Superinsulation

Thermal Insulation. In addition to their low thermal conductivity, as discussed in the section above, siUca aerogels can be prepared to be highly transparent in the visible spectmm region. Thus, they are promising materials as superinsulating window-spacer. To take further advantage of its... 

Superinsulation has been developed, primarily for use in space, where protection is needed against external temperatures near absolute zero. Superinsulation fabric consists of multiple sheets of aluminized mylar, each about 0.005 cm (about 0.002 in.) thick, and separated by thin spacers with about 20 to 40 layers per cm (about 50 to 100 layers per in.). 

More advanced insulations are also under development. These insulations, sometimes called superinsulations, have R that exceed 20 fthh-°F/Btu-m. This can be accomplished with encapsulated fine powders in an evacuated space. Superinsulations have been used commercially in the walls of refrigerators and freezers. The encapsulating film, which is usually plastic film, metallized film, or a combination, provides a barrier to the inward diffusion of air and water that would result in loss of the vacuum. The effective life of such insulations depends on the effectiveness of the encapsulating material. A number of powders, including silica, milled perlite, and calcium silicate powder, have been used as filler in evacuated superinsulations. In general, the smaller the particle size, the more effective and durable the insulation packet. Evacuated multilayer reflective insulations have been used in space applications in past years. 

Silica aerogels, a newly developing type of material, also have been produced as thermal insulations with superinsulation characteristics. The nanometer-size cells limit the gas phase conduction that can take place. The aerogels are transparent to visible light, so they have potential as window insulation. The use of superinsulations at present is limited by cost and the need to have a design that protects the evacuated packets or aerogels from mechanical damage. 

The typical volume of a dewar is 1001. Figure 5.1 shows the design of a commercial vacuum-superinsulated dewar for liquid 4He. Such dewars are made from aluminium or stainless steel. The evaporation rate of a good dewar is about 1% per day. 

To reduce the radiation power input, several solutions are possible inserting n thermally floating shields between the cold and hot surfaces, the transmitted power is reduced by a factor (n + 1). The practical realization is the so-called superinsulation used in the dewar of Fig. 5.1 a few layers of a thin metallized insulating foil about 4 xm thick is used. To prevent thermal contact between adjacent layers, the material is often corrugated or a thin layer of fibreglass cloth is inserted between layers. 

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. 

Some of them use two reservoirs (liquid N2 and 4He, see below), but most metallic dewars do no longer use LN2 (which produces vibrations in boiling) to cool radiative shields. Instead superinsulation is used. 

Physical situations that involve radiation with conduction are fairly common indeed. Examples include heat transfer through superinsulation made up of separated layers of very reflective material, heat transfer and temperature distributions in satellite and spacecraft structures, and heat transfer through the walls of a vacuum flask. 

A certain superinsulation material having a thermal conductivity of 2 x 0 4 W/m - °C is used to insulate a tank of liquid nitrogen that is maintained at - 320°F 85.8 Btu is required to vaporize each pound mass of nitrogen at this temperature. Assuming that the tank is a sphere having an inner diameter (ID) of 2 ft, estimate the amount of nitrogen vaporized per day for an insulation thickness of 1.0 in and an ambient temperature of 70°F. Assume that the outer temperature of the insulation is 70°F. 

A superinsulating material is to be constructed of polished aluminum sheets separated by a distance of 0.8 mm. The space between the sheets is sealed and evacuated to a pressure of 10"5 atm. Four sheets are used. The two outer sheets are maintained at 35 and 90°C and have a thickness of 0.75 mm, whereas the inner sheets have a thickness of 0.18 mm. Calculate the conduction and radiation transfer across the layered section per unit area. For this calculation, allow the inner sheets to float in the determination of the radiation heat transfer. Evaluate properties at 65°C. 

At very low cryogenic temperatures, the best insulation is a vacuum jacket that is silvered to eliminate radiative heat leaks. Various kinds of superinsulation exist and are used to store and transport volatile cyrogens like liquid helium. In one type a fine insulating powder is placed in the vacuum jacket in another type a metallized variety of Kapton film is used. 

Consider steady heat transfer between two large parallel plates at constant temperatures of Ti = 300 K and Tz - 200 K that are t = 1 cm apart, as shown in Fig. 1-41. Assuming the surfaces to be black (emissivity e = 1), determine the rate of heat transfer between the plates per unit surface area assuming the gap between the plates is (a) filled with atmospheric air, (b) evacuated, (c) (illed. vvith urethane insulation, and (d) filled with superinsulation that has an apparent thermal conductivity of 0 00002 W/m K... 

Discussion This example demonstrates the effectiveness of superinsulations andy xplains why they are tiie insulation of choice in critical applications de spite theifvhigh cost. 

SlC Why is the thermal conductivity of superinsulation orders of magnitude tower than the thermal conductivity of ordinary insulation ... 

Consider a 3-m-diameter spherical tank that is initially filled with liquid nitrogen at 1 atm and I96°C. The tank is exposed to ambient air at I5°(. with a combined convection and radiation heal transfer coefficient of 35 W/m °C. The temperature of the thin-shellcd spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Determine the rate of evaporation of the liquid nitrogen in the tank as a result of the he.ii transfer from the ambient air if the tank is (<7) not insulated, h) insulated with 5-cm-thick fiberglass insulation k = 0.035 W/m C), and (c) insulated with 2-cm-lhick superinsulation which has an effective thermal conductivity of 0.00005 W/m C. 

Oil average, superinsulaled homes use just 15 percent of the fuel required to heat the same size conventional home built before the energy crisis in the 1970s. Write an essay on... 

Thus at least prima facie it seems that there seems to be no difficulty in principle in achieving c = 0, and even if not perfectly then for all practical purposes also b = 0. The main concern is whether or not a = 0 is achievable, namely whether or not superinsulation exists with respect to heat as it does with respect to electricity [23], Probably the best that we can do at this point is to admit that we do not know that this is an open question [23]. 

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