam insulation is used in the middle section, and a high-performance, low temperature insulation is used on the outer side near ambient conditions.

Analyses can be performed including the effects of temperature variations of thermal conductivity. However, the most frequent approach is to assume that the thermal conductivity is constant at some temperature between the two extremes experienced by the insulation.

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