Thermal insulation

Conventionally, the thermal conductivity at room temperature is less than 0.2W / (m · K) of the material known as thermal insulation materials, or insulation materials, insulation materials. Some thermal insulation materials use their natural conditions, such as asbestos, cork and sawdust, but most of the thermal insulation materials are specially processed after the products, such as rock wool, slag cotton, microporous calcium silicate, foam concrete , Expanded perlite, expanded vermiculite and foam. These man-made thermal insulating materials are porous materials. The air in the pores is at rest and the thermal conductivity of the air is low, so the thermal conductivity of these materials is very low and has good thermal insulation properties. In the choice of thermal insulation materials, in addition to thermal insulation properties, but also must pay attention to the mechanical properties of materials, high temperature resistance and the ability to absorb moisture.
For the thermal insulation of the pipe, increasing the thickness of the insulation will increase the thermal resistance. However, the thickness of the insulation layer increases, so that the outer convection heat transfer surface area increases, which will reduce the convective heat transfer resistance, so thick insulation layer does not necessarily reduce the heat loss. The outer diameter of the smallest pipe insulation layer corresponding to the maximum total thermal resistance under the condition of a given thermal insulating material (ie, thermal conductivity k given) and a given convective heat transfer coefficient h is referred to as the critical insulation diameter, It is equal to 2k / h. Therefore, in the thermal insulation design of the pipe insulation layer diameter must be greater than the critical insulation diameter.
In order to store liquid nitrogen, liquid helium and other ultra-low temperature liquid, there is a very low temperature (less than -250 ℃) of the special thermal insulation technology. By the multi-layer interval of high reflectivity materials composed of adiabatic structure, and the use of the principle of the heat shield greatly weakened the radiation heat transfer between layers. The entire system is pumped into a vacuum to minimize air heat. The thermal conductivity of this super thermal insulation can be as low as 0.3 mW / (m · K). The heat shield is a thin plate that is added between the two radiative heat transfer surfaces. This layer of sheet in the whole system does not take any heat, but in the heat flow path to add a thermal resistance. The high reflectivity of the material made of the heat shield has a high thermal resistance. For example, a black plate with a blackness of 0.05 is provided between two parallel plates with a blackness of 0.8, which reduces the amount of heat transfer to 1/27. The installation of multi-layer shield effect is more significant. The principle of the heat shield is also widely used in high temperature furnace thermal insulation and so on.

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