Evacuated powders

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. ... 

Foam Insulation Since foams are not homogeneous materials, their apparent thermal conductivity is dependent upon the bulk density of tne insulation, the gas used to foam the insulation, and the mean temperature of the insulation. Heat conduction through a foam is determined by convection and radiation within the cells and by conduction in the solid structure. Evacuation of a foam is effective in reducing its thermal conductivity, indicating a partially open cellular structure, but the resulting values are stiU considerably higher than either multilayer or evacuated powder insulations. 

Thermal insulation is available over a wide range of temperatures, from near absolute zero (-273 C) ( 59.4°F) to perhaps 3,(1()0°C (5,432°F). Applications include residential and commercial buildings, high- or low-temperature industrial processes, ground and air vehicles, and shipping containers. The materials and systems in use can be broadly characterized as air-filled fibrous or porous, cellular solids, closed-cell polymer foams containing a gas other than air, evacuated powder-filled panels, or reflective foil systems. 

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. 

The radiation contribution for highly evacuated powders near room temperature is larger than the solid-conduction contribution to the total heat transfer rate. On the other hand, the radiant contribution is smaller than the solid-conduction contribution for temperatures between 77 and 20 or 4 K. Thus, evacuated powders can be superior to vacuum alone (for insulation thicknesses greater than 0.1 m) for heat transfer between ambient and liquid nitrogen temperatures. Conversely, since solid conduction becomes predominant at lower temperatures, it is usually more advantageous to use vacuum alone for reducing heat transfer between two cryogenic temperatures. 

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. 

In recent years there have been many new developments in sophisticated vacuum-insulation systems. These new developments have revolutionized the small-container field, where cryogenic fluids, down to and including helium, are now economically stored in small quantities. In addition, the superior thermal performance of these materials has made it possible to build trailers, trucks, and railroad tank cars with a much larger capacity than would be possible with evacuated powder systems. Multilayer insulations have been readily applied to factory-fabricated vessels utilizing conventional methods, such as spiral wrapping. 

Opacified evacuated powders - Contain metallic flakes to reduce radiation conform to irregular shapes. 

Values of apparent thermal conductivities,], of evacuated powders between room and liquid nitrogen temperatures are given in Table II. 

Table II. Apparent Thermal Conductivities of Evacuated Powders...

B. J. Hunter, R. H. Kropschot, J. E. Schrodt, and M. M. Fulk, "Metal Powder Additives in Evacuated-Powder Insulation," Advances in Cryogenic Engineering, VoL 5, K.D. Timmerhaus (ed.). Plenum Press, Inc., New York (1960). 

Metal Powder Additives in Evacuated-Powder Insulation (5) 146 Effect of Condensation on Conventionally Insulated Cryogenic Tanks (5) 157 A Study of Condensing-Vacuum Insulation (5) 162... 

Table 6.8 lists some thermal conductivities of evacuated powders used in large Dewars for LH2. Since the dielectric powders are transparent to room temperature radiation, the effectiveness of the powders can be increased significantly by the addition of metal powders (e.g., Al, Cu, Ag) which reflect the radiation. Since the metal powders conduct, their addition is an optimum at certain levels. 

Available insulations were divided into three main categories of solid, evacuated powder, and high vacuum, A... 

The evacuated powder type insulations considered were perlite, silica-aerogel, and peach pit charcoal. While the evacuated powder provides excellent insulation in certain cryogenic applications, it was not best adapted for this specific application due to very high installation cost. The factors responsible for these high costs are the additional number of filling and vacuum pumping ports which would be needed, plus the additional vacuum manifold which would be necessary for the full length of the transfer line. 

We have established that of the solid insulations and evacuated powders, glass fiber Type insulating material is... 

Answer by author Yes, but as pointed out in the paper, evacuated powder was not considered competitive because of cost, installation problems, etc. 

The choice of insulation for a particular application is usually a compromise in which such factors as economy, convenience, weight, ruggedness, and volume are considered along with the effectiveness of the insulation [11]. Vacuum insulation is perhaps the oldest means of achieving effective minimum heat transfer to cryogenic liquids [12]. In the past few years, however, many other types of insulation have been proposed and tried. These Include evacuated powders [13] (C-3), fibrous materials [14.15] (C-1, C-2, and C-6), solid foams [14-16] (F-1) and sup>erinsulations (D-1. D-2, D-3, D-4, and D-5). 

METAL POWDER ADDITIVES IN EVACUATED-POWDER INSULATION... 

The purpose of this paper is to discuss the results of some recent experiments using evacuated powders as insulation for very low temperatures. Particular emphasis is given to the mechanics of radiant heat transfer through evacuated powders to which metal particles of various sizes were added as opacifiers. A powder with an apparent mean thermal conductivity of 1.7 uw/cm- K between 300 and 20 K is reported. 

In recent years the increasing use of liquefied gases in industry, as rocket propellants, and as low-temperature baths, to mention only a few. and the practical results of transporting gases in liquid form, have necessitated a search for more efficient schemes for low-temperature insulation. Evacuated powders are one scheme that shows promise. Powder insulation, in either evacuated or unevacuated systems, refers to the use of materials with a low bulk solid conduction, in a finely divided state. Examples are silica aerogel (Santocel, Cab-0-Sil,Syloid), calcium silicate (Microcel), diatomaceous earths, perlite, etc. 

M.M.Fulk, "Evacuated powder insulation for low temperatures," in Progress in Cryogenics, K. Mendelssohn (ed.) Academic Press, New York (1959). 

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