liquefied natural gas (Liquefied Natural Gas) is "LNG". After a series of treatments such as low temperature condensation, natural gas in the ultra-low temperature (-162℃) atmospheric pressure state liquefaction, become liquefied natural gas.

 The application of LNG has a special significance for China. The low price, cleanliness, flexibility and many advantages of LNG are needed by China's economic development. The internationalization of LNG is needed by China's strategic energy security, and LNG itself is playing a huge role in optimizing China's energy structure and rational new layout.

For minus 162℃ of LNG, its transportation and storage have always been a difficult problem. The emergence of vacuum low temperature insulation technology provides a technical guarantee for the safe storage and transportation and reliable use of low temperature liquid. The principle and classification of this technique will be briefly introduced.

 The purpose of adiabatic heat

 The purpose of low-temperature adiabation is mainly to reduce the heat conduction from the environment and LNG, that is, to reduce the cold loss. For cryogenic liquid storage and transportation equipment, it can reduce the loss of gasification, or create conditions for long-term storage and long-distance transportation. Especially for LNG liquefaction units and LNG storage containers, if there is no good insulation, liquefaction and storage are difficult to achieve.

Therefore, low temperature adiabatic has not only economic but also technical significance. In addition, the equipment and pipeline can also avoid dew and frost on the outer surface, and can also avoid the feeling of human skin is cold and even frostbite in contact with it. This is necessary to improve the working conditions and to prevent accidents.

 Classification of the vacuum for adiabatic conditions

 Low temperature insulation is divided into ordinary insulation and vacuum insulation. Ordinary insulation is a use of earlier traditional insulation method, it is in the equipment, container, pipe outside of laying solid porous insulation material, and in the gap of the insulation material is filled with air under atmospheric pressure (or other gas). The adiabatic performance of this insulation method is poor, but its structure is simple and the cost is low, so it is widely used in the case that the insulation requirements are not high. Now the pipeline of LNG liquid station uses this insulation method.

Vacuum insulation is the insulation structure made into a closed sandwich, the internal space to a certain degree of vacuum, to reduce the inflow of heat. There are three basic types of vacuum insulation: high vacuum insulation, vacuum porous insulation and multilayer vacuum insulation.

❶ high-vacuum insulation

Also known as pure vacuum adiabatic, it simply draws the sandwich space to 1.3810^-3Vacuum of the Pa. The common vacuum vacuum uses this insulation method. In the vacuum sandwich, there is only the radiation heat transfer between the two walls and the heat conduction of the residual gas in the sandwich. If the inner surface of the sandwich is polished, or coated with a layer of reflective material, a vacuum sandwich with good reflective performance is formed, and the radiation heat transfer can be reduced. This adiabatic structure is not very good, but its simple structure, light weight and small heat capacity has been widely used since the beginning of the last century.

❷ Vacuum and porous thermal insulation

 It is filled with a porous insulation material, and then pumped to a certain vacuum. Considering from the convenience of the process, it is generally filled with powder material or fiber material, so this thermal insulation mode is also called vacuum powder insulation or vacuum fiber insulation.

The adiabatic material in the vacuum sandwich weakens the radiation heat transfer between the walls, so its adiabatic performance is better than the high vacuum adiabatic, the requirement of vacuum can be reduced, generally up to about 1.33Pa. If a certain proportion of highly reflective metal powder, such as aluminum powder or copper powder, is added to the powder or fiber material, to reduce the radiation heat transfer between the powder particles or fiber in the material is called light resistance, the thermal insulation performance can be greatly improved. Vacuum powder and vacuum fiber insulation are now widely used in the storage and transportation equipment of LNG liquid.

❸ Multilayer vacuum is adiabatic

 It is embedded in a vacuum sandwich with a number of radiation screens, used to reduce radiation heat transfer between the walls. There are two basic forms of multilayer vacuum insulation, one is to use metal foil as a radiation screen, the screen is filled with low thermal conductivity spacing; one is to use single-sided aluminum spray polyester film as a radiation screen, and pressed into ripples to form a concave and convex shape to reduce the contact heat transfer between the screen. Multilayer vacuum insulation is one of the best insulation methods with current insulation performance, which is mostly used in LNG, liquid hydrogen and liquid helium storage and transportation containers, and is also used in small liquid oxygen and liquid nitrogen containers. Its insulation performance and safety are the guarantee of safe storage and transportation and reliable use of low temperature liquid.

 In the manufacture of a multilayer vacuum insulation vessel, the performance of the vacuum system is to achieve high vacuum (less than 10^-3Pa) the key of vacuum process is one of the bottleneck technologies of manufacturing high quality high vacuum multi-layer insulation vessel. In view of the sandwich vacuum of multilayer vacuum insulation vessel, there have been many studies on the processes involving helium mass spectrum leak detection, internal / external heating, vacuum extraction, nitrogen replacement and so on, and many application achievements have been made in industrial production.

 The structure of multi-layer vacuum insulation vessel is shown in the following figure, mainly including inner bladder, shell, insulation layer, liquid level gauge, pipeline valve and other structures. When manufacturing, weld the inner tank and liquid level meter, and then wrap the inner tank and fit with the shell; after closing, vacuum the dissection space between the inner tank and the shell until the vacuum degree is reached.

In order to obtain the high vacuum of the low temperature insulation vessel, the vacuum system is mainly used for heating and vacuum inside the low temperature storage tank. Internal heating can increase the kinetic energy of the sandwich gas and accelerate the release of the inner bladder, shell and sandwich insulation material. In the vacuum process, the vacuum pump and diffusion pump are used to pump the container interlayer until the process vacuum requirements are met.

 Evaluation of the adiabatic performance

 The adiabatic performance of the adiabatic structure can be evaluated by its effective thermal conductivity (or apparent thermal conductivity, including convection and radiation heat transfer). The smaller the value, the better the adiabatic performance.

The following figure shows the variation range of the effective thermal conductivity of the various adiabatic modes. As can be seen from the figure, the multilayer vacuum insulation has much higher thermal insulation performance than other thermal insulation modes, while the non-vacuum insulation mode has the worst thermal insulation performance.