Multilayer insulations

The U.S. military specification, M1L-P-27201B, requires 95% para content, 99.995% minimum hydrogen by difference, 50 vppm maximum total imputities, 9 vppm maximum combined nitrogen, water, and volatile hydrocarbons, 1 vppm maximum combined oxygen and argon, 39 vppm maximum helium, 1 vppm maximum carbon monoxide and dioxide, and a 10/40 micrometers nominal /absolute particulate filtration level. Liquid hydrogen is stored in double-walled vessels with evacuated pedite or multilayer insulation and transported in similarly insulated 50,000-L trailers or 900,000-L barges. 

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. 

For a highly evacuated (on the order of 1.3 X 10 Pa) multilayer insulation, heat is transferred primarily by radiation and solid conduction through the spacer material. The apparent thermal conductivity of the insiuation material under these conditions may be determined from... 

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. ... 

MLI A high-vacuum, multilayered insulation used in cryogen storage and transfer. 

The difficulties encountered with the use of multilayer insulation for complex structural storage and transfer systems can be minimized by the use of evacuated powder insulation. This substitution in insulation materials, however, incurs a 10-fold decrease in overall thermal effectiveness of the insulation system. Nevertheless, in applications where this is not a serious factor and investment cost is a major factor, even unevacuated powder insulation with still a lower thermal effectiveness may be the proper choice of insulating material. Such is the case for large LNG storage facilities. 

Cryogenic fluid transfer lines are generally classified as one of three types uninsulated, foam-insulated lines, and vacuum-insulated lines. The latter may entail vacuum insulation alone, evacuated powder insulation, or multilayer insulation. A vapor barrier must be applied to the outer surface of foam-insulated transfer lines to minimize the degradation of the insulation that occurs when water vapor and other condensables are permitted to diffuse through the insulation to the cold surface of the lines. 

Fig. 1-7 Apparent thermal conductivities of typical cryogenic insulation material (a) multilayer insulations (b) opacified powders (c) glass fibers (cf) powders (e) foams, powders, and fibers, according to Ref. 1. [1 Btu in/h ft2 ° F = 144 mW/m °C]...

Membranes from many sources have been studied. One of these is myelin, the multilayered insulation that surrounds the axons of many nerve cells. 

Kapton polyimide is used extensively on spacecraft in flexible substrates for lightweight, high-power solar arrays because of its inherent strength, temperature stability, excellent insulation properties, UV stability and IR transparency. It is also used in conjunction with Teflon FEP in multilayer insulation blankets for thermal control insulation because of its superior optical properties, including low solar absorptance. In these multilayer insulation blankets, aluminium (or gold) is typically applied to Kapton due to its low emissivity [1]. 

Figure 1. Insulation design for pressure vessel. The figure shows a vacuum space for obtaining good performance from the multilayer insulation, instramentation for pressure, temperature and level, and a vapor shield for reducing hydrogen evaporative losses.

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