Evacuated insulation

Liquid water at 25°C is introduced into an evacuated, insulated vessel. Identify the signs of the following thermodynamic functions for the process that occurs AH, AS, AG, ATwater, AS SUIT, ASumv. 

Liquid water at 25°C is introduced into an evacuated, insulated... 

Caps R, Heinemann U, Ehrmaimtraut M, Fricke J (2001) Evacuated insulation panels filled with pyrogenic silica powders — Properties and Applications, High Temperatures - High Pressures 33(2) 151-156... 

The storage vessel is insulated either by a pure vacuum alone or by one of several evacuated insulations such as perlite, santocel, fiberglas or a combination of fiberglas and metal foil. Li recent months improvements of the latter type of insulation have resulted in superior insulating systems which are fabricated by custom lamination of various reflectors and separating materials. The typical storage vessel is shown in Fig. 2. 

Work done by a system (Figure 6.3A, next page). Consider the reaction between zinc and hydrochloric acid as it takes place in a nearly evacuated, insulated container attached to a piston-cylinder assembly. We define the system as the reaction mixture, and the surroundings as the container, piston-cylinder, outside air, and so forth. In the initial state, the internal energy is the energy of the reactants (metallic zinc and... 

Transportation and Storage of H2 Table 6.8 Thermal conductivities (>-) of some evacuated insulating powders 99... 

Batteries developed for electric vehicles employed evacuated insulation to minimize thickness and weight. Both ABB and SPL used a double-walled, evacuated thermal enclosures with either a fiber board or microporous insulation. Chemical gettering agents were placed within the enclosure to maintain the needed levels of vacuum. This type of system was the only identified design that adequately minimized heat loss while providing the necessary load-bearing capability. 

Fleinemann, U. (2008) Influence of water on the total heat transfer in evacuated insulations. Int.. Thermophys., 29, 735-749. 

We have investigated the possibility of making an evacuated insulation in flat panel form. Because in many insulations a large portion of the thermal conductivity is contributed by air, its removal from the insulation would give substantial improvement in insulation efficiency. The problem was to make a vacuum space in the form of a flat panel. The walls of the vacuum space required strong supports against the external atmospheric pressure, and the solid conduction of the supports had to be very low. 

In evacuated insulations one has only solid conduction and radiative heat transfer. The method of separately measuring these two components is illustrated in Fig. 5. This illustration consists of three parts. The experimental setup is shown schematically at the upper left. The temperature distribution across the glass fiber specimen for various amounts of radiation is illustrated on the right, and at the bottom are the equations which were used to solve for the two components of heat transfer — conduction by radiation and conduction by the solid glass fibers. Looking at the temperature—distance plot on the right, the upper solid curve represents the temperature distribution one would have if heat transfer were solely by radiation. In this case the temperature distribution is nonlinear... 

The second type, suitable for smaller tanks, is evacuated insulations. The latest versions are composed of multi-layer reflective insulation MLI wifli extremely low k values, provided the vacuum is maintained. 

Table 3.1 Effective thermal conductivities of gas-purged and evacuated insulations between 300 and 77 K, in mW/m K...

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. 

Storage tanks for large volumes (>1000 m ) of LNG, Hquid nitrogen and Hquid oxygen often use unevacuated pedite insulation that is pressuri2ed with bod-off gas from the tank s Hquid. Tanks for similar volumes of Hquid usually insulated with evacuated pedite. Smaller tanks for LNG, Hquid... 

At very low temperatures with hquid air and similar substances, the tank may have double walls with the interspace evacuated. The weh-known Dewar flask is an example. Large tanks and even pipe hues are now built this way. An alternative is to use double walls without vacuum but with an insulating material in the interspace. Perlite and plastic foams are two insulating materials employed in this way. Sometimes both insulation and vacuum are used. 

Vacuum Insulation Heat transport across an evacuated space (1.3 X lO"" Pa or lower), is by radiation and by conduction through the residual gas. The heat transfer by radiation generally is predominant and can be approximated by... 

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. ... 

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. 

Thermal insulation in use today generally affects the flow of heat by conduction, convection, or radiation. The extent to which a given type of insulation affects each mechanism varies. In many cases an insulation provides resistance to heat flow because it contains air, a relatively low thermal conductivity gas. Ill general, solids conduct heat the best, liquids are less conductive, and gases are relatively poor heat conductors. Heat can move across an evacuated space by radiation but not by convection or conduction. 

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. 

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